فصلنامه فیزیک زمین و فضا
سال سی و هشتم شماره 1 (بهار 1391)

When an alternating electromagnetic field propagates through the ground, it induces electric currents in any conductor in its path. The strength of the induced currents mainly depends on the electrical resistivity of the conductor concerned and the frequency with which the primary field is alternating. Generally speaking, the currents are stronger for smaller resistivity and higher frequency. Airborne Electromagnetic (AEM) surveys can be used extensively for mapping and interpretation of geological targets. If the data were available for many frequencies, the resolution of such maps will be better. With modern AEM equipment, the secondary magnetic field of the current induced in the ground is measured at a rate of ten readings per second and for several frequencies simultaneously. The large amount of data collected during a survey must be converted into a set of apparent resistivity and centroid depth values which will provide at least a rough picture of the vertical resistivity distribution at each sampling point. The centroid depth for each frequency is an estimate of the depth in the half-space at which there is maximum induced current flow. This is the depth for which there will be maximum sensitivity to resistivity variations. Here we have used a complex transfer function c (or generalized skin depth) derived from data for the secondary magnetic field measured by a dipole system with a small coil spacing "r" at a height "h" above the ground. It is known that the data measured above layered ground with AEM systems which satisfy h≥3.3 r, can generally be interpreted in terms of an equivalent half-space. The complex transfer function ‘c’ has useful properties: for a uniform or layered ground, the real part of c yields the ‘centroid depth’ (z*) and the imaginary part of c yields the apparent resistivity (ρa) as a function of frequency. So, if the function ρa(z*) is known over a broad frequency range, it yields a smoothed approximation of the true resistivity distribution ρ(z) without an initial model. The relations between ρa(z*) and ρ(z) are studied for a number of multilayer models. We also examine the method on real data which are collected over Kalat-e-Reshm area in southeast of Semnan province. Results show that the method can determine the existence of conductive and resistive structures but it cannot deliver detailed information about subsurface structure. Thus, it is recommended to use this method as a reconnaissance method not an exact detailed method.

Recent studies show that electromagnetic methods are robust and widespread geophysical methods that can be employed to delineate the electrical conductivity of earth materials. In this paper Magnetotelluric and Radio-magnetotelluric methods have been used to map the Earth's structures. The Magnetotelluric (MT) method is a frequency-domain electromagnet (EM) sounding technique used to investigate the electrical structure of the earth's surface. The method exploits naturally existing EM fields as a signal source. The MT method has a wide range of applications, from shallow investigations (geotechnics, ground water, and environment) to moderate and deep target in exploration of natural resources (mineral, geothermal and petroleum) depending on frequency band used. In general the Radio-magnetotelluric (RMT) uses the MT-principles by employing artificial transmitters far off the measuring site in the frequency range from about 10 kHz up to 300 kHz. This is called Controlled source RMT method. At the same time we can take the advantage of using signals from powerful communication transmitters for submarines in the very low frequency range (VLF) from 10 to 30 kHz. In this study we used only the latter case. Data were collected in year 2000 at one site located on Midsommar Island, west of Stockholm, Sweden. This work has been done under the coverage of Björkö Energy Project which has been designed to map the structure at depth with geophysical methods and by drilling. Combined data including RMT data which reflects the characteristics of the uppermost part of the earth and MT data reflecting the characteristics of the deeper parts of the earth’s upper crust at the island are considered. Hence, a good coverage from surface to deeper part of the earth can be obtained. For RMT measurements radio signals with frequency ranges from 15 to 30 kHz have been used while in MT method three bands have been applied with frequency ranges from 256 Hz to 8192 Hz, 8 Hz to 256 Hz, and 0.25 s to 8 Hz for band 1 to band3, respectively. In some frequencies a weak correlation were observed due to the noisy data which were collected mostly from band 3. So these data were put aside and we placed more emphasis on the rest. MT and RMT data, resulted from 1D inversion, had been processed in order to obtain resistivity variation with depth. As a result, electrical structures from upper parts of the crust to 3000 meters depth have been plotted for this site. RMT data are being used to evaluate the characteristics of structures from the surface to 200 meters depth, which demonstrate decrease in the electrical resistivity. This parameter increases down to 600 meters depth then there is a decline in the electrical resistivity in deeper parts. From 900 meters to 1300 meters, a conducting zone has been detected by using MT data. Inversion results correlate perfectly with borehole data which had been drilled to 964 meters depth on the island. From the surface to 200 meter depth low electrical resistivity about 300 can be detected which interpreted as brackish water. From 200 meters to 670 meters the electrical resistivity increases to about 5000and the lithology of this part indicates Jotnian sandstone with sharp boundary. From 670 meter to 900 meter the electrical resistivity exceeds. From 900 meters to 960 meters depth the electrical resistivity decreases to about 600 most probably due to the electrical nature of the breciated quartz lithology at that depth. From this depth down to 1300 meter a conducting zone is detected probably due to the abundant fractures in rocks which contain fluids and the MT data confirms this by showing resistivity lows. In regard to the frequency gap between MT and RMT data, it is suggested to combine another electromagnetic method such as, CSMT which data can be controlled better with less noisy data while filling this gap. Professor Laust Pedersen from Uppsala University is apprecitaed for providing the data to be reprocessed and reported.

