Geophysics
Asghar Rastbood; Mohsen Sahebi-Ilekhchi
Abstract
Very Long Baseline Interferometry (VLBI) is one of the essential approaches in geodesy. Significant capabilities of this technique are creation of International Celestial Reference Frame (ICRF), determination of Earth orientation parameters (EOPs), coordinates of station with high accuracy and light ...
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Very Long Baseline Interferometry (VLBI) is one of the essential approaches in geodesy. Significant capabilities of this technique are creation of International Celestial Reference Frame (ICRF), determination of Earth orientation parameters (EOPs), coordinates of station with high accuracy and light deflection. Many attempts could be done to increase accuracy of VLBI data, including technical improvement of equipment, physical models improvement, using optimal data processing methods and increasing number of stations. In this study, in order to reduce the error of EOPs in CONT17 campaign, the appropriate position for a VLBI station in the Middle East is investigated. The results showed that mean error of EOPs in the Legacy-1and Legacy-2 reduced approximately 6.64% and 13.86% by adding two stations in Oman and Egypt countries, respectively.
Geophysics
Reza Ghanati; Zahra Tafaghod Khabaz
Abstract
Inversion modeling of Vertical Electrical Sounding (VES) data is formulated as a nonlinear problem and solved using multiple depth layers of fixed boundary and a few depth layers of the variable boundary. The model parameters in Smooth inversion are only the resistivity values. However, in the block ...
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Inversion modeling of Vertical Electrical Sounding (VES) data is formulated as a nonlinear problem and solved using multiple depth layers of fixed boundary and a few depth layers of the variable boundary. The model parameters in Smooth inversion are only the resistivity values. However, in the block inversion, the thickness of each layer is also added to the model parameters. Due to the non-linearity of the inverse problem, the determination of an appropriate initial model is very significant. A technique has been adopted to estimate a proper starting model for the block inversion strategy. Thus, from the solution of the smooth inversion, the second derivative of cumulative resistivity is calculated, then using the difference of turning points, the initial values of the model parameters are determined. The proposed algorithms are first tested on data derived from two artificial models and then implemented on two real data sets. The numerical experiments demonstrate that the smooth inversion of the geo-electrical sounding data due to less dependency on the initial model confronts fewer challenges proportional to the block inversion strategy. However, in the case of block inversion, an appropriate choice of the initial model makes it possible to determine the boundaries of the layers with more certainty.
Geophysics
Hamidreza Samadi; Roohollah Kimiaefar; Alireza Hajian
Abstract
One of the most important issues in seismology is to accurately determine the epicenter of earthquake, mainly because the accuracy of dependent studies such as seismic risk analysis, fault mechanism, depends on the accuracy of mentioned parameter. Double difference method is one of the conventional relative ...
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One of the most important issues in seismology is to accurately determine the epicenter of earthquake, mainly because the accuracy of dependent studies such as seismic risk analysis, fault mechanism, depends on the accuracy of mentioned parameter. Double difference method is one of the conventional relative earthquake relocation methods which is used vastly by seismological centers and it is used as the basis of an algorithm that relocates earthquake events about 0.004 second. In the proposed method, Adaptive Neuro-Fuzzy Inference System (ANFIS) is trained based on the former relocated events and generalized for new earthquakes. In the training procedure, the hypocentral information, for the main event and the nearest one (determined by K nearest neighbor search), and the velocity model of the region are considered as inputs and hypocentral information are set as target for the network. Here, with reference to the information of about 8000 earthquakes, as the training events, 490 earthquakes were relocated in about 0.1 (s) where RMS for predicating latitude and longitude of the events were 0.002 and for the depth parameter, RMS was 0.45. It is concluded that the proposed method could be considered as a powerful algorithm for near real time relocation of earthquakes.
Geophysics
Mohammad Fouladi; Mirsattar Meshinchi Asl Meshinchi Asl; Mahmoud Mehramuz; Nima Nezafati
Abstract
Geophysical methods have been developed to study the particulars of physically diffusible fields in the ground. In geophysical studies, the goal is usually to detect inhomogeneities inside the earth using physical values measured at the earth's surface. Geophysicists attempt to reconstruct the Earth's ...
