فهرست مطالب

پژوهش های ژئوفیزیک کاربردی - سال چهارم شماره 2 (پیاپی 8، پاییز و زمستان 1397)
  • سال چهارم شماره 2 (پیاپی 8، پاییز و زمستان 1397)
  • تاریخ انتشار: 1397/08/09
  • تعداد عناوین: 20
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  • عارف زینال پور، رضا قائدرحمتی *، علی مرادزاده، محمدرضا رحمانی صفحات 171-186
    در این مقاله اکتشاف منابع زمین گرمایی در منطقه بوشلی استان اردبیل با استفاده از داده های مگنتوتلوریک (MT) مورد بررسی قرار گرفته است. این مطالعه بر اساس داده های 60 ایستگاه مگنتوتلوریک در منطقه ای به وسعت 90 کیلومترمربع در بوشلی، جنوب شرقی شهرستان نیر، صورت گرفته است. ابتدا پردازش داده ها با استفاده از الگوریتم هایی مبتنی بر روش های پایدار و مقاوم در برابر نوفه صورت گرفته است. سپس تحلیل ابعادی داده ها بر اساس پارامترهای چولگی، چولگی حساس به فاز، بیضی وارگی و اندیس های وزنی نرمال شده مورد توجه واقع شده است. بر اساس این تحلیل ها امتداد ساختارهای منطقه بیشتر دوبعدی و با جهت شمالی- جنوبی تشخیص داده شد. در مرحله بعد عملیات مدل سازی وارون یک و دوبعدی روی داده ها در طول 12 پروفیل انجام گرفته است. بر اساس مقاطع مقاومت ویژه به دست آمده از این مدل ها و همچنین اطلاعات زمین شناسی، ساختارهای احتمالی منطقه شناسایی و تفسیر شده اند. نتایج مقاطع یک و دوبعدی مقاومت ویژه با تلفیق اطلاعات زمین شناسی، یک سیستم زمین گرمایی را نشان می دهد. موقعیت سه بخش اصلی این سیستم شامل سنگ پوش، مخزن و منبع داغ زمین گرمایی به خوبی نشان داده شده است. بخش فوقانی این سیستم یک منطقه با مقاومت ویژه حدود 10 اهم متر به عنوان پوشش رسی مخزن به خوبی نشان داده شده است. مخزن زمین گرمایی با مقاومت ویژه بیشتر (حدود 100 اهم متر) ، در زیر این پوشش رسی قرار گرفته است. همچنین مقاطع مقاومت ویژه به دست آمده از مدل سازی دوبعدی داده ها، موقعیت منبع داغ زمین گرمایی را در زیر مخزن زمین گرمایی در عمق بیشتر از 3000 متر، به خوبی نشان می دهند. نتایج این مطالعه موقعیت مخزن زمین گرمایی را در منطقه جنوبی محدوده مورد نظر با کشیدگی به سمت جنوب منطقه نشان می دهد.
    کلیدواژگان: بوشلی جنوب سبلان، پردازش داده ها، داده های مقاومت ویژه و فاز، روش مگنتوتلوریک، مخزن زمین گرمایی، وارون سازی یک و دوبعدی
  • مصطفی عباسی *، علی غلامی صفحات 187-197
    در این مطالعهراهکاری برای وارون سازی خطی AVO در یک قالب بیزی ارائه گردیده است. هدف، به دست آوردن توزیع پسین سه پارامتر سرعت موج P، سرعت موج S و چگالی سنگ است. وارون سازی بکار رفته در این مطالعه بر مبنای مدل همامیختی و یک تقریب خطی از معادله زوپریتس به نام مدل تباین ضعیف است. در این مسئله با ارائه یک رابطه تحلیلی برای توزیع پسین پارامترهای هدف، یک روش کارآمد با تفکیک پذیری قابل قبول در وارون سازی تصادفی داده های لرزه ای فراهم شده است. به منظور بررسی هرچه بهتر عملکرد این روش، خروجی های آن با نتایج حاصل از وارون سازی همزمان پیش از برانبارش که یک روش رایج در وارون سازی داده های لرزه ای است؛ مقایسه شده است. آزمایش های انجام شده روی داده​های مصنوعی نشان می​دهد که این روش پارامترهای هدف را تقریبا به طور کامل بازیابی می کند. این روش همچنین روی داده​های واقعی مربوط به یک میدان نفتی در خلیج مکزیک نیز پیاده​سازی شده است؛ که نتایج حاصل از آن انطباق قابل​قبولی با داده​های چاه نشان می​دهد. به علاوه این که در مقایسه با روش وارون سازی هم زمان پیش از برانبارش، نتایج بسیار بهتری مخصوصا در مورد دو پارامتر سرعت موج S و چگالی ارائه می دهد.
    کلیدواژگان: وارون سازی پیش از برانبارش، وارون سازی بیزی، AVO خطی، وارون سازی همزمان پیش از برانبارش، تباین ضعیف، عدم قطعیت
  • رضا احمدی *، محمد صادق امیری بختیار صفحات 199-210
    اشباع شدگی آب (Sw) سنگ مخزن یکی از پارامترهای پتروفیزیکی مهم است؛ که تاثیر زیادی بر دقت تخمین میزان نفت اولیه مخزن دارد. به دلیل اهمیت زیاد این پارامتر در محاسبات اقتصادی توسعه مخزن، تعیین دقیق آن اجتناب ناپذیر است. در پژوهش حاضر برای تخمین این پارامتر، مدل رگرسیون ماشین بردار پشتیبان شامل 5 متغیر ورودی یعنی داده های چاه نگاری پرتو گامای طبیعی، تخلخل نوترونی، چگالی کپه ای سازند، زمان گذر امواج صوتی و مقاومت ویژه الکتریکی حقیقی و پارامتر Sw به عنوان تک خروجی برای سه حلقه چاه در یکی از میدان های نفتی بزرگ سازند آسماری واقع در جنوب غرب کشور ایران مورد استفاده قرار گرفته است. به منظور مقایسه نتایج تخمین با واقعیت به طور بصری، ستون چینه شناسی و اشباع شدگی آب و هیدروکربور سازند نیز توسط نرم افزار Geolog برای چاه های مورد مطالعه ترسیم شده است. از تعداد کل 1211 داده نقطه ای موجود برای سه حلقه چاه، حدود 80 درصد به عنوان داده های آموزشی و حدود 20 درصد به عنوان داده های آزمون انتخاب شدند. عملکرد الگوریتم از طریق اعتبارسنجی متقابل بر اساس معیارهای مختلف همانند ترسیم نمودار پراکندگی مقادیر اندازه گیری های آزمایشگاهی Sw توسط مغزه ها در مقابل مقادیر تخمینی با استفاده از داده های چاه نگاری سه حلقه چاه توسط مدل SVR و محاسبه پارامترهای آماری معرف خطا، نیز اعتبارسنجی شده است. نتایج تحقیق نشان می دهد که مدل مذکور از قابلیت بالایی برای تخمین میزان Sw سنگ مخزن با استفاده از داده های چاه نگاری برخوردار است. به گونه ای که داده های آموزشی را با ضریب تعیین همبستگی عالی بیش از 87 درصد و داده های آزمون را با ضریب تعیین همبستگی مطلوب بیش از 76 درصد تخمین زده است.
    کلیدواژگان: اشباع شدگی آب (Sw)، رگرسیون ماشین بردار پشتیبان (SVR)، سازند آسماری، داده های چاه نگاری
  • علیرضا گودرزی *، فرهاد ملائی صفحات 211-223
    توان تفکیک داده های لرزه ای به دلیل اثرات جذب فرکانسی، تضعیف دامنه و تداخل امواج کاهش می یابد و به عنوان یک چالش اساسی در مطالعات لرزه نگاری مدنظر است. افزایش توان تفکیک با روش های مختلفی صورت می پذیرد؛ اما مهم ترین و شناخته شده ترین روش، روش واهمامیخت است؛ که به عنوان یک روش متداول برای افزایش توان تفکیک، با تقویت یا بازیابی فرکانس های بالا منجر به فشرده سازی می شود. این مقاله روشی بر پایه تبدیل هیلبرت را در فضای موجک گسسته مختلط ارائه می دهد. نخست داده ها با تبدیل موجک گسسته مختلط تجزیه می شوند؛ سپس با استفاده از پوش هیلبرت محاسبه شده ضرایب موجک در تمامی مقیاس ها تقویت شده و با انجام وارون تبدیل موجک و افزایش فرکانس های بالای موجود در داده ها بدون هیچ تقریب یا تخمینی، داده با فرکانس بالا به صورت فشرده ارائه می شود. یکی از بارزترین نتایج این روش کاهش اعوجاج نتایج نسبت به دیگر روش های موجود در این زمینه است. نتایج حاکی از برتری روش ارائه شده نسبت به روش تبدیل موجک گسسته غیر کاهشی مشابه است؛ زیرا تبدیل موجک گسسته مختلط اثرات جانبی کمتری را نسبت به روش غیر کاهشی دارد و دلیل این امر ارتقای زمان- فرکانس بالاتر نسبت به روش مذکور است. نکته حائز اهمیت دیگر، ضروریات محاسباتی است؛ به نحوی که اعمال روش در حیطه موجک گسسته مختلط با محاسبات کمتری مواجه است. زیرا فرآیند کاهش با فاکتور 2 در آن صورت می پذیرد؛ در حالی که روش تبدیل موجک گسسته غیر کاهشی هیچ کاهشی را در ضرایب ارائه نمی دهد؛ اما روش تبدیل موجک گسسته مختلط نسبت به روش تبدیل موجک گسسته مرسوم افزونگی بالاتری دارد؛ زیرا از دو موجک بهره می برد که به صورت همزمان داده ها را آنالیز می کنند.
    کلیدواژگان: تبدیل موجک، توان تفکیک، داده های لرزه ای، تبدیل هیلبرت
  • کیانوش سلیمانی، علیرضا عرب امیری، ابوالقاسم کامکار روحانی، محمود شمس الدینی نژاد *، فرجاللهمومنی صفحات 225-235
    در این پژوهش به بررسی کارایی روش سونداژزنی ژئوالکتریکی در تشخیص لایه معدنی در ذخایر بوکسیت کارستی دارای توپوگرافی شدید سطح زمین پرداخته می شود. بدین منظور 21 سونداژ ژئوالکتریکی در کنار نقاط حفاری شده در یکی از ذخایر بوکسیت جاجرم در طول یک خط، طراحی و برداشت گردید. داده های برداشت شده نخست به وسیله منحنی های استاندارد تفسیر شده و سپس نتایج حاصله به عنوان مدل اولیه به وسیله نرم افزارIX1D ، مدل سازی یک بعدی شد. نتایج مدل سازی یک بعدی، وجود حداقل چهار لایه ژئوالکتریکی را در منطقه مورد بررسی نشان می دهد. جهت درک بهتر و جامع تر، این نتایج به صورت یک مقطع دوبعدی نشان داده شد. بررسی نتایج وارون سازی یک بعدی نشان داد که به دلیل تباین کافی مقاومت ویژه بین بوکسیت و سنگ کف دولومیتی، تشخیص کمر پایین لایه معدنی ممکن است؛ اما از آنجایی که اختلاف مقاومت ویژه بوکسیت کائولنی با لایه های بالایی آن اندک است؛ تشخیص کمر بالای ماده معدنی و در نتیجه تعیین ضخامت لایه معدنی مشکل است. به منظور بررسی صحت و دقت نتایج حاصل از برداشت های ژئوفیزیکی در محدوده معدنی، این نتایج با نتایج حاصل از حفاری های اکتشافی در منطقه مذکور مورد مقایسه قرار گرفتند. در این مقایسه مشخص شد 5 سونداژ دارای خطای تخمین بالای 50 درصد، 10 سونداژ خطای تخمین بین 10 تا 30 درصد و 6 سونداژ خطای تخمین کمتر از 10 درصد دارند. ضریب همبستگی رتبه ای کندال تائو بین تخمین عمق لایه ها به روش ژئوالکتریکی و عمق به دست آمده از حفاری، مقدار 0. 486 به دست آمد؛ که نشان دهنده همبستگی نسبتا خوبی بین عمق تشخیص کنتاکت دولومیت و بوکسیت در تفسیر نتایج سونداژ و نتایج حاصل از داده های حفاری اکتشافی در منطقه است.
    کلیدواژگان: ژئوالکتریک، مدل سازی، بوکسیت کارستی، حفاری، مقاومت ویژه
  • محمد شاهی فردوس، رسول حمیدزاده مقدم، راشد پورمیرزائی * صفحات 237-250
    در مطالعه حاضر یک روش اعتبارسنجی مبتنی بر ماتریس وضوح مدل، ماتریس وضوح داده و کوواریانس واحد برای مدل ژئوفیزیکی مقاومت ویژه ارائه شده است. در این مطالعه وارون سازی داده ها با استفاده از روش وارون تعمیم یافته انجام گرفت. همچنین برای به دست آوردن داده های محاسباتی از روش تفاضل محدود استفاده شد. روش پیشنهاد شده پس از کد نویسی در محیط متلب، به وسیله یک مدل مصنوعی دارای نوفه مورد ارزیابی قرار گرفت و سپس برای پردازش داده های واقعی استفاده شد. داده های واقعی در محدوده اندیس معدنی همیج واقع در شهرستان بیرجند، با استفاده از آرایه دوقطبی- دوقطبی و با کمترین فاصله الکترودی 20 متر در جهت شمال- جنوب برداشت شد. در ادامه اعتبار سنجی مدل به دست آمده برای منطقه مورد مطالعه با استفاده از ماتریس وضوح داده، ماتریس وضوح مدل و ماتریس کوواریانس واحد انجام شد. نتایج حاصل از ماتریس وضوح داده و ماتریس وضوح مدل نشان می دهند که روش وارون تعمیم یافته برای مدل سازی داده های مقاومت ویژه محدوده همیج به خوبی عمل کرده و مدل ارائه شده دارای صحت بالایی است. همچنین نتایج حاصل از ماتریس کوواریانس واحد نشان می دهد برخی از پارامترهای مدل دارای دقت پایین تری می باشند؛ که در تفسیر نتایج باید به آن ها توجه شود. در پایان داده های اندیس همیج با استفاده از نرم افزار Res2dinv نیز پردازش شد و خروجی نرم افزار با نتایج به دست آمده از روش پیشنهاد شده در این مطالعه مورد مقایسه قرار گرفت. نتایج این مطالعه نشان می دهد استفاده از سه ماتریس به کار برده شده برای اعتبار سنجی و یافتن بهترین پارامترهای مدل از عملکرد مناسبی برخوردار است.
    کلیدواژگان: ماتریس وضوح داده، ماتریس وضوح مدل، ماتریس کوواریانس واحد، وارون تعمیم یافته، تفاضل محدود، مقاومت ویژه
  • افسانه نصرآبادی* ، محمدرضا سپهوند، زهرا لیموچی صفحات 251-265
    در این مقاله ساختار سرعتی پوسته شمال شرق ایران به دلیل داشتن پتانسیل لرزه خیزی بالا با استفاده از روش برگردان همزمان توابع گیرنده و پاشندگی سرعت گروه و فاز امواج رایلی مورد بررسی و مطالعه قرار گرفت. برای تعیین توابع گیرنده از روش تکرار واهمامیخت در حوزه زمان و سه سال داده دورلرز (ژانویه 2012 تا دسامبر 2014) با بزرگای 5/5 در چهار ایستگاه لرزه نگاری باند پهن متعلق به مرکز ملی شبکه لرزه نگاری باند پهن ایران (INSN) و مرکز لرزه نگاری کشوری (IRSC) ، استفاده شد. با توجه به وابستگی توابع گیرنده و پاشندگی امواج سطحی به پارامترهای متفاوت و وجود خطا در الگوی ساختاری حاصل از برگردان مستقل هر کدام از این داده ها، تلاش شد با برگردان همزمان این داده ها خطای مدل سرعتی حاصل به حداقل برسد. منحنی های پاشندگی سرعت گروه و فاز موج رایلی از مطالعه ی بر روی ساختار پوسته و گوشته ی بالایی فلات ایران در بازه ی دوره ی تناوبی 10 تا 100 ثانیه تامین شده است. با توجه به وابستگی توابع گیرنده و پاشندگی امواج سطحی به پارامترهای متفاوت و وجود خطا در الگوی ساختاری حاصل از برگردان مستقل هر کدام از این داده ها، تلاش شد با برگردان همزمان این داده ها خطای مدل سرعتی حاصل به حداقل برسد. نتایج نشان می دهد که میانگین ستبرای پوسته در ایستگاه شاهرود (SHRO) 44 کیلومتر، در ایستگاه سبزوار (SBZV) 40 کیلومتر، در ایستگاه جرخشک (JRKH) 40 کیلومتر و در شمال منطقه مورد مطالعه، در زیر ایستگاه مراوه تپه (MRVT) 38 کیلومتر است. به طور کلی شمال شرق ایران از پوسته نازکی با ضخامت میانگین 40 کیلومتر برخوردار است. مقدار خطا در تعیین عمق موهو با توجه به مدل سازی مستقیم داده ها 2± کیلومتر است.
    کلیدواژگان: شمال شرق ایران، کپه داغ، ساختار پوسته، توابع گیرنده، برگردان همزمان
  • علی آدیم، محمدعلی ریاحی، مجید باقری * صفحات 267-275
    برای بهینه سازی تصمیمات حفاری و برنامه ریزی محل چاه ها در مناطق فشار بالا، تخمین فشار منفذی بسیار ضروری می باشد. به عبارت دیگر در مراحل مختلف مهندسی نفت، پیش بینی قابل اعتماد فشار منفذی، قبل از حفاری بسیار مهم می باشد. پیش بینی فشار منفذی، جهت انتخاب مناسب لوله جداری و وزن گل حفاری بسیار مهم می باشد. هدف اصلی این مطالعه تخمین فشار منفذی به روش های ایتون و باورز و مقایسه دقت و کارایی آنها می باشد. جهت دستیابی به این هدف، در روش باورز، محقیقین جهت ارتباط مستقیم بین سرعت و تنش موثر تلاش کردند. یکی از این مدل ها توسط باورز گسترش داده شد. داده های سرعت و چگالی موجود در محل چاه ها با استفاده از تکنیک ترکیبی شبیه سازی گوسی پی در پی و کوکریجینگ هم مختصات، در فواصل بین چاه ها پراکنده شد. سپس با استفاده از رابطه بین سرعت و تنش موثر، مکعب تنش موثر و رابطه بین چگالی و فشار روباره، مکعب فشار روباره تولید می شود. در نتیجه فشار منفذی سازند با توجه به رابطه ترزاقی تخمین زده می شود. در روش دیگر )ایتون( فشار منفذی با استفاده از اطلاعات چاه نگاری با به کار بردن روش پیش بینی ایتون با اصلاحات مورد نظر در محل چاه ها تخمین زده می شود. در این روش، خط روند زمان گذر صوتی با استفاده از روش ژانگ و با توجه به لیتولوژی تقسیم بندی می شود. نتایج این مطالعه، نشان می دهد که فشار منفذی تخمین زده شده به وسیله روش اصلاح شده ایتون با ضریب نمایی 0. 4 بیشترین شباهت را با داده های فشار اندازه گیری شده دارند.
    کلیدواژگان: فشار منفذی، روش ایتون، روش باورز، مقاومت صوتی، شبیه سازی گوسی پی درپی، کوکریجینگ هم مختصات
  • رشید چمبری، امین روشندل کاهو *، مهیار یوسفی، مهرداد سلیمانی صفحات 277-292
