Magnetotelluric

Due to the big consuming of ground water in agricultural activities, the reservoir water of Kharga Oasis is noticeably decreased. The demand of drilling deeper wells is increased and most of them reached to the basement rocks. The well data have been observed since 40 years. Over-exploitation of reservoir systems has caused a decline in water table in many parts in the area under studied between 60-80 m. Hot springs are found in the southern part of Kharga City of water temperature 45° C. Magnetotelluric (MT) measurements are carried out in the northern part of Kharga oasis, in order to image the deep part of its reservoir water, using available data of hydro-geology and boreholes information. The results of MT analysis in 1D and 2D show that the sediments of Quaternary and Paleocene (less than 1 km.) have low resistivity value, while the high resistivity value belongs to the basement complex of Pre-Cambrian rocks at depths greater than 1 km.
Keywords: Kharga Oasis Reservoir water (KOR), hydro-geology and magnetotellurics

Intisari. Pemodelan resistivitas 2-D pada penelitian ini dilakukan dengan 2 data MT yang berbeda. Data pertama adalah data yang dirotasi berdasarkan modus tipper strike dari seluruh frekuensi pada setiap titik pengukuran sehingga tiap titik pengukuran dirotasi pada sudut yang berbeda. Data kedua adalah data yang dirotasi berdasarkan modus tipper strike dari seluruh frekuensi dan seluruh titik pengukuran sehingga seluruh titik pengukuran dirotasi pada sudut yang sama. Data kedua memberikan hasil nilai rms error global yang lebih kecil dibandingkan pendekatan pertama sehingga interpretasi dilakukan menggunakan model kedua. Model kedua menunjukkan adanya benda konduktif vertikal miring pada kedalaman ~8 km di bawah permukaan laut dan diinterpretasikan sebagai intrusi magma yang naik melalui sistem sesar Sumatera. Model dan interpretasi tersebut dapat menjelaskan penyebab anomali negatif gravitasi pada area penelitian serta hasil analisis geokimia yang menunjukkan adanya kandungan air yang berasal dari magma pada manifestasi air panas di sekitar area penelitian. Batas kedalaman zona alterasi smektit diperkirakan berada pada kedalaman 500 m di bawah permukaan laut. Metode magnetotellurik (MT) merupakan salah satu metode geofisika yang banyak diterapkan pada eksplorasi panasbumi serta studi mengenai sesar. Metode tersebut memiliki kelebihan antara lain: dapat memetakan distribusi resistivitas bawah permukaan hingga orde puluhan kilometer tanpa sumber buatan, peralatan yang relatif praktis sehingga dapat diterapkan pada kondisi medan yang cenderung sulit, serta konsep geoelectric strike pada metode MT yang seringkali berhubungan dengan persebaran struktur-struktur geologi di bawah permukaan. Menurut Siripunvaraporn et al (2004), pemodelan resistivitas 2-D metode MT pada umumnya dimulai dari mengumpulkan informasi mengenai struktur geologi regional kemudian mengambil data MT dengan orientasi titik pengukuran yang memotong jurus dari struktur tersebut.

1-Abstract The Fayoum–Cairo district lies in the south western part of Cairo, which is affected by several earthquakes. According to the Egyptian Network Seismology of National Research Institute of Astronomy and Geophysics (NRIAG), the last one occurred in the 31 st July, 2005 (Magnitude = 4.2 in Richter scale). Magnetotelluric (MT) can offer good opportunity to detect crustal fluids along faults due to their high electrical conductivity anomalies. The targets of these measurements are various. Its always desired to determine the location(s) of active fault(s) to study the connection of the Fayoum–Cairo seismicity and MT resistivity structure. A single MT profile was carried out across the area in order to investigate the deep resistivity structures. Beside RTP aeromagnetic anomaly map was used to delineate the shallow structures.

The general problem of magnetoteluric (MT) data is due to the difference of coordinate system between the measurement strike and the subsurface condition. Whereas, Magnetotelluric is a passive method that used natural electromagnetic (EM) field, that affect the MT data ambiguity level. MT data processing refers to coherency parameters. Where the coherence is a real dimensionless number indicating the suitability between the two fields (electric and magnetic). Qualitatively, impedance rotation can be used as a controller of MT data quality in terms of the lack of splitting data that occurs between TE and TM. Rotation process is done with two types of rotation, namely principal axis (strike angle) and user defined (fix angle). We have tested on one MT station by applying a new data processing technique, that is rotating (fix angle & strike angle) data prior to XPR, resulting in five different types of data for comparison is XPR (no rotation), XPR PREfix rotation, XPR POST-fix rotation, XPR PRE-strike rotation, and XPR POST-strike rotation. The result of data analysis of all conventional and non-conventional techniques, with angle correction of 25.5 degrees, we decided that the XPR PREfix rotation data as the best data qualitatively reviewed (lack of splitting between TE and TM to from high frequency to lower frequencies) and quantitative (coherence 89,936%).