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Geophysical methods have been developed to study the particulars of physically diffusible fields in the ground. In geophysical studies, the goal is usually to detect inhomogeneities inside the earth using physical values measured at the earth's surface. Geophysicists attempt to reconstruct the Earth's interior structure using such data. The present study, which is known as the Third- order Moving Average, is the solution of an inverse problem for simultaneous estimation of the shape and depth of residual magnetic anomalies. The method was applied to the synthetic model, with and without noise, which Superiority this method will show over the Second-order Moving Average. Also, tested by the instance of field data in Geological Survey of Iran (GSI). This method, calculates a nonlinear relationship between depth and shape factor, at seven points with successive window length. Finally, the results showed that this method is very accurate for non-noise data and in has good agreement with the noisy data. which the problem, will showed the superiority of the Third-order Moving Average Method over the Second-order Moving Average method for estimating the shape and depth of buried Magnetic Anomalies. It also showed that it is suitable for real data with 5% error.
Geophysics
Ghazal Janghorban; Seyed Mohammad Abtahi Forooshani; Keytash Moshtaghian; Hooshang Asadi Harooni; Hamzeh Sadeghi Sorkhani; Mohammad Hajheidari
Abstract
Kuh-e Lakht epithermal gold index is located on Urumieh-Dokhtar Volcanic Belt in Isfahan province. Geological studies indicate the presence of an epithermal gold mineralization system in the area. The geophysical exploration in the area includes magnetometry, resistivity, and induced polarization ...
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Kuh-e Lakht epithermal gold index is located on Urumieh-Dokhtar Volcanic Belt in Isfahan province. Geological studies indicate the presence of an epithermal gold mineralization system in the area. The geophysical exploration in the area includes magnetometry, resistivity, and induced polarization surveys. Then, we implemented Li and Oldenburg algorithm for 3D inversion of the magnetic data. The depth of the largest estimated magnetic susceptibility obtained from data inversion coincides with the average depth of the magnetic sources obtained from Euler deconvolution. Furthermore, we compared the estimated 2D resistivity and electrical changeability models and the estimated magnetic susceptibility. The comparison denotes the correlation of the estimated magnetic susceptibility variations with the reduction of resistivity, high electrical chargeability, and alteration zones. Combining the estimated magnetic susceptibility model with the geochemical analysis of the exploratory boreholes in the area indicates that the mineralization often occurred at the maximum variation of the magnetic susceptibility. Nonetheless, the results indicate the possibility of mineralization along the zones with large magnetic susceptibility variations.
Geophysics
safieh farrokhi mogaddam; Abbas Alli Alli akbari bidokhti; Farhang Ahmadi givi; Mojtaba Ezam
Abstract
Salinity and water temperature are considered as important characteristics of water of different seas. The absolute values and variability of them in time and space determine life sustainability conditions, development of plants and animals and also types of human activity in seas. Water temperature ...
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Salinity and water temperature are considered as important characteristics of water of different seas. The absolute values and variability of them in time and space determine life sustainability conditions, development of plants and animals and also types of human activity in seas. Water temperature and salinity are important because of their effects on chemical and biochemical reactions of hydro-organisms. In this research the effects of climate change on physical characteristics of Persian Gulf including water temperature and salinity has been studied using numerical simulations and some observations. To carry out numerical simulation, the 1-dimension oceanic numerical model PROBE was used. The input data of the model includes meteorological information obtained from ECMWF and also initial values of the station. To simulate the vertical turbulence, the relations of Axcell-Liungman, Omstedt, Rodi, Pacanovski and Marchuk have been used for a station in the Persian Gulf and the results have been compared with some measurements and similar studies. The results of investigations show that limits of seawater and salinity variability model outputs are similar but the Omstedt relation shows more detailed changes in comparison to the others. The numerical results indicate that the temperature increase of the Persian Gulf over a decade can be as large as one to two degree Celsius, with some salinity increase as well.
