JOURNAL OF RESEARCH ON APPLIED GEOPHYSICS
Year:2017 | Volume:3 | Issue:1 |Papers Count:10

Geophysical methods are effective tools in archaeological investigations. Sensitive magnetometers have been used for many years to locate and characterize archaeological sites. Magnetometry is one of the most widely geophysical methods in this application as it contains no harmful environmental effects. Magnetization contrast in ancient sites is created by natural causes and human activities. Studies have shown that topsoil has generally a higher magnetic susceptibility than most bedrocks and sub-soils. Vegetation fires and fermentation effects increase background magnetic susceptibility, and oxidation-reduction cycles associated with alternate wetting and drying of the soil; all of these tend to convert the iron compounds to strongly magnetic oxide maghaemite. In this paper, subsurface structures have been investigated in the Tepe-Hissar ancient area of Damghan. In this regard, synthetic and real magnetic data and fuzzy filters have been used to investigate these subsurface structures. Moreover, normalized derivatives respect to vertical derivative (NAVD or NA) filter has been introduced that enhance edges in potential field methods....

Deconvolution is considered as a successful tool in seismic exploration for increasing the temporal resolution of the data. Gabor deconvolution is proposed to treat the non-stationarity issue of the problem by breaking it into several stationary sub-problems via a Gaussian window, solving them independently, and then, recombining/projecting the sub-solutions into an approximate solution to the original nonstationary problem. The projected Gabor deconvolution has recently been proposed by the second author as an improvement over Gabor deconvolution. In the projected Gabor deconvolution, the sub-problems are projected to a unified problem in time domain, and then, the resulting problem is solved. This modification brings useful advantages over the Gabor deconvolution including an improved convergence property, more efficiency for sparse deconvolution, more flexibility for incorporating prior information in the presence of noise, and more reflectivity structure via a least-squares method. In this paper, we propose a method for sparse and non-sparse deconvolution of non-stationary seismic signals in the presence of Gaussian and spike-like random noises. Numerical tests using simulated and field data are presented to show high performance of the proposed method for generating accurate and stable reflectivity models from nonstationary seismograms....

Today, the significance of the velocity model estimation in seismic migration as well as time to depth conversion of seismic sections is very clear to every geophysicist. From a practical point of view, approaches such as well logging techniques or direct observations cannot provide a reliable description of regional scale physical properties of the earth. Seismic imaging is a tool to describe the earth physical properties. Today, one of the imaging techniques, which is highly welcomed by geoscientists is the full waveform inversion (FWI). The full waveform inversion-an efficient method in seismic imaging-uses all available information in the seismograms, including the amplitude, phase, and the first arrival time so as to judge the physical properties of the earth. In fact, FWI is represented as an optimization problem by defining a misfit function, which measures differences between the observed waveforms and the computed waveforms. In FWI, the size of the model space parameters will not allow us using global minimization algorithms. Therefore, we minimize the misfit function by local optimization methods. A waveform inversion problem is usually solved repetitively by the gradient-based solutions. In this paper, we have compared two gradient optimization algorithms of Gauss-Newton algorithm based on the main diagonal of the pseudo Hessian matrix (GN-DPH) and the limited memory-BFGS (LBFGS). Although the L-BFGS does not need the computation of Hessian matrix, but according to the numerical tests on synthetic models, we have found that GN-DPH algorithm results are more accurate than L-BFGS after 40 iterations.....

Gravimetric survey, together with other geophysical methods, could be used for analysis and interpretation of geological structures. For a more comprehensive interpretation, we could employ geophysical data modeling methods. One of these methods is the method of data inverse modeling, which is used to determine the model parameters from the obtained data. In this method, we consider two types of model parameters, namely, physical and geometrical parameters. Consequently, there are two types of inverse modeling for interpretation of potential field data. In the first type, the geometrical parameters are considered to be constants and physical parameters are considered to be variables which need to be determined, while in the second type, physical parameters are constants and geometrical parameters, such as depth, should be calculated. In the present study, gravity data were measured in 380 points in Mahallat area, and then, necessary corrections and appropriate filtering and processing, were applied on the acquired data to determine surface trends. After combining the gravity data with the existing geological information, an attempt was made to find out the effective surface and deep trends on local hot springs, and hence, to determine the possible presence of geothermal reservoirs in the area....

Challu granitoid pluton is located in southeast of Damghan and northern part of central Iranian structural zone. Intrusion of the pluton into volcanic and volcanic-sedimentary rocks and the resultant hydrothermal fluids has caused alteration and Fe mineralization. Generally, two different phases of propylitic and argillic alterations are identified in the studied pluton. Magnetic susceptibility of fresh and altered rocks at the Challu granitoidic pluton is measured by magnetic fabric technique. The average measured magnetic susceptibility has been obtained about 28872± 3410 μ SI for monzodiorite and 21487 ± 3916 μ SI for quartzdiorite. Circulation of hydrothermal fluids throughout the intrusive body has caused the main mineral compositions variation and changed the magnetic properties of minerals as the average measured magnetic susceptibility for the above rocks has, respectively, been reduced to 25117± 988 μ SI and 6262± 1577 μ SI, due to propylitic and argillic alteration. Different kinds of opaque minerals in the pluton have also been identifined by thermomagnetic curves as magnetic susceptibility changes with temperature. These results show how magnitude of magnetic susceptibility decreases during alteration due to removal or reducing in the size of magnetite or its convertion to hematite.