It is important to explain the geological structures around active faults, especially to estimate the fault activity. Magnetotelluric sounding (MTS) method was applied to investigate active subsurface structures which are covered by the relatively thin Quaternary layers in Dahshour-Qatrani region. MT profiles were carried out across suspected fault(s), where one of the damaging earthquake (such as the October 12, 1992 Dahshour) occurred in the southern westpart of Cairo.

There are currently three types of algorithms in use for regularized 2-D inversion of magnetotelluric (MT) data. All seek to minimize some functional which penalizes data misfit and model structure. With the most straightforward approach (exemplified by OCCAM), the minimization is accomplished using some variant on a linearized Gauss-Newton approach. A second approach is to use a descent method [e.g., nonlinear conjugate gradients (NLCG)] to avoid the expense of constructing large matrices (e.g., the sensitivity matrix). Finally, approximate methods [e.g., rapid relaxation inversion (RRI)] have been developed which use cheaply computed approximations to the sensitivity matrix to search for a minimum of the penalty functional. Approximate approaches can be very fast, but in practice often fail to converge without significant expert user intervention. On the other hand, the more straightforward methods can be prohibitively expensive to use for even moderatesize data sets. Here, we present a new and much more efficient variant on the OCCAM scheme. By expressing the solution as a linear combination of rows of the sensitivity matrix smoothed by the model covariance (the "representers"), we transform the linearized inverse problem from the M-dimensional model space to the Ndimensional data space. This method is referred to as DASOCC, the data space OCCAM's inversion. Since generally N M, this transformation by itself can result in significant computational saving. More importantly the data space formulation suggests a simple approximate method for constructing the inverse solution. Since MT data are smooth and "redundant," a subset of the representers is typically sufficient to form the model without significant loss of detail. Computations required for constructing sensitivities and the size of matrices to be inverted can be significantly reduced by this approximation. We refer to this inversion as REBOCC, the reduced basis OCCAM's inversion. Numerical experiments on synthetic and real data sets with REBOCC, DASOCC, NLCG, RRI, and OCCAM show that RE-BOCC is faster than both DASOCC and NLCG, which are comparable in speed. All of these methods are significantly faster than OCCAM, but are not competitive with RRI. However, even with a simple synthetic data set, we could not always get RRI to converge to a reasonable solution. The basic idea behind REBOCC should be more broadly applicable, in particular to 3-D MT inversion.

The Southern Granulite Terrain with exposed Archean lower crustal rocks is studied using various geophysical tools. The crustal structure derived from seismic reflection and refraction/wide-angle reflection studies is used to understand the tectonic evolution of the region. Deep seismic reflection section along the Kolattur-Palani segment shows an oppositely dipping reflection fabric near the Moyar-Bhavani shear zone, which is interpreted as a signature of collision between the Dharwar craton and another crustal block in the south. The thickened crust due to collision was delaminated during the orogenic collapse and modified the central part, covering the Cauvery Shear Zone system, located between the Moyar-Bhavani and Karur-Oddanchatram shear zones. The delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the Moho. The velocity model of the region indicates crustal thickening at the boundary of the Dharwar craton and Moyar-Bhavani shear zone and thinning further south. Back-scattered seismic wave field with negative moveout and the Moho-offset indicate the spatial location and strike-slip nature of the shear zones. Present study suggests that the late Archean collision and suturing of the Dharwar craton with the southern crustal block at the Moyar-Bhavani shear zone may be responsible for the evolution of late Archean granulites. Late Neoproterozoic rifting is observed along the paleo-fault zones. The seismic studies constrained by gravity, magnetic and magnetotelluric data suggest that the Moyar-Bhavani and Karur-Oddanchatram shear zones of the Cauvery Shear Zone system mark terrane boundaries/suture zones.