    امروزه شناسایی و اکتشاف گنبدهای نمکی به منظور ذخیره سازی مواد هیدروکربنی و اکتشاف منابع هیدروکربنی از اهمیت ویژه ای برخوردار است. از میان روش های ژئوفیزیکی، روش لرزه نگاری بازتابی یکی از بهترین روش هایی است که قابلیت شناسایی گنبد نمکی را داراست. هرچند تعیین دقیق مرزهای گنبد نمکی به دلیل میرایی شدید انرژی لرزه ای در نمک و تفاوت سرعت بالای انتشار موج در نمک نسبت به رسوبات دربرگیرنده آن، باعث شده است که شناسایی مستقیم گنبدهای نمکی و تعیین مرز آن ها از روی داده های لرزه ای بازتابی کمی مشکل باشد. نشانگرهای لرزه ای به عنوان ابزاری برای استخراج ویژگی های داده های لرزه ای می تواند به تفسیر و شناسایی گنبدهای نمکی و تعیین مرزهای آن کمک بسزایی کند. با توجه به این که هر نشانگر به تنهایی دارای اطلاعات مجزایی است؛ لذا ترکیب و تلفیق نشانگرها یکی از ابزارهایی است که می تواند اطلاعات جامعی از هدف موردنظر در اختیار مفسر قرار دهد؛ بنابراین امروزه تکنولوژی ترکیب چند نشانگری با اهداف مختلف در لرزه شناسی مورد استفاده قرار می گیرد. هدف از تحقیق حاضر، تلفیق نشانگرهای لرزه ای با استفاده از روش فازی داده محور به منظور شناسایی مرز گنبد نمکی در محیط GIS است. در این راستا ابتدا تعدادی از نشانگرهای لرزه ای بافتی متداول بر روی داده لرزه ای به منظور شناسایی و تعیین محدوده گنبد نمکی اعمال گردید و نتایج هرکدام به صورت جداگانه مورد برسی قرار گرفت. سپس با استفاده از روش های وزن دهی فازی پیوسته، نشانگرهای مورد بررسی، فازی شدند. در مرحله بعد این لایه ها با استفاده از عملگرهای فازی تلفیق شده و در نهایت یک مدل واحد که حاوی اطلاعات تمام نشانگرهای مجزا است و مرزهای جانبی گنبد نمکی را با دقت بیشتری تعیین کرده است، به دست آمد. نتایج حاصل از داده های واقعی نشان می دهد که استفاده از روش وزن دهی فازی و تلفیق نشانگرها با استفاده از عملگر فازی توانسته مرزهای جانبی گنبد نمکی را به خوبی تعیین کند.
    کلیدواژگان: ماتریس هم رخداد سطح خاکستری، نشانگر بافتی، گنبد نمکی، GIS، وزن دهی فازی، توابع لجستیکی
  • میلاد فرشاد *، احمدرضا مختاری مبارکه صفحات 293-304
    تحلیل سرعت یکی از مهم ترین مراحل پردازش داده های لرزه ای است؛ چرا که بسیاری از مراحل پردازش از جمله تصحیحات برونراند، برانبارش و مهاجرت زمانی و عمقی را تحت تاثیر قرار می دهد. روش های متفاوتی برای ساخت مدل سرعتی از داده های لرزه ای معرفی شده است. متداول ترین روش تحلیل سرعت استفاده از معیار شباهت است. این معیار و سایر انواع آن با اندازه گیری دامنه لرزه ای در امتداد مسیرهای هذلولی، سعی در به دست آوردن طیف سرعت دارند. از مشکلات اصلی این معیار زمان گیر بودن محاسبات با توجه به ابعاد داده ها و عدم کارایی مناسب در صورت وجود تغییرات دامنه با دورافت یا تغییرات قطبش آن است. بدین منظور از معیار شباهت AB استفاده می شود؛ که این معیار نیز تفکیک پذیری بسیار پایینی دارد. از طرفی محدود بودن باند فرکانسی رخدادهای لرزه ای به علت اثر موجک لرزه ای، باعث کاهش تفکیک پذیری زمانی می شود. در این مقاله درجات بالاتر تبدیل رادون هذلولی واهمامیختی برای افزایش تفکیک پذیری و مقابله با مشکل تغییرات دامنه با دورافت معرفی شده است. به علاوه، به منظور به دست آوردن طیف سرعت با وضوح بالا از الگوریتم سریع آستانه گذاری با تکرار و برای کاهش حجم محاسبات تبدیل رادون، از حوزه قطبی-لگاریتمی استفاده شده است. اجرای این الگوریتم روی مثال های مصنوعی عاری و حاوی نوفه و همچنین روی داده های واقعی مربوط به خلیج مکزیک، افزایش چندین برابر تفکیک پذیری را نسبت به روش معمول شباهت و شباهت AB در به دست آوردن طیف سرعت نمایش می دهد.
    کلیدواژگان: تحلیل سرعت، تبدیل رادون هذلولی، تبدیل رادون واهمامیختی، تبدیل رادون در حوزه قطبی- لگاریتمی، درجات بالاتر تبدیل رادون، AVO
  • صادق مقدم *، مژده آزادی، اصغر آزادی، محمد جعفری شمس آبادی صفحات 305-321
    امروزه روش های ژئوفیزیکی به عنوان روش هایی موفق در تعیین پارامترهای ژئوتکنیکی و مشخصات لایه های زیرسطحی عمل کرده اند. هدف از این تحقیق، ارائه مدلی بسیار کاربردی و معمول از مهندسی ژئوفیزیک کاربردی در علوم عمرانی و ژئوتکنیک و نوآوری در ارائه اطلاعات اولیه و در عین حال حائز اهمیت در شکل گیری سازه های عظیم است. بررسی سلامت سازه ای شمع های درجاریز، نوع رفتار آن ها از نظر یکپارچگی در زمان اجرا و پس از آن، از مهم ترین چالش های موجود در مطالعات ژئوتکنیک است؛ چراکه سازه شمع های درجاریز، همزمان با حفاری در داخل خاک شکل می گیرد و درنتیجه ابهامات بیشتری در ارتباط با کیفیت و ابعاد واقعی شمع به وجود می آید. در این تحقیق به منظور تکمیل مطالعات ژئوتکنیکی در کنترل یکپارچگی و استحکام شمع های اجرا شده و از طرفی بررسی پاسخ ساختگاه مورد مطالعه به لرزه های با منابع مختلف، آزمایش های پاسخ ضربه PIT، روش های لرزه ای سطح به عمق (Down hole) و بین گمانه ای (Cross hole) در محدوده مطالعاتی انجام گرفته اند. با توجه به نتایج به دست آمده از آزمایش پاسخ ضربه، نمودار فراوانی سرعت و نقشه سرعت امواج لرزه ای سازه های بتنی به منظور به تصویر کشیدن استحکام سازه های زیرسطحی به دست آمده است. همچنین نمودار سرعت شمع های مورد آزمایش برحسب سیمان مصرفی در محدوده مورد مطالعه به دست آمده اند که رابطه خطی میزان سیمان مصرفی و استحکام سازه های مورد نظر با ضریب R2=0. 67 تقریب زده شده است. در ادامه با توجه به جنس و بافت خاک، سرعت متوسط موج برشی در 12 متر اول 2 گمانه به دست آمده است که بر اساس آیین نامه 2800 ایران در تیپ II در نظر گرفته شده اند؛ در نهایت با توجه به نتایج آزمایش بین گمانه ای در 10 جفت گمانه، ضخامت سازه های شمع با تقریب قابل ملاحظه ای و با تفاوت قابل اغماض با نمونه مشاهده شده در بازرسی مستقیم، بین 5/0 تا 8/0 متر محاسبه شده اند. با توجه به نتایج این تحقیق و نظر به لرزه خیزی ناحیه توصیه می شود طراحی عملیات بارگذاری در شمع های L18-1 و L9-11 اجرا شود.
    کلیدواژگان: آزمایش پاسخ ضربه، روش لرزه نگاری سطح به عمق، روش لرزه نگاری بین گمانه ای، کلارآباد
  • بهروز اسکوئی *، سید محمدجواد روحانی، صفیه امیدیان، میثم عابدی صفحات 323-337
    دو توده کمانی شکل پلور با جنس بازالت و تراکیت بازالت در 75 کیلومتری شمال شرق تهران و جنوب آتش فشان دماوند واقع شده اند. محل خروج آن ها هنوز مشخص نیست و از نظر زایش، نحوه قرارگیری و پراکندگی جزء مباحث حل نشده زمین شناسی می باشند و نمونه های مشابه آن ها در شرق آتش فشان دماوند نیز وجود دارد. این دو توده از نظر جنس، سن و منبع تغذیه کننده با گدازههای دماوند اختلاف دارند. با برداشت داده های مغناطیسی، جهت تجزیه وتحلیل ساختارهای زمین شناسی مورد نظر با اعمال روش‏های ترکیبی سیگنال تحلیلی- اویلر، واهمامیخت اویلر و تحلیل طیف توان، شاخص ساختاری و عمق گدازههای مولد بیهنجاری مغناطیسی برآورد شد. حداکثر عمق چشمه در هر دو کمان تقریبا کمتر از 95 متر از سطح زمین است. با توجه به محل چشمه ها در هر دو کمان، به نظر می رسد مجرای خروج بازالت‏ها حدودا در این عمق، زیر سطح کنونی آن ها واقع شده است. با انجام مدل سازی سه بعدی، نتایج حاکی از ریشهدار بودن ساختارهای مورد تجسس دارد. هر چند در کمان چپ، ارتباط آنومالی سه بعدی با توده قابل نمایش نبوده و احتمالا توده از وسعت بیشتری برخوردار بوده و فرسایش رودخانه ای در این مسیر بخشی از توده را در سطح از بین برده است. به دلیل طول کوتاه پروفیل ها و عمق کم توده های شناسایی شده، در مورد روند صعود ماگما و محل احتمالی مخزن نمی توان اظهارنظر کرد که متعاقبا با توسعه شبکه برداشت و همچنین به کارگیری اطلاعات تکمیلی حاصل از سایر روش های ژئوفیزیکی می توان اطلاعات دقیق تری ارائه کرد. وضعیت آنومالی های زیرسطحی و عمق آن ها نشان می دهد که بازالت ها ریشه عمقی برجا دارند و از محل دیگری به این ناحیه روان نشده‏اند.
    کلیدواژگان: بازالت‏های پلور، روش طیف توان، سیگنال تحلیلی، مدل سازی سه بعدی، واهمامیخت اویلر
  • وحید ریاحی کجور، نرگس افسری * صفحات 339-349
    برآورد کاهندگی امواج لرزه ای با توجه به فاصله یکی از مهم ترین بررسی های مورد نیاز هر منطقه است. از کاربردهای فراوان آن می توان به برآورد خطر زمین لرزه، شبیه سازی جنبش نیرومند زمین، تعیین روابط کاهندگی اشاره کرد. هدف از این پژوهش، برآورد ضریب کیفیت و چگونگی جذب امواج برشی حاصل از زمین لرزه در البرز مرکزی محصور به عرض جغرافیایی 34 تا 38 درجه عرض شمالی و 50 تا 56 درجه طول شرقی، با استفاده از روش کاهندگی طیفی است. به همین منظور از داده های ثبت شده از زمین لرزه 28 می سال 2004 میلادی فیروزآباد-کجور و پسلرزه های آن، توسط شبکه های لرزه نگاری ساری و سمنان، وابسته به مرکز لرزه نگاری کشوری برای برآورد فاکتورکیفیت موج برشی (QS) استفاده شده است. در این مطالعه ضریب کیفیت موج مستقیم S برای هفت باند فرکانسی 2-1، 4-2، 6-3، 8-4، 12-6، 16-8 و 24-12هرتز با بسامدهای مرکزی 1/5، 3، 4/5، 6، 9، 12 و 18 هرتز در ناحیه البرز مرکزی برآورد شده است. طبق نتایج به دست آمده، تابع فاکتور کیفیت موج برشی (QS) برای مولفه های شمالی- جنوبی (N-S) به صورت QS=76. 61f 0. 8 و برای مولفه شرقی- غربی (E-W) QS=70. 35f 0. 85 به دست آمد. طبق نتایج بدست آمده از برآورد فاکتور کیفیت موج برشی (QS) در منطقه، مقدار این فاکتور با افزایش فرکانس افزایش یافته و رابطه وابستگی فرکانسی ضریب کیفیت امواج برشی برای مقادیر میانگین QS دو مولفه افقی بر حسب فرکانس نیز به صورت QS =73. 54f 0. 83 به دست آمده است؛ که مقدار Q0برآورد شده (کمتر از 200) با زمین ساخت و لرزه خیزی منطقه مطابقت دارد.
    کلیدواژگان: البرز، فاکتور کیفیت Qs، جذب، موج برشی
  • محمد رسول نیک بخش، میرستار مشین چی اصل* ، محسن اویسی موخر، حمیدرضا سیاه کوهی صفحات 351-363
    مهم ترین هدف در تفسیر داده های مغناطیسی، محاسبه عمق و تعیین هندسه (شاخص ساختاری) بی هنجاری مغناطیسی است. به همین منظور روش های متعددی ابداع و پیشنهاد شده است. کاربردی ترین روش، استفاده از روش سیگنال تحلیلی است. در این تحقیق روش جدیدی برای تفسیر داده های دوبعدی مغناطیسی پیشنهاد شده است. در این روش از ترکیب سیگنال تحلیلی و گرادیان با بهبود روش سیگنال تحلیلی برای تخمین عمق و شاخص ساختاری بی هنجاری های مغناطیسی استفاده شده است. به دلیل حساس بودن روش حاضر به نوفه از روش فراسو برای کاهش اثر نوفه بکار برده شده است. به طور کلی روش سیگنال تحلیلی یک روش مشتق گیری است که این روش باعث تقویت دامنه نوفه ها می شود. داده های واقعی اندازه گیری شده همواره با نوفه همراه اند، بنابراین برای شبیه سازی داده های واقعی باید داده هایی تولید شوند که به نوفه آلوده باشند. به عبارت دیگر باید به داده های مصنوعی مقداری نوفه اضافه شود. در داده های مصنوعی نوفه دار، جواب ها ناپایدار هستند. برای کاهش اثر نوفه در ایجاد ناپایداری، از فیلتر ادامه فراسو استفاده می شود. استفاده از این تابع سیگنال تحلیلی نیازی به اطلاع از جهت مغناطیس شدگی ندارد در نتیجه استفاده از آن در مواقع وجود مغناطیس شدگی بازماند مفید است. برای بررسی دقت روش سیگنال تحلیلی بهبود یافته از یک مدل دایکی شکل در عمق های متفاوت استفاده شده است. نتایج به دست آمده توسط روش پیشنهاد شده بر روی داده های مصنوعی نوفه دار و بدون نوفه نشان می دهد که روش سیگنال تحلیلی بهبود یافته خطایی کمتر از 8 درصد در محاسبه عمق و شاخص ساختاری بی هنجاری های مغناطیسی دارد. از این روش برای تعیین عمق کانسار آهن خلیل آباد واقع در استان کرمان، شمال غرب شهرستان سیرجان استفاده شد و نتایج آن با نتایج روش تخمین عمق اویلر و اطلاعات گزارش های حفاری مورد مقایسه قرار گرفته است.
    کلیدواژگان: سیگنال تحلیلی بهبودیافته، بی هنجاری مغناطیسی، تخمین عمق، شاخص ساختاری
  • فرهاد خوشبخت * صفحات 365-375
    مهم ترین نمودار چاه پیمایی، نمودار ‏مقاومت الکتریکی است؛ که برای تعیین نوع و مقدار هیدروکربن موجود در مخازن ‏مورد استفاده قرار می گیرد. ‏ در روش های سنتی تفسیر نمودارهای ‏مقاومت، از چارت بوک استفاده می شود. چارت بوک مدلی تئوری از رفتار ابزار نمودارگیر ‏است؛ که با استفاده از مدل های یک بعدی کامپیوتری تهیه می شود. این کدهای کامپیوتری ‏معادله ماکسول را در ‏فضایی ساده سازی شده و با صرف نظر از شیب لایه ها، رخنه ناهمگن سیال حفاری، اثر لایه های مجاور و زاویه چاه حل می کنند؛ که در مخازن پیچیده از نظر خواص و هندسه با خطای زیگردابی همراه است. برای لحاظ کردن پیچیدگی های مخزن و افزایش دقت تفسیر، از روش های عددی نظیر روش های اجزاء محدود و تفاضل محدود برای مدل کردن دو و سه بعدی نمودارهای مقاومت استفاده می شود. ‏ هدف اصلی این مقاله، مدل سازی ابزار القایی آرایه ای ‏AIT‏ است؛ تا رفتار این ابزار در شرایط مختلف چاه و مخزن واکاوی و ‏شناخته شود. کسب این آگاهی، امکان تفسیر دقیق تر داده های این نمودار را فراهم کرده و در کنار آن، با شناخت نقاط قوت و ‏ضعف ابزار ‏AIT، به کارگیری آن در چاه های ایران با روال منطقی و به صورت بهینه انجام شود. ‏برای راستی آزمایی، نمودار مقاومت الکتریکی در یک مدل چند لایه بازسازی و با مقاومت واقعی مقایسه شد. در مدل چند لایه، ابزار ‏AIT‏ طراحی شده، مقاومت لایه های ضخیم را با دقت اندازه گیری کرده است؛ ولی برای لایه های نازک انطباق کامل وجود ندارد. برای شناخت بهتر رفتار ابزار ‏AIT‏ در طراحی و شرایط مختلف، آنالیز حساسیت بر روی ویژگی های اصلی آن ‏شامل فرکانس، ‏فاصله گیرنده-فرستنده و تغییرات مقاومت سیال درون چاه انجام شد؛ تا مشخص شود تغییر این ‏پارامترها چه اثری در رفتار ابزار ‏AIT‏ دارد. ‏ در فرکانس های بالا به دلیل تشدید پدیده اثر پوسته ای، علی رغم وابستگی خطی مقاومت با فرکانس، ابزار ‏توانایی اندازه گیری مقاومت سازند را از دست می دهد. با افزایش فاصله گیرنده-فرستنده، مقدار مقاومت برداشت شده ‏تغییر چندانی نشان نمی دهد؛ حال آن که این افزایش، قدرت تفکیک عمودی را تا حد زیگردابی کاهش می دهد. ‏به دلیل رفتار غیرخطی، ابزار ‏AIT در چاه های با رسانندگی سیال حفاری بیش از 1 زیمنس/متر قابل برداشت نیست. ‏
    کلیدواژگان: مقاومت الکتريکي، ابزار نمودارگيري، مخزن، اشباع هيدروکربن، آناليز حساسيت
  • محمد رضایی* صفحات 377-385
    روش مغناطیس سنجی یکی از روش های پرکاربرد ژئوفیزیکی است. آشکارسازی لبه ساختارهای زیرسطحی یکی از اهداف مهم تفسیر داده های مغناطیس سنجی است. روش های متعددی برای آشکارسازی لبه ساختارهای زیرسطحی با استفاده از داده های میدان پتانسیل ارائه شده است؛ که در بین این روش ها، روش زاویه تمایل و انحنای تانسور گرادیان می توانند مرز ساختارهای زیرسطحی را به شکل کمی تعیین نماید. در این مقاله کاربرد روش انحنای تانسور گرادیان برای آشکارسازی لبه ساختارهای زیرسطحی با استفاده از داده های مغناطیس سنجی، مورد بررسی قرار گرفت. اعمال این روش بر روی داده های حاصل از مدل مصنوعی و داده های مغناطیس سنجی کانسار مس پرفیری قاهان نشان داد که مقدار ویژه کوچک ماتریس انحنای تانسور گرادیان می تواند مرز توده هایی که بی هنجاری مثبت مغناطیسی ایجاد می کند؛ را به صورت کمی تعیین نماید و مقدار ویژه بزرگ این ماتریس می تواند مرز توده هایی که بی هنجاری منفی مغناطیس ایجاد می کند را به صورت کمی تعیین نماید. همچنین نتایج نشان داد که روش انحنای تانسور گرادیان مرز ساختارهای زیرسطحی را با دقت بیشتری نسبت به روش زاویه تمایل تعیین می نماید و حساسیت این روش به نوفه موجود در داده ها نسبت به روش زاویه تمایل کمتر است.
    کلیدواژگان: مغناطیس سنجی، آشکارسازی لبه، تانسور گرادیان، زاویه تمایل، ساختارهای زیرسطحی، قاهان، مس پرفیری
  • فاطمه رضوی راد، احمد قربانی * صفحات 387-400