It is well established that the seismic ground response of surface topographies may differ from those of free field motion during earthquakes. Complex nature of seismic wave scattering by topographical structures can only be solved accurately and economically using advanced numerical methods under realistic conditions. Among the numerical methods, the boundary element is a powerful numerical technique for analyses of linear and homogeneous materials for both bounded and unbounded domains. In this paper, the algorithm of seismic wave scattering by homogeneous media using time-domain three-dimensional boundary element method has been presented. Three-dimensional traction elastodynamic kernels for both cases of constant and linear variation of displacement have been presented. The convoluted kernel for constant time variation contains apparent singularities in the wave fronts, while in the linear time-convoluted elastodynamic traction kernel, apparent singularities in the wave's front disappear and a well-behaved kernel is resulted. Behavior of the constant and linear time-convoluted elastodynamic traction kernel have been investigated numerically. Kernel values were calculated at the central point of the three boundary elements considering different time steps. The boundary elements are in different condition of symmetry with respect to the source point. The kernel values in the case of constant time-convoluted elastodynamic traction kernel tend to the elatstostatic fundamental solution as expected, Whereas the kernel values in the case of linear time-convoluted elastodynamic traction kernel is equal to the elastostatic fundamental solution if the time step would be greater than time required to shear waves passed the receiver point on the element. Presented constant and linear time-convoluted elastodynamic traction kernels are casaul and have time-translation property which could be used for optimization of numerical algoritms. The presented elastodynamic kernels have been used instead of temporal integration for wave scattering analyses of homogeneous medium using BE algorithm. Seismic wave scattering by a semi-spherical canyon subjected to incident compressional and shear waves has been analyzed. The semi-spherical canyon has a radius of 200m, shear wave velocity of 800m/s, Poisson’s ratio of 0.25, and mass density of 2.00gr/cm3. The incident wave of the Ricker type has the predominant frequency of 3.0Hz. Calculated results in time-domain are presented as well as comparison of results with other transformed-domain methods in term of dimensionless frequency, which shows the efficiency of the presented boundary element algorithm for solution of seismic wave scattering by homogeneous media in time domain as well as the accuracy of elastodynamic kernels.

While a crystalline material is heated from room temperature to around 500°C, a weak but measurable light will be emitted. This light is known as thermoluminescence (TL)‎ and is based on storage of energy from ionizing radiation in many naturally occurring TL minerals, including quartz and feldspar. The source of radiation is the radioactive materials such as uranium, thorium, potassium and cosmic radiation. The physical bases for this phenomenon in the simplest model can be expressed as follow: In nature all minerals are exposed to radiation, which leads to ionization of the atoms as electrons and holes. Freed electrons may become trapped in structural defects. The number of trapped electrons will be accumulated up to the time that the material is heated sufficiently. Heating stimulate the atoms and electrons can be released and recombine with the ‘holes’. Some of these holes are called 'the luminescence center's in the mineral crystal lattice which recombination of electron with them, results to an emission of light which is called thermoluminescence. When mineral is heated to high temperature, it loses all its previously acquired TL signal, and sets luminescence clock to zero. After cooling, the natural radioactivity causes thermoluminescence to build up again and thus the amount of TL induced is proportional to the time that has elapsed since the minerals were fired. For dating in laboratory, it is necessary to measure equivalent dose and dose rate. The intensity of the radiation damage in crystal lattices is a measure of the Equivalent Dose (DE)‎ which the mineral has received since last “resetting” by exposure to heat. DE is obtained by means of a TL measurement. The dose rate (DR)‎ is a measure of radiation dose per unit of time absorbed by mineral. The equation for obtaining an age is: Age (ka)‎ = DE (Gy)‎/DR (Gy/ka)‎ Two approaches are employed to determine the DE: the additive-dose method and the regeneration method. In this study, we used the first method. In additive-dose method, a number of nearly equal portions of the sample (aliquots)‎ are divided into groups; one is reserved for measurement of the natural TL signal only, while the others are given various doses of laboratory radiation. Then, all the aliquots are measured together and the luminescence intensity is plotted against laboratory radiation dose; this forms the sample’s dose response. The DE is determined from the intercept of the fitted line with the dose. The dose rate is calculated from an analysis of the radioactive elements in both the sample and its surroundings. These are determined using the measured concentrations of radioactive elements (uranium, thorium, potassium-40) within the sample and its surroundings, which are, in turn, converted into dose rates using standard conversion factors and formulae. Contribution of cosmic rays is also determinated. There are different methods for obtaining concentrations of radioactive elements such as γ-Spectrometer, ICP Mass Spectrometer, Notrun Activation, α counting and flame photometry. Thermoluminescence dating began in 1960- 1970, with age determination of pottery and other fired material in archeology and then it was employed in the other science such as paleoseismology, paleoclimatology, geology, archeology and geography. Iran is an ancient country which is located on the belt of earthquake, so most of ancient monuments and old civilizations have been destroyed by earthquake or other natural disasters. Dating can be a device to relate paleoseismology, paleoclimatology and archeology in Iran. Therefore, having knowledge of dating methods, especially luminescence, is so important for experts in geography, archeology, geology, seismology students and etc. In addition to an introduction to TL dating, this study has dated five potteries from Iran national museum in a range of 1000 to 4000 years.

