Magnetotelluric

A detailed three-dimensional (3-D) numerical electrical resistivity model of the Bathurst No. 12 deposit (New Brunswick) has been constructed using available geological and geophysical information. This model facilitates studies of the resolution capabilities of audio-magnetotellurics (AMT) at locating and defining mineral targets, and of methods of optimizing data acquisition. Different conditions were analyzed: presence of overburden, dimensions and positions of the ore body, and varying data sampling. The behavior of 3-D EM fields is compared with 2-D ones. The 3-D and 2-D responses are similar at high frequencies, but at low frequencies only those responses for current flow perpendicular to the body (TM mode in 2-D) are reasonably alike. The 2-D inversions show that the position and top of the 3-D ore body can be well resolved, but not the bottom and resistivity of the body.

Steeply-dipping mineralized bodies present a particular problem for audio-magnetotelluric (AMT) exploration. Such targets have little observable effect on currents flowing perpendicular to their dominant strike, thus TM-mode AMT data are not useful for target detection or delineation. The TE-mode and vertical-field transfer function data do detect such targets, and appropriately-designed surveys can define their geometries. The anomalous responses due to such targets are greater in the magnetic fields than in the electric fields. Conducting overburden suppresses the high frequency response, moving the anomaly maxima to lower and lower frequency with increasing overburden conductance. For optimal resolution of target geometries, all five AMT components must be measured at each and every site.

Structural features of volcanic and hydrothermal systems can be used to infer the location of magma chambers or productive geothermal areas. The Hengill volcanic triple-junction complex has a well-developed geothermal system, which is being exploited to extract hot fluids that are used for electrical power and heat production. In the framework of the I-GET project, a 4month temporary seismological network including seven highdynamic broadband instruments was deployed and 1D transient electromagnetic soundings (TEM) and 3D magnetotelluric (MT) surveys were performed to improve understanding of the relationships between structural features, seismic activity and fluid production at the Hengill geothermal system. The MT and TEM data set is analyzed elsewhere. The analysis of the seismological data set allowed the detection and classification of more than 600 earthquakes, among which long-period (LP) earthquakes were observed for the first time in this area. This work focuses first on a joint inversion for the 3D velocity structure and determination of the locations of the hypocentres from about 250 local volcano-tectonic earthquakes with clear Pand S-wave arrival times. The results confirm those from earlier tomography studies in this area. Integrating the seismic velocity and resistivity models in a semi-quantitative approach by crossplotting the resistivity model with the velocity ratio V P /V S delineates a structural body with a high seismic velocity ratio and low resistivity that is interpreted as the main heat source of the geothermal system.

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

The MT interpretation procedure begins with a set of sounding data in the frequency domain. The overall quality of these data can be variable both as a function of frequency and location. Many simple interpretation procedures, such as the assessment of static distortion, act directly on the sounding data. A number of response characteristics, such as the location (in frequency) and number of turning points, are important to the interpretation. Localised scatter (noise) in the response estimates can produce false gradients which degrade the quality of the inferences made from the data. This study considers how the D + solution can be used to process the raw sounding data to provide a number of interpretational advantages. Although the D + solution has strict formal roots in 1D inverse theory, it is used here simply to enhance those data attributes, particularly that of physical validity, which lead to a more meaningful assessment of data characteristics. The data considered are 84 broadband array soundings from the Paraná basin, Brazil. The advantages provided by the D + processed data set are demonstrated by using the raw and processed data in two main interpretational procedures. The first procedure concerns the ability of the data to provide quantitative assessments of the influence of static distortion. The second procedure concerns the application of transform methods which attempt to recover a resistivity/depth or reflectivity profile directly from the sounding data.

Measurement of apparent resistivity of a half space * The primary magnetic field (H p ) produced by the Tx extends into the Earth * The H p oscillates, secondary currents (I S ) are induced in the Earth. * The (I S ) flow at right angles to the direction of the primary magnetic field.

A method is described for filtering magnetotelluric (MT) data in the wavelet domain that requires a minimum of human intervention and leaves good data sections unchanged. Good data sections are preserved because data in the wavelet domain is analyzed through hierarchies, or scale levels, allowing separation of noise from signals. This is done without any assumption on the data distribution on the MT transfer function. Noisy portions of the data are discarded through thresholding wavelet coefficients. The procedure can recognize and filter out point defects that appear as a fraction of unusual observations of impulsive nature either in time domain or frequency domain. Two examples of real MT data are presented, with noise caused by both meteorological activity and power-line contribution. In the examples given in this paper, noise is better seen in time and frequency domains, respectively. Point defects are filtered out to eliminate their deleterious influence on the MT transfer function estimates. After the filtering stage, data is processed in the frequency domain, using a robust algorithm to yield two sets of reliable MT transfer functions.