    در این مطالعه، وابستگی رسانایی الکتریکی مجازی ( ) به رسانایی سیال پرکننده ی منافذ ( ) بر روی اندازه گیری های قطبش القایی طیفی برداشت شده بر روی نمونه های ماسه و ماسه سنگ تمیز یکی از مخازن نفتی ایران و همچنین ماسه سنگ حاوی رس برداشت شده از یکی از آبخوان های ماسه سنگی شمال غرب انگلستان بررسی شده است. بخش حقیقی رسانایی الکتریکی ( ) یک رابطه ی خطی با رسانایی سیال اشباع کننده ی منافذ ( ) نشان می دهد. همچنین تغییرات رسانایی الکتریکی مجازی ( ) با شوری و در نتیجه رسانایی الکتریکی سیال اشباع کننده ی منافذ ( ) به صورت یک رابطه ی خطی مثبت است. به منظور تعیین زمان رهایی ( ) داده های طیفی، مدل کول-کول بر روی اندازه گیری های انجام شده در چهار درجه ی شوری مختلف بر روی نمونه های آبخوان ماسه سنگی برازش شد. زمان رهایی حاصل از مدل کول- کول برای بیشتر نمونه ها با افزایش رسانایی الکتریکی سیال پرکننده ی منافذ به شکل خطی افزایش می یابد. البته برای دو نمونه زمان رهایی با افزایش رسانایی سیال اشباع کننده ی منافذ، کاهش نشان می دهد؛ که این کاهش ممکن است به دلیل تفاوت در اندازه ی منافذ و همچنین محتوی رس متفاوت نمونه ها باشد. رفتار رسانایی مجازی ( ) نیز با بارپذیری نرمالیزه ( ) ، که هر دو از پارامترهای اندازه گیری قطبش هستند، قابل مقایسه است. به عبارت دیگر بارپذیری نرمالیزه نیز با افزایش شوری سیال و در نتیجه رسانایی الکتریکی سیال اشباع کننده ی منافذ ( ) ، افزایش می یابد. به منظور توصیف وابستگی قطبش سطحی به رسانایی الکتریکی سیال، پارامتر قطبش پذیری فصل مشترک دانه- سیال بر واحد (و) نیز در این مطالعه مورد بررسی قرار گرفت. قطبش پذیری فصل مشترک دانه- سیال بر واحد (پارامترهای و) مفهومی است؛ که اختلاف بین بزرگی قطبش (یا) بین نمونه های با میزان یکسان را توضیح می دهد. در این مطالعه نشان داده شد پارامترهای قطبش پذیری (و) کاملا به رسانایی سیال اشباع کننده ی منافذ وابسته هستند. به عبارت دیگر با افزایش رسانایی سیال اشباع کننده ی منافذ، میزان قطبش پذیری نمونه ها نیز افزایش می یابد. بیشترین مقادیر پارامترهای قطبش پذیری (و) در شوری های بالا مشاهده شده است؛ که شاهدی بر بیشینه ی قطبش پذیری کوارتز غالب ماسه های سیلیسی است. بارپذیری نرمالیزه نیز یک رابطه ی توانی مثبت با رسانایی الکتریکی سیال اشباع کننده ی منافذ نشان داده است.
    کلیدواژگان: قطبش القایی طیفی، شوری، رسانایی الکتریکی مجازی، بارپذیری نرمالیزه، زمان رهایی، مدل کول- کول، رسانایی سیال
  • پیوند حیدرنژاد صنمی *، علی نجاتی کلاته صفحات 401-412
    آنالیز طیفی داده های مغناطیس هوابرد جهت تخمین عمق نقطه ی کوری می تواند اطلاعات ارزشمندی از گرادیان دمای سطحی در مقیاس ناحیه ای و تمرکز انرژی زمین گرمایی در سطح عرضه کند. در این تحقیق از داده های مغناطیس هوابرد شمال غرب ایران به منظور تخمین عمق کوری به دو روش مرکزیابی و مدل سازی پیشرو استفاده شده است. برای این کار ابتدا نقشه شدت میدان مغناطیسی کل تهیه شد، بعد از حذف اثر IGRF از داده ها فیلتر برگردان به قطب مغناطیسی به داده ها اعمال شد. ابعاد بهینه پنجره ها 100×100 کیلومتر انتخاب شده و پنجره ها بر روی نقشه شدت میدان مغناطیسی کل اعمال شدند. سپس طیف توان میانگین شعاعی برای هر پنجره محاسبه شد و عمق کوری به دو روش مرکزیابی و مدل سازی پیشرو تخمین زده شد. در تخمین عمق به روش مرکز یابی لگاریتم طیف توان بر مبنای عدد موج رسم شده و عمق بالا و مرکز هر بلوک با استفاده از برازش خط راست به قسمت های مختلف نمودار طیف توان انرژی انجام می شود. مدل سازی پیشرو نیز با زیر برنامه ای که در محیط Matlab برنامه نویسی شده است؛ انجام می شود. با توجه به نتایج عمق کوری بین 14 تا 22 کیلومتر برآورد می شود. با توجه به نقشه تراز تغییرات عمق کوری حاصل از روش مدل سازی پیشرو قسمت های شرق قله ی سهند با توجه به کاهش مقادیر به دست آمده ی عمق کوری و همچنین با توجه به تمرکز چشمه های آب گرم به عنوان نواحی مستعد برای برنامه ریزی آتی برای اکتشاف منابع زمین گرمایی معرفی شد.
    کلیدواژگان: زمین گرمایی، طیف توان شعاعی، مدل سازی پیشرو، مرکز یابی، نقطه کوری
  • مصطفی موسی پور یاسوری *، وحید ابراهیم زاده اردستانی صفحات 413-427
    در تفسیر داده های گرانی سنجی از گرادیان های اول و دوم قائم به طور گسترده استفاده می شود. گرادیان های قائم به نوفه حساس هستند. دقت در محاسبه گرادیان های قائم به طور مستقیم بر روی دقت تفسیرها اثر می گذارد. به همین دلیل محاسبه دقیق و بدون نوفه گرادیان قائم بسیار حائز اهمیت است. متداول ترین روش برای محاسبه گرادیان قائم تبدیل فوریه است. وجود نوفه اندک در داده ها باعث می شود که گرادیان های قائم محاسبه شده با استفاده از تبدیل فوریه حاوی نوفه شدیدی باشند. در این مقاله از تبدیل کسینوس برای محاسبه گرادیان های قائم استفاده شده است. در داده های عاری از نوفه نتایج تبدیل فوریه و تبدیل کسینوس کاملا یکسان است؛ اما در داده های حاوی نوفه، تبدیل کسینوس عملکرد بهتری از تبدیل فوریه دارد. علت این بهبود با استفاده از نسبت سیگنال به نوفه بررسی شده است. مقدار این نسبت در تبدیل کسینوس بزرگ تر از تبدیل فوریه است و به همین دلیل در محاسبه گرادیان های قائم با استفاده از تبدیل کسینوس نوفه کمتری وارد می شود. این روش بر روی داده های مصنوعی دارای نوفه گوسی امتحان شده است. گرادیان های اول و دوم قائم بدست آمده از تبدیل کسینوس در مقایسه با تبدیل فوریه نوفه کمتری را نشان می دهد. همچنین این روش بر روی داده های واقعی معدن منگنز صفو اعمال شده و نتایج قابل قبولی از آن به دست آمده است. نمونه ای از کاربرد گرادیان ها در تفسیر داده های گرانی، استفاده از آنها در تعیین لبه داده ها است. برای تعیین لبه داده های مصنوعی و داده های واقعی از سیگنال تحلیلی استفاده شده است. سیگنال تحلیلی حاصل از گرادیان های تبدیل کسینوس در مقایسه با سیگنال تحلیلی حاصل از گرادیان های تبدیل فوریه، حاوی نوفه کمتری است و کیفیت بهتری دارد.
    کلیدواژگان: گرادیان های قائم، تبدیل فوریه، تبدیل کسینوس، کاهش نوفه، گرانی سنجی، معدن منگنز صفو
  • روح الله کیمیایی فر، حمیدرضا سیاهکوهی، علیرضا حاجیان *، احمد کلهر صفحات 429-440
    فیلتر پخش ناهمسانگرد می تواند به عنوان یک روش کارآمد سطح نوفه تصادفی را در بسیاری از داده های کاهش دهد، هرچند در استفاده از این فیلتر برای داده های لرزه ای با سطح نوفه بالا، باید جانب احتیاط را در مورد ظهور رویداد های غیرواقعی در مقطع رعایت نمود. در این مقاله، به عنوان یک راه حل به منظور مقابله با این مسئله، با معرفی یک چهارچوب هوشمند خودکار، خروجی بهینه فیلتر، برای هر نقطه از داده های ورودی، از طریق شبکه استنتاج عصبی- فازی تطبیق پذیر استخراج می شود. آموزش شبکه عصبی-فازی، با استفاده از خروجی فیلتر پخش ناهمسانگرد و نیز خوشه بندی فازی و توسط الگوریتم C- Mean تعیین می گردد. آزمایش های انجام شده در این تحقیق نشان می دهد که در مقام مقایسه با فیلتر پخش ناهمسانگرد مرسوم، روش ارائه شده به صورت محسوس، در دستیابی به مقاطع مصنوعی با نسبت سیگنال به نوفه بالاتر، حداکثر به میزان 32% عملکرد فیلتر پخش ناهمسانگرد را ارتقا داده است. در داده های حقیقی نیز، علاوه بر تضعیف نوفه های مقطع، نسبت به حفظ رویدادهای همدوس مقطع، دقیق تر عمل کرده است.
    کلیدواژگان: تضعیف نوفه تصادفی، فیلتر پخش ناهمسانگرد، شبکه استنتاج عصبی- فازی تطبیق پذیر، خوشه بندی فازی
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  • Aref Zainalpour, Reza Ghaedrahmati*, Ali Moradzadeh, Mohamadreza Rahmani Pages 171-186
    Summary: The exploration of geothermal resources in the Bushli area of Ardebil Province utilizing magnetotelluric (MT) data is presented in this paper. This study is performed on 60 MT stations in an area surface of 90 square kilometers of Bushli area, located in southeast of Nir district. The data has been processed using algorithms based on robust methods, which are resistant to noise. After that, dimensionality analysis has been applied to the MT data having appropriate limits of frequencies related to all stations. Considering dimensionality analysis results, the regional structures are mostly identified as two-dimensional structures with north - south strike direction. Regional structures have been interpreted following one dimensional (1D) and two dimensional (2D) inverse modeling applied on the data. The results of 1D and 2D inverse modeling integrated with geological data indicate that the upper part of the geothermal reservoir is composed of a low resistivity area at the depth of 500 to 2000 meters. Final results have shown that the location of the geothermal reservoir extends to the southern parts of the study area.
    Introduction
    MT method is an electromagnetic (EM) method that uses natural EM fields, generated from Earth's magnetosphere for mapping deep subsurface structures. This method plots the electrical conductivity distribution beneath the earth surface by recording vertical and horizontal components of magnetic and electric fields from ground surface. High penetration depth of EM fields in MT method has made it applicable to deep target explorations such as geothermal and hydrocarbon resources. Hence, due to these options, this method has special status among other geophysical methods. Preliminary studies on the geology of Bushli area, which is located in southwest of Nir and Sareyn hot springs, have shown a relatively good potential for geothermal resources in this area that could be a preferred alternative for fossil fuels and future energy supply. Methodology and Approaches : MT method is widely utilized for surveying geothermal areas. In thermal areas, the electrical resistivity is extremely lower than that of areas with colder subsurface temperature. The selected MT survey lines are located in the area crossing over the hydrothermally altered zones and different geological structures. The data was acquired along 12 survey lines crossing the Bushli hot springs with a total of 60 MT stations in a frequency range of 1000 Hz to 0.001 Hz. Spacing between MT stations was almost considered 500 m constantly, for a better resolution. At first, 60 MT sounding time series data were reviewed. Then, the acquired raw data were analyzed using methods resistant to noise (i.e. robust methods), and also, outlier elimination method in order to achieve high quality apparent resistivity and phase data at each desired frequency. Specialized software was utilized for this purpose such as Mapros for processing and WinGLink for 1-D and 2-D smooth inverse modeling. Mapros has plenty of different robust methods for processing, and the preferred processing procedure, used in this paper, was to use least squares weighted functions. The Rodi and Mackie computer code and Occam smoothing algorithm were also used for 2D inversion and forward modeling, respectively. This algorithm seeks the minimum possible structure model subjected to an appropriate fit for the data, and it uses a code for 2D inversion from Rodi and Mackie (2001) in a way that this algorithm searches simultaneously for the model with the lowest overall RMS misfit and the smallest lateral and vertical conductivity gradients respectively. Apparent resistivity and phase data of TE+TM (joint) mode along each survey line were modeled in this study. Results and Conclusions: Considering the results obtained from 2D inversion and the geological information, the following conclusions were acquired: the thick surface layer with resistivity of 100-500 ohm-m along the north-south survey lines was also observable along the west-east survey lines. In some stations, a very conductive layer was seen on top of the surface that could be interpreted as the top soil saturated by penetrated water. Below this layer, there was a decline of resistivity with depth observable along the whole survey lines. This conductive layer (<10 ohm-m), showing variable thicknesses along the profile, was most naturally interpreted as the limestone, related to late Permian, of Ruteh formation acting as system reservoir. Below this conductor, a very resistive zone (>250-300 ohm-m) was observed. This resistive and intrusive mass was interpreted as the bed rock zone and a heat source that was mostly formed from granite and granodiorite related to first age of geology with high enthalpy. According to the models and electrical vertical sections and also horizontal resistivity maps at different depths, the geothermal reservoir was designated at a depth of 2500 to 3000 meters. Furthermore, resistivity map showed that the location of the geothermal reservoir continued to the south of the area. This was probably due to the significant properties of eastern parts of the area like the existence of many faults as well as low height of this part of the area compared to neighboring parts that caused the appearance of numerous hot springs in the area.
    Keywords: Bushli Area Sabalan, Data Processing, Apparent Resistivity, Phase Data, Magnetotelluric Method, Geothermal Reservoir, One-, Two-Dimensional Inversion
  • Mostafa Abbasi *, Ali Gholami Pages 187-197
    Summary: In this study, a novel approach for linearized amplitude versus offset (AVO) inversion in a Bayesian framework is presented. Objective is to estimate the posterior distribution of three elastic parameters, P wave velocity, S wave velocity and rock density. The methodology is based on the convolutional model and a weak contrast linearized approximation of Zoeppritz equation for PP waves. In this study, assuming a priori Gaussian distribution for input parameters, and also, a Gaussian distribution for seismic misfit function, Bayesian equation also yields a Gaussian distribution for posterior parameters, which can be analytically computed. This analytic solution is a rather fast approach for inversion of elastic parameters along with their uncertainty distribution. This methodology is tested on both synthetic and field data sets and in both cases yields reasonable solutions. In the current study, assuming a weak contrast model for rock properties, a linearized AVO approximation of Zoeppritz equation is used in a Bayesian framework to invert prestack seismic data for the above-mentioned three elastic parameters. The methodology is tested on both synthetic and field data sets. The results show preferably good matches with the true data.