The Earth’s gravity field dedicated missions, CHAMP, GRACE and GOCE have opened up a new era for the study of the Earth’s gravity field. The missions measure different functionals on the Earth’s gravitational potential which are represented in terms of the Stokes Coefficients or the so-called geopotential models. The mapping process of the observations into the coefficients is called recovery of the Earth’s gravity field. Different approaches have been introduced by the research institutions and universities for recovery of the field. In principle, these methods can be categorized into two integral- and acceleration-based groups. The integral-based approach utilizes the integral form of the satellite equation of motion, i.e., Jacobi’s integral whereas the acceleration method is set up based on the Newton’s gravitational equation. The main difference between the methods is the scalar and vectorial form of the observation equations. Moreover, the Jacobi’s integral is only need the position and velocity observations where the acceleration approach is written based on the position, velocity and acceleration measurements. This papers aims to numerically compare the methods for the recovery of the Earth’s gravity field in the GRACE mission. The vectorial form the high-low satellite-to-satellite observations in the GRACE mission is used for the recovery purposes where the observation equations in the integral approach are derived from the scalar form the observations. Our numerical investigation shows that the acceleration approach can lead to a higher accuracy if the full form of the observation is used. However, the recovery can be done with comparable accuracy to the integral approach only using the radial component of the acceleration differences in the high-low mode. The aliasing problem is the next point that has been addressed in this paper. The recovered geopotential models are usually affected by the high-degree frequency of the gravitational field. The acceleration approach is much more sensitive to the presence of the omission error due the imperfect modeling of the high-frequency constituents. Using the numerically derived second-order time derivative of the GPS-based position observations is might be the main reason of the higher sensitivity.

The mathematical aspect of cartographic mapping is a process which establishes a unique connection between points of the earth’s sphere and their images on a plane. It was proven in differential geometry that an isometric mapping of a sphere onto a plane with all corresponding distances on both surfaces remaining identical can never be achieved since the two surfaces do not possess the same Gaussian curvature. In other words, it is impossible to derive transformation formulae which will not alter distances in the mapping process. Cartographic transformations will always cause a certain deformation of the original surface. These deformations are reflected in changes of distances, angles and areas. One of the main tasks of mathematical cartography is to determine a projection of a mapped region in such a way that the resulting deformation of angles, areas and distances are minimized. It is possible to derive transformation equations which have no deformations in either angles or areas. These projections are called conformal and equiareal, respectively. Since the transformation process will generally change the original distances it is appropriate to adopt the deformation of distances as the basic parameter for the evaluation of map projections. In 1861 an English astronomer, G. B. Airy, made the first significant attempt in cartography to introduce a qualitative measure for a combination of distortions. His measure of quality was designed to be an equivalent to the variance in statistics. A more realistic evaluation of the deformations at a point was suggested by German geodesisit, W. Jordan, in 1896. In 1959, Kavraisky recommended a small modification of the mean square deformations of Airy and Jordan by the logarithmic definition of linear deformation. Such altered mean square deformation are called Airy-Kavraisky and Jordan-Kavraisky. Using the above two mentioned criterions we can compute the mean square deformation of distances at a point. The evaluation and comparison of map projections of a closed domain is done by integration of the above two criterions. In this paper the first measure was used as the qualitative measure of map projections. The two criterions should lead to similar results but the application of the Airy-Kavraisky criterion in the computation process is much simpler. This is the main reason for its selection as the basis of finding the best projection. Optimization process was done in irregular domain of Iran for Lambert conic, Mercator cylindrical and stereographic azimuthal conformal projections. At first a grid composed of 165 points was created in the region. The scale factor was computed for the center of grid elements. The boundaries consist of a series of discrete points. Since the optimization domain is not regular like a spherical trapezoid, spherical cap or a hemisphere, so the minimization of the criterion leads to a least squares adjustment problem. For the Lambert conformal conic projection the optimization process will determine four unknown parameters: the geographic coordinates of metapole (φ0, λ0) and the projection constants C1 and C2. For other map projections the number of unknown parameters is three (φ0, λ0, C). The following table shows the numerical results of Airy-Kavraisky criterion after optimization. In this table shows the Airy-Kavraisky criterion before optimization and shows this criteria after optimization. Computational results show a decrease in Airy-Kavraisky criterion after optimization. This study of optimization of cartographic projections for small scale mappings was conducted to investigate the general approaches for obtaining the best projections using the Airy-Kavraisky measure of quality.

The mathematical aspect of cartographic mapping is a process which establishes a unique connection between points of the earth’s sphere and their images on a plane. It was proven in differential geometry that an isometric mapping of a sphere onto a plane with all corresponding distances on both surfaces remaining identical can never be achieved since the two surfaces do not possess the same Gaussian curvature. One of the main tasks of mathematical cartography is to determine a projection of a mapped region in such a way that the resulting deformation of angles, areas and distances are minimized. Most large-scale national topographic maps are based on conformal map projections such as transverse Mercator and Lambert conformal conic projections. The essential condition in every conformal map-projection is the infinitesimal similarity. Chebyshev studied conformal map projections, using the oscillation of the logarithm of the infinitesimal scale function as a measure of distortion. Chebyshev’s criterion states that the conformal map projection on Ω with minimum distortion is characterized by the property that the infinitesimal-scale σ is constant along the boundary of Ω. The oscillation in Ω of the logarithm of the infinitesimal-scale function associated to this best Chebyshev conformal map projection (or simply Chebyshev projection) will be called the minimum conformal distortion associated with Ω. Then we consider how to quantify the minimum conformal distortion associated with geographical regions. The minimum possible conformal mapping distortion associated with Ω coincides with the absolute value of the minimum of the solution of a Dirichlet boundary-value problem for an elliptic partial differential equation in divergence form and with homogeneous boundary condition. If the first map is conformal, the partial differential equation becomes a Poisson equation for the Laplace operator. The Dirichlet BVP could be solved by the finite element method (FEM). The FEM method is a procedure used in finding approximate numerical solutions to BVPs/PDEs. It can handle irregular boundaries in the same way as regular boundaries. It consists of the following steps to solve the elliptic PDE: 1- Discretize the (two-dimensional) domain into subregions such as triangular elements, neither necessarily of the same size nor necessarily covering the entire domain completely and exactly. 2- Specify the positions of nodes and number them starting from the boundary nodes and then the interior nodes. 3- Define the basis/shape/interpolation functions for each subregion. As a particular case, we consider the region of Iran in this paper. Conformal mapping equation in this region is solved for Mercator as the base map projection. To solve this equation three approaches are used: Finite Element Method (using Matlab Partial Differential Equation, PDE, Toolbox for square domain and Femlab code for arbitrary irregular domain), Fourier Method and Harmonic Polynomials. At the end, graphs associated with logarithm of the infinitesimal-scale function and also obtained results for coefficients of harmonic polynomials associated with the best Chebyshev projection over the region of Iran are presented.The minimum conformal distortion associated with square boundary domain estimated as 9.232×10-3 using finite element method and 9.243×10-3 using Fourier method. Also for the region of Iran with real domain, the value of this quantity estimated as 2.381×10-3 using finite element method and 2.462×10-3 using harmonic polynomials. Computations show that the results of three approaches are very close to each other. So for determination the best Chebyshev’s projection for a geographic region, the three mentioned approaches give the same results.