Geophysics
Ata Eshaghzadeh; Alireza Hajian; Roghayeh Alsadat kalantari
Abstract
Potential field data is the assembled of effects of all underground sources. Computing regional-residual anomaly is a critical step in modeling and inversion in the gravity method. Existence quantitative noise in corrected gravity data is unavoidable. In this paper, we present a novel separation method ...
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Potential field data is the assembled of effects of all underground sources. Computing regional-residual anomaly is a critical step in modeling and inversion in the gravity method. Existence quantitative noise in corrected gravity data is unavoidable. In this paper, we present a novel separation method based on a Singular Value Decomposition (SVD) analysis of gravity dataset. With the SVD, a matrix of bouguer gravity data can be decomposed to a series of eigenimages. The number of required eigenimages or threshold for the reconstruction of the regional and residual (local) anomalies maps and noise distribution map from bouguer anomaly is determined based on the derived singular values by SVD. To reconstruct the data set by eigenimages may lose negligible information. We have considered which this value is equivalent with the mean of the variance of the resulted matrixes by eigenimages. The efficiency of the Singular Value Decomposition method was tested with the noisy synthetic gravity data of a hybrid model of the sphere as a local anomaly and deep-seated sloping plane as a regional anomaly. The separation results are satisfactory. The proposed method was applied on gravity field dataset of the Qom area, Iran.
Geophysics
Vahid Maleki; mohammad reza Hatami; Ali Asghar Mottaghi
Abstract
In this study, we investigate the seismicity of eastern Alborz based on precise location of the earthquakes during the last two decades. In order to reduce the effect of outliers and low quality data on earthquake location results, during phase reading, weighing has been done. Manual readings of seismic ...
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In this study, we investigate the seismicity of eastern Alborz based on precise location of the earthquakes during the last two decades. In order to reduce the effect of outliers and low quality data on earthquake location results, during phase reading, weighing has been done. Manual readings of seismic phases for the desired area have been provided with a seismic data bank in which 5461 phases S and 8523 phases have been read, compared to seismic phases read in The Iranian Seismological Center (IRSC) shows a significant increase. Initial location is based on phase picking and initial weights using a nonlinear probabilistic method (Lomax et al, 2000). After calculation of initial location, the double difference method (waldhauser et al, 2000) is used to enhance the earthquake location quality. The earthquake waveform correlation tool is used to re-find the earthquakes. Transparency correlations are performed for events with a similarity of over 70% in a given time window for phase P and S. As a result, using this technique, the phase reading error for the same events reaches its lowest level, resulting in seismic events in re-location with a much higher accuracy than the initial location and location based only on the catalog data. Results of waveform cross correlation shows that in this area, earthquakes with high similarity occurring at in different times. Investigating the location of earthquakes on the Firoozkouh fault and then Chashm fault and Astaneh Fault in the cross sections.
Geophysics
Marzie Shabani; Abolfazl Rezaei; Zohre Masoumi
Abstract
The water table in the Zanjanrood catchment has severely declined recently. Therefore, determination of the groundwater potential zoning map is of great importance for optimal management of water resources. Here, we used seven effective criteria including lithology, slope, drainage, lineament, rainfall, ...
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The water table in the Zanjanrood catchment has severely declined recently. Therefore, determination of the groundwater potential zoning map is of great importance for optimal management of water resources. Here, we used seven effective criteria including lithology, slope, drainage, lineament, rainfall, spring density and landuse to asses groundwater potential. The analytical hierarchical analysis process was used for weighting the criteria. Overlay analysis was implemented using TOPSIS model to prepare the groundwater potential map in four categories of priority including very-good, good, low and poor. In general, the very-good category corresponds with the first priority while the lowest potential of groundwater match with the fourth priority. The high-discharged production wells and the geoelectrical investigations (resistivity and induced polarization) were applied to verify the model. The spatial distribution of the high-discharge production wells has completely coincided with both the very-good and good priorities in the area. The results of resistivity and IP models also are in good agreement with those from the TOPSIS model. Overall, the results suggest there is no rich aquifer in mountains of the Zanjanrood catchment. Furthermore, the most important aquifer in the Zanjanrood catchment is located around the Zanjanrood River where a severe water table decline has occurred.