    Introduction: Inversion of seismic AVO is a way to estimate elastic parameters from prestack seismic data. This technique can be solved in both nonlinear (Dahl and Ursin, 1991) and linear (Smith and Gildow, 1987) approaches. Lortzer and Berkhout (1993) also used the same methodology as presented here, but they used the relative contrast of elastic parameters instead of their absolute values.

    Methodology and Approaches: Assuming a Gaussian distribution for elastic parameters and seismic noise, and also, a linearized formulation for forward modeling, distribution for posterior parameters will also be Gaussian. Based on this methodology, the mean and covariance matrices of prior distribution are estimated from well data. Then using the linearized formulation of AVO, the mean and covariance matrices of observed seismic data are estimated. Having the statistical parameters of prior and likelihood functions, the statistical parameters of posterior distribution is analytically yielded based on Bayesian formulation. The covariance matrix of posterior distribution gives an estimate of the uncertainty in the elastic parameters.

    Results and Conclusions: A Bayesian AVO inversion method was proposed and tested on both synthetic and field data sets. In case of synthetic data, the estimated parameters fitted the true values almost exactly. The result for the field dataset was also reasonable and matched the well log data relatively well except in some locations where prestack seismic data were not preconditioned very well. The initial models used for this methodology does not need to be detailed at all and very simple initial models such as constant or linear values lead to good estimation of the posterior distribution. Therefore, this approach can be a good choice for generation of pseudowells where not a rich dataset is available.
    Keywords: Prestack Inversion , Bayesian Inversion , Linearized AVO , Prestack Joint Inversion , Weak Contrast , Uncertainty
  • Reza Ahmadi*, Mohammad, Sadegh Amiri Bakhtiar Pages 199-210
    Summary: Water saturation (Sw) of a hydrocarbon reservoir is an important petrophysical parameter having a great impact on the accuracy of primitive estimation of the reservoir. Due to highly importance of this parameter dealing with the economic calculations of the reservoir, it must be estimated precisely. Although experimental analysis of core samples taken from a reservoir leads to very useful information about Sw of the reservoir, this experimental method is highly expensive and time consuming; and therefore, this method is applicable only for a small number of wells in a field. To overcome this problem, an intelligent pattern recognition method, known as support vector regression (SVR), has been employed in the current research to estimate Sw from well logs data of 3 wells in one of the largest oil fields of Iran. The performance of the algorithm has also been validated through different criteria. The results of this research indicate that the SVR model can estimate Sw from well logs data accurately, in which the determination coefficients of 87 and 76 percent have been obtained from the training and test steps, respectively. Introduction: Generally in most commonly hydrocarbon reservoirs, Sw is estimated using well logs data through applying Archie's fundamental empirical relation. However, this relation is just satisfied for clean sandstone formations (without clay minerals). So far several empirical models have been proposed to measure Sw using well logs data. The main disadvantage of these models is their formation dependency, which makes the models specific and not comprehensive to be applied in a variety of other formations. In addition to empirical methods, several linear regression techniques have also been applied to estimate this parameter using well logs data. These techniques cannot estimate Sw appropriately due to the complexity of the parameter features. Resistivity and porosity logs are the most important well logs used to estimate Sw by Archie's relation. The porosity of a formation can be very accurately determined through sonic, density and neutron logs. However, resistivity logs are very sensitive to the presence of shale and other clayey impurities in formations. Their effects can be adjusted by means of gamma ray (GR) log. Therefore, to estimate Sw,, employimg an intelligent method using appropriate well logs data will be useful. The oil reservoir, studied in this research, is located in Asmari formation in southwest of Zagros Mountain. Overall this formation in the investigated region has been formed from a sequence comprising of carbonate rocks (limestone and dolomite), sandstone and shale. Methodology and Approaches: In the current research, to estimate Sw, SVR method has been applied to well logs data from 3 wells in one of the largest oil fields of Iran. In this study, appropriate well logs data comprising of GR, neutron porosity, formation bulk density, sonic transit time and true resistivity from deep induction log (ILD) have been used. Moreover, Sw values measured from cores in the laboratory are available for whole depth of the wells. In order to employ SVR to estimate Sw, the model needs to be trained using appropriate input and output data in MATLAB environment. In the current research, the input consists of 5 variables (well logs data) while the output is only the Sw parameter. From 1211 data points (containing 5 variables of well logs data and Sw parameter measured by core) available from the 3 wells, about 80 percent (i.e. 988 samples) were selected for training and the remaining 20 percent (i.e. 223 samples) were chosen for test. To compare the estimated values with the measured ones for the reservoir in the study region, visually, chart of lithology, water and hydrocarbon saturations of the formation were also depicted for the 3 wells by means of Geolog software. Results and Conclusions: The performance of the algorithm has been validated through different criteria such as scatter plot of Sw values from cores versus the estimated Sw values from well logs data of 3 study wells by means of SVR model as well as computing statistical parameters indicating the accuracy of the results. Furthermore, the results of the research revealed that the SVR model can estimate Sw using well logs data accurately so that it has estimated the training and test data with the determination coefficients of 87 and 76 percent, respectively. As a result, the proposed method, i.e. SVR, is an accurate, fast and cost-effective method to evaluate the petrophysical parameter Sw.
    Keywords: Water Saturation (Sw) , Support Vector Regression (SVR) , Asmari Formation , Well Logs Data
  • Alireza Goudarzi*, Farhad Mollaei Pages 211-223
    Summary: The resolution of seismic data decreases due to tuning effect, attenuation, and absorption and has always been one of the challenges for the interpreters. We can perform resolution enhancement in many ways, and in this regard, spiking deconvolution is the most critical approach. The ideal method to increase the resolution is to magnify or retrieve weak high frequency signals to provide a broad frequency spectrum, and therefore, substantially compressed signals. In this paper, a Hilbert-wavelet derived method has been investigated for this purpose. First, the input data using the wavelet transform (WT) are decomposed and enhanced by the Hilbert transform (HT) in the wavelet domain. The inverse WT yields compressed data based on current frequencies without any estimate and approximations. The Lack of significant distortions of recovered frequencies makes this process more distinctive than introduced methods. In this study, complex wavelet transform (CWT) and undecimated discrete wavelet transform (UDWT) are used. In fact, UDWT provides increased resolution, but on the opposite side, CWT presents impressive results. It deliveres fewer artifacts because of its time-frequency representations due to its unique combination of shift-invariant. This approach has been proposed and implemented to shot gathers after preliminary processing.