In the present study, the Q factor is obtained for the Central Alborz in Iran using the Lg coda method. The energy of a seismic wave decays while passing through a ‘real’ medium such as the earth which is not completely elastic. The decay in energy is due to non-elastic phenomena and is called intrinsic attenuation and is characterized with the Q parameter, the large values of which represent small values of attenuation and as Q approaches zero the pertaining attenuation will become quite strong. Therefore Q could be considered as a measure of elasticity of the media The objective of this study is to determine the Lg coda Q from ground motion recorded at 32 short-period stations in the central Alborz, north central Iran. Lateral variations of Q and its frequency dependence were estimated using 1020 high quality Lg waveforms in the frequency range between 0.3-7.0 Hz. In the studied area, the factor resulted from this research, on average, is given by:. The lateral variation of correlates well with the large scale tectonic units of the studied area. Damavand volcano and its surrounding region is also characterized with a significant gradient in values, going from anomalously low values in the western side to the average values in the eastern side. The current seismicity in the Damavand area is mostly confined to the south-western part, which is characterized with anomalously low values and sharp gradient of. In Damavand region, has a relatively higher gradient than that of the surrounding region. It sharply declines moving from east to west. The map correlates well with the large-scale tectonic units of the studied area and also several clear trends corresponding to different characteristics of seismic activity and attenuation field. Most of Q factor variations can be attributed to the lateral heterogeneity as well as the severity of the crustal velocity gradient. Since the seismicity in the area is quite shallow (earthquake depths mostly are less than 30 km), the results can be attributed to the average of upper part of the 30 km of the crust of the study area.

Most geologic changes have a seismic response but sometimes this is expressed only in certain spectral ranges hidden within the broadband data. Spectral decomposition is one of the methods which can be utilized to help interpreting such cases. There are several time-frequency methods including: short-time Fourier transform (STFT), continuous wavelet transform (CWT), Wigner-Ville distribution (WVD), S-transform, and matching pursuit decomposition (MPD). In this paper, we use the MPD method. This method is newer than the other time-frequency methods used in exploration seismology. Mallat and Zhang (1993) have improved time and frequency resolution simultaneously by using MPD method. In this method, a signal is decomposed into constructive wavelets. Time and frequency properties of wavelets are used locally for spectral decomposition. Pursuits are the algorithms which search the best time-frequency matching between the signal and a linear combination of selected wavelets from wavelet dictionary. Matching pursuit which is an iterative procedure optimizes signal estimation by each new wavelet chosen from a dictionary. These wavelets combined linearly to obtain the best match with the signal. A signal should expand to waveforms which their time-frequency properties could be matched to local structures. Such waveforms are called time-frequency atoms. There are many approaches to match wavelets of dictionary to a seismic signal and to obtain time-frequency spectrum in matching pursuit decomposition. The base of all approaches is the Mallat and Zhang’s algorithm. However, computing time of the original algorithm is very high due to many iterations and that is why particular conditions have been applied in different researches to limit the matching pursuit algorithm for obtaining the lower performing time. In this work, particular conditions are rather similar to Wang’s (2007) method. On seismic data, layer thickness is described on the basis of the seismic travel time. When a layer with different properties has a thickness by one-fourth wavelength, top and base reflections will interfere constructively. For thin layers less than tuning thickness, combined seismic amplitude decreases with thickness (when reflection coefficients of the top and of the base are opposite). Generally, spectral decomposition has many applications in interpretation of seismic sections and so there will be extra needs to study and develop them. Thin layers and many stratigraphic hydrocarbon reservoirs are beneath the threshold of the vertical seismic resolution (tuning thickness) and because of their low thicknesses, they are not resolvable. For this reason, mapping the small-scale geological structures is one of the important interpretational cases. When the thickness of a thin layer decreases pick frequency slightly increases. In this work, this issue has been used to detect thin layers by time-frequency spectrum and by single-frequency sections obtained from MPD. In this paper, we investigated the performance of the matching pursuit decomposition for time-frequency analysis of seismic sections to delineate and detect thin layers on synthetic data (including simple thin layer model and also wedge model) and real data. It is observed that the interpretation of thin layers is simpler by single-frequency sections. It is shown that for a simple thin layer if considerable frequency in single-frequency sections increases, ability in resolving layer interfaces would be increased. In the wedge model, as the frequency increases resolution threshold of layer interfaces moves to a lower thickness and therefore it would be possible to detect lower thickness layers. The tuning thickness has been decreased from 19 meters in original seismic section to 12 meters in 80 Hz frequency section. In the real data, it is shown that when a thin layer is not resolvable in a seismic section it might be detected using the MPD method. In this case, by providing 20, 40, 60, 80 and 100 Hz single-frequency sections when high frequency sections are studied, interfaces of thin layer are appeared gradually. It is concluded that time-frequency sections are useful instruments to detect and delineate thin layers.

