فصلنامه فیزیک زمین و فضا
سال چهل و یکم شماره 4 (زمستان 1394)

We have investigated an optically stimulated luminescence (OSL) dating study in the Nimbluk lakebed in Khorasan, northeast Iran. Two samples of the lake-bed sediments from ~1 m below the land surface are successfully dated at 7.3-9.9 ka. All necessary experiments have been performed to choose the most suitable procedure for dating quartz extracts using single aliquot regeneration method (SAR). We have employed weighted histogram, unweighted histogram and central age model (CAM) for equivalent dose determination. Although, these results of the ages do not allow us to determine the timing of desiccation, the results suggest that the early part of the Holocene was much wetter than today. This provides valuable palaeo-environmental data in the region.

Iran is one of the most tectonically active regions on the Alpine-Himalayan earthquake belt. Eastern Iran, nowadays, is one of the most active regions of the country. The occurrence of several destructive earthquakes during the past 50 years provides the evidence for the seismic activity in this region. The earthquakes are mostly concentrated around the Lut-block. There are strike-slip fault systems with nearly north-south strike, east and west of the Lut-block. The fault system located in the west part of the Lut-block includes Tabas, Nayband, Lakarkuh, Gowk and Sabzevaran faults. This system is of great importance since it has generated destructive earthquakes such as Dasht-e-Bayaz. Since understanding the focal mechanism of the fault responsible for earthquake is one of the most important parameters, the accuracy to calculate the focal mechanism is extremely vital. Therefore, we have calculated focal mechanisms of the 34 recent earthquakes happened on this system using full moment tensor inversion. The waveform data from 8 broad-band stations, operated by International Institute of Earthquake Engineering and Seismology (IIEES), was used in this study. Appling ISOLated Asperities (ISOLA) package for the full moment tensor inversion using the local and regional data enables us to achieve a higher accuracy in determined focal mechanisms, in comparison with other methods which use teleseismic data. As the magnitude of these events are all smaller than 5.5 (the biggest one equals 5.2), it was not possible to obtain the focal mechanism of almost all of these events through CMT solutions using teleseismic data. The obtained focal mechanisms show that the main mechanism of the Nayband-Gowk-Sabzevaran system is right-lateral strike-slip with a reverse component. The trends of the three main stress axes were also calculated using the 32 focal mechanisms and the stress inversion technique. The results show that the second stress axis (σ2) is nearly vertical, which is one of the characteristics of the strike-slip regimes.

In this research, we have used 102 strong motion recordings from 2012 Ahar-Varzaghan earthquakes (Mw=6.5 and Mw=6.3) to study the form of attenuation of shear wave Fourier amplitude spectra of those two events. The analysis is carried out in a broad-band frequency range from 0.1 to 20 Hz. A bilinear shape for geometrical spreading is assumed based on nonparametric regression of the data. The hinge point of the bilinear shape is around 60 km away from the earthquake source; the geometric spreading forms for the first and second segments are R-0.9 and R-0.5, respectively. The results of this study show that there is considerable dependency of the rate of geometrical spreading on frequency. If only frequencies above 1 Hz are considered, the first segment of geometrical spreading will have a slope steeper than R-1. In contract, for lower frequencies it has a gentle slope. The associated quality factor for the assumed shape of geometrical spreading (appropriate for frequencies logarithmically spaced between 0.1 to 20 Hz) is Q(f)=148 f 0.62. The estimated Q(f) in this study agrees well with the other estimated shear wave quality factors in the region; however, if the whole attenuation model (consisted of geometrical spreading and quality factor) is considered, there will be conspicuous differences between different models.

New attenuation relationships for rock and soil in Alborz, have been developed in this study. When the quantity of usable ground-motion data is inadequate in the magnitude and distance ranges, development of an empirical prediction equation is deficient. Due to lack of data, the two well-known simulation techniques, point source and finite-fault models have been used to generate more than ten thousands of strong motions as input data. The stochastic finite-fault modeling that can be used to predict regional groundmotion for large faults has been developed based on subdividing the fault surface into smaller subsources, as stochastic point sources. The model incorporates the seismological information obtained from recorded data of northern Iran to provide new information on source and path effects. In this study, the uncertainty due to inherent variability in earthquake source, path, and site effects has been considered. The results include the attenuation relationships that are validated by statistical analysis to compare the estimated ground motion with those of recorded data at the observed stations in Alborz region.