    Introduction: We can characterize a seismic trace as a convolution of two unknown discrete time series: a source wavelet and reflectivity series. The reflectivity series can constitute the unknown geology. The objective of geophysical data processing is to get information of data to prepare interpretable results (Yilmaz, 2001). Researchers have stated various methods to enhance the resolution and frequency bandwidth, such as inversion-based and spectral methods. An additional approach for improving the resolution is deconvolution. Nevertheless, the efficiency of this method relies on many considerations, including noise and source type. Therefore, we cannot obtain the desired results perfectly (without any damage to the signal) (Yilmaz, 2001). Fourier-based methods cannot deal with non-stationary signals and suffer from side effects such as the frequency leakage. One method that has recently been implemented to geophysical data, is discrete WT to consider unstable properties of seismic data. The enhancement of the seismic bandwidth has been achieved based on several types of WTs (Rusu et al., 2011; Zhou, 2004; Rawat & Surinder, 2010; Ferner et al., 2012).

    Methodology and Approaches: The CWT has been presented by Kingsbury (2001) and implemented by Goudarzi et al. (2014) for seismic noise attenuation. The wavelet in comparison with the Fourier analysis can handle wide ranges of signals. A deficiency of the DWT refers to signal representation at the discrete number of decomposition levels. Each level has twice the frequency content than the previous level. Pinar (1985) and Rao et al. (1982) determined fault parameters by applying the HT on gravity and magnetic data. In this paper, a Hilbert-wavelet derived method for this purpose has been examined. First, the input data using the wavelet transform (WT) are decomposed and enhanced by the HT in the wavelet domain. The inverse WT yields compressed data based on the current frequencies without any estimate and approximations.

    Results and Conclusions: Seismic resolution enhancement is an essential step in geophysical data processing. It is implemented for the thin layer identification as a necessity for hydrocarbon exploration. A data-oriented approach has been proposed for improving the seismic resolution using DWT and HT. We have used this method on synthetic data and three real sections. The results have shown that UDWT and CWT improve the seismic data resolution. However, CWT results are better than UDWT results for this purpose. The advantage of CWT method compared to UDWT is prominent. CWT produces fewer artifacts compared to UDWT due to the process because the CWT is less sensitive to abrupt changes than other similar WTs. CWT has considerably lower computational cost than UDWT due to less redundancy that is important for the seismic exploration industry. The results of the real and synthetic examples illustrate that the proposed method is beneficial for resolution enhancement of post-stack seismic data. We suggest that the method could be applied to shotgather after further filtering. This approach compresses the signal, and consequently, precise velocity analysis yields to a more accurate stacked section.
    Keywords: DWT, Resolution , Seismic Data, Hilbert Transform
  • Kianoush Soleimani, Ali Reza Arab, Amiri, Abolghasem Kamkar, Rouhani, Mahmoud Shamsaddini Nejad*, Farajollah Momeni Pages 225-235
    Summary: In this research, the efficiency of electrical sounding method in recognizing the mineral layer in karstic bauxite deposits having coarse topography is investigated. For this purpose, 21 electrical sounding locations beside drilled points along a survey line in one of Jajarm bauxite deposit have been designed and surveyed. First, the acquired data have been interpreted using standard curves, and then, the obtained results have been modelled by IXID software. Because of enough resistivity contrast between the bauxite and dolomite bed rock, the recognition of basement is possible. Despite this, due to low resistivity contrast between the bauxite and upper layers, the recognition of upper layers, and thus, determination of the bauxite layer thickness is difficult. In order to investigate the accuracy of the results of the geoelectrical surveys in the mineralized area, these results were compared with the results of exploration drilling in the study area. After comparison, it was identified that estimation error of the results of 5 sounding locations were above 50%,, estimation error of the results of 10 sounding points were between 10% and 30%, and estimation error of the results of 6sounding locations was less than 10%. Kendall rank correlation coefficient between estimated depth of basement by geoelectrical method and the achieved depth of drilling results was 0.486 that revealed a good correlation between contact of recognized dolomite depth and bauxite from the interpretation of sounding results and from the results of drilling data in the study area.

    Introduction: Bauxite exploration methods like exploration of most of minerals have divided into direct and indirect methods. Different drilling methods such as borehole drilling, core drilling and trench drilling are different kinds of direct methods, which are expensive and taking very much time. Among indirect exploration methods, we can refer to geophysical methods, which their applications are spreading because they are cheaper and faster, i.e. have higher operation speed. The major question of this research is whether electrical resistivity method is effective in bauxite exploration or not? and also, how much is the accuracy of modeling results and interpretation of electrical sounding surveys in determination of the depth and thickness of bauxite layer in the study area?

    Methodology and Approaches: In this research, resistivity method has been used for recognition of bauxite layer in Jajarm bauxite mine. After preliminary studies and inspection of Jajarm bauxite mine, one part of that area has been selected as an area for planning and acquisition of geoelctrical surveys. This area has coarse topography that limits the planning of survey network. Finding the depth and thickness of the bauxite layer is the purpose of this research. As the resistivity in vertical direction changes more than resistivity in horizontal direction in the area, vertical changes in resistivity represent the properties of bauxite layer in the vertical direction, and thus, the Schlumberger electrode array was used for geoelectrical sounding surveys. The distance between successive sounding points in survey lines according to exploration boreholes was considered 25 meters. Geological and exploration reports as well as the positions of the bauxite outcrops show the dip of bauxite layers. Considering this point, 21 electrical sounding points having 25 m distances from each other were surveyed. In this research, first, electrical sounding data were interpreted by standard curves, and then, the achieved results as the primary model were modeled by IXID software. Kendall rank correlation coefficient between estimated depth of basement as a result of modeling electrical sounding data and the achieved depth of drilling results was 0.486 that revealed a good correlation between the basement depth from the interpretation of sounding results and from the results of drilling data in the study area.

    Results and Conclusions: Five geoelectrical layers have generally been recognized in the study area as a result of the interpretation of all the acquired electrical sounding data, and also, the geological evidences from the area. Insufficient resistivity contrast between the bauxite unit and its overlying layer cause that these two units cannot be separable, and thus, determination of the thickness of the bauxite layer using geoelectrical method seems to be difficult. Although there is sufficient resistivity contrast between the bauxite unit and its underlying karstic dolomite basement, the estimated contact depths between these two layers from the interpretation of sounding data in 5 sounding locations, based on the drilling results in these locations, are not acceptable because of artificial noises from different human factors such as drilling equipment, drilling debris and other human installation, and also, because of geological noises such as varying thickness and dip of soil and rock layers and the existence of coarse topography in the study area. In general, based on the obtained results from the interpretation of the electrical sounding data, we can say that the accuracy of the geophysical method in estimation of subsurface information in this research is acceptable. We can also conclude that the geoelectrical method in the estimation of bauxite and basement layers has high accuracy.
    Keywords: Geoelectric , Modeling , Karstic Bauxite , Drilling , Resistivity
  • Mohammad Shahi Ferdos, Rasoul Hamidzadeh Moghadam, Rashed Poormirzaee* Pages 237-250
    Summary: The resistivity method is frequently used in fields of engineering geology and exploration of mineral resources. The simplicity of the equipment, the low cost of the survey in comparison with other methods, and the abundance of interpretation methods makes it as a popular geophysical method. There are many methods for inversion of resistivity data. Validation of inverted models is an important step in modeling. In general, validation of the resistivity results is performed by calculating the difference between observed and estimated values, but in this study, a validation technique based on data resolution matrix and model resolution matrix is proposed. The applied method for validation of the results has not been used so far, in this research work, resistivity and induced polarization data have been collected using dipole-dipole electrode array in Hamyj copper deposit located near the city of Birjand. The results of data resolution matrix and model resolution matrix have shown that generalized inversion is a suitable method for processing of resistivity data, because both data and model resolution matrix have been close to an identity matrix.

    Introduction: After gathering the resistivity data, application of a suitable inversion method for finding an adequate subsurface model is very important. Visual and analytical methods are used for the interpretation of resistivity data over simple structures such as faults. However, these methods require a certain degree of symmetry and they are suitable only for simple geological conditions. Generalized inversion is one of the important modelling techniques to invert geophysical data. In current study, generalized inversion is used for inversion of resistivity data. Normally the validation of the resistivity results is performed by calculating the difference between observed and estimated values, i.e. error function, but data resolution matrix and model resolution matrix are suitable tools for validation of the results. This method for validation of the results has not been used so far. In this research work, unit covariance matrix has been used to identify the correctness of the each parameter. Moreover, data resolution matrix describes the accuracy level of the estimated values. The covariance of the model parameters depends on the covariance of the data and the way that the error is mapped from data to model parameters. This mapping is just dependent on the data kernel and the generalized inversion, which is independent of the data.

    Methodology and Approaches: The proposed technique was tested on synthetic and real datasets. To explore the capability of the applied method more, 10 percent noise was also added to the synthetic data. The results of synthetic dataset showed the capability of the applied technique in the absence and presence of noise. For collecting the real resistivity data, an electrode spacing of 20 m has been used. Then inversion of the resistivity data acquired along a survey line was carried out using the generalized inversion method. Finite difference method was used for forward modelling, and also, perturbation approach was used for calculation of the Jacobin matrix in the inversion process. The results showed the presence o a fault in the study area. Furthermore, the results had a good correlation with the geological evidence from the study area. In this research, the code of the generalized inversion method has been written in MATLAB.

    Results and Conclusions: In the present study, a new method for the inversion of resistivity data has been proposed. The proposed method, which is a, generalized inversion method, has been tested on synthetic and actual datasets. The results have shown that the generalized inversion method is a successful technique in the inversion of the resistivity data, because both data resolution matrix and model resolution matrix have been close to an identity matrix. The results, obtained from applying the unit covariance matrix, have shown that the variance of some data is not zero. In other words, the field datasets acquired from the southeast of the survey line, have less accuracy. Finally, we can conclude that these three matrixes for the validation of the model and finding the best model parameters are very useful.
    Keywords: Data Resolution Matrix , Model Resolution Matrix , Unit Covariance Matrix , Generalized Inversion , Finite Difference , Resistivity
  • Afsaneh Nasrabadi*, Mohammad Reza Sepahvand, Zahra Limochi Pages 251-265
    Summary: Crustal velocity structure beneath four broadband seismic stations located in northeast of Iran, including Shahrood (SHRO) and Maraveh Tappeh (MRVT) stations set up by Iran National Seismic Network (INSN) and Sabzevar (SZBV) and Jarkhoshk (JRKH) stations set up by Iranian Seismology Center (IRSC), have been investigated by joint inversion of P receiver function and Rayleigh wave phase and group velocity dispersion curves. A three-year teleseismic data (2012 -2014) with epicentral distance of 25o-90o and magnitude more than 5.5 have been used to determine the receiver functions by iterative deconvolution in time domain proposed by Ligorria and Ammon (1999). Iterative deconvolution in time domain to determine the receiver functions are more stable with noisy data in comparison to frequency domain. The fundamental mode of Rayleigh wave group and phase velocity dispersion curves have been provided by the study on the structure of crust and upper mantle of the Iranian Plateau for the period interval of 10-100 seconds made by Rahimi (2010). A combined inversion of body wave receiver functions and Rayleigh wave velocities increases the uniqueness of the solution over the separate inversions, and also, facilitates explicit parameterization of the layer thickness in the model space. Moho discontinuity depth is one of the most important parameters for investigation of crustal structure. The results of this study indicate an average crustal thickness varying from 38 km beneath Maraveh Tappeh (MRVT) station in north of the study region up to 44 km beneath Shahrood (SHRO) station in west of the region. Moreover, the results of this study suggest that the average crust thickness beneath Sabzevar (SZBV) and Jarkhoshk (JRKH) stations located in center and east of the study region is 40 km. In general, northeastern Iran region has a thin crust compared to the crusts in the other areas investigated in this research work. It has also been shown that the joint inversion method can cause ±2 kilometers of error.
    Introduction: Iran is situated in one of the world's seismic regions and the possibility of destructive earthquakes in most regions of the country has given great significance to recognition of Iranian seismic nature from a seismic and seismotectonic standpoint. The seismicity within Iran suggests that much of the deformation is concentrated in the Zagros, Alborz and Koppeh Dagh mountains, and in east of Iran, surrounding Central Iran and the Lut desert. The aim of this research is to study the crustal structure and Moho discontinuity of northeastern Iran region, Binalood mountains and Koppeh Dagh by the analysis of receiver function and surface waves dispersion.

    Methodology and Approaches: Receivers functions are time series obtained from three-component seismometers, and are created by deconvolving the vertical component from the radial and transverse components of the seismogram to isolate the receiver site effects from the other information contained in a teleseismic P and S wave. The depth-velocity trade-off in receiver function causes nonuniqueness in the inverse problem. However, by incorporating information of absolute shear wave from dispersion estimates and joint inversion of these two datasets, this shortcoming can be compromised. To determine the receiver functions, we have used iterative deconvolution in time domain, proposed by Ligorria and Ammon (1999) that is more stable with noisy data in comparison to frequency domain. We have processed teleseismic events with epicentral distance of 25o-90o and magnitudes more than 5.5 that are recorded at a three-year time interval of 2012 to 2014. We have set the parameter a of the Gaussian filter to 1.00, which gives an effective high frequency limit of about 0.5 in the P wave. In order to eliminate the source, path and instrument effects, deconvolution of the vertical component from the horizontal components of the seismograms has been used. All receiver functions have been grouped by azimuth (<10◦) and distance (<15◦), and in order to improve the signal-to-noise ratio, the individual receiver functions within each group have been stacked. The fundamental mode of the Rayleigh wave group and phase velocities dispersion curves have been provided from the study carried out by Rahimi et al., (2014) on the structure of crust and upper mantle of the Iranian Plateau for the period interval of 10-100 seconds. Joint inversion of two independent data sets has been performed by considering appropriate weighting parameter obtained from Herrmann and Ammon program (2003). Minimizing standard error between real and predicted data is the criteria for getting the desired final and close to the earth real model. The inversion package requires that the real velocity structure is represented by a set of flat-lying, homogeneous, isotropic velocity layers. The starting model comprises of the layers having 1-km thick as the top 6 km of the model space, 2-km thick between the depths of 6 and 66 km, and 4 km thick between the depths of 66 and 78 km. The starting velocity for each layer in the model has been Vp=8.0 km/s, which equates to upper mantle velocity.