Geophysics
Mojtaba Naghavi; Habib Rahimi; Ali Moradi
Abstract
In this study, we have used recorded local earthquakes by 17 permanet seismic stations to separate intrinsic and scattering attenuation in North-West of Iranian pleateau. Intrinsic and scattering attenuation can be applied as useful tools to study the geodynamic and tectonic characteristics of a region. ...
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In this study, we have used recorded local earthquakes by 17 permanet seismic stations to separate intrinsic and scattering attenuation in North-West of Iranian pleateau. Intrinsic and scattering attenuation can be applied as useful tools to study the geodynamic and tectonic characteristics of a region. They also represent thermal, compositional and deformational characteristics of the crust and upper mantle. The wave attenuation has strong correlation with seismicity and heterogeneity of medium and is regularly used in the study of tectonically active regions of the world. Single backscattering and coda normalized methods are used to estimate the coda Q (Qc) and Qs respectively, using 14,969 earthquakes which are recorded by the stations. The results show this region is very active region tectonically and seismically. Due to low values of Quality factor and thus high attenuation values of body and shear waves in North West part of Iran, amplitude of the propagated waves are decreased severely in the interested area.The intrinsic attenuation and the Coda wave attenuations curves around the North Tabriz fault are closer in comparison with entire northwestern Iran region and Tabriz city, indicating a strong attenuation of the earthquake waves around this fault system. Similarly, these curves are closer in Tabriz city than those calculated for the northwestern region of Iran which expresses the overriding intrinsic attenuation from the effect of dispersion. The attenuation effect of seismic waves reduces the damages caused by earthquakes at appropriate distances of faults at the time of earthquake occurrence.
Geophysics
Jafar Jaghouri; Hossein sadeghi; Sayyed Keivan Hosseini
Abstract
The shear wave quality factor is one of the key parameters for earthquake and engineering seismology studies. In the present study, this factor was investigated in the region of the 2012 Ahar-Varzaghan twin earthquakes. We used a generalized inverse method based on the shear wave windows of the aftershocks ...
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The shear wave quality factor is one of the key parameters for earthquake and engineering seismology studies. In the present study, this factor was investigated in the region of the 2012 Ahar-Varzaghan twin earthquakes. We used a generalized inverse method based on the shear wave windows of the aftershocks of these two earthquakes. 2860 records of broadband three-components and 540 records of short-period single-component from 1,650 aftershocks with a magnitude of 0.2 to 3.0 were used. The records of the aftershocks obtained within 29 days by a temporal seismic network by the Earthquake Research Center - Ferdowsi University of Mashhad at five broad-band and five short-period stations. Since the generalized inverse method needs a reference site to remove the source effect, the choice of the reference site was done using the results of the H/V spectral ratio method at the broad-band stations, as well as the topographic location of the stations. The frequency dependence of the was regressed in the range of 0.1 to 10 Hz and as Q_s=43 f^1.52 . This low value and high frequency-dependency can be attributed to the high heterogeneity of the crust and the active seismicity of the region.
Geophysics
zahra hajeb; zahra mousavi; zohreh masoumi; Abolfazl Rezaei
Abstract
The arid and semi-arid climate with a low rainfall along with the population growth has been lead to overexertion from groundwater resources over recent two decades in Iran. This over-extraction has been led to the significant groundwater depletion and water level decline in most parts of Iran as well ...
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The arid and semi-arid climate with a low rainfall along with the population growth has been lead to overexertion from groundwater resources over recent two decades in Iran. This over-extraction has been led to the significant groundwater depletion and water level decline in most parts of Iran as well as to the significant land subsidence in many aquifers throughout Iran. In this study, we investigated the subsidence occurred over the Jafarabad and Qanavat in Qom plain using the radar interferometry technique in conjunction with hydrogeological measurements. We used ENVISAT ASAR images from 2003 to 2010, in descending orbits to produce interferograms. Once all interferograms are corrected from topographic and flatten phase, we obtain the mean velocity map of the Qom plain region based on SBAS time series analysis method. The mean velocity map reveals 35 and 28 mm/yr of going away deformation in the line of sight direction in Jafarabad plain and Qanvat plain, respectively. Zoning map of subsidence resulted from the InSAR analysis in conjunction with the water level measurements and the thickness of fine-grained sediments (clay and silt) across the aquifers indicated that the maximum subsidence occurred in the places that not only have a higher water level decline but also have the thicker thicknesses of fine-grained sediments. This work clearly highlights the influences of mismanagement of water resources and over-extraction of groundwater in the arid areas.