The Kazerun Fault System (KFS) is a right-lateral strike slip fault system in the middle part of the Zagros seismogenic zone in Iran. Historical and instrumental earthquake data catalogs of this fault system show good evidence of fault interactions and seismic migrations. This study provides evidence for the migration of seismicity in the middle part of the Zagros region along the segments of the KFS, as well as, among the Kazerun fault segments. North Anatolian Fault System (NAFS) is similar to the Kazerun Fault System (KFS) and there are also interactions among the segments of the NAFS. In this paper, we have described the fault interactions and seismic migrations in the KFS and NAFS based on the spatiotemporal analysis of the earthquake data of these two regions in a period of 5 years (from 2005 to 2010). The obtained results indicate that these migrations mainly occur along the trend of these fault systems. Additionally, we found a good agreement between these seismicity patterns and the overall ongoing plate tectonic movements in these parts of the World.

Refractor ambiguities are big problem in seismic refraction method especially in seismic engineering. There can be hidden subsurface geological phenomena such as hidden faults and shear zones which are not simply predicted by the travel-time graph or some geophysical methods. Head wave amplitudes are used to show the resolution of refractor ambiguities and the existence of anisotropy in complex geological area. Wave amplitude is proportional to the square root of energy density; it decays as 1/r. In practice, velocity usually increases with depth, and causes further divergence of the wave front and a more rapid decay in amplitudes with distance. Amplitudes measured from first peak to first trough and corrected for geometric spreading, can be demonstrated some subsurface information such as anisotropy. Meanwhile, amplitudes are not commonly study by researchers in seismic refraction studies, because of being the very large geometric spreading components due to variations related to wave speeds in the undulated refractor. The variations in amplitudes are described with the transmission coefficient of the Zoeppritz equations. This variation in velocity and density produces head wave amplitude and head coefficient changes in refractor, even with refractors exhibiting large variations in depth and wave speeds. The head coefficient can be approximately calculated by the ratio of the specific acoustic impedance in the overburden layer in the refractor. This study shows that there is a relationship between the amplitude and the seismic velocity which the lower the contrast in seismic velocity and/or density, the higher the amplitude and vice versa.

Since hydrocarbon sources have an important role in development of industry and technology, exploration of them has been lionized by human. The seismic reflection method is one of the most applicable investigative methods to identify the hydrocarbon reservoirs, but in some cases this method does not work well because of geology conditions and wave attenuation in depth. Thus, some exploration methods such as magnetotelluric can help us reach better results and lead to better interpretation of such reservoirs in compound with seismic methods. The Magnetotelluric (MT) method is suitable to map electrical resistivity in hydrocarbon explorations. This method has been widely used in exploration of conventional energy and also the renewable energy such as geothermal resources and a powerful tool to investigate different kinds of geological structures under the earth's surface. MT method usually focuses on the deeper geologic targets than the other EM methods. MT provides an excellent image of subsurface formations in the areas covered by high-velocity carbonate. The investigated area is located in southeastern part of the most prolific oil province of Iran, the Khuzestan in Dezful Embayment. Oil reservoirs of Iran have been contributed by Mesozoic and Cenozoic evaporated sediments. Multiple petroleum systems exist in the investigated area, since at least two proven source rocks exist within the area: The Kazhdumi and Pabdeh shale sediments. Tree main groups of reservoirs are recognized in the Khuzestan basin: the Khami Group, the Bangestan Group and the Asmari Formation. All the tree groups of reservoirs are recognized in investigated area with excellent fracture permeability and locally primary porosity. A formational interpretation of 2D inversion of MT data is used to demarcate hydrocarbon prospective formations underneath carbonated sediments of south west Iran. MT measurements are made in southwest Iran. The sites were distributed in two profiles of approximate SW-NE direction. Profiles are called P1 and P2, respectively. The MT experiment was carried out by deploying 63 sites with about 600 m spacing with 40 frequency values in seven decades and the period ranged 0.003-2000 (s). The dimensionality and the best geoelectrical strike estimation were carried out using tensor decomposition and phase tensor analysis. The ellipticity, phase tensor and skew angle are other measured parameters. NLCG inversion algorithm is used to inverse two MT data profiles. Both TE and TM modes with both MT polarizations were jointly inverted using the NLCG algorithm. The NLCG algorithm attempts to minimize an objective function that is the sum of the normalized data misfits and the smoothness of the model. The obtained MT sections show three anticlines underground. Seh Qanat anticline is the most important one in the investigated area. The lithological log of Seh Qanat Deep-1 (SQD-1) borehole is applied to interpret MT sections. This borehole has been drilled on Seh Qanat Anticline with total depth of 2876 meters. It could be detected a boundary of Asmari formation that is the primary shallow oil target in the investigated area. Other formations such as Sarvak, potentially are reservoir of hydrocarbon resources.