In this paper, some recent topics on the modeling of magnetotelluric data are introduced. The focus is on the handling of real field data for two-dimensional resistivity modeling. First, the removal of the effects of near surface heterogeneity is reviewed. It covers telluric distortions (phase mixing and static shift) and magnetic distortions using conventional Groom-Bailey type 3D/2D model (three-dimensional local anomaly underlain by regional two-dimensional structure). The extension of a 3D/2D distortion model for multi-site, multi-frequency is a new development. Magnetic distortion seems to be less significant for land observations, but significant for sea floor data, where the regional magnetic field is weak due to seawater. In special cases involving for example, distortion due to topography and bathymetry, explicit removal is possible. There are some schemes proposed for a 3D/3D model (three-dimensional local anomaly underlain by regional three-dimensional structure). Along with the removal of the distortion, it is important to recognize the dimensionality of the dataset prior to modeling. A property using strike estimates for each site is an indicator for dimensionality which is unaffected by local distortion. Mapping the local strike or a rose diagram is an effective visualization of the dimensionality. Two-dimensional inversion is becoming routine. For the fast calculation of derivatives, approximate calculation, reciprocity or conjugate gradient methods are used. In order to incorporate a priori information and to overcome the intrinsic ill-posed nature of the inversion problem, imposing constraints on the model structure is important. A proper tradeoff between the data fit and constraints should be optimized to obtain minimum structures that are required by the field data. However, the choice of constraints is rather subjective and depends on the geological situations. For field data, two-dimensional inversion has limits on modes, area, and period range. Special care must be taken for the structure outside the profile. Two-dimensional inversion incorporating anisotropy is interesting and becoming popular, but the structure may not be unique. Future development in three-dimensional inversion for real datasets should take the above points into consideration.

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.

Magnetotelluric method (MT) offers opportunity to detect crustal fluids along faults due to their high conductivity anomaly. Supposing that fluids deposited minerals in the conductive fractures (faults, dykes) decreasing the resistivity, the high seismicity in the area can be explained by the presence of these fluids. MT measurements were carried out in the period range 0.001-420 s crossing the Kalabsha fault (Aswan, Egypt) and Remirermont fault (Southern Vosges, France). In these work we detect geoelectrical resistivity anomalies of the Earth's crust and link them to local seismic activity. Seismic events having magnitude (M<5) are found along fault zones in Kalabsh and Remirermont. The goals of our measurements are various. We would like to determine the precise location of the active faults, to study the connection of the Remiremont & Kalabsha seismicity to the MT resistivity structures, and to support the idea of the influence of the fluid-bearing conducting faults in the Remiremont & Kalabsha areas to the earthquake. These applications afford the unusual opportunity to study the percolation of water into the faults system and its effect on the seismicity, to reveal geological structures and the stress field covered by thin Quaternary formations. Data are analysed by 2D simultaneous inversion of both polarizations. The resulting models are compared with the local seismicity map. Our MT model reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in active regions.

We describe two extensions to the three-dimensional magnetotelluric inversion program WSINV3DMT (Siripunvaraporn, W., Egbert, G., Lenbury, Y., Uyeshima, M., 2005, Three-dimensional magnetotelluric inversion: data-space method. Phys. Earth Planet. Interiors 150, 3-14), including modifications to allow inversion of the vertical magnetic transfer functions (VTFs), and parallelization of the code. The parallel implementation, which is most appropriate for small clusters, uses MPI to distribute forward solutions for different frequencies, as well as some linear algebraic computations, over multiple processors. In addition to reducing run times, the parallelization reduces memory requirements by distributing storage of the sensitivity matrix. Both new features are tested on synthetic and real datasets, revealing nearly linear speedup for a small number of processors (up to 8). Experiments on synthetic examples show that the horizontal position and lateral conductivity contrasts of anomalies can be recovered by inverting VTFs alone. However, vertical positions and absolute amplitudes are not well constrained unless an accurate host resistivity is imposed a priori. On very simple synthetic models including VTFs in a joint inversion had little impact on the inverse solution computed with impedances alone. However, in experiments with real data, inverse solutions obtained from joint inversion of VTF and impedances, and from impedances alone, differed in important ways, suggesting that for structures with more realistic levels of complexity the VTFs will in general provide useful additional constraints.

Particle Swarm Optimization (PSO) is one of nature-inspired optimization algorithms that adopts swarm (insects, school of fish, flock of birds etc.) behaviour in search for food or common target in a collaborative manner. The particles (or agents) in the swarm learn from their neighbours as well as themselves regarding the promising area in the search space. The information is then used to update their position in order to reach the target. The search algorithm of a particle is dictated by the best position of that particle during the process (individual learning term) and the best particle in its surroundings (social learning term) at a particular iteration. In terms of optimization, the particles are models defined by their parameters, while the promising area in the model space is characterized by a low misfit associated with optimum models. Being a global search approach, PSO is suitable for nonlinear inverse problem resolution. The algorithm was applied to a simple minimization problem for illustration purpose. The application of PSO in geophysical inverse problem is demonstrated by inversion of synthetic magnetotelluric (MT) data associated with simple 1D models with satisfactory results in terms of model recovery as well as data misfit.