The seismic anisotropy is a subject of interest for seismologists and geologists. The information of seismic anisotropy has significant role in geology interpretations (Savage, 1999). In this study, we have investigated the upper crust anisotropy in Bam area by means of shear wave splitting Sg phase. We have selected more than three hundred aftershocks from IIEES local temporary seismic network that had been installed after 26 December 2003 Bam earthquake. In the present study, due to using local waveform data, type of Sg phase and shallow depth of events, the estimated values of seismic anisotropy could be related to heterogeneities within upper crust of Bam area. The shear wave, upon entering the anisotropic region, splits into two phases with polarizations and velocities that caused properties of the anisotropic media. The phases, polarized into fast and slow components, progressively split in time as they propagate through the anisotropic media. This split is preserved in any isotropic segments along the ray path and can be observed as a time delay (δt) between the two horizontal components of motion. Polarity and amplitude are strongly affected by the azimuth of arrival. The orientation of anisotropy is estimated trough measuring the azimuth of fast component (φ). The magnitude of anisotropy is estimated by measuring the time split (δt) between the fast and slow components of motion. Our aim in this study is to calculate the magnitude (t) of anisotropy and direction () of the fast wave as the main parameters of seismic wave anisotropy in Bam area in south east of Kerman Province. For measuring anisotropy parameters, we have used the Teanby et al. (2004) shear wave splitting technique. This method can be divided into three main groups, where the search for the optimal pair of splitting parameters is based on: (1) the minimization of a penalty function which represents the difference between observed and predicted transverse components (e.g., Vinnik et al., 1989); (2) the maximization of the cross-correlation between the fast and slow components or linear particle motion (e.g. Bowman and Ando, 1987; Levin et al.,1999); and (3) the minimization of energy on the corrected transverse component reassembled from the optimal fast and slow components (Silver and Chan, 1988 and 1991). The results for 15 seismic stations show two perpendicular main directions for shear wave anisotropy. These two dominant seismic anisotropy directions as given in table 1, can be considered as geological fabric and the principal stress directions. In the present study, one of the main seismic anisotropy directions is perpendicular to the faults trend for the nearest seismic stations on the fault border. Therefore, some of our results indicated that the polarization of the fast split shear wave is parallel to direction of the maximum horizontal stress. The second of the main seismic anisotropy directions is parallel to the faults trend, especially for that stations far a way fault border. The size of anisotropy is about 0.034 to 0.1 S.

Any 3D seismic survey can have an acquisition footprint. Acquisition footprint is an expression of the surface geometry (most common on land data) that leaves an imprint on the stack of 3D seismic data. Often we recognize it as amplitude and phase variations on time slices, which of course display the amplitudes within our data set at a specified two way time (Cordsen, 2004). On the other hand, acquisition footprint is often used to describe amplitude stripes that appear in time slices or horizon slices produced from 3D seismic data volumes. Although acquisition design of a 3D survey has a major influence on the nature and severity of a footprint, improper data processing techniques such as the use of incorrect normal moveout (NMO) velocities can also create footprint (Cordsen, et al., 2000). More seriously, on horizon slices, footprint can interfere with and confuse stratigraphic patterns. Many different contributions to the generation of acquisition footprint are possible. These can be divided into two main categories: (1) geometry effects: line spacing, fold variations, wide versus narrow patch geometry, source generated noise and variations of offset and azimuth distribution. (2) non-geometry effects: topography, culture, weathers, surface conditions and processing artifacts. In this article we study the effects of these parameters for 3D seismic survey in AHWAZ oil field and calculate acquisition footprint noise in this field. Most of the time the acquisition footprint is based on the source and receiver line spacing and orientations. The larger the line spacing, the more sever the footprint. In land situations where access is very open and, therefore, the lines are very regularly spaced, we may be able to recognize the footprint very clearly. Because the geometry is regular, the footprint also will have the same periodicity. Fold variation themselves are the simplest form of an acquisition footprint. Fold changes with offset (or rather mute distance from the source point); each offset range, therefore, has differing fold contributions (Cordsen, 1995). Because each individual bin of a 3D survey has changing offset distributions, the CMP stack of all traces in a bin will display bin-to-bin amplitude variations. This variation in itself can produce an acquisition footprint. Generally it has been thought that acquisition footprint is far worse in the shallow part of the seismic and therefore, of course, the geological section, mainly because the fold is lower, and amplitude variations necessarily are far more dramatic. Offset limited fold variations alone may produce a recognizable footprint. The higher the fold, the better the signal to noise ratio; therefore, less footprint is evident. Wide recording patch geometries are far more accepted these days than narrow patch geometries (Cordsen, et al., 2000). The reasons are numerous and ranges from reduction in acquisition footprint (particularly that due to back-scattered shot noise) to improved statics solutions and the availability of large channel capacities on seismic recording crews (also leading to higher fold). In addition to the impact of the fold variations, acquisition footprint is made worse by source generated noise trains that penetrate our data sets. The lower the signal to noise ratio is, the worse the footprint will be. Unfortunately, the noise typically has a low frequency content that is much less affected by attenuation. Therefore the noise becomes more prominent relative to the signal content deeper in the section. Our experiences have shown that acquisition footprint problems can be just as prevalent in the deep section as they are in the shallower section. If surface access is poor because of topography variations, tree cover, towns, etc., we irregularize the geometry by moving source points to locations of easier access, and therefore mask the acquisition footprint. It is still present, however. The footprint is just so much harder to identify. Weather and surface conditions may also impact the recorded amplitudes. One can model an acquisition footprint by creating a stack response on either synthetic or real data. We stack the data in a 3-D cube and display the resulting seismic data over a small time window. The best input is a single NMO and static corrected, offset sorted 2D (or 3D) CMP gather. These traces will be applied to each CMP Bin in the recording geometry. In summary, we should attempt to minimize footprints by employing proper seismic acquisition and processing techniques, but if a footprint persists in the stacked data, there are ways to filter the data and mitigate its effect on geological interpretation. In this article we optimized acquisition parameters in order to minimize acquisition footprint noise for 3D seismic survey in AHWAZ oil field and finally with 3D modeling by OMNI software we saw the intensity of this noise in our seismic sections.