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%).

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.

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.

A robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory, as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle. This review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data, and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy. To this end, we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years: eastern North America, the Canadian Shield, Australia, and southern Africa. Based on this full suite of results, we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle, usually to depths of at least 200 km and perhaps more. An implication of these results is that tectonic plates are, at most, only partially coupled to the underlying mantle. The results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years, it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy. We suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics, magnetotellurics, gravity, and geodesy.

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 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.

Geoscience modelling plays a vital role in mapping and tracking Earth's resources. Magnetotellurics, which maps the electrical resistivity of the subsurface, is a useful and costeffective technique for sensing large areas at depth. However, due to the inherent difficulty in sensing deep strata, models produced using MT have a degree of uncertainty. Geoscientists can reduce this uncertainty by producing multiple alternative models, and using multiple modelling techniques and settings, to correlate robust model features with field data responses. Population-based evolutionary search techniques are of interest to MT modelling because they offer an alternative to deterministic techniques, and are able to produce multiple models for analysis. Unfortunately, evolutionary techniques have not been successfully applied to 3D MT modelling. In this work we describe a new, more compact, representation of MT models using volumetric functions. Using this representation we successfully apply evolutionary search techniques to 3D MT modelling for both artificial and real models and show how the development of large scale features during modelling can be correlated with the model's fit to field 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.

We consider 2D earth models consisting of laterally variable layers. Boundaries between layers are described by their depths at a set of nodes and interpolated laterally between nodes. Conductivity within each layer is described by values at a set of nodes fixed within each layer, and is interpolated laterally within each layer. Within the set of possible models of this sort, we iteratively invert magnetotelluric data for models minimizing the lateral roughness of the layer boundaries, and the lateral roughness of conductivities within layers, for a given level of data misfit. This stabilizes the inverse problem and avoids superfluous detail. This approach allows the determination of boundary positions between geological units with sharp discontinuities in properties across boundaries, while sharing the stability features of recent smooth conductivity distribution inversions.

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

Within the framework of the Southern African MagnetoTelluric EXperiment (SAMTEX) a focused study was undertaken to gain better knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe Craton into Botswana, with the overarching aim to increase our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover

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.

St Helens, Adams and Rainier-are located on the margins of a mid-crustal zone of high electrical conductivity 1-5 . Interconnected melt can increase the bulk conductivity of the region containing the melt 6,7 , which leads us to propose that the anomalous conductivity in this region is due to partial melt associated with the volcanism. Here we test this hypothesis by using magnetotelluric data recorded at a network of 85 locations in the area of the high-conductivity anomaly. Our data reveal that a localized zone of high conductivity beneath this volcano extends downwards to join the mid-crustal conductor. As our measurements were made during the recent period of lava extrusion at Mount St Helens, we infer that the conductivity anomaly associated with the localized zone, and by extension with the mid-crustal conductor, is caused by the presence of partial melt. Our interpretation is consistent with the crustal origin of silicic magmas erupting from Mount St Helens 8 , and explains the distribution of seismicity observed at the time of the catastrophic eruption in 1980 (refs 9, 10).

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of Cood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate seismic activity, the most recent one being a swarm-type activity in the Palghar region, about 120 km north of Mumbai, that started in November 2018 and has produced a few thousand micro-earthquakes and a 4.3 magnitude earthquake since then. We have carried out a mag-netotelluric (MT) study along a 35-km long proBle across the seismic zone to delineate the subsurface structure to understand the possible cause for the seismic activity. Broadband MT data were acquired at 18 sites with average station spacing of 2 km. Impedance tensors were analyzed for distortion and dimensionality, decomposed into TE-and TM-mode, and inverted by a 2D inversion algorithm. The geoelectric structure yields an assemblage of highly resistive and moderately conductive blocks in the uppermost crust resting on a major listric-type fault, that possibly reaches the surface at the West Coast Fault from a depth of about 15 km beneath the Panvel Cexure zone. In conjunction with the regional Bouguer gravity anomaly and the seismicity distribution, we infer that the upper crustal heterogeneities coupled with the basement fault and low rheological strength of the fractured upper-to-middle crust might be leading to triggering of the seismicity in the region.

Surficial bodies can severely distort magnetotelluric (MT) apparent resistivity data to arbitrarily low frequenties. This distortion, known as the MT static shift, is due to an electric field generated from boundary charges on surficial inhomogeneities, and persists throughout the entire MT recording range. Static shifts are manifested in the data as vertical, parallel shifts of log-log apparent resistivity sounding curves, the impedance phase being unaffected. Using a three-dimensional (3-D) numerical modeling algorithm, simulated MT data with finite length electrode arrays are generated. Significam.staticshifts are produced in this simulation; however, for some geometries they are impossible to identify.