The main purpose of this study in the High Central Zagros area near the townships of Lordegan and Yasuj was to investigate the nature of the High Zagros Fault (HZF) and the structural styles in the adjacent foreland. A second task was to determine whether the prolific petroleum plays productive further out in the Zagros Foreland would extend into the study area and if new petroleum plays could be recognized.

1] Magnetotelluric data from a 150-km-long profile crossing the Banggong-Nujiang suture (BNS), central Tibet, acquired as part of the International Deep Profiling of Tibet and the Himalaya (INDEPTH) project, have been examined for crustal and upper mantle structure. Strike and dimensionality analyses demonstrate that regional-scale electrical structures are two-dimensional and oriented approximately parallel to surface geological strike. As seen elsewhere in Tibet, the double thickness crust is generally characterized by resistive upper crust (hundreds to thousands of ohm meters) overlying conductive middle and lower crust (tens to hundreds of ohm meters), but in detail, there are lateral variations at all levels. Regionally, a northward transition from thick ($45 km) to thin ($15 km) resistive upper crust coincides with (1) the surface trace of the BNS, (2) a prominent strand of the Karakorum-Jiali fault system, (3) northward decrease in upper mantle seismic velocities and increase in attenuation, and (4) pronounced northward onset of seismic polarization anisotropy. The latter two seismological features have been taken to mark the northern limit of Indian mantle lithosphere thrust beneath southern Tibet. On the basis of our electrical model, we speculate that (1) the resistive upper crustal root beneath the Neogene Lunpola and Duba basins was produced by crustal shortening localized along the northern edge of the Lhasa terrane; (2) the low midcrustal resistivity beneath the BNS reflects enhanced Neogene melting and/or metamorphic dewatering of relatively fertile subduction zone complex rocks; (3) observed steep upper crustal low-resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the BNS; and (4) these strike-slip and extensional fault arrays are surface manifestations of lithosphere-penetrating shear localized along the northern edge of the underthrust Indian plate. Citation: Solon, K. D., et al. (2005), Structure of the crust in the vicinity of the Banggong-Nujiang suture in central Tibet from INDEPTH magnetotelluric data,

Magnetotelluric data were obtained along a 450 km, almost west-east profile at approximately 31.5°S, which extends from La Rioja to Santa Fé provinces in central Argentina. The profile crosses two main crustal domains that were juxtaposed during the Early-Middle Cambrian Pampean Orogeny: the Pampean terrane to the west and the Río de la Plata craton to the east. The electrical resistivity structure of the crustal domains together with their boundary is presented. Through dimensionality analysis of the data, it was demonstrated that regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallel to the surface geological strike. The resistivity model shows a subvertical limit approximately along the eastern border of the Sierra Chica de Córdoba. To the east, the shallower structure is the Chaco-Paranense basin extending to a depth of 6 km with resistivities between 1 and 30 Ohm-m, whereas below the basin the ca 2.1-2.3 Ga Río de la Plata craton shows resistivities in a range of 300-10,000 Ohm-m. The Pampean terrane presents a 6 km layer with a resistivity higher than 10,000 Ohm-m whereas below this layer the resistivity values range from 50 to 200 Ohm-m. Based on both the geological information and the magnetotelluric results, the sharp lateral discontinuity observed in the resistivity model to the east of the Sierras de Córdoba is conjectured to represent the boundary between the Río de la Plata craton and the Pampean terrane which may correspond to the Early Cambrian suture.

Iceland is very active tectonically as it is crossed by the Mid-Atlantic Ridge and its associated rift zones and transform faults. The high-temperature geothermal systems are located within the neo-volcanic zone. A detailed comparison of the main features of the resistivity models and well data in exploited geothermal fields has shown that the resistivity structure of Iceland is mainly controlled by alteration mineralogy. In areas where the geothermal circulation and related alteration take place at depths of more than 1.5 km, the investigation depth of the DC and TEM methods is inadequate and the MT method appears to be the most suitable survey method. MT soundings were carried out to determine the deep structure between two neighboring Quaternary geothermal fields: the Hengill volcanic complex and the Brennisteinsfjoll geothermal system, both known as high-temperature systems. MT data were analyzed and modeled using 1D and 2D inversion schemes. Our model of electrical conductivity can be related to secondary mineralization from geothermal fluids. At shallow depths, the resistivity model obtained from the MT data is consistent with the general geoelectrical models of high-temperature geothermal systems in Iceland, as revealed by shallow DC and TEM surveys. The current MT results reveal the presence of an outcropping resistive layer, identified as the typical unaltered porous basalt of the upper crust. This layer is underlain by a highly conductive cap resolved as the smectite-zeolite zone. Below this cap a less conductive zone is identified as the epidote-chlorite zone. A highly conductive material has been recognized in the middle of the profile, at about 5 km depth, and has been interpreted as cooling partial melt representing the

Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such pervasively high conductance suggests that partial melt and/or aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow.