    Results and Conclusions: The results of this study suggest that the average crust thickness beneath Shahrood (SHRO) station, located in west of the study region is 44 km and the average crust thickness beneath Sabzevar (SZBV) and Jarkhoshk (JRKH) stations located in center and east of the study region is 40 km. Furthermore, the crust thickness beneath Maraveh Tappeh (MRVT) station located in north of Koppeh Dagh region in is 38 km. In general, northeastern Iran region has a thin crust compared to the crust in other areas of northeast of Iran.
    Keywords: Northeast of Iran , Koppet Dag , Crustal Structure , Receiver Function , Joint Inversion
  • Ali Adim, Mohammad Ali Riahi, Majid Bagheri* Pages 267-275
    Summary: Pore pressure is defined as the pressure of the fluid inside the pore space of the formation that is also known as the formation pressure. When the pore pressure is higher than hydrostatic pressure, it is referred to as overpressure. Knowledge of this pressure is essential for cost-effective drilling, safe well planning, and efficient reservoir modeling. To optimize drilling decisions and well planning in overpressure areas, it is essential to predict pore pressure in the first step. Before drilling, reliable prediction of pore pressure is critically important at different stages of petroleum engineering investigations. Pore pressure prediction has an important application in proper selection of casing and reliable mud weight. Currently pore pressure prediction in carbonate reservoir is still far from satisfaction, and there is no specific method widely accepted. The existing methods and theories in the pore pressure prediction community are almost all based on the shale properties. Although these methods are not the proper way to predict pore pressure in carbonates and may probably lead to dangerous errors, they are still used in the field practice of carbonate reservoirs. Introduction: The generation of abnormal pore pressure in carbonate formation is different from a reservoir to another reservoir because of different depositional and geological conditions. Although most studies currently point out that under-compaction mechanism is the dominant mechanism of abnormal pore pressure generation, the role of this mechanism in carbonate formations needs further study and discussion. Over-pressure can be identified by direct and indirect methods. Direct methods such as measured pressure data obtained from drill stem test (DST), repeat formation tester (RFT), modular dynamic test (MDT) and mud weight, and they are reliable evidences that reflect the over-pressure phenomenon in permeable reservoirs. Methodology and Approaches: Pressure coefficient, which is the ratio of the actual fluid pressure versus corresponding normal hydrostatic pressure at the same depth, has also been proposed and applied to investigate over-pressure. On the contrary, indirect methods, such as well logging, seismic and rock mechanic data, can identify abnormal pressure zones based on various response anomalies compared with the normal pressure system. The main objective of this study is to determine pore pressure using Eaton’s and Bowers’ methods and to compare their accuracy and usage. To obtain this goal in Bowers’ method, researchers try to directly link the formation velocity to the effective stress. One of these models was developed by Bowers (1995). Results and Conclusions: Sonic and density data in the distance between wells have been predicted using a combination of sequential Gaussian simulation and collocated cokriging techniques. Then, effective stress cube and overburden pressure cube have been predicted using velocity to effective stress transform and density to overburden pressure relation. Eventually, formation pore pressure has been predicted using Terzaghi’s method. Another way to estimate pore pressure is used from well logging data by applying Eaton's method (1975) with some modifications. In this way, sonic transient time trend line is separated by lithology changes determined from Zhang’s method. Our results show that the best correlation with the measured pressure data is obtained by the modified Eaton’s method with the Eaton’s exponent of about 0.4. Pore
    Keywords: Pore Pressure , Eaton’s Method , Bowers’ Method , Acoustic Impedance, Sequential Gaussian Simulation , Collocated Cokriging
  • Rashid Chambari, Amin Roshandel Kahoo*, Mahyar Yousefi, Mehrdad Soleimani Monfared Pages 277-292
    Summary: Complex structures in seismic images introduce ambiguities in structural and stratigraphic interpretation of seismic data. On the one hand, using high quality seismic image could resolve some of structural ambiguities of seismic image. On the other hand, some seismic attributes, which extract additional information from seismic data, could be used in interpretation step. However, vast number of introduced attributes brings another ambiguity that is which attributes would be appropriate for the specified purpose. Thus, several methods have been introduced to select appropriate attributes for a special case and furthermore, integrate attributes to extract as much as possible information and make their contribution in one single image. In this study, we have introduced a novel strategy for integration of seismic texture attributes in GIS environment. In the proposed strategy, each attribute is considered as a separate information layer, and then, these attributes undergo fuzzification process, and subsequently, are weighted by conventional functions and are integrated in GIS environment. This strategy has been applied on a complex seismic data containing a salt dome. Results of the application this novel strategy has proved that this strategy could image salt boundary and internal reflection of salt much better than conventional integration methods. Introduction: Seismic attributes could be used as an appropriate tool to extract as much as possible information from seismic data. By introducing vast number of attributes in recent decades, we find out that the problem moves to define a strategy on how select the most relevant attributes for each special interpretation and how it could be more useful if these attributes could be combined to obtain more and more precise and accurate information. Subsequently, numerous methods have been introduced for integration of best selected attributes to extract as much as possible information. Fuzzy methods are among conventional and most popular methods in this regard. In this study, we introduce a strategy to integrate attributes related to the family of gray level co-occurrence matrix (GLCM) texture analysis tool in GIS environment. Methodology and Approaches: The GLCM group of attributes gives different information from seismic data. These attributes could be more used by application of an appropriate integration method. In mineral exploration, a strategy is used by integration of different exploration information. The same strategy has been applied here for the GLCM attributes to obtain a final image for further geological interpretation purposes. These attributes undergo a fuzzification step performed by logistic function. Subsequently, they are weighted by conventional fuzzy weighting method. Converting the information layers (weighted attributes, known as an information layers) to binary domain, we then integrate these information layers by fuzzy operators. Finally, a unique map would be obtained which contains the most level of information of the target study. This strategy has been applied on a seismic image containing nearly horizontal layers and a complex geological structure of a salt dome. Results and Conclusions: We extracted conventional attributes from the seismic data. These attributes were energy, cluster prominence, entropy, variance, similarity, dissimilarity, intensity and contrast. Structural interpretation based on these attributes could not be performed precisely. For instance, boundary of salt dome, accurate truncation position of layers in conjunction with salt boundary, exact width of salt and internal reflection within the salt were imaged differently in each attribute image. Thus, they were not precise for further structural interpretation. Therefore, they were processed by the proposed method to obtain an appropriate seismic attribute image. Initially, each attribute image, known as an information layer, was processed by fuzzification function and was prepared for suitable weighting. Then, these information layers were weighted in GIS environment prepared for further integration. Based on the final goal of interpretation, which is the most accurate separation of the salt and surrounding sedimentary media in the seismic image, higher weights were allocated to those attributes containing more information of the salt structure. As midway result, the images of fuzzy GLCM attributes could better separate the salt and surrounding sedimentary media from each other, and as a consequence, they could better image the salt boundary and internal reflection within the salt. Finally, these results were integrated by the proposed method and the final image was considered for any further structural interpretation.
    Keywords: Gray Level Co-Occurrence Matrix (GLCM), Texture Attribute, Salt Dome , GIS , Fuzzy Weighting, Logistic Functions
  • Milad Farshad*, Ahmadreza Mokhtari Mobarakeh Pages 293-304
    Summary: Velocity analysis is one of the most important stages of seismic processing. The most conventional method for velocity analysis is to calculate the semblance coefficients. Traditional semblance has some shortcomings, for instance, low resolution in time and velocity direction, high computational cost, and having trouble in presence of amplitude variation-with-offset (AVO) phenomenon. In order to compensate for the latest shortcoming, AB semblance has been proposed. However, this method has approximately twice the lower resolution than traditional semblance. On the other hand, due to effects of the source wavelet, seismic events have band-limited nature, which leads to a decrease in temporal resolution. Recently, the deconvolutive Radon transform has been introduced to overcome the latest problem. In this paper, we have developed the deconvolutive hyperbolic Radon transform for AVO preserved velocity analysis. We have also used the log-polar domain in order to reduce computational cost. We tested this method on both synthetic and real field data sets to show resolution improvement in the proposed method. Introduction: Velocity analysis is one of the most important steps in seismic data processing and interpretation. It affects many processes of seismic data processing, such as NMO correction, stacking, and time and depth migration. There are several methods for calculation of velocity spectra. The most conventional method for velocity analysis is based on moveout of reflection events, which uses the coherency. Semblance is the most conventional coherency measure, which is defined as normalized output to input energy ratio of a windowed hyperbola. Although it is functional in most circumstances, it has some problems such as low resolution in time and velocity direction, and lack of velocity peak in the presence of strong variations of amplitude along seismic events (AVO) or polarity reversals. In order to overcome the later problem, AB semblance has been introduced. However, it suffers from lower resolution compared to conventional semblance. In this papr, high-order sparse deconvolutive Radon transform has been employed to increase the resolution of velocity spectra. Methodology and Approaches: Since the basic functions in Radon transform are independent of the data which is to be analyzed, the Radon transform is a non-adaptive transform. Due to effects of the source wavelet, seismic events have band-limited nature, while basis functions of Radon transform are Dirac delta functions in time. This leads to a decrease in resolution. Recently, the deconvolutive Radon transform has been introduced in order to overcome these problems. This method can be defined as a blend of deconvolution and Radon operators. The conventional Radon transform sums the amplitudes along trajectories for a specific time and velocity values and does not consider AVO characteristics of seismic data. To avoid this drawback, high-order Radon transform has been introduced . This method combines the conventional Radon transform with the orthogonal polynomial transform. As a consequence, it can estimate the seismic data more accurately by using a few polynomial coefficients to represent its AVO characteristics. In this paper, we have used higher order of deconvolutive Radon transform to overcome the AVO problem and enhance the resolution in time direction. Moreover, by applying fast iterative shrinkage-thresholding algorithm (FISTA), a high-resolution velocity panel is obtained. The main step of this algorithm involves the computation of the forward and adjoint operators, which in the case of hyperbolic Radon transform can be very timeconsuming. In order to reduce the computational costs, we have rewritten the Radon transform in log-polar coordinates. By doing so, the main computational parts are attributed to compute convolutions, which can be computed rapidly in frequency domain. In order to use frequency domain, samples in log-polar coordinates must be chosen on an equally spaced grid; Thus, interpolating is required for switching between log-polar and time-offset coordinates. Results and Conclusions: In this paper, we have developed the deconvolutive hyperbolic Radon transform for AVO preserved velocity analysis. This method is based on higher order of deconvolutive hyperbolic Radon transform. This method significantly improves the resolution of velocity analysis in presence of AVO. The method has been examined by applying it on real field and synthetic data. The results from these tests have confirmed the above claims.
    Keywords: Velocity Analysis , Hyperbolic Radon Transform , Deconvolutive Radon Transform , Radon Transform in Log-Polar, Coordinates , High-Order Radon Transform , AVO
  • Sadegh Moghadam*, Mozhdeh Azadi, Asghar Azadi, Mohamad Jafari Pages 305-321
    Summary: Recently, geophysical methods have been used successfully to determine geotechnical characteristics or parameters of subsurface layers. In this research, the integrity of vertical piles, the dynamic elastic parameters of soil and the thickness of the piles in a residential construction site in Kelarabad, located in west of Mazandaran, Iran, were delineated using pile integrity testing (PIT), downhole and crosshole methods. Introduction: A pile is a slender element cast in the ground or driven into it. In building piles, integrity determination of the product at the end is very difficult because it is not possible to evaluate the integrity directly. However, primary controls such as sampling of concrete before pouring in the well or record the volume of the poured concrete for the pile is carried out in all pile building projects. Despite this, there are always some problems like collapsing the wall of the well and making a thin region in the pile section or occurring a disorder or pausing in drilling or concrete pouring operation that makes further pile integrity tests necessary. One of these tests is impulse response non-destructive test. The purpose of integrity testing is to discover such flaws before they can cause any damage. This method covers the procedure for determination of the integrity of individual vertical piles by measuring and analyzing the velocity and force response of the pile induced by an impact device usually applied axially and perpendicularly to the pile head surface. The impactor is usually a 1-kg sledge hammer with a built-in load cell in the hammer head. Response to the input stress is normally measured using a geophone. This receiver, namely geophone, is preferred to accelerometers because of its stability at low frequencies and its robust performance in practice. By processing the obtained data and interpretation of them, some indexes are obtained that are useful to evaluate the integrity of the pile. Furthermore, construction of foundation systems for civil structures often requires detailed information of the site soil properties. Bore logs provide soil samples for soil type classification and laboratory testing to determine the strength and consolidation parameters with respect to depth. Downhole and crosshole methods have been developed to measure shear wave velocity and consolidation parameters with regard to depth. The downhole seismic investigations require only one borehole to provide shear and compressional velocity wave profiles. This method uses a hammer source at the surface to impact a wood plank and generating shear and compressional waves. The energy from this impact is then received by a pair of matching three component geophone receivers, which have been lowered downhole and are spaced 3 m apart. Crosshole seismic investigations are performed to provide information on dynamic soil and rock properties for earthquake design analyses of structures, liquefaction potential studies, site development, and dynamic machine foundation design. These investigations determine shear and compressional wave depth versus velocity profiles. Other parameters, such as Poisson's ratios and moduli, can be determined from the measured shear and compressional wave velocities. Methodology and Approaches: The evaluation of PIT surveys was carried out based on the information from 80 piles in the field site. In addition, crosshole seismic test was carried out in 20 boreholes drilled in the investigated site in order to evaluate the thicknesses of the piles. On the other hand, to measure the vertical changes in seismic velocity and geotechnical parameters, the downhole test was also carried out in 2 boreholes. Results and Conclusions: According to the results of the PIT, the frequency chart, velocity map of the concrete structures and the velocity graph of the tested piles in terms of consumable cement have been obtained to illustrate the strength of the pile structures. Using the velocities of P and S waves and geotechnical parameters from 2 boreholes, the dynamic elastic parameters of soil have been computed. After applying conventional pre-processing methods, travel time curves have been obtained. On the basis of seismic downhole surveys carried out in 2 boreholes, the ground has been classified as class II in terms of the Iranian code of practice (Standard No. 2800). Finally, according to the results of crosshole test, the thicknesses of the pile structures with an acceptable approximation of 0.5 to 0.8 m have been calculated.
    Keywords: Pile Integrity Testing (PIT) , Downhole , Crosshole, Kelar-Abad
  • Behrooz Oskooi*, Seyed Mohammad Javad Rouhani, Safieh Omidian, Maysam Abedi Pages 323-337