MRS method is one of the new geophysical methods that directly detects the Groundwater. By using this method, one can determines some parameters of aquifer (porosity, permeability and transmissivity). The purpose of this research is the application of MRS method in the conductive ground. Combination of MRS and RS method is used for better detection of the aquifer in the region of Abyek located at southeast of Qazvin province. Processing MRS sounding data, EN/IN and S/N are equal to 2.66 and 1.14 respectedly for 8sq-75 sounding. The external noise in 8sq-75 is 3 times of the internal noise, so the parameters in this sounding are not considerable. As we can see the amount of permeability and transmissivity (8.5 × 10 -6 m2/s) are very low in this sounding. For the sounding of sq-150-1 the EN/IN and S/N were 1.29 and 1.4 respectively. The external noise and internal noise is almost equal and this sounding is reliable. The signal and the noise in this sounding showed equal amount, so there is a little amount of water bearing in the aquifer exist upto 150 m depth beneath this sounding. The amount of transmissivity in this sounding is 8.3 × 10-4 m2/s. The EN/IN for sounding sq-150-2 is 1.29, and S/N is 1.6 which is better than sq-150-1 and 8sq-75 soundings in noise level condition. The EN/IN and S/N for 8sq-37-5 are 1.05 and 1.64 which is the best of all in low level noise situation. The transissivity of the sounding 150sq-1 showed the highest of all among the 4 soundings, so this is the best in water bearing of all in this region. Inversion of MRS soundings showed, the region is in the low level of water bearing aquifer, because the transmissivity is less than 0.015 m2/s, so the water production is not efficient. The comparison of the MRS method and RS method determined that the aquifer layers are located at very low resistivity zones. For 8sq-75 sounding the aquifers layer have 4-5 ohm-m resistivity. The aquifers in 150sq-1 sounding have almost 1 ohm-m resistivity. The aquifers in 150sq-2 sounding have almost 0.5 ohm-m, which is the lowest of all in this region. The aquifers in 8sq-37-5 sounding have resistivity from 1.5 ohm-m to 7 ohm-m. In the present work, influence of the conductivity on MRS phase, has been applied in the three regions, namely Abyek Qazvin with low resistivity (5 Ω-m), Hamedan with medium resistivity (40 Ω-m) and the southwest of Iran with high resistivity (approximately 300 Ω-m) were compared with each others. If the resistivity of the subsurface decreases, the phase variation will be increased and amplitude of signal will be decreased. In the low resistivity zone (Abyek Qhazvin) has 270 degree phase variation, while in the medium resistivity zone (Hamedan) has 183 degree variation and in high resistivity zone (Lorestan) has 34 degree variation. The signal amplitude for low resistivity zone (Abyek Qhazvin) is 21 nvolt, for medium resistivity (Hamedan) is 34 nvolt and for high resistivity zone is 381 nvolt.

The causes of wintertime precipitation decrease in the Southern Coast of Caspian Sea (SCCS)‎ compared to its counterpart in autumn, is investigated with the use of synoptic stations daily dataset of SCCS and gridded NCEP/NCAR and NOAA dataset. The structure of atmospheric circulation and synoptic and physical conditions over the Caspian Sea region which are dominate in the winter and autumn precipitation events are analyzed. The results revealed that the anticyclone which is located over the north and/or west of Caspian Sea is highly reinforced compared to its counterpart in autumn in the all wintertime precipitation synoptic patterns. Also, there is a considerable increase of anticyclonic circulation intensity in lower troposphere over the Caspian Sea in all patterns, which the mean value of it in wintertime high pressure pattern is reached to twice of its value in autumn. The investigation of vertical velocity over the Caspian Sea region indicates an outstanding Sea-Saw behavior between the northern and southern Caspian Sea during autumn. As such, the maximum ascend and descend of air flow is seen at 45˚ and 37.5˚ N, respectively. During winter, as the autumn Sea-Saw pattern is weakened, the frequency and intensity of ascending air is decreased over the SCCS. The repeated suitable settlements of anticyclone centers over the northern Caspian Sea in autumn, and their considerable meridional variations of them in winter caused the obvious seasonal differences in ascending air and hence in precipitation occurrence. The investigation of SST and latent heat flux over the Caspian Sea showed that there is a significant positive Correlation between the precipitation amounts in SCCS and the SST and latent heat flux. As the cold period prevails, because of the SST decrease there is a decrease in the evaporation over the Caspian Sea and consequently there is a decrease in the precipitation of SCCS. The investigations also revealed that there is a strong and significant negative correlation between the northerly wind magnitude at the lower troposphere and SST, latent heat flux, and precipitation of SCCS during the cold period. Therefore, the more the intensity of northerly winds increase over the Caspian Sea, the more latent heat values, SST and precipitation intensity decrease in return. The results showed that the anticyclones which are settled over the northern and western parts of Caspian Sea are stronger during winter. Consequently, the pressure of lower troposphere is considerably increases over the southern part of Caspian Sea. Such an increase is associated with the noticeable meridional variability of wintertime anticyclones. The changing of dominate wind direction in the most important pattern for precipitation in the Caspian region, and also the decrease of SST and latent heat flux. The aforementioned conditions are caused by the decrease of amount, intensity and numbers of precipitation days during winter.