A simple impedance tensor analysis of four magnetotelluric soundings recorded over the ECORS section of the Rhine Graben shows that for periods shorter than about 30 s, induction dominates over channelling. For longer periods, 2-D induction galvanically distorted by surface heterogeneities and/or current chanelled in the Graben can explain the observations; the role of chanelling becomes dominant at periods of the order of a few hundred seconds. In the area considered, induction appears to be controlled by inclusions of saline water in a porous limestone layer (Grande Oolithe) and not by the limits of the Graben with its crystalline shoulder (Vosges). The simple analysis is supported by tipper analyses and by the results of schematic 2-D modelling.

Magnetotelluric studies over the Damoh±Jabalpur±Mandla±Anjaneya pro®le in central India have delineated Vindhyan sediments which are about 5 km thick in the Damoh± Katangi region. The crust below the Vindhyan sediments shows the characteristics of lower crust, as observed from the relatively lower resistivity of about 200 V m and high seismic velocities (P-wave velocities of 6.5 km s x1 compared with 5.8±6.2 km s x1 in the surrounding region). It is conjectured that the upper crust may have been completely eroded in the uplift and erosion process and thus the Vindhyan sedimentation has occurred directly over the lower crust. An anomalous conductivity is observed at depths of 10±12 km in the Vindhyan crust. The conductance of more than 1200 S observed here may be due to either the serpentinization of the ma®c and ultrama®c rocks or the presence of grain boundary graphites. The thickness of the Deccan traps is about 100 m near Jabalpur and decreases near Mandla. On the south of Mandla, the Archaean crust is exposed. Two crustal conductors are delineated below the Deccan volcanics with a resistivity of about 30 V m. The ®rst on the immediate south of Jabalpur seems to mark the southern boundary of the Jabalpur horst block. The second conductor was delineated about 40 km southeast of Jabalpur, coincident with a positive gravity anomaly of about 30 mGal. Deep seismic sounding studies do not show any signi®cant density contrast associated with this conductive feature. It is proposed that the gravity high may be due to the upwarp of the Moho. The high electrical conductivity is attributed to the¯uids in the upper crust.

Two-dimensional resistivity analysis of magnetotelluric data has been done at "Z" geothermal area which is located in southern part of Indonesia. The objective is to understand subsurface structure beneath reasearch area based on 2-D modeling of magnetotelluric data. The inversion finite element method were used for numerical simulations which requires discretization on the boundary of the modeling domain. The modeling results of magnetotelluric data shows relativity structure dissemination: 0-10 ohm.m in a thickness of 1 km (Clay Cap), 10-100 ohm.m with 1-2 km depth respectively (reservoir zone), and on a scale of 100-1000 ohm.m in a depth of 2-3 km (heat source zone). The result of relativity structure can be used to delineate an area with geothermal prospect around 12 km 2 .

Saudi Arabia is a desert country with no permanent rivers or lakes and very little rainfall. Ground water aquifers are the major source of water in Saudi Arabia. In the Riyadh region, several Wadies including Wadi Nisah store about 14 · 10 6 m 3 of water, which is extracted for local irrigation purposes. In such areas, the water wells are as shallow as 200-300 m in depth. The importance of Wadi Nisah is because the subsurface water aquifers that are present there could support the region for many years as a water resource. Accordingly, in this study, we performed a Magnetotelluric survey using a portable broadband sounding system (MT24/LF) to evaluate the ground water aquifer at great depths. We collected 10 broadband Magnetotelluric sounding stations (1 station/day) with an interval of about 2-3 km reaching a profile length of about 25-30 km along Wadi Nisah. Additionally, we used available gravity data to image the subsurface structure containing the aquifer.

Previously proposed methods of area selection for diamond-prospective regions have predominantly relied on till geochemistry, airborne geophysics, and/or an appraisal of tectonic setting. Herein we suggest that a novel, deep-probing geophysical technique—electromagnetic studies using the natural-source magnetotelluric (MT) method—can contribute to such an activity. Essentially, diamondiferous regions must have (1) old lithosphere, (2) thick lithosphere, and (3) lithosphere that contains

In this paper, we compare and contrast the results of field experiments with the dipole-dipole-induced polarization (IP) and controlled-source audio-magneto- tellurics (CSAMT) methods, along the same survey pro- files, at a test area that was subject to extensive drilling and detailed geological investigation. The ore bodies are interbedded between two series of dacitic tuff. The depth and thickness of the

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.