Inverse problem is one of the most important problems in geophysics as model parameters can be estimated from the measured data directly using inverse techniques. In this paper, applying different inverse methods on integration of S-wave and GPR velocities are investigated for estimation of porosity and water saturation. A combination of linear and nonlinear inverse problems are solved. Linear least-squares and conjugate gradient are used as linear techniques, whereas grid search and Newton methods are selected as nonlinear ones. It is understood that vS depends on density and Lame Constant (shear modulus) and vGPR on dielectric constant. This combination seems to be logical. Shear modulus is related to porosity using Bruggeman’s rule. Density and dielectric constant is also related to porosity and water saturation. This implies that vS and vGPR are bivariate functions of porosity and water saturation, which are our unknown model parameters. The model parameters are estimated to minimize the cost functional ora system of the equations. In order to convert the nonlinear problem into the linear form, taking logarithm and changing variables were used. The problem was convex, which was inferred from the linear form, so there was just one local minimum as the global minimum of the problem. The grid search method shows that porosity and water saturation cannot be estimated by vGPR or vS uniquely. The results of the four methods were compared with each other and a good agreement was observed.

In this paper an indirect method is presented to detect potential geothermal sites in Kerman province, southeast Iran. Geothermal heat flux is one of the main parameters to be investigated in geothermal exploration programs. However, few direct heat flux measurements are available for Iran. Given the proved relation between Curie depths and heat flux, magnetic data can be used to calculate the Curie depths in the areas where few or no direct heat flow measurements are available. The method presented here uses an iterative forward modeling approach to calculate the Curie depth in Kerman Province. It has used the satellite magnetic crustal field model of MF5 obtained from CHAMP mission. The equivalent source magnetic dipole method was used to estimate the magnetic crustal thickness from the observed induced field. The obtained Curie map reveals an area with very low Curie depth in the southeast Kerman. The area may be considered as a potential geothermal site. Geological evidence confirmed our findings for the probability of a geothermal site in the area.

One of the main goals of interpretation of gravity data is to detect location and edges of the anomalies. Edge detection of gravity anomalies is carried out by different methods. Terracing of the data is one of the approaches that help the interpreter to achieve appropriate results of edge detection. This goal becomes a complex task when the gravity anomalies have smooth borders due to gradual change of density contrast. In this article terracing of data has been inspected using the profile curvature method. The synthetic data are used to assess the accuracy and efficiency of the method in edge detection of gravity anomalies. The results of this research have been compared with the results of other methods such as first vertical derivation, analytic signal, tilt angle, horizontal gradient of tilt angle, and laplacian second derivative. Two real data set are also used to show the applicability of the method.

The S-transform has widely been used in the analysis of non-stationary time series. A simple method to obtain depth estimates of gravity field sources is introduced in this study. We have developed a new method based on the spectral characteristics of downward continuation to estimate depth of structures. This calculation procedure is based on replacement of the Fourier transform with the S-Transform in traditional downward formula. We expect the localized estimation of the depth of anomalies using the S-transform spectrum rather than FFT spectrum. Likewise in the wavelets which don’t have a direct relationship with wave numbers, the S-Transform corresponds to wave number instead of scale or pseudo wavenumber. This is the main advantage of using S-transform instead of wavelets. This advantage will lead to easier and more precise calculation of depth estimation. Synthetic examples indicate the usefulness of this method. The method was applied to field examples producing reasonable results comparable to some common methods such as wavelet-based source characterization and Euler deconvolution. It is possible to average the local spectra over the wavenumber axis that leads to the spectrum referenced to position axis. The depth of anomaly can be computed in any point of the profile by using localization of spectrum. Thereby, we can analyze distinguished traces of shallow and deep anomalies while the lateral effects are also considered.