Geophysics
Mohammadreza Jamalreyhani; Abdolreza Ghods; Seyyed Khalil Mottaghi; Esmail Shabanian; Morteza Talebian; Beijing Chen
Abstract
One of the main issues in Zagros is the extent that basement and the thick sedimentary layer participate in the observed seismicity. During Nov. 2013, five magnitude ~5 Mw events hit a region in the western end of Lurestan arc of Simply Folded Belt (SFB) of Zagros. We used the Iran-China temporary network ...
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One of the main issues in Zagros is the extent that basement and the thick sedimentary layer participate in the observed seismicity. During Nov. 2013, five magnitude ~5 Mw events hit a region in the western end of Lurestan arc of Simply Folded Belt (SFB) of Zagros. We used the Iran-China temporary network and data from regional permanent networks to relocate the events using a multiple event relocation method, calculate regional moment tensor and centeriod depths for the five moderate events and estimate a velocity model for the two nearby seismic stations using joint inversion of the receiver functions and dispersion curves. Our results show a seismic cluster of about 30 km long and 15 km width. The moment tensor solutions of the five moderate events indicate an almost pure thrust mechanism. The aftershock cloud indicate a low angle east dipping fault plane (i.e., , dip in the range of 23 o -39 o ) as the causative fault plane. The calculated centeroid of the five moderate events are about 14 km and the focal depth of all events calculated by multiple event relocation are about 10-16 km. The results indicate that the thickness of sedimentary column is less than 8 km and thus showing the whole seismic cluster happened within the basement. This observation proves wrong the hypothesis of vertical separation of mainshock and aftershocks in Ssimply folded belt FB of Zagros (Nissen et al. 2011) and implies that in the Lurestan arc the sedimentary layers are deformed aseismically.
Geophysics
Afsaneh Nasrabadi; Fateme Azimi; MohammadReza Sepahvand
Abstract
Crustal velocity structure and Moho discontinuity depth have investigated beneath 7 the broadband seismic stations, AFRZ, TKDS, TPRV, TNSJ, ANAR, KRSH of the Iranian Seismological Center (ISC) and YZKH of Iranian National Seismic Network (INSN) located in the center of Iran by joint inversion of receiver ...
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Crustal velocity structure and Moho discontinuity depth have investigated beneath 7 the broadband seismic stations, AFRZ, TKDS, TPRV, TNSJ, ANAR, KRSH of the Iranian Seismological Center (ISC) and YZKH of Iranian National Seismic Network (INSN) located in the center of Iran by joint inversion of receiver functions and Rayleigh waves group velocity dispersion. Three years (2012 to 2014) teleseismic waveforms (with epicentral distance 25o-90o) for computation receiver functions by iterative approach in time domain have been processed. The Rayleigh waves group velocity dispersion curves were incorporated into our joint inversion scheme from an independent surface wave tomography study. Receiver function is response of local structure of ground (located beneath the three–component broadband seismic station) to teleseismic P-wave, that is sensitive to seismic discontinuities. Since there is very little absolute-velocity information contained in the receiver function, its inversion for shear-wave velocity structure is non-unique (velocity-depth trade-off). On the other hand, dispersion curves are sensitive to the average velocity structure of the upper layers rather than to seismic discontinuities. So the non-uniqueness problem can be solved by combining receiver function inversion with surface-wave dispersion. Results from joint inversion in center of Iran indicates that Moho discontinuity depth depth beneath AFRZ, TKDS and TPRV stations is 40 Km, beneath TKDS 42 Km, beneath ANAR is 38 Km and beneath KRSH and YZKH stations are 44 Km. It was shown that the joint inversion method can cause ±2 kilometers of error. The average Moho depth is about 42±2 kilometers beneath center of Iran.