In the Llucmajor aquifer system (Majorca Island, Spain) some geothermal evidences have appeared. This phenomenon is not isolated to Majorca and it is present in other areas, where it can be associated with structural conditions, especially to the extensional event suffered by the island after the Alpine Orogeny. However, the origin of this anomaly in Llucmajor is not well known, and there is no surface geological evidence of these structural conditions. With the aim of delineating the geoelectrical structure of the zone and identifying the geological structure that allows the presence of this anomaly, an audiomagnetotelluric (AMT) survey was carried out. The AMT data was processed using a Wavelet Transform-based scheme. Dimensionality analysis indicates that the geoelectrical structure is mainly 3D. The 3D model was obtained by trial and error forward modeling, taking accounting of the responses from the determinant of the impedance tensor. The model shows a vertical resistivity distribution with three horizons associated with different units: on the top, a shallow high resistive media related to an unconfined shallow aquifer; in the middle, a conductive layer related to the aquitard, and below it, another resistive media related to the confined deeper aquifer. The intermediate horizon shows a sudden thinning beneath the thermal anomalous zone that can be identified as a weakness zone (fault or fracture) connecting both aquifers. An exploratory well was drilled after the AMT survey and reached almost 700 m in depth. This allowed correlating the resistivity distribution of the 3D model with data logging and lithology obtained from the well, showing a proper agreement between them.

The Aravalli Delhi Mobile Belt (ADMB) in NW India includes the Proterozoic Aravalli and Delhi rifts and fold belts. A seismic profile and gravity modelling across the ADMB shows several dipping reflections and suggests a thick crust (45-46 km) and a high density (3.04 g cm−3) dome-shaped body in the lower crust. This high density body in the lower crust may represent underplated lower crust caused by extension during evolution of the Proterozoic rifts. Part of the gravity high observed over the ADMB is attributed to the upthrust blocks of high density lower crustal granulite rocks. The gravity low along the eastern margin of the ADMB is modelled as being due to a westerly dipping (45°) low density body in the crust, in accordance with seismic reflectors which may represent remanents of the subducted crustal rocks. The Hindoli group of rocks above this body is characterized by high conductivity, indicating a high fluid content and a fractured nature which may be related to the collisional tectonism of this region. A sharp gravity high along the western margin of the ADMB is attributed to the basic rocks of an ophiolite melange. The gravity low and high west of the ADMB are attributed to plutonic activity and bimodal calc-alkaline volcanics in linear basins and are related to arc and back-arc rift magmatism, respectively. The ADMB, therefore, represents a Proterozoic collision zone between the Bundelkhand craton towards the east and the Rajasthan craton towards the west with a plausible subduction towards the west along the eastern margin of the ADMB. The thrusting is mainly confined to the obducted block towards the west, although some thrusting is inferred even in the subducted block. The central Indian Suture (CIS) is located at the southern margin of the Satpura Mobile Belt (SMB) in central India and separates high grade metasediments and granulite towards the north from low grade Proterozoic volcanogenic successions towards the south. A seismic profile and gravity modelling across the CIS show a thick crust with a thicker crust north of the CIS compared to the south and a contact in the lower crust whose projection on the surface coincides with the CIS. The CIS at the surface coincides with a low density (2.65 g cm−3) and conductive body that is interpreted to be fluid-filled and fractured in nature. The high density (2.81 g cm−3) body modelled in the upper crust north of the CIS coinciding approximately with a highly conductive body and dome-shaped seismic reflectors may represent an upthrust block from the lower crust as high density granulite rocks are exposed in this region. Short wavelength gravity highs and lows south of the CIS represent exposed metavolcanics and granite intrusions which represent tholeiitic series rocks to calc-alkaline series rocks associated with island arcs and back arc magmatism. Therefore, the large wavelength gravity low south of the CIS is modelled as a low density body below the Moho, which conforms with seismic reflectors and may represent crustal rocks subducted towards the south. Thrusting in this case is indicated in the subducted block towards the north of the CIS. The bipolar gravity anomaly across the CIS, therefore, indicates a collision zone between the Bundelkhand craton towards the north and Bhandara-Deccan cratons towards the south.