Abstract: The frequency-normalised impedance (FNI) function is introduced to give a physical significance to the magnetotelluric data. The FNI function is separated into its real and imaginary parts that have the same denominator to estimate their values in the high- and low-frequency limits. The behaviour of the apparent resistivity definitions and the phase is explained theoretically by considering the real and the imaginary parts of the FNI function. The oscillations on the apparent resistivity curves are explained by using ascending and descending types two-layer curves of the FNI function. The concept of the reciprocal geoelectric section is developed by using the properties of the FNI function. It has been shown that the apparent resistivity curves of a geoelectric section and those of corresponding to the reciprocal geoelectric section are symmetric. The axis of symmetry is the horizontal axis that intersects the vertical axis at unity. The FNI function assists the interprete...

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.

The devastating (M, 6.2) Latur earthquake of September 29, 1993 in South India has claimed an estimated 11,000 human lives. With an I,,, of VIII, the earthquake was felt to an average distance of 750 km. More than I25 shocks were reported to have been felt during August l992-March 1993. Out of these, during October-November 1992, several shocks of M > 2.0 were recorded at the NGRI seismic station at Hyderabad which is the closest (220 km) to the epicentre. No such shocks occurred for at least 8 months before the Latur earthquake. The aftershocks were monitored by a network of up to 21 stations between October 8, 1993 and January 31, 1994. A majority of the aftershocks occurred within a IO-km radius from the main shock. On the basis of the location of aftershocks in the first few days, a plane dipping at an angle of 45" towards the southwest and striking at 135" is inferred to be the fault plane which extends to a depth of 4.5 km and on projection meets the surface in the vicinity of the observed surface rupture. Assuming the aftershock zone to be the rupture zone, the stress drop is estimated to be 7 MPa with a maximum displacement of I .7 m for the main earthquake. A unique discovery is the high concentration of helium in the soil in the immediate vicinity of the surface rupture indicating that the rupture extends to the surface from a depth of a few km. A detailed broadband magnetotelluric (MT) investigation revealed the presence of an anomalously high conductive zone at a depth of 6-10 km. Observation of a PC phase, lagging behind the Pg phase by about 0.6 to 0.8 s in the seismograms of aftershocks, indicating a low-velocity zone at 7 to IO km depth, is consistent with the MT results. This highly conductive low-velocity layer is inferred to be fluid-filled. The main stress regime in Peninsular India is NE compressive stress due to plate tectonic movement. Erosion of the basalt cover in the Deccan Plateau may be adding additional compressive stress in the region. The existence of a low-velocity highly conductive fluid-filled layer will enhance stress concentrations in the uppermost brittle part of the crust causing the earthquake. 0 1998 Elsevier Science B.V. All rights reserved.

The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the Hellenic Subduction Zone (HSZ). A Long Period Magnetotelluric (LMT) study was carried out in the southern part of the Greek mainland to study the deep electrical characteristics of the HSZ and examine whether prominent modelled features correlate with structures identified by the seismic methods. The study comprised collection, processing and modelling of magnetotelluric (MT) data in the period range 100-10000 s from ten sites located along a 250 km NE-SW trending profile. The dimensionality of the data was examined at a pre-modelling stage and it was found that they do not exhibit three-dimensional (3-D) features. The latter enabled to construct both one-dimensional (1-D) and two-dimensional (2-D) models. The proposed geoelectric model for HSZ was based on 2-D modelling, since it had better maximum depth resolution of about 400 km, and revealed structures not detected by 1-D modelling attempts. The model structure which was related to the African and Euro-Asian lithosphere is relatively resistive (N 800 V-m) and has an average thickness of 150-170 km. Although the bottom of the lithosphere is adequately resolved, the Wadati-Benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature. The modelled structure associated with the subducting part of the African lithosphere penetrates a relatively conductive (b200 V-m) asthenosphere with a dip angle of 428. Intermediate electrical resistivities (200-800 V-m) are attributed to the ascending melting part of the lithosphere below the region of the Hellenic Volcanic Arc (HVA) and to a dipping zone below the south-western part of the profile, at 170-220 km depths. D

Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.

Ad&n. A., Kaikkonen, P., Hjelt, SE., Pajunpti, K., Szarka, L., Verii, J. and Wallner, A., 1982. Magnetotelluric and audiomagnetote~luric measurements in Finland. In: E.S. Husebye (Editor), The Structure of the Lithosphere-Asthenosphere in Europe and the North Atlantic. Tectonophysics. 90: 77-90. Joint Finnish-Hungarian MT (magnetotelluric) and AMT (audiomagnetotelluric) measurements were carried out in Finland in the framework of the international ELAS project. The conditions for MT measurements are favorable at these latitudes. Five MT and 150 AMT stations gave information on the electrical conductivity distribution in the area: AMT results guided the choice of MT sites with minimal near-surface distortion effects and helped the interpretation of the MT soundings; the MT measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near-surface dyke structures. This result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area.