    Summary: Polour basalts are located 75 km away from Tehran. It is situated in northeast of Tehran and south of Damavand volcano. The basalt emission point has not been distinguished yet, whereas its occurrence is a debatable topic among geologists. The procedure of formation, geological setting and the way of ascending have been always as controversial issues. Petrologically, Polour basalts are basalt–basalt trachyte rocks. Similar units are found at the east of Damavand volcano and as expected, the composition, age and feeder source are different from Damavand volcano. Polour basalts occurred as arc-shaped bodies at shallow depth in central part of study region, where they mostly consist of old terraces and alluvial fans. Because of having considerable amount of magnetic susceptibility, magnetometric survey might delineate the location of such geological units. Various depth estimation techniques have been employed to determine the depth of causative sources associated with the basaltic units in the region of interest. Introduction: Several well-known techniques have been developed to estimate the depth of causative sources in potential field studies. Among them, AN-EUL, Euler deconvolution and analysis of power spectrum methods are the most efficient ones, which provide valuable pieces of information about the geometry of the prospect geological sources. Methodology and Approaches: AN-EUL technique, as a combination of the analytic signal and the Euler deconvolution methods, is an automatic algorithm for simultaneous estimation of depth, location and geometry of the subsurface sources in the potential field studies. The derivation of the main equations of this technique is based on the substitution of the derivatives of the Euler homogeneous equation into the analytic signal of the potential field data. Location of sources can be approximately estimated from the position of the maximum value of the analytic signal amplitude, and subsequently, the formulae of depth and structural index (SI) estimation are calculated at this point. An important advantage of the AN-EUL method is that it is not restricted only to idealized sources (i.e. having integer structural index). Its wider applicability means that the SI can be a fractional number that indeed describes sources with various arbitrary shapes. Analysis of power spectrum is also one of the methods used widely to estimate the depth of geological structures. The sources of magnetic anomalies within a region are assumed to average out so that spectral properties of an ensemble of sources are equal to the average of all causative sources responsible for potential field anomalies. This approach is advantageous since it is (1) statistically oriented, (2) averaging source depths over a region containing complex patterns of anomalies, (3) less affected by interference effects due to overlapping anomalies and high-wavenumber noise than other methods because it is based entirely on analyzing the wavelengths of the anomalies, (4) independent of the directional attributes of the magnetization of the sources and the geomagnetic field, and finally (5) it can be used to study a wide range of depths by varying the window involved in the data analysis. Results and Conclusions: Employing the above-mentioned methods for estimation of the depth and geometry of subsurface anomaly to the collected ground-based magnetometric data leads to obtain valuable information about the subsurface anomaly. Thus, in this study, the depths and structural indices of the subsurface anomalies have accurately been estimated that have been strongly in good agreement with the geological information of the study area. The maximum depth on both arcs is about 95 m below the surface topography. In addition, the 3D model from the area reveals the basalt root in both bodies amazingly is from separated sources. Due to the short length of profiles, that causes to have only shallow depths of identified bodies, the trend of creeping magma from the chamber is unknown. Subsurface anomalies and their depths indicate that the Polour basalts have deep roots and have not flown on the ground surface. By proving the existence of the anomalies as signatures of basalts, the depths and structural indices of the anomalies have accurately been estimated.
    Keywords: Basalts of Polour , Structural Index, Power Spectrum Method, Analytic Signal , 3D Modeling, Euler Deconvolution
  • Vahid Riyahi Kojor, Narges Afsari* Pages 339-349
    Summary: The attenuation of seismic waves with respect to distance is one of the most important parameters in seismological and earthquake engineering studies. The purpose of this study is to estimate quality factor of shear waves (QS) of seismic data of the earthquake occurred in the Central Alborz at 34º to 37º north latitude and 50º to 56º east longitude, using the spectral drop. For this purpose, the data from earthquake of May 28, 2004 Firouzabad-Kojur and its aftershocks, recorded by seismic networks of Sari and Semnan, affiliated with the Institute of Geophysics of Tehran University have been used; and the quality factor of direct S wave (QS) at the seven central frequencies of 1.5, 3.0, 4.5, 6.0, 9.0, 12.0 and 18.0 Hz has been estimated. The estimated frequency-dependent relationship of QS on N-S and E-W components are QS=76.61f 0.8 and QS=70.35f 0.85, respectively. The mean values of QS for the two components indicate a relation with frequency that is QS=73.54f 0.83. The results show an increase in QS with increasing frequency, and Q0 value for this area corresponds with the seismotectonic study of the region. Introduction: When seismic waves pass through the earth they interact with inhomogeneous, anisotropic and non-elastic environments. Obviously, by knowing the effect of these factors on seismograms, we can obtain a considerable amount of information from within the earth. The energy of waves from a seismic source decreases with increasing distance from the source, and consequently, the seismic wave amplitude will decrease. It is mainly due to the geometry of propagation of seismic waves and partly due to the anelastic properties of the material through which they travel. The energy loss of seismic waves due to non-elastic effects is named intrinsic absorption. The inverse of attenuation represents quality factor. By calculating the quality factor in each region, we can understand the rate of seismic activity in that region. Moreover, this quantity has many applications in seismological and earthquake engineering studies. The study region is located at longitude 50°-56° E and latitude 34°-38° N in north of Iran, and in terms of seismotectonic provinces of Iran, it is located in Central Alborz (Mirzaei et al., 1998). The Alborz Mountain of northern Iran form a belt of active crustal deformation along the southern side of the Caspian Sea within the broad Arabian-Eurasia continental collision zone. In this study, the quality factors of shear waves (QS) have been estimated in the Central Alborz. Methodology and Approaches: There are many different methods for estimating the quality factor. In this study, the spectral decay method (Anderson and Quas, 1988) have been used for estimation the quality factor of shear waves (QS) at the seven central frequencies of 1.5, 3.0, 4.5, 6.0, 9.0, 12.0 and 18.0 Hz. For this estimation, shear waves on horizontal components (E-W and N-S) have been analyzed (Yoshimoto et al., 1993; Chung et al., 2001; Kim et al., 2004; Rahimi et al., 2010a). The bandwidth of each frequency band is 2/3 of its central frequency. To determine the shear wave window on the seismogram, the start of the S wave is determined by observation, and then, the end of the window is determined using the Kinoshita (1994) algorithm. Furthermore, the shear wave velocity in the study region can be considered as an average of 3.58 km/s (Afsari et al. 2015). With the first-order fit (with slope b) in the least squares, QS is obtained in each frequency band for the horizontal components E-W and N-S, according to the QS = - (πf/βb). Results and Conclusions: In this study, we have analyzed the attenuation of the S waves using the data recorded of the 2004 Firouzabad-Kojur earthquake and its aftershocks. The mean QS values for the seven central frequencies have been determined. The estimated frequency-dependent relations for E-W and N-S components are expressed as QS=70.35f0.85 and QS=76.61f0.8, respectively.. Finally, the mean QS-frequency relationship of the two horizontal components of the studied range is expressed as QS=73.54f 0.83. In the Central Alborz region, the values of Q at 1.0 Hz are less than 200 for the frequencydependent relationships of QS. This implies high attenuation of the wave S at the studied frequencies and distances. A strong correlation between n and the level of tectonic activity of the region has been observed by several investigators (Aki, 1980; Gupta and Ashwani, 1988). The value of n in the estimated QS relation for the study region indicates that the area is active. Morover, the results obtained in this study are consistent with previous studies carried out in the active seismotectonic zones in Iran and elsewhere in the world.
    Keywords: Alborz, Quality Factor Qs, Attenuation , Shear Wave
  • Mohammad Rasool Nikbakhsh, Mirsttar Meshinchi Asl*, Mohsen Oveisy Moakhar, Hmid Reza Siahkoohi Pages 351-363
    Summary: This paper presents a new method for interpretation of two dimensional (2D) magnetic anomaly data. The new method uses a combination of analytic signal and its total gradient to estimate the depth and nature -i.e. structural index- of an isolated magnetic source. However, the proposed method is sensitive to noise. In order to lower the effect of noise, upward continuation technique is applied to smooth the anomaly. Tests on synthetic noise-free and noise-corrupted magnetic data show that the new method can successfully estimate the depth and nature of the causative source. For practical application of the method, it was applied to measured magnetic anomaly data from Khalilabad area. For validity of the method, it was tested on a synthetic example with and without random noise. After adding 5%, 10% and 15% random noise to the synthetic data, the maximum error for the model parameters was seen to be less than ±3%. Moreover, it was found that the inversion results of magnetic data from an area in northeast Sirjan was in good agreement with the results from Euler deconvolution of the analytic signal of the magnetic data. Introduction: An important goal in the interpretation of magnetic anomaly data is to obtain the depth and the geometry or structural index of the causative source. To this end, a large number of methods exist to accomplish this goal. The analytic signal method is a popularly used method for this purpose. Using the analytic signal method, we can calculate both the depth and structural index of causative sources. However, the improved analytic signal method, presented in his paper, needs the computation of third-order derivatives of the magnetic anomaly and requires data of high precision or strict filtering. Methodology and Approaches: In this paper, we present an improved analytic signal method to interpret 2D magnetic anomaly data. The proposed method uses a combination of the analytic signal and its total gradient to estimate the depth and the structural index of the causative source. The feasibility of the proposed method to compute the source parameters is displayed on synthetic and measured magnetic anomalies. Results and Conclusions: We have proposed a new method to estimate the depth and the structural index of the causative source using the ratio of analytic signal to its total gradient. Our method provides two linear equations to estimate individually the depth and nature of a magnetic source. The feasibility of the new method is demonstrated on synthetic magnetic anomaly with and without random noise. For noise-free data, the structural index and the depth, estimated by the new method, are consistent with the theoretical values. For noise-corrupted data, our approach can provide reasonable results by applying upward continuation technique to lower the effect of noise. Application of the method on measured data indicates that the results of the method are in god agreement with the results computed by the Euler deconvolution of analytic signal method.
    Keywords: Improved Analytic Signal , Magnetic Anomaly, Depth Estimation , Structural Index
  • Farhad Khoshbakht * Pages 365-375
    Summary: Resistivity logs are among the most important logs since their measurements are directly related to hydrocarbon saturation. Conventionally, Chartbooks as theoretical response of logs are used for interpretation of electrical logs. One dimensional (1D) computer codes are the cornerstones of chartbooks. These codes solve Maxwell’s equations by ignoring layer inclination, non-uniform mud invasion, shoulder bed effect and well deviation. Simplifying the geometry of the model introduces significant amount of error in the interpretation of resistivity logs. Consequently, it is necessary to utilize complex numerical models for interpretation of the logs. The main objective of this paper is to present numerical modeling of array induction log or tool (AIT). The numerical modeling enables us to understand the response of the tool in different well and reservoir situations. Proper understanding of the response of the tool improves the accuracy of interpretation of the logs and reduces the risk of logging jobs in the harsh conditions. For verification of the model, the resistivity logs were constructed in a multi-layer model. The modeled resistivity logs were in good agreement with the true formation resistivity values in thick layers but conversely, in the thin layers, the performance of the model was not acceptable. Sensitivity analysis was performed on the frequency, the transducer-receiver spacing of AIT and the resistivity of mud. In higher frequencies, due to skin effect, the tool is not able to measure the true resistivity of the layer. Increasing the transducer-receiver spacing does not affect the accuracy of the AIT while it decreases the vertical resolution of the tool. In wells where the conductivity of mud is below 1 S/m, the AIT tool cannot be run. Introduction: Wireline logs indirectly measure the petrophysical properties of hydrocarbon reservoirs. The most important logs are electrical logs because they determine the amount of hydrocarbon in the pore space of the study reservoir. Since the invention of electrical logs in 1927, many researchers have investigated new methods to improve the accuracy and precision of the interpretation of electrical logs. Chartbooks, forward modeling, inversion and three dimensional (3D) numerical modeling are the main approaches for the interpretation of resistivity logs. In this paper, AIT, as a modern resistivity log, which is frequently used in oil industry, was numerically modeled to understand its behavior in hydrocarbon reservoirs. The results of the paper improve interpretation methods and give geometrical properties of reservoirs, and also, optimize the log acquisition plans. Methodology and Approaches: Ansys-Maxwell electromagnetic simulator has been used to numerically model AIT. It numerically solves the Maxwell’s equations in a specific geometry and calculates the vectors of magnetic field. All details of the tool including number of coils, distance of transmitter and receivers, materials of coils, frequencies and electrical current have been identical to the real tool. The modeled AIT tool has also been calibrated in a multi-layers formation model. The calibrated model has been utilized to calculate the geometrical factor of shallow, medium and deep arrays. Moreover, sensitivity analysis has been performed on the frequency, transmitter-receiver distance of the tool and the resistivity of borehole mud. Results and Conclusions: Numerical modeling of AIT enables us to analyze and predict the behavior of AIT in different logging conditions, formation properties and geometry. In this research, it has shown that the AIT accurately measures the resistivity of thick layers but it lost its accuracy in measuring the resistivity of thin beds. The sensitivity analysis, made in this study, has demonstrated that the measured resistivity linearly depends on the frequency of transmitter. Skin effect masks the response of AIT in high frequencies and leads to improper measuring of resistivity. Precise modeling of AIT helps us to design a domestic version of the array induction tool, which is customized to the characteristics of Iranian reservoirs and borehole conditions.
    Keywords: Resistivity , Logging Tool , Reservoir , Hydrocarbon Saturation, Sensitivity Analysis
  • Mohammad Rezaie * Pages 377-385
    Summary: Edge detection of subsurface structures is an important objective in interpretation of magnetic data. In this paper, curvature gradient tensor (CGT) of magnetic data has been used along with tilt angle method to detect edges of subsurface structures. Application of these methods on synthetic and real gravity data has shown that the CGT of magnetic data, compared to the tilt angle method, can determine the edges of subsurface structures better. Introduction: The main objective of the interpretation of magnetic data is to extract information about subsurface structures. Edge detection is an important means to image the edges of subsurface structures. Therefore, edge detection has traditionally been an important objective in the interpretation of magnetic data. There are various methods for edge detection. Tilt angle method is a traditional method that can detect edges of subsurface structures quantitatively. The value of Tilt angle is zero above the edges of subsurface bodies. The curvature gravity gradient tensor (CGGT) has also been used to interpret subsurface geological structures quantitatively. The eigenvalues of CGGT are zero above edges of subsurface bodies. In this paper, the CGT of magnetic data has been used for edge detection of subsurface magnetic bodies. The results of using the CGT of magnetic data have been compared with the results obtained from applying Tilt angle method on the data. Methodology and Approaches: In order to obtain the CGT of magnetic data, at first, the magnetic data are reduced to pole (RTP). Then, horizontal vector gradients of the gradient tensors are computed from the RTP data using a Fourier transform technique. Then, the eigenvalues of the CGT of magnetic data are obtained. The small eigenvalue can only be used to detect the edges of bodies with positive susceptibility contrast, and the large eigenvalue can only be used to determine the edges of bodies with negative susceptibility contrast. As an example, chromite ore has positive density contrast with the host rock and produce positive gravity anomaly. Finally, the tilt angle method is also applied to compare its results with those of the CGT of magnetic data. Results and Conclusions: The robustness of the method used for the enhancement of edge detection is tested with a magnetic anomaly map caused by two prisms of synthetic bodies with positive and negative susceptibility contrast. The results have shown that the zero contour of the small eigenvalue of the CGT of magnetic data compared to the zero contour of the tilt angle method can better detect the edges of synthetic bodies with positive susceptibility contrast. Moreover, the zero contour of the large eigenvalue of the CGT of magnetic data compared to the zero contour of the tilt angle method can better detect the edges of synthetic bodies with negative susceptibility contrast. The Tilt angle method is also more sensitive to noise than the CGT of magnetic data. The CGT method has been applied to real magnetic data from Qahan porphyry copper deposit in Markazi Province, Iran. The results have indicated that the large eigenvalue of the CGT can determine the edges of porphyry deposit and the small eigenvalue can outline positive magnetic anomalies caused by propylitic alteration. However, the tilt angle method has not been capable of finding the edges of the porphyry deposit.