Ice can form on aircraft surface at 0° C or colder temperatures when liquid water is present. This can happen both on the ground and at the flight levels of the aircraft. Aircraft icing has not only an impact on flight safety but has also an economic impact. According to the report of research department of the FAA (Federal Aviation Administration)‎, from 1982 to 2001, 13% of accidents due to aircraft icing resulted in loss of human life, about 30 cases in 20 years. This research refers to case studies in Mehrabad airport during winter in order to determine the threshold values of some important indices in aircraft icing forecast. In the first case study, aircraft engine failed and emergency landing occurred due to aircraft icing. Second case study is considered because of domination of a very cold outbreak in northern parts of Iran and all Mehrabad flights were cancelled. In addition to study the synoptic and thermodynamic weather patterns, the relevant MM5 outputs including vertical velocity, liquid water content (LWC)‎ and relative humidity are surveyed and their thresholds are compared with other studies in this area including Ellrod and Bailey in 2007 and Henderson in 2006. Synoptic conditions are analyzed using GFS data with 1° resolutions in latitude and longitude directions (http://nomads.ncdc.noaa.gov/data.php)‎. The initial and boundary conditions come from the operational 12Z runs of global forecasting system (GFS)‎ of NCEP (National Center for Environmental Prediction)‎ for MM5 (Meso scale Model version 5) model(Dudhia 1993; Grell et al. 1994) to survey the NWP model outputs. Satellite images as the other reference in this study, are taken from METEOSAT-7 which provides one image every 30minutes over Iran. Thermodynamic charts are drawn by RAOB software using Mehrabad station upper air sounding data to determine the threshold values of instability indices such as LI, KI, SI, and SWEAT. The most important results of this research show that large scale patterns are not a suitable factor to determine the type and severity of icing. Relative humidity and vertical velocity as the outputs of the MM5 regional numerical weather prediction model does have a rather good consistence with the observations and their thresholds are quite similar with other thresholds gained from the other parts of the world and the small differences can be due to topographical and boundary conditions in the regional model. The liquid water content values do not match the available thresholds and require more research. Thermodynamic charts show temperature and dew point deficits values can show the aircraft icing conditions but the thermodynamic indices such as LI, KI, SI, and SWEAT are not suitable and did not satisfy icing conditions. It can be said that due to shallow stratiform clouds which are reported in Mehrabad station observations provides threshold values so that the dew point deficit is less than 3°C, temperature between zero and -10°C, vertical velocity -0.2 hPa/s, relative humidity more than 75% in 700 hPa level are good diagnosis in the first guess in aircraft icing and they make easy icing forecast for the Mehrabad airport area.

Evaporation and evapotranspiration are two major components of hydrological cycle which are very important for agricultural studies as well as water resources management. So far, various methods have been addressed for the estimation of daily reference crop evapotranspiration (ET0). Among them, FAO Penman-Montieth 56 (PMF-56) (Allen et al., 1998) is widely used as a standard method, particularly for arid and semi-arid regions. The major drawback to this method is the fact that the required weather data are not usually available for majority of the study sites. Pan coefficient (Kpan) method (Eq.1) is an alternative procedure which can be used for such conditions. ET0 = KPan (1) To estimate the pan coefficient (KPan), many works have been performed by various researchers (e.g. Cuenca, 1989; Allen and Pruitt, 1991; Snyder, 1992; Orang, 1998; Ranghuwanshi and Wallender, 1998). In a humid region, Imark et al (2002) used Frevert et al (1983) and Snynder (1992) methods for estimation of KPan and ET0. They suggested that Frevert method performs more reliable ET0 estimation close to PMF-56 than other Kpan estimators. Sabziparvar et.al (2009) introduced Orang (1998) and Ranghuwanshi and Wallender (1998) methods as the most accurate Kpan estimators for warm arid and cold semi-arid regions of Iran. Another research work conducted for north Spain suggested that the daily ET0 which estimated by Artificial Neural Network (ANN) method performs more accurate results than empirical and semi-empirical relations. Aims and Scope: The main aim of this study is to assess Kpan values as estimated by ANN and ANFIS (Adaptive Neuro-Fuzzy Interface System) predictors against empirical estimators such as Cuenca, Orang, Pereira, Snyder, Raghuwanshi and Wallender methods. The comparison is made by using the statistics such as coefficient of determination (R2), Root Mean Square Error (RMSE) and Mean Absolute Error (MAE). Summary and In this research, the performance of different pan models (Cuenca, 1989; Allen and Pruitt, 1991; Snyder, 1992; Orang, 1998; Ranghuwanshi and Wallender, 1998) for better estimation of pan coefficients for the selected sites in warm arid climate of Iran is compared with Artificial Neural Network (ANN) and Adaptive Neuro-Fuzzy Interface System (ANFIS) results. For this purpose, a ten-year daily measured pan data (1996-2005) were used. Having considered the shortage of lysimeter data, we applied the FAO recommended alternative approach of Penman-Montieth FAO-56 for the determination of daily ET0 at the study sites. In ANN and ANFIS methods, wind speed, relative humidity and fetch distance were applied as the input of the verified networks. The pan coefficients as estimated by PMF-56 method were also used as the input of the intelligent networks. Model validation was presented by using Root Mean square Error (RMSE), coefficient of determination (R2), and Mean Absolute Error (MAE) criteria. The results showed that the ANFIS method performs more accurate pan coefficient and reference daily evapotranspiration values compare to other approaches. For the selected ANFIS method, the mean values of R2, RMSE and MAE were 0.83, 0.97 (mm day-1) and 0.74 (mm day-1) respectively. Among the empirical pan models, Cuenca and Snyder methods are recommended for prediction of Kpan in warm arid climates. In this work, we assumed that the synoptic weather sites of Shiraz and Kerman are representatives of agricultural fields in warm arid climate. This assumption might affect the estimated Kpan values. To remove some weaknesses, of the fetch distances, using evaporation pan and lysimeter instruments inside the agricultural fields, in addition to other weather instruments such as wind recorder (anemometer) are recommended for more reliable results.