Mt Ruapehu is an active andesite cone volcano, which marks the southern termination of the Kermadec volcanic arc. Results from 40 broad-band magnetotelluric soundings have been analysed using the phase tensor. This approach provides a way of determining dimensionality, allowing for distortion removal, and visualizing data in a 3-D situation. The phase tensor analysis suggests that the shallow resistivity structure is largely 1-D in character, but that the deeper structure requires a 3-D interpretation. 1-D inversions show that at sites on Ruapehu a shallow conductive layer lies between a high resistivity layer, of a few hundred metres thickness, and higher resistivity layer corresponding to basement greywacke. The low resistivity layer is contiguous with the waters of the highly acidic Crater Lake, and thus is believed to be the hydraulically controlled upper limit of a zone of acid alteration overlain by dry volcanic rock and ash. To the southwest of the volcano the conductive layer merges with a surface conductor associated with Tertiary sediments. Following initial 2-D inversions, the deep resistivity structure has been derived through 3-D inversion of data from 38 sites. This indicates the existence of a dyke-like low resistivity zone that persists to at least 10 km depth and extends from beneath the summit of Ruapehu to the northeast where it appears to connect to a poorly constrained region of high conductivity, which lies outside the network of measurement sites. The low resistivity dyke-like feature may be identified with a volcanic feeder system, which also supplies the other volcanoes of the Tongariro Volcanic Centre and marks the conduit by which hot gases and (occasionally) magma reach the surface.

and Antonio L. Padilha (DGE-INPE, [email protected]) Copyright 2004, SBGf -Sociedade Brasileira de Geofísica Este texto foi preparado para a apresentação no I Simpósio Regional da Sociedade Brasileira de Geofísica, São Paulo, 26-28 de setembro de 2004. Seu conteúdo foi revisado pela Comissão Tecno-científica do I SR-SBGf mas não necessariamente representa a opinião da SBGf ou de seus associados. E proibida a reprodução total ou parcial deste material para propósitos comerciais sem prévia autorização da SBGf. ____________________________________________________________________

1] A long-period magnetotelluric data set was obtained in 2005 along a two-dimensional profile across the western part of the late Archaean-early Proterozoic Gawler Craton, South Australia. The study is aimed at delineating the electrical conductivity structure of the crust and upper mantle underneath an east-west trending profile extending from the Gawler Range Volcanics in the east, crossing the Nuyts Domain and the highly prospective Meso-Proterozoic Fowler Domain, and terminating in the Eucla Basin to the west. The resistivity model shows a very electrically resistive crust and upper mantle underneath the Nuyts and Fowler Domain, possibly representing the cratonic root of the Gawler Craton extending to depths of ∼160 km. The resistive cratonic root is closer to the surface underneath the Fowler Domain compared to the Nuyts Domain which supports findings from outcrops of metasediments of higher metamorphic grade in the Fowler Domain. A subvertical conductor marks the western terminus of the Fowler Domain and is imaged to upper mantle depths. On the eastern side of the Fowler Domain, another subvertical conductor extends to similar depths. These features spatially coincide with the Tallacootra and Coorabie shear zone at the surface, respectively, and their higher conductivity is likely due to a reduction in grain size of olivine associated with an increase in influence of grain boundary diffusion and thus enhanced conductivity. A comparison of the results with other surveys across mobile belts worldwide shows a more resistive response of the interpreted mobile belt of the Fowler Domain raising questions as to the nature of the domain.

Broadband and long-period magnetotelluric soundings were collected along a 560 km E-W profile in a region in central Brazil subjected to Neoproterozoic collision tectonics and Archean to Cretaceous magmatic events. The profile crosses the northeast part of Phanerozoic sediments and volcanics of the Parana basin, the southern extension of the Neoproterozoic metasedimentary rocks of the Brasilia belt, locally pervaded by Cretaceous alkaline magmas of the Alto Paranaiba igneous province (APIP), and Neoproterozoic sedimentary cover and Archean exposed basement of the southern Sao Francisco craton. 2D conductivity structures derived by joint inversions of the TE and TM polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area. A gravity-defined suture zone beneath the Parana basin related to the Neoproterozoic collision of the Sao Francisco and Parana blocks is detected in the models as a subvertical con...