A nonlinear inversion technique using a fast method is developed to estimate successively the depth, shape factor and amplitude coefficient of a buried structure using residual gravity anomalies along a survey line. By defining the anomaly value at the origin and the anomaly value at different points on the survey line, the problem of depth estimation is transformed into a problem of solving a nonlinear equation of the form f (z) = 0. Knowing the depth of the anomaly source, we can estimate the shape factor, and finally, the amplitude coefficient of the anomaly. Using the amplitude coefficient, we can also compute the radius of the anomaly. This technique is applicable for a class of geometrically simple anomalous bodies, including semi-infinite vertical cylinder, infinitely long horizontal cylinder, and sphere. The efficiency of this technique is demonstrated using the gravity anomaly due to a theoretical model with and without random errors. Finally, the applicability of the technique is illustrated using the residual gravity anomaly of a salt dome, situated near Miyaneh, northeast of Iran. The interpreted depth and other model parameters are in good agreement with the known actual values of the parameters. The estimated depth, radius, shape factor and the amplitude coefficient values of the salt dome are 64. 63 m, 34. 8 m, 1. 43 and-472 mGal, respectively....

In this paper, 3D inversion of gravity data for determination of subsurface density distribution is made using geostatistical co-kriging method. Co-kriging is a mathematical interpolation and extrapolation tool. It uses the spatial correlation between the secondary variables and a primary variable to improve the estimation of the primary variable at un-sampled locations. The Co-kriging method gives weights to the data so as to minimize the estimation variance (the co-kriging variance). In this paper, the primary variable is density, (estimated by ρ *) and the secondary variable is gravity g. For determination of kernel matrix, the subsurface area is divided into large number of rectangular blocks of known sizes and positions. The unknown density contrast of each prism is the parameter that should be estimated. In addition, the weighting matrix has also been used in order to improve the depth resolution. Preconditioned conjugate gradient method has been used for inversion. The computer program has been written in MATLAB and tested on synthetic gravity of a rectangular prism model. The results indicate that the geometry and density of the reconstructed model are close to those of the original model. The gravity data acquired in an area, which includes concealed manganese ore bodies (Safoo mine site), in northwest of Iran. The results show a density distribution in the subsurface from the depth of about 5 to 35-40 m. These results are in good agreement with the results of the borehole drilled in the site.

In this study, satellite images, and aeromagnetic and ground magnetic data were analyzed to explore the geothermal potential in Sirch-Golbaf area in Kerman province. Thermal anomalies and apparent thermal inertia images were studied using MODIS, Landsat and ASTER images. As a result, two thermal anomalies were obtained; one in the southeastern part of the study area and the other one in Jooshan location. Magnetic investigations also detected two anomalies; a deep magnetic anomaly in the southern part of the area and another magnetic anomaly near Jooshan hot spring in the northeastern part. These magnetic anomalies were correlated with thermal anomalies.

Reflection seismic data is often contaminated by a variety of coherent and incoherent noises that influence the reliability of the seismic data to provide a better understanding of the hydrocarbon reservoir characteristics. The groundroll noise is characterized by dispersive wave, low frequency, high amplitude that propagates along and near the surface of the earth and will often obscures the seismic reflection data. Removal of this type of noise is an essential part of land seismic data processing. Most of ground-roll noise removal or attenuation methods are based on transforms. Radial trace transform and wavelet transform are two of the most important and common transforms, which are used to attenuate ground-roll noise. Various methods have been introduced by different authors to attenuate the ground-roll noise. In this paper, we have used the wavelet transform on radial basis to attenuate the ground-roll noise. The method is based on the joint application of the two-dimensional discrete wavelet and radial transforms in order to eliminate the coherent noise, especially ground-roll noise. Since the radial trace transform reassign the linear events in the x-t domain into vertical events in the r-t domain, it provides a pleasant framework for two-dimensional (2D) discrete wavelet transform. The de-noised signal can be obtained by suppressing the vertical band of one-level, 2D discrete wavelet transform of the radial transformed seismic gather followed by transforming back the filtered radial-wavelet transform into the x-t domain. To evaluate the efficiency of the proposed method to eliminate the ground-roll noise, we have tested the method on a real reflection seismic gather from an oilfield in southwest of Iran. We have also compared the obtained results with those from the f – k filtering for ground-roll noise suppression from seismic data. The results obtained from applying the proposed method on the real data test prove that the proposed method for ground-roll noise attenuation is as good as, or better than, those produced using f – k filtering.

Cleaning of contaminated lands containing unexploded ordnance (UXO) and mine fields demands special consideration in the war-torn countries. Some geophysical methods have been used for detecting these destructive remnants of war. Nowadays, airborne GPR has been considered as a fast and effective tool for UXO detection. In this study, the feasibility of airborne GPR for detection of anti-personal metallic landmines has been investigated by numerical simulation of back scattered GPR waves from two artificial physical models A and B. Model A is a column of a two-layer model comprising of a layer of soil under a layer of air. In model B, an anti-personal metallic landmine has been buried in the center of model A at a depth of 10 cm beneath the soil. By using the numerical finite-difference time-domain (FDTD) method, the radargram and central traces of back scattered GPR waves from both physical models A and B have been simulated in various altitudes (0-10 meters) and operating frequencies of the transmitted GPR wave (100-1500 MHz). To do this, MATLAB and REFLEXW software packages have been employed. Moreover, several signal processing techniques including signal energy analysis, multi resolution wavelet transform, and time-frequency analysis have been performed instead of using the noise removal and trace extraction, which are related to the target. As a result, detection of intended mine becomes possible by applying airborne GPR up to altitude of 10 m and using operation frequency of transmitted GPR wave of 550 MHz to over 1000 MHz.