The spontaneous generation of inertia–gravity waves (IGWs) in the idealized simulation of vortical flows is investigated using the isentropic two-layer model on a sphere. The contour-advective semi-Lagrangian (CASL) algorithm is applied to solve the primitive equations using the potential vorticity (PV), velocity divergence, and acceleration divergence as the prognostic variables. The CASL algorithm consists of a Lagrangian part and an Eulerian part. While the Lagrangian part is attributed to PV equation and solved by contour advection, the Eulerian part includes the prognostic and diagnostic equations for the grid-based variables of velocity divergence, acceleration divergence and the depth field. The fourth-order compact differencing and spectral transform are used, respectively, in latitudinal and longitudinal directions and time stepping is carried out using a three-time-level semi-implicit scheme. The model is set up using 256 grid points in both latitudinal and longitudinal directions, the upper- and lower-layer potential temperatures of, respectively, 280 K and 310 K, and the same horizontal mean depth of 5 km for the two layers. A balanced, zonal jet is used as the initial state and a very small perturbation is added to it as a trigger instability. In order to determine the IGWs more accurately, the Bolin–Charney balance relations are used to decompose the flow into a balanced part controlled by PV and an unbalanced part representing free inertia–gravity waves. In this study, the analysis of the velocity divergence, acceleration divergence and PV points to breakdown of balance and generation of two wave packets of IGWs where sharp PV gradients appear to contribute significantly to the generation and organization of the wavepackets of IGWs. Application of the CASL algorithm helps to capture fine-scale structures in PV and to quantify IGWs generated by vortical flows more accurately. With regard to balanced initial conditions used, these gravity waves are spontaneously generated from the breakdown of the balance. The first wave packet is found on the downstream side of the trough similar to the mesoscale waves described by Zhang in 2004. The second wave packet is identified on the upstream side of the trough similar to the wave packet described by Plougonven and Snyder in 2007 in idealized simulations of a baroclinic life cycle dominated by cyclonic behavior. It seems that the propagation direction of the first wave packet is the same as that of the Zhang, but the second wave packet propagates perpendicular to the wave packet of Plougonven and Snyder. By determining the characteristics of the waves, the magnitude of the intrinsic frequency of both wave packets are found to be larger than those of the previous studies and is near to the results of Wang and Zhang in 2007 for their low static stability experiment. Further, a short-time Fourier transform is used to determine the dominant absolute frequency and verify frequency characteristics of the packets obtained by the dispersion relation. In this method, the time–frequency analysis for each signal is provided by applying a moving time window to the signal and taking the fast Fourier transform. The more accurate results obtained by this method for the intrinsic frequency confirm the estimates based on the local dispersion relation.

Using a Magnetic Property Measurement System (MPMS), we measure magnetic susceptibility continuously at frequencies from 0 to 1000 Hz for three different superparamagnetic (SP) grain sizes of magnetite (5nm, 7 nm and 10 nm) suspended in hydrocarbons (Ferrofluids). Our results confirm non-zero values of frequency-dependent susceptibility in the fine tail of nano-sized grains and provide new empirical data for the degree of interaction between SP grains (nano particles) affecting magnetic susceptibility and frequency-dependent susceptibility.

We use high-resolution electrical resistivity imaging to delineate the geometry of the landslide and discontinuity surface between slide mass and bedrock in Roudbar region, Ghazvin-Rasht-Anzali railway (the tunnel no. 2 at Km. 108+825). The Roudbar region characterized by high geological hazard and shows a complete panorama of mass movements. In this area, different landslide types predisposed and tightly controlled by the geostructural characteristics. Geoelectrical images are produced from dipole-dipole, schlumberger and combined resistivity profiling data along arrays spanning selected profiles positioned perpendicular and parallel to the tunnel route in landslide area. Regarding the surface geology, geomorphology, integration and comparison of dipole-dipole results with other geophysical data delineated the geometry and characteristics of the landslide and geological structures.