Broadband and long-period magnetotelluric soundings were collected along a 560 km E-W profile in a region of central Brazil subjected to Neoproterozoic collision tectonics and Archean to Cretaceous magmatic events. The profile crosses the northeastern portion of Phanerozoic sediments and volcanics of the Paraná basin, the southern extension of the Neoproterozoic metasedimentary rocks of the Brasília belt, locally pervaded by Cretaceous alkaline magmas of the Alto Paranaíba igneous province (APIP), and Neoproterozoic sedimentary cover and exposed Archean basement of the southern São Francisco craton. 2D conductivity structures derived by joint inversions of the TE and TM polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area. A gravity-defined suture zone beneath the Paraná basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths, which strengthen the interpretation of a Neoproterozoic collision of the São Francisco craton and a continental block beneath the sedimentary basin. Deep underthrusting of organic graphite-bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity. A similar conductivity signature beneath the sedimentary covered region of the São Francisco craton can be tentatively interpreted as another similar cryptic suture zone. Other isolated high-conductivity anomalies at midcrustal depths below the Paraná and APIP provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of Cretaceous mafic-ultramafic volcanics. A very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the APIP volcanic complex, coincident with a zone of low velocity defined by seismic tomography. Geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures. Thus, interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity. The seismic and conductivity anomalies are probably triggered by the same source mechanisms (incipient melting) because they are plausibly related to the same magmatic process. Conductivity anomalies at crustal and mantle depths in the southern segment of the São Francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history. Consequently, the enhanced conductivity in the lower crust can be genetically related to upwelling volatile-rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low-degree carbonatitic melts from deeper-sourced metasomatic processes. tal collisions, allochtonous terrains of all sizes are now comprised into large collage areas which display the typical imprint of the Neoproterozoic Brasiliano/Pan-African orogenic cycle . The magmatism includes the extensive Paraná flood basalt province and several alkaline mafic to ultramafic provinces surrounding the large Paleozoic Paraná basin . Intruding the southern portion of the Neoproterozoic Brasília fold belt and bounded by the Paraná basin to the southwest and the limits of the exposed São Francisco craton to the northeast, the Cretaceous Alto Paranaíba igneous province (APIP) is the most important of these alkaline provinces. APIP magmas are 0301-9268/$ -see front matter

Airborne very low frequency (VLF) data are routinely collected by national agencies and commercial companies together with other passive geophysical measurements of the static magnetic field and radiometric data. The purpose of this paper is to demonstrate that both standard three-component VLF and tensor VLF (TVLF) data contain a lot of useful quantitative and qualitative information about the electrical conductivity distribution in the upper few hundred meters of the crystalline basement. We first give a new derivation of the fundamental transfer functions (the tipper) used in the TVLF technique. We then show that the tipper can be estimated from simultaneous measurements of the wave magnetic fields from at least two transmitters with somewhat different frequencies, and present a simple model by which the maximum error introduced by the difference in frequencies can be found. Single transmitter scalar VLF maps emphasise those conductive structures that have dominant strikes in the direction of the transmitter. Multiple transmitter transfer functions are dependent only upon the underlying conductivity structure. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic magnetotelluric (MT) soundings. In such cases three-component VLF measurements can be modelled easily upon appropriate rotation of the co-ordinate system to ''strike'' co-ordinates. Single frequency transfer functions (tippers) have real and imaginary parts that carry information on not only lateral contrasts in conductivity, as usually stated in text books, but, taken together, they provide a robust tool for determining the background conductivity level away from distinct conductors, and they can also be used to discriminate between deep and shallow conductors. Based upon simulations using multi-frequency data, it can be concluded that such a new development would dramatically increase the resolving power of airborne VLF measurements.

Saudi Arabia is a desert country with no permanent rivers or lakes and very little rainfall.
Ground water aquifers are the major source of water in Saudi Arabia. In the Riyadh region, several
Wadies including Wadi Nisah store about 14 · 106 m3 of water, which is extracted for local irrigation
purposes. In such areas, the water wells are as shallow as 200–300 m in depth. The importance of
Wadi Nisah is because the subsurface water aquifers that are present there could support the region
for many years as a water resource. Accordingly, in this study, we performed a Magnetotelluric survey
using a portable broadband sounding system (MT24/LF) to evaluate the ground water aquifer at
great depths. We collected 10 broadband Magnetotelluric sounding stations (1 station/day) with an
interval of about 2–3 km reaching a profile length of about 25–30 km along Wadi Nisah. Additionally,
we used available gravity data to image the subsurface structure containing the aquifer.

Magnetotelluric method (MT) offers opportunity to detect crustal fluids along faults due to their high conductivity anomaly. Supposing that fluids deposited minerals in the conductive fractures (faults, dykes) decreasing the resistivity, the high seismicity in the area can be explained by the presence of these fluids. MT measurements were carried out in the period range 0.001-420 s crossing the Kalabsha fault (Aswan, Egypt) and Remirermont fault (Southern Vosges, France). In these work we detect geoelectrical resistivity anomalies of the Earth's crust and link them to local seismic activity. Seismic events having magnitude (M<5) are found along fault zones in Kalabsh and Remirermont. The goals of our measurements are various. We would like to determine the precise location of the active faults, to study the connection of the Remiremont & Kalabsha seismicity to the MT resistivity structures, and to support the idea of the influence of the fluid-bearing conducting faults in the Remiremont & Kalabsha areas to the earthquake. These applications afford the unusual opportunity to study the percolation of water into the faults system and its effect on the seismicity, to reveal geological structures and the stress field covered by thin Quaternary formations. Data are analysed by 2D simultaneous inversion of both polarizations. The resulting models are compared with the local seismicity map. Our MT model reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in active regions.