Geophysics
Abbas Gholamzadeh; Nasim Haydarpoor
Abstract
The main purpose of this paper is studying sedimentary layer in the Qeshm Island. We use aftershocks of 27 Nov. 2005 Qeshm Island (Mb=6.0), to perfume 2D tomography of Rayleigh waves. We have analyzed surface wave data for 856 aftershocks, consist 9821 waveforms. The dispersion curves are calculated ...
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The main purpose of this paper is studying sedimentary layer in the Qeshm Island. We use aftershocks of 27 Nov. 2005 Qeshm Island (Mb=6.0), to perfume 2D tomography of Rayleigh waves. We have analyzed surface wave data for 856 aftershocks, consist 9821 waveforms. The dispersion curves are calculated in the period range between 0.1 seconds and 10 seconds, which correspond to the shallow structures of uppermost crust including sedimentary layer. Surface wave tomography has also been performed to estimate the two-dimensional group velocity maps of Rayleigh waves in the region. The isolated surface wave fundamental modes by Herman and group velocities dispersion curves obtained by it, using Yanovskaya-Ditmar linear inversion method for estimation of 2D tomography maps. Based on the ray coverage, the area divided into cells with dimension 1 Km ×1 Km. in every cell and for distinct periods the velocities were calculated. The minimum dimension of distinct heterogeneities estimated as about 3 km. Low-velocity anomalies in tomography maps can be related to the accumulation of salt plugs and some soft sediment in the area. In periods up to 4 sec, which related to shallower depths, a distinct lineation observed that may relate to a fault. It seems that this fault has cut off the sediments. Into the deeper sediment, this fault disappeared which means the sediments were softer and unbreakable.
Geophysics
Shahrokh Pourbeyranvand; Mohammad Tatar
Abstract
The Alborz, as one of the important seismotectonic provinces in Iran, has a great vulnerability from natural disasters, especially seismic risk point of view, because of the existence of Tehran megacity in its southern edge. The importance of this area has caused a relatively dense GPS network around ...
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The Alborz, as one of the important seismotectonic provinces in Iran, has a great vulnerability from natural disasters, especially seismic risk point of view, because of the existence of Tehran megacity in its southern edge. The importance of this area has caused a relatively dense GPS network around it. In this study, the earthquake focal mechanism data obtained from different resources was used for stress tensor inversion in the Central Alborz. On the other hand, interpolation of the GPS vectors in rectangular grids and differentiation in the center of each grid cell was used for the study of strain rate in this area. The results showed special variation in principal axes of stress and strain rate, and also the changes in areal and maximum shear strain rates in different parts of the Central Alborz, which shows the geodynamic complexity of the study area. Finally, the angular differences between the maximum horizontal stress and strain rate compressive axis directions were calculated in the location of the clusters of the earthquakes. These calculations showed that the extent of the strain partitioning in the middle parts of the mountain range, where strike-slip motions on Mosha and Firuzkuh faults are observed, has more significant effect on the current deformation processes in the Central Alborz. Meanwhile, in the Northern parts of the range, where dominantly sip slip motions on Khazar Thrust and North Alborz fault occurs, deformation partitioning plays a minor role in the region.
Geophysics
mohammad rasool nikbakhsh; Mir Sattar Meshinchi Asl; Hamidreza Siyahkohi; Mohsen Oveisy Moakhar
Abstract
An important goal in mining exploration is the estimation of the depth and the thickness of the causative source. According to this simplification, several methods have been developed for interpreting magnetic field anomalies. In this article, the Hilbert transform has been used to calculate the ...