As a first deep geophysical survey, thirteen magnetotelluric and magnetovariation soundings in the period range of 0.0085-6000 s have been carried out in the Methana Peninsula and Trizina area (Greece), which form the western part of the active Hellenic Volcanic Arc. Data have been processed using robust techniques and further analysed using decomposition methods in order to find the regional azimuth. By using the smooth inverse of Smith and Booker (J. Geophys. Res. 96, 3905-3922, 1991), the 2-D modelling has resolved a low-resistivity area (<30 m) at depths of 2-3 km beneath the volcanics, whereas elsewhere the resistivity is much higher (>100 m). Parkinson induction arrows also at the range 0.01-0.1 s point towards the centre of the peninsula. The conductivity anomaly is interpreted as being connected with the volcanic history of Methana, as a cooler remnant of former magmatic activity. The contrast with the much higher resistivities suggested at depths >5 km, is discussed in terms of fluid mobility, limitation of the MT method and extensional processes in the southern Aegean.

Electrical anisotropy in southern Africa, inferred from the analysis of magnetotelluric (MT) data recorded as part of the Southern African MT Experiment (SAMTEX), is compared with seismic anisotropy inferred from an SKS shear-wave splitting study in the same region. Given the vastly varying penetration depths in the survey area, electrical anisotropy is derived in terms of approximate depth, rather than frequency. Electrical anisotropy directions for crustal depths (<35 km) show more distinct variability than those for upper mantle depths, and, not surprisingly, appear to be strongly related to large-scale geological structures. Our results for upper lithospheric mantle depths (>45 km) are not consistent with the fast-axis directions inferred from the SKS analyses. Upper mantle electrical results appear to be mostly a consequence of the geometry of large-scale geological structures and provide evidence that some crustal structures are distinct at depth, while others seem to be confined to the crust. Our results indicate that the causative region for the seismic anisotropy in the lithospheric mantle has either a correspondingly weak electrical anisotropic signature, or is more prominent at greater lithospheric depths than those we investigate here.

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.

Data from 20 magnetotelluric (MT) soundings in the Coast belt of southwestern British Columbia, Canada, in the vicinity of volcanic Mounts Meager and Cayley (approximately 150 km north of Vancouver), are analysed to determine both the internal electrical conductivity structure of the Garibaldi volcanic belt and the regional conductivity structure. Galvanic distortion effects on the data are removed using Groom-Bailey tensor decomposition, and the regional 2D responses are determined in a survey-consistent reference coordinate frame for 1D and 2D modelling and inversion. The remaining unknowns, the site gain at each location, are estimated by requiring the long-period asymptotes of the E-polarization p 0 curves to be the same, and alternatively are derived as part of the 2D inversion procedure. Positive correlations are observed between an increased temperature gradient at a depth of about 200 m, a zone of enhanced electrical conductivity and drilling results. The zone of enhanced conductivity is interpreted to be due to montmorillonite-dominated clay alteration minerals which form the cap rock of the geothermal reservoir. Below this cap, the zone is electrically resistive owing to the chlorite and sericite which form the reservoir itself. The volcanic belt is underlaid by highly resistive (more than 10000 tI m) crystalline rock, beneath which is a deep conductive region (less than 70 tI m) beginning at a depth of 12 km. The cause of this crustal zone of enhanced conductivity is thought to be free saline water possibly released by metamorphic devolatilization from the downgoing Juan de Fuca slab. Within this conductive layer is a region of high conductivity directly beneath the volcanic belt, which we interpret as the magma source body for the belt. Sensitivity studies imply that the upper crustal resistive zone underlies the whole complex, although a weakly conductive, thin, vertical channel, representative of a magma conduit from the deep source body to the near surface, cannot be excluded.

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.

In this paper we present recent results for the solution of the three-dimensional (3D) magnetotelluric (MT) inverse problem. Compared with the models considered in our previous work, we are considering a more complicated 3D model. Here we outline the basics of our solution, and show the results obtained for this model. The results are encouraging, and show that the software works. However, some unresolved problem remains, as usual. This problem is directly inherent in the physics of the 3D MT inverse problem, rather than related to some bugs of our solution. We strongly believe that it can be successfully resolved, if we apply more constraints on the model parameters, in addition to those imposed by traditional Tikhonov-type regularization.