Lut 009 meteorite was found during a trip to Lut Desert of Iran in March,2012, at 30°20.38' N, 59°09.04' E. Chemical compositions of equilibrated olivine (Fa19.3 ± 0.5) and orthopyroxene (Fs16.7 ± 0.6) show that the meteorite sample belongs to H group of ordinary chondrites, while the texture (chondrule petrography and plagioclase size) suggests a petrologic type of 4. The Lut 009 has been very weakly shock altered and has a shock stage of S2. Fe-Ni is completely weathered whereas less than 5 percent of troilite is still present. Therefore, the meteorite has a weathering grade of W4. Magnetic susceptibility is log χ =4.75 (χ in 10-9 m3/kg) and, thus, consistent with a W4 H ordinary chondrite. Here we report description of Lut 009 in the first extended study on a meteorite from Lut Desert. Along with this sample, in-progress investigations of other meteorites from the desert will open a window into the characteristics of meteorite concentrations in this region.

This study investigates the contributions of Atlantic Ocean to June-August rainfall over Uganda and western Kenya (KU). The study has utilized the datasets including precipitation from the Global Precipitation Climatology Centre, North Atlantic Oscillation Index (NAOI), South Atlantic Ocean Dipole Index (SAODI), ERA-interim reanalysis, and the Atlantic Ocean Sea Surface Temperature (SST). Singular value decomposition (SVD), composite analysis and correlation analysis are used to achieve the objective of the study. Results show that the recent extreme rainfall events of June - August (JJA) season were experienced in 2007 (above normal) and 2009 (below normal). Further analysis reveals that there are significant coupled modes of variability; the first mode explains 32% whereas the second mode explains 16% of the total covariance. The first SVD mode captures the positive phase of the South Atlantic Ocean Dipole (SAOD) over Atlantic Ocean. This is associated with positive anomaly of rainfall in most parts of KU. The second SVD mode captures the negative phase of SAOD. The North Atlantic Ocean Index (NAOI) exhibits a significant positive correlation of coefficient ≥ 0.3 with the mean JJA rainfall anomaly over most parts of KU at 95% confidence level. The correlation between the mean JJA rainfall over most parts of KU and NAOI is higher compared to that with SAODI. The dominant moisture source in the region during JJA season is the Atlantic Ocean and the Congo rainforest. The findings from this study provide insight into the influence of Atlantic Ocean on the mean JJA rainfall over KU. The study recommends further research on the utilization of NAOI and SAODI as predictors of the JJA seasonal rainfall over the study area. The production of the JJA seasonal rainfall forecast in the region will enhance better utilization of water resources in the region

This study investigates the atmospheric circulation associated with extreme rainfall events over the coastal West Africa. The rainfall data of this study were obtained from the Global Precipitation Climatology Centre (GPCC), spanning from 1981 to 2010. The atmospheric datasets were also obtained from the ERA-Interim reanalysis. The study employed the Z-Index to categorize dry and wet years into seven distinct grades. The analyses focused on the summer monsoon rainfall season experienced in July to September (JAS). The extreme drought years were identified to be 1982 and 1983, while extreme wet years were pointed out to be 1999 and 2007. The area of study was dominated by anomalous westerly moisture transport, characterized by convergence at low level during wet years. The major source of moisture over the study area is Atlantic Ocean. Dry and wet years are characterized by positive and negative low-level geopotential height anomalies, respectively. Although the results of this study do not give a diagnosis of the reported rainfall variability, the information herein can be useful in the monitoring and update of seasonal forecasts. Accurate and reliable seasonal forecasting is beneficial in that it helps us minimize loss of lives and destruction of property.