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An important goal in mining exploration is the estimation of the depth and the thickness of the causative source. According to this simplification, several methods have been developed for interpreting magnetic field anomalies. In this article, the Hilbert transform has been used to calculate the depth and thickness of 3-D thin plate anomalies. The Hilbert-Fourier transform performs an important role in analytic signals. Since the total magnetic fields anomalies function has the characteristics necessary for an analytic function, i.e. its real and imaginary parts form a Hilbert transform pair, the function can be used to interpret networked data in terms of three-dimensional origins. The Hilbert transform does not change the amplitude of a function but shifts the phase by and for positive and negative phase values, respectively. This paper uses a two-dimensional Hilbert transform and a 3-D analytic function to calculate the depth of a thin three-dimensional plate modeled based on the method of Talwani for noisy data and without noise data. The results show that the estimated depth values derived from the Hilbert transform method are associated with an error of less that 3% for data without noise, and an error of 8% for data of 15% noise. . This method was also tested on the real magnetic anomaly data from the Kheirabad iron mine located at 5 km NE of Golgohar, Sirjan, Iran. The results were compatible with the Euler method and with drilling information of the mine. The obtained depth is in good agreement with the actual depth, which confirms the application of the Hilbert transform for the interpretation of field data and estimation of magnetic anomalies depths.
Geophysics
Ali Ramzani; Mir Sattar Meshinchi Asl; Mohsen Oveisy Moakhar
Abstract
Geophysical surveys have been carried out to characterize the structure and to better understand the karstic water reservoirs in the Quchan area. Deep 2-D resistivity tomography and magnetic method have been used to detect the most promising zones for new water-well siting. Magnetic method has a common ...
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Geophysical surveys have been carried out to characterize the structure and to better understand the karstic water reservoirs in the Quchan area. Deep 2-D resistivity tomography and magnetic method have been used to detect the most promising zones for new water-well siting. Magnetic method has a common usage in the interpretation of the tectonic framework, faults and fractures that are triggering parameters of karstification phenomena. Therefore a high resolution magnetic survey was carried out as the first detection approach at selected sites in the studied region with the aim of probable buried fault exploration for karstic zone detection. The geomagnetic results detect magnetic anomalies in NW-SE direction agreed with the known faults system in the area. As the second approach, a Deep Electrical Resistivity Tomography (DERT) process has been designed according to magnetic results. Tree sections of 2-D electrical tomography using the Wenner-Schlumberger array has been carried out along a survey line of 100 m and an investigation depth of about 250 m with resolution of 5 m. The 2-D inversion modeling provides suitable information about the exploration targets and essential geological formations. The studies illustrate that low resistivity areas on the sections related to karstic zones. We used this combination of geophysical methods in this study to prove that these geological phenomena can be detected by such quick, economic and confident sequence of geophysical methods.
Geophysics
Mohammad Reza Ebrahimi; Mohammad Tatar
Abstract
Impoundment of a reservoir and changes in the lake levels can trigger / induce the seismicity. In this research, the strength changes are modeled across the Golestan fault, due to loading effect of Gotvand-e Olya reservoir which is located in the Zagros Mountain of SW Iran. The formulas based on 3D Boussinesq ...
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Impoundment of a reservoir and changes in the lake levels can trigger / induce the seismicity. In this research, the strength changes are modeled across the Golestan fault, due to loading effect of Gotvand-e Olya reservoir which is located in the Zagros Mountain of SW Iran. The formulas based on 3D Boussinesq solutions were used to calculate normal and shear stresses on a given fault plane. Geometry and mechanism of the Golestan fault were precisely determined using local earthquakes recorded by a local seismological network. For a better analysis, we did the calculations in three different dips of 55, 60 and 65 degrees for the Golestan fault. Since the lake is located on the hanging wall of the Golestan fault, it is expected that loading effect of the reservoir causes delay on occurrence of earthquakes and make the fault more stable. Analysis of induced stresses confirms this too, the results in all dips in most parts of the fault from 7 km to 20 km depth, which are in closer distance to the reservoir, indicate on stabilization. Only in a small part of the fault, located at shallow depths between 5 and 7 km, the destabilizing effect is observed. The calculated strengths, which leads to destabilizing the Golestan fault ranges between -0.243 bars and 0. The observed seismicity in vicinity of the Golestan fault for before and after the impoundment confirms the the results of stress modelling in different parts of the fault.