    Keywords: Magnetic Method , Edge Detection , Gradient Tensor , Tilt Angle , Subsurface Structure, Qahan , Porphyry Copper
  • Fatemeh Razavirad, Ahmad Ghorbani* Pages 387-400
    Summary: In this study, the influence of pore fluid salinity on the complex electrical conductivity responses of sandstone and sand samples is analyzed. The investigated samples in this study include clean sandstone and sand samples provided by an Iranian offshore oil company and sandstone samples containing clay minerals that have taken from a sandstone aquifer located in northwest of England. All samples have been saturated with sodium chloride solution. The fluid electrical conductivity ( ) of sodium chloride solution has gradually been increased from 57 mS/m to 6000 mS/m. The expected linear relation between and the real component of electrical conductivity ( ) of the saturated samples has been observed. It is also observed that the imaginary component ( ) increases with the increase of salinity, and consequently, the increase of fluid conductivity. To determine the relaxation time ( ) of spectral induced polarization (SIP) data, ColeCole model has been fitted on the measurements carried out at four different salinities of sandstone aquifer samples. The Cole-Cole relaxation time increases with the increase of fluid conductivity for all samples except two samples. The behavior of is also comparable to as both parameters measure the polarizability. In other words, normalized chargeability also increases with the increase of fluid salinity. In this study, the dependence of polarizability on fluid salinity is also demonstrated. Maximum values of polarizability are observed at high salinity for quartz dominated siliceous material. Introduction: The induced polarization (IP) geophysical method measures the low-frequency electrical properties of rocks and soil materials. This method has appeared as a potentially powerful tool for subsurface imaging due to the dependence of the measurements on rock or soil internal surface area. Given the increasing interest in using IP measurements to deduce rock textural properties such as permeability, further studies of the dependence of IP measurements on pore fluid composition are needed. In SIP, a phase lag between the current and the electrical field provides complementary information to electrical conductivity measurements. The conductivity and the phase can be rewritten into a complex conductivity (or a complex resistivity) that can be measured over a broad range of frequencies, typically from 1 mHz to few tens of kHz in the laboratory and from 10 mHz to 10– 100 Hz in the field. Methodology and Approaches: SIP measurements were collected in a fully saturated state for each sample at four different salinities. All samples were saturated with a NaCl solution starting with the lowest salinity and then fluid electrical conductivity ( ) of sodium chloride solution was gradually increased from 57 mS/m to 6000 mS/m. The electrical impedance was measured using a four-electrode array. Two silver coils were used as current electrodes to inject an alternating sinusoidal current. The voltage and the phase shift between the applied current and measured voltage was determined using Ag-AgCl nonpolarized potential electrodes. Electrical measurements were taken using the ZEL-SIP04-V02 impedance meter in a frequency range of 2 mHz to 45 kHz (Forschungszentrum Julich GmbH). Results and Conclusions: Our investigation has shown that the fluid chemistry is an important parameter that controls the polarizability of the mineral-fluid interface. Real and imaginary components of conductivity increase with the increase of saturating fluid conductivity. It has also been demonstrated that normalized chargeability show a positive power law relationship with fluid conductivity. The results of Cole-Cole model fitted on data have indicated that relaxation time increases with salinity for all samples except two samples. We have also shown that polarizability parameters are controlled by fluid conductivity.
    Keywords: Spectral Induced Polarization , Salinity, Imaginary Conductivity , Normalized Chargeability, Relaxation Time, Cole-Cole Model, Fluid Conductivity
  • Peyvand Heidarnejad, Sanami*, Ali Nejati, Kalate Pages 401-412
    Summary: Geothermal energy can be a good replacement for any kind of energy we use today. Estimation of curie point depth (CPD) is one of the first steps in geothermal exploration. Spectral analysis of aeromagnetic data can provide important information about temperature distribution in depth. In this study, we attempt to estimate the CPD using centroid method and forward modeling in northwest of Iran. The reduced-to-pole (RTP) aeromagnetic data were divided into 11 overlapping windows of the size of 100 100  km. In the centroid method, the average depth to the top of the deepest crustal block, t Z was first computed by linear fitting to the second longest wavelength segment of the power spectrum of aeromagnetic data. Then, depth to the centroid of the deepest crustal block, o Z was computed by linear fitting to the longest wavelength segment of power spectrum of aeromagnetic data. In forward modeling, the modelled spectra were fitted to the observed spectrum iteratively and the depth to the top and bottom of the block was finally estimated. According to the obtained results, the CPD is shallow in the west part of the area and as a comparison of the results with the locations of hot springs in the area, we conclude that the west part of the area has a good potential for geothermal exploration. Introduction: Estimation of CPD using spectral analysis of aeromagnetic data is one of the first steps in primary geothermal exploration. The goal of estimation of CPD is to know the thickness of crust and temperature distribution in depth from a set of known aeromagnetic observations measured on the surface. Spectral analysis of aeromagnetic data can also help in the constraining of the temperature within the crust based on identifying and mapping the depth of curie isotherm. Above the curie temperature (580 C  ), magnetic minerals lose their ferromagnetism. This means that deeper layers at greater temperatures are essentially non-magnetic. This curie isotherm interface can be detected through a number of spectral magnetic methods. In this paper, geothermal exploration in northwest of Iran is attempted by determining the depth to the magnetic bottom. We estimate the magnetic bottom using two different methods that are centroid method and forward modeling of the spectral analysis of aeromagnetic data. These methods are used to determine CPD or magnetic bottom in northwest of Iran. Methodology and Approaches: Aeromagnetic data of the area were taken from the Geological Survey of Iran. This data were corrected for the international geomagnetic reference field (IGRF1976). In this study, centroid method and forward modeling of the spectral analysis of the aeromagnetic data were used to determine CPD. To use these methods, the RTP aeromagnetic data were divided into 11 overlapping windows of size of 100 100  km (overlapped 50% with the adjacent windows). The two dimensional (2D) power spectrum of aeromagnetic data for each window was computed by Oasis Montaj software using fast Furrier transform (FFT) method. The biggest advantage of 2D power spectrum is that the depth of sources is easily determined by measuring the slop of power spectrum when the centroid method is used. Determination of CPD using centroid method can be carried out in two steps. Firstly, the centroid depth of the deepest magnetic source is estimated from the slop of the longest wavelength part of the spectrum divided by the wave number and the depth to the top of the magnetic source is similarly derived from the slope of high wave number portion of the power spectrum. Then, the bottom of magnetic source is obtained. Using forward modeling for estimation of CPD or magnetic bottom has another advantage. That is the depths to the top and bottom of magnetic source can be determined together. Another advantage of forward modeling is that it allows one to fit iteratively the position matching the adjacent part of the slope more precisely and explore the model space. Based on the fitting the modeled spectra with the observed, one may accept or reject the results more confidently in this overall subjective process of fitting specific part of the spectra. Results and Conclusions: An attempt has been made to calculate the depth to the bottom of magnetic sources from the aeromagnetic data in northwest of Iran using two spectral methods. CPD has been calculated by the centroid method and forward modeling. The results show that the CPD varies from 15 to 23km in the study area. In forward modeling, the calculated power spectrum is fitted iteratively with the measured power spectrum using MATLAB software. A conclusion is that by using forward modeling, the depths to the top and bottom of magnetic source are estimated together and with this approach, CPD can be estimated better. Because of that, the CPD, estimated by forward modeling, is considered the CPD of study area. According to the obtained results of forward modeling, the CPD is shallow in west part of area. By comparing the results with the locations of hot springs in the area, we conclude that west part of area has a great potential for geothermal exploration. Consequently, it is proposed that the results of this study to be integrated in GIS environment with all available geological, geophysical, geochemical and other information layers. This additional information will facilitate selection of optimum sites for geothermal exploration.
    Keywords: Geo Thermal , Radially Power Spectra , Forward Modeling, Centroid Method , Curie Point
  • Moustafa Mousapour Yasoori*, Vahid Ebrahimzadeh Ardestani Pages 413-427
    Summary: First and second vertical gradients are widely used in the interpretation of gravity data. Vertical gradients are sensitive to noise. Accuracy of vertical gradient calculation directly effects the accuracy of interoperations. Therefore, accurate and without noise calculation of vertical gradient is vital. The most common method to calculate vertical gradients is to use Fourier transform. Low noise in the gravity data causes that the vertical gradients, calculated by Fourier transform, have severe noise. In this research, we have used discrete cosine transform (DCT) to calculate vertical gradients. Results of DCT and Fourier transform are completely equal when the gravity data are noise free, but in the case of noisy data, DCT has better performance than FFT. This improvement is investigated by using signal to noise ratio (SNR). The SNR of the results of DCT compared to Fourier transform is larger, therefore less noise enters in the calculation of vertical gradients by using DCT. We have tested these two transforms on the synthetic data containing Gaussian noise. First and second vertical gradients are calculated by DCT and Fourier transform. The results have shown that DCT in comparison with Fourier transform is less sensitive to noise. Moreover, these two transforms are used for calculating first and second vertical gradients of gravity data obtained from Safo manganese mine. The results have shown that less noise enters in vertical gradient map obtained using DCT. Edge detection of anomalies is one of the usage of gradients in the interpretation of gravity data. Analytic Signal has been used for edge detection of anomalies in the cases of real and synthetic gravity data. Vertical gradient of analytic signal calculated by DCT, compared to Fourier transform, has less noise and better quality. Introduction: The first and second vertical gradients are used to distinguish the difference between two adjacent anomalous bodies, reduce the effects of interference of the amplitudes of anomalies, separate the local field superimposed on the background determine, and to determine the location and dimensions of the anomalies. The gradient data are used in direct interpretation and inversion and inputs of many interpretations. Vertical gradients are more sensitive to noise than bouguer map, and second vertical gradients is more sensitive to noise than first vertical gradients. Methodology and Approaches: Generally, the relation between cosine and sine transforms with Fourier transform is equal to ‘F=C + iS’. Where F, denotes Fourier transform and C and S denote cosine and sine transforms, respectively. If the function is positive or constant, imaginary part of the Fourier transform is equal zero. In other words, Fourier transform and DCT will be equal. It can be assumed that noise is a function that is added to gravity signal. Because of the nature of noise, Fourier transform and DCT of noise will not be equal. Then, the signal to noise ratios for DCT and Fourier transform of noisy gravity signals are not the same. Hence, less noise enters the gradient map when calculating vertical gradients using DCT. Results and Conclusions: In this research, we have used DCT to calculate vertical gradients. We analytically have proven that DCT in noise-free data has equal results with Fourier transform results. However, the DCT of noisy data have shown less noise enters the data compared to Fourier transform. We have examined this issue by using synthetic data containing Gaussian noise. A comparison between vertical gradients obtained by DCT and Fourier transform indicates that DCT is less sensitive to noise. We have obtained similar results by using real data. The results of analytic signal calculated by DCT and Fourier transform in both cases of real and synthetic data have been compared. Edge detection calculated by using DCT has shown less noise due to less noise in the DCT and also has better quality compared to using Fourier transform.
    Keywords: Vertical Gradients, Fourier Transform, Cosine Transform, Reduction Noise, Gravity , Safo Manganese Mine
  • Roohollah Kimiaee, Hamid Reza Siahkoohi, Alireza Hajian*, Ahmad Kalhor Pages 429-440
    Summary: Anisotropic diffusion filtering (ADF) is widely used as an efficient method in random noise attenuation problems, and various modifications to its original version have been proposed. The main reason could be the thought that ADF preserves edge features with acceptable performance beside noise attenuation procedure. In seismic data processing, however, it should be noticed that using ADF could cause severe changes (artifacts) in the zones that are highly contaminated with random noise. In this paper, the optimum value is derived, by introducing an automatic framework based on two artificial intelligence (AI) algorithms, adaptive neuro-fuzzy inferences (ANFIS) and fuzzy c-mean clustering (FCM). The neuro-fuzzy network is trained using original data, successive ADF values are calculated for each data point, and FCM output is obtained in a weighted averaging manner adapted with estimated noise level. The trained network is, then, generalized to all data, and thus, the ANFIS optimized version of ADF, called here AOADF, is achieved. Comparison of the results of the ADF and AOADF experiments reveals that in synthetic common mid-point (CMP) gathers, the proposed method improves peak signal to noise ratio (PSNR) value, 40% higher than ADF (in the best case) and in real CMP and common offset sorted gathers, the performance of AOADF is considerably higher than ADF, in terms of random noise attenuation without adding unwanted artifacts and preserving continuity of coherence components. Introduction: As an inevitable phenomenon in seismic data acquisition, random noise affects the processing and interpretation results of seismic reflection data. Although it is expected that random noise decreases dramatically by increasing the stack fold, observation of random noise at far offsets and later arrivals, especially in relatively deep acquisitions, could be considered as a common case. This paper intends to enhance the signal to noise ratio (SNR) of the seismic reflection data by attenuating background random noise and preserving reflection data, utilizing powerful potential of the ANFIS and FCM in model discrimination and the ability of ADF in random noise attenuation. The proposed method mainly seeks to improve the ability of ADF in the zones when the input data is highly contaminated with random noise, and thus, ADF output usually severely cause unwanted artifacts. Achieving latter goal is mainly based on increasing the weight of FCM output value, in the averaging scheme designed for optimum output calculation. Methodology and Approaches: In the interior of a segment in the input data, the nonlinear isotropic diffusion behaves almost like the linear diffusion filter, but one should consider that at edges, diffusion is inhibited. Therefore, noise at edges cannot be eliminated successfully by the mentioned process. As a solution to this problem, anisotropic models do not only take into account the modulus of the edge detector, but also its direction. ANFIS, as a neural-fuzzy system, combines the learning capabilities of neural networks with the functionality of fuzzy inference system. FCM is a robust method for analysis of data and construction of models, more natural than hard clustering, in almost all problems. Data on the boundaries between several classes are not forced to fully belong to one of the classes, but rather are assigned membership degrees between 0 and 1 indicating their partial membership. In the method adapted in this research, at the very first stage, ADF with some different diffusion coefficients (5 to 80) are calculated for a small part of input data. At the next step, the standard deviations of ADF values are compared to each other. The point with higher standard deviation could be considered as noise related data point and vice versa. Hence, sorting, and then, selecting the first, let us say five percent of data, could relatively promise that we have selected one percent of dataset, which are less likely to be masked by random noise. Using the set of selected data and a weighted average of initial ADF values, original values of input data and output of FCM analysis, the training pairs for ANFIS network will be constructed. The AOADF output will be automatically achieved by generalizing the trained network to all data. Results and Conclusions: Although ADF is a strong method for random noise attenuation in many problems, in seismic data processing, ADF could cause artifacts in the zones that are highly contaminated with random noise. In this research, as the comparison of the results of AOADF and ADF on synthetic and real seismic datasets indicates, the AOADF method considerably performs better in random noise attenuation and in preserving the continuity of the coherence events without adding significant artifacts.
    Keywords: Anisotropic Diffusion , ANFIS , Random Noise Attenuation, Seismic , Fuzzy C-Mean Clustering