Magnetotelluric Investigations Research Articles

Deep electrical resistivity imaging of crustal structures along the Satluj River valley, northwest Himalaya, India

Magnetotelluric (MT) investigations along the Nahan- Kourick Chango transect in the Satluj river valley, northwest Himalayas, provide insight into the subsurface electrical properties. In the geoelectrical model, medium to highly metamorphosed crystalline rocks in the Lesser and the Higher Himalayas are observed as high resistive features. At a depth approximately 5–15 km, an intra-crustal low resistive layer is consistently present along the entire profile. The geometry of this dipping intra-crustal layer signifies the presence of the Main Himalayan Thrust, which represents the base of the accretionary wedge where all the major thrusts of the Himalayan tectonics sole down. The geometrical differences and distinct resistivity of this layer compared to the adjacent regions of Garhwal-Kumaon and Nepal Himalaya indicate the variability of tectonic processes in this part of the Himalaya. Another significant observation is the existence of a thin, highly conducting layer at a shallow depth (approximately 1–3 km). The spatial extent of this layer, along with the bending extent of the main central thrust toward the south, suggests its role in the southward translation of crystalline sheets during Himalayan tectonics.

Preliminary magnetotelluric investigation of crustal magma plumbing system beneath the Wulanhada volcanic field, northern China: Implications for the Magma reservoir and pathway

The Wulanhada Volcanic Field (WVF) in northern China consists of a few tens of well-preserved monogenetic volcanoes, where Holocene eruptions have occurred, but is rarely known. Understanding the internal structure of the crustal magma plumbing system beneath the WVF is of great significance for the study of volcanic eruption mechanisms and the assessment of volcanic risks and hazards. We present the crustal electrical resistivity models across the WVF obtained from two-dimensional (2-D) and three-dimensional (3-D) inversion of magnetotelluric profile data, which show some robust and meaningful features. Two high-resistivity anomalies (>1000 Ω·m) occupy almost the entire upper and middle crust. They may serve as an impermeable barrier to impede the ascent of deep magmatic fluids and/or melts. However, there is also a sub-vertical anomaly with moderate resistivity separating two high-resistivity anomalies. We inferred that it may represent a crustal-scale buried fault that provided a pathway for magma ascent during past eruptions and controlled the spatial distribution of the Holocene volcanic group in the WVF. A remarkably high-conductivity anomaly (3–10 Ω·m) in the middle-lower crust is explained to be the presence of ~0.8–7.5% partial melts. It may represent a magma reservoir that exists as a crystal-rich mush zone. Considering the presence of the magma reservoir and pathway beneath the WVF, the volcanic risks and hazards of future eruptions should not be ignored. The results shed light on the structure of the buried volcanic edifice in the WVF and need to be verified by more specific work in the future.

Role of fluid on seismicity of an intra-plate earthquake zone in Western India: an electrical fingerprint from magnetotelluric study

The magnetotelluric (MT) investigation carried out in Koyna Seismogenic Zone (KSZ), an intra-plate earthquake region in Western India, along an E–W profile brings out moderately conductive (~ 700–1000 Ωm) near vertical features within the very high resistive (> 20,000 Ωm) granite/granite-gneiss basement. Occurrences of these anomalous moderate conductors are corroborated with sensitivity analysis. The alignment of earthquake hypocenters along the resistive–conductive boundary signifies the moderate conductor as basement fault. The conversion of resistivity values to the ratio of seismic P- to S-wave velocity (vp/vs) suggests that the moderate conductivity of the fault zone (as compared to the surrounding basement) appears due to the presence of fluid in the fault zone. Geophysical evidences reveal ~ 2.5–3.6 vol% fluid in the fault zone with ~ 1.8–2.6% interconnected porosity, which migrates along the structural boundary and invades the mechanically strong basement to nucleate the brittle failure within it. The present study proposes two mechanisms for the seismicity in the Koyna region. First: the meteoric water circulation due to the loading–unloading of nearby Koyna reservoir acts as potential fluid source for this triggered seismicity, which has also been suggested by previous studies. Second: the fluid circulation due to a deep-seated source. The present MT study brings out a conductive feature below 20 km depth which is thought to be emerged due to the dehydration of amphibole bearing rocks. The fluid generated from dehydration might act as a probable source to the triggered seismicity; since the conductive feature has a linkage to the upper crust.Graphical

Magnetotelluric investigations at Andean volcanoes: Partial melt or saline magmatic fluids?

Knowledge of the architecture of active magmatic systems is important for both volcanic hazard assessment and evaluating potential geothermal energy production and metals recovery from magmatic fluids. Increasingly, magmatic systems are imaged using the magnetotelluric method to detect electrically conductive partial melt and/or saline magmatic fluid reservoirs. We review recent magnetotelluric studies at eight Andean volcanoes, revealing electrical conductivity anomalies with variable magnitudes and locations. Six of the studied volcanoes exhibit three main electrical conductivity anomalies, located at shallow (<3 km), intermediate (≈5 km), and deep (>10 km) depths. The shallow anomalies are often thin and laterally extensive, consistent with clay cap alteration layers, while the deep anomalies are generally interpreted as partial melt reservoirs. The intermediate depth anomalies, although also often attributed to partial melt, have less clear origins. By analysing laboratory-derived electrical conductivity relationships, we show that the intermediate depth anomalies are generally most consistent with saline magmatic fluids stored in porous rock. However, other geophysical and petrological data suggest that localised partial melt also exists at intermediate depths. Therefore, the intermediate depth anomalies likely represent mixed melt and saline magmatic fluid systems, such as those responsible for forming magmatic-hydrothermal alteration zones and copper porphyry deposits. At individual volcanoes, refining the generalised three layer model proposed here by using additional geophysical or petrological data is key to constraining the resources and/or hazard potential of the magmatic system.

Neoproterozoic and Cretaceous crustal structures revealed by a 3-D magnetotelluric investigation of the onshore Espírito Santo Basin and the contiguous Araçuaí Orogen (SE Brazil)

SUMMARY The study region in southeastern Brazil is composed of two domains: the Cretaceous onshore Espírito Santo Basin and the Neoproterozoic Araçuaí Orogen. In this region, associations among geological evolution, observed structures and geophysical data are limited. Therefore, this study aimed to further investigate this region using magnetotelluric (MT) data obtained from 22 W-E spanning stations over the Espírito Santo State. Because the dimensionality analysis revealed a substantial 3-D characteristic of the data, a 3-D MT inversion using the ModEM code was performed to generate a regional geoelectric model, and potential and well data were used as qualitative support for the interpretation. The resistivity profile obtained from the model revealed a resistive pattern for the Precambrian terrain and a conductive pattern for the sedimentary rocks of the onshore Espírito Santo Basin. The basement of the basin was delimited in the resistivity model, revealing consistency with the basement depths from the well data. Furthermore, three crustal conductors inserted in a resistive Precambrian orogen were identified. These conductors may be related to the recent circulation of saline fluids associated with plume action during the Upper Cretaceous and Cenozoic. However, the structures in which these fluids circulate exhibit different genesis along the resistivity profile. The conductors to the west of the profile may be related to plutons and dykes associated with the NNW Colatina Lineaments generated by the gravitational collapse of the orogen (530–490 Ma). Conversely, the conductor to the east under the Espírito Santo Basin appears to be related to the primary detachment rift fault generated during the Cretaceous breakup of Gondwana.

Magnetotelluric investigations over geothermal provinces of India: an overview

Magnetotelluric (MT) and audio-magnetotelluric (AMT) studies are sensitive to the geothermal fluids filling the faults and/ or fracture zones of the geothermal system. In India, MT/AMT studies have been carried out in NW Himalayas, central, eastern, and western India. In other areas, detailed MT/AMT studies need to be expedited. This review paper presents the art of geothermal exploration in India by using MT/AMT techniques and identifies potential zones that can be exploited for power generation and direct application. Reservoir characteristics, carbon emissions reduction methods, and levelised cost factor are also discussed.

Magnetotelluric Investigations of the Kīlauea Volcano, Hawaii

AbstractIn 2002 and 2003 a collaborative effort was undertaken between Lawrence Berkeley National Laboratory, Sandia National Laboratories, the U.S. Geological Survey (USGS) Menlo Park, the USGS Hawaiian Volcano Observatory, and Electromagnetic Instruments Inc. to study the Kīlauea volcano in Hawaii using the magnetotelluric (MT) technique. The work was motivated by a desire to improve understanding of the magma reservoirs and conduits within Kīlauea and the East and Southwest Rift zones, which has implications for understanding Kīlauea's plumbing system. An improved understanding of the rift zones has implications in understanding large‐scale landslides that are generated in the Hilina Slump, which produce significant impacts on coastal communities. Up to eight stations operated simultaneously, with multiple remote reference sites, and data were processed using multi‐station robust processing techniques. In total, data were acquired at 70 sites over the Southwest and East rift zones. Good to excellent quality data were obtained even in the harshest conditions, such as those encountered on the fresh lava flows of the East Rift Zone, where electrical contact resistances are on the order of 100 kΩ. A three‐dimensional (3D) MT model study was done to guide interpretation of the observed MT measurements. Synthetic modeling demonstrates that conductive bodies in the upper 3 km can be spatially resolved where MT station sampling is good. Resistivity anomalies in the 3D inversions have a high degree of spatial correlation with previously published seismic velocity anomalies beneath Kīlauea. Melt fractions between 0.096 and 0.117 are calculated for the Kīlauea and Puʻuʻōʻō low resistivity anomalies, respectively.

Investigation of the electrical resistivity structure of the subsurface at Mogod valley in central Mongolia: Insight is using 1D Magnetotelluric inversion

In this paper, we report a preliminary result of the Magnetotelluric investigation of the Mogod area. The Mogod region is one of the most prominent fields for geophysical study since the region includes young and active faults and geothermal activities. We conducted magnetotelluric measurements at 20 sites during geophysical field seasons in 2018-2021 as a pilot survey to understand data property and the electromagnetic noise level for the detailed electromagnetic studies. During the fieldwork, we used Lemi Magnetotelluric instruments and measured all three orthogonal components of the magnetic field and the horizontal components of the electric field. For the data processing, we used Matlab code by using the M-estimate regression method, and estimated the magnetotelluric transfer function with a lownoise level. The electrical resistivity model of the subsurface of survey layout shows us the existing resistivity anomalies at shallow-depth, and thickness of the upper crust approximately 11-17 km. Here, suggest that the thickness of the upper crust is 17 km and crust is 40 km with local magnetotelluric measurements. Additionally, the electric conductor appears in the southwest of Mogod region, we interpret that conductor play as a source of geological activity of Mogod region, and it might be the signature of a remanent fluid.

Investigation of the electrical resistivity structure of the subsurface at Mogod valley in central Mongolia: Insight is using 1D Magnetotelluric inversion

In this paper, we report a preliminary result of the Magnetotelluric investigation of the Mogod area. The Mogod region is one of the most prominent fields for geophysical study since the region includes young and active faults and geothermal activities. We conducted magnetotelluric measurements at 20 sites during geophysical field seasons in 2018-2021 as a pilot survey to understand data property and the electromagnetic noise level for the detailed electromagnetic studies. During the fieldwork, we used Lemi Magnetotelluric instruments and measured all three orthogonal components of the magnetic field and the horizontal components of the electric field. For the data processing, we used Matlab code by using the M-estimate regression method, and estimated the magnetotelluric transfer function with a lownoise level. The electrical resistivity model of the subsurface of survey layout shows us the existing resistivity anomalies at shallow-depth, and thickness of the upper crust approximately 11-17 km. Here, suggest that the thickness of the upper crust is 17 km and crust is 40 km with local magnetotelluric measurements. Additionally, the electric conductor appears in the southwest of Mogod region, we interpret that conductor play as a source of geological activity of Mogod region, and it might be the signature of a remanent fluid.

The interrelationship between electrical resistivity and VP/VS ratio: A novel approach to constrain the subsurface resistivity structure in data gap areas in a seismogenic zone

Large man-made water reservoirs promote fluid diffusion and cause critically stressed fault zones underneath to trigger earthquakes. Electrical resistivity is a crucial property to investigate such fluid-filled fault zones. Therefore, we carry out magnetotelluric (MT) investigation to explore an intraplate earthquake zone, which is related to artificial reservoir-triggered seismicity. However, due to surface access restrictions, our data set has a gap in coverage in the middle part of the study area. This data gap region coincides with the earthquake hypocenter distribution in that intraplate earthquake zone. Therefore, it is vital to fill the data gap to obtain the electrical signature of the active seismic zone. To compensate for the data gap, we have developed a relation that connects resistivity with the ratio of seismic P- to S-wave velocity ([Formula: see text]). Using this relation, we estimate a priori resistivity distribution in the data gap region from known [Formula: see text] values during the inversion to compensate for the data gap. A comparison study of the root-mean-square misfits of inversion outputs (with and without data gap filled) proves the effectiveness of the established relation. The inversion output obtained using the established relation brings out fault signatures in the data gap region. To examine the reliability and accuracy of these fault signatures, we occupy a portion of the data gap with new MT sites. We compare the inversion output from this new setup with the inversion output obtained from the established relation and observe that the electrical signatures in both outputs are spatially correlated. Furthermore, a synthetic test on a similar earth model establishes the credibility and robustness of the derived relation.

Magnetotelluric evidence for trapped fluids beneath the seismogenic zone of the Mw6.0 Anjar earthquake, Kachchh intraplate region, Northwest India

Lithospheric rheological heterogeneities across a rift play a significant role in intraplate deformation and seismic activity. The Kachchh rift in the northwestern part of the India has not only experienced lithospheric stretching but also witnessed large magnitude devastating earthquakes. Here, we present the results of a magnetotelluric (MT) investigation carried out in the southeastern part of the Kachchh intraplate region illuminating the seismogenic zone of the 1956 Anjar earthquake (Mw 6.0). The two-dimensional geoelectrical model derived from the MT data acquired at 21 sites along a NE-SW profile reveals assemblage of conductive and resistive zones in the upper and the mid crustal depths in the close vicinity of the major fault zones and the hypocentral depths. The results reveal a very high conductive zone near the Moho, affirming the presence of an upper mantle fluidized zone. Unlike the South Wagad Fault, the Kachchh Mainland Fault and the Katrol Hill Fault do not have a downward lower crustal extension, and terminate at the mid crustal depths. We suggest that the presence of trapped fluids in the brittle-ductile transition zone (12–15 km) and their seepage to the resistive plutons might be a possible mechanism for triggering the 1956 event as well as the current seismic activity in the region.

Magnetotelluric investigation in the swarm prone intraplate Talala region of Saurashtra, Gujarat, western India

The Talala region of Saurashtra has experienced three moderate seismic events of magnitudes Mw5.0, Mw4.8 (on 6th November 2007), and Mw5.1 (on 20th October 2011) accompanied by swarms. The local earthquake data suggested two epicentral trends (ENE- WSW and NNW-SSE). The focal mechanism of all three earthquakes exhibit strike-slip mechanism with nodal planes sympathetic to these two trends. Based on low gravity values and ambiguous seismic reflections, the ENE-WSW trending fault was inferred and named as Girnar fault. The Magnetotelluric (MT) investigation has been carried out (i) to locate and characterize the Girnar fault and (ii) to ascertain the cause of seismicity in the region. The MT data is acquired along the ~62 km long NW-SE oriented profile, starting from the Sendarda village (NW) to the Hadmatiya village (SE) in the Junagadh district (Saurashtra), India. The regional strike of N19°E is obtained from the MT data. The data is subjected to 2D modeling and two conductors are identified along the profile, the first conductor is located between Ajeb and Aambalgadh village in the northern part of the profile with a resistivity range from 20 to 3500 Ohm.m and the second conductor is ~15 km south of the first conductor near Hirenvel village with a resistivity range from 50 to 3500 Ohm.m. The second conductor continues vertically down to 12–15 km depth and inferred as the Girnar fault. The shallow clustered seismicity (~5 km) near the second conductor advocates that it may be related to stresses induced by intense monsoon on the critically stressed Girnar fault, which has fractured the basement. The basement got fractured either from the local stress accumulation near the structural boundary or from the movement of the fluids to the less permeable crust.

Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor amplitude

Kusatsu-Shirane volcano hosts numerous thermal springs, fumaroles, and the crater lake of Yugama. Hence, it has been a particular study field for hydrothermal systems and phreatic eruptions. On 23 January 2018, a phreatic eruption occurred at the Motoshirane cone of Kusatsu-Shirane, where no considerable volcanic activity had been reported in observational and historical records. To understand the eruption process of this unique event, we analyzed seismic, tilt, and infrasound records. The onset of surface activity accompanied by infrasound signal was preceded by volcanic tremor and inflation of the volcano for ~ 2 min. Tremor signals with a frequency band of 5–20 Hz remarkably coincide with the rapid inflation. We apply an amplitude source location method to seismic signals in the 5–20 Hz band to estimate tremor source locations. Our analysis locates tremor sources at 1 km north of Motoshirane and at a depth of 0.5–1 km from the surface. Inferred source locations correspond to a conductive layer of impermeable cap-rock estimated by magnetotelluric investigations. An upper portion of the seismogenic region suggests hydrothermal activity hosted beneath the cap-rock. Examined seismic signals in the 5–20 Hz band are typically excited by volcano-tectonic events with faulting mechanism. Based on the above characteristics and background, we interpret that excitation of examined volcanic tremor reflects small shear fractures induced by sudden hydrothermal fluid injection to the cap-rock layer. The horizontal distance of 1 km between inferred tremor sources and Motoshirane implies lateral migration of the hydrothermal fluid, although direct evidence is not available. Kusatsu-Shirane has exhibited unrest at the Yugama lake since 2014. However, the inferred tremor source locations do not overlap active seismicity beneath Yugama. Therefore, our result suggests that the 2018 eruption was triggered by hydrothermal fluid injection through a different pathway from that has driven unrest activities at Yugama.

Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan

Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this shallow seismicity. We also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity. We find that (1) implosive NDC earthquakes occur at the craters that were active during the 1990–1991 eruptions, which are currently buried by the approximately 200-m-thick Heisei-Shinzan Lava Dome; (2) the detected NDC earthquakes generally possess downward P-wave arrivals, although upward P-wave arrivals are observed at a few seismic stations; (3) the tensile axes of the NDC earthquakes show various slip directions; (4) repeated NDC earthquakes are in some cases detected within 20 s of the initial event; and (5) a high-resistivity zone exists beneath the NDC earthquakes. We conclude that the implosive NDC earthquakes were generated by the collapse of small, randomly oriented, vapor-filled voids at the two buried craters, with active volcanic conduits still present beneath these craters. The shallow DC earthquakes consist of two clusters that are located 2–3 km below the NDC earthquakes. The focal mechanisms of the shallower cluster (approximately 2 km depth) indicate various fault mechanism, whereas those of the deeper cluster (approximately 3 km depth) indicate strike-slip faulting. The top of the shallower cluster corresponds to the base of the conductive zone, which suggests that the DC earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer.

Magnetotelluric investigation of the Precambrian crust and intraplate Cenozoic volcanism in the Gour Oumelalen area, Central Hoggar, South Algeria

SUMMARY The Tuareg Shield was assembled by oceanic closures and horizontal movements along mega-shear zones between approximately 20 terranes during the Pan-African Orogeny (750–550 Ma). Although there is an ongoing debate about its origin, the exhumation of the Tuareg Shield is assumed to be related to Cenozoic intraplate volcanism. The Gour Oumelalen is a key region of the Tuareg Shield and is located in the northeastern part of the Egéré-Aleksod terrane, corresponding to the eastern boundary of the Archean–Palaeoproterozoic microcontinent LATEA (Central Hoggar). The eastern boundary of the study area corresponds to a Neoproterozoic suture zone separating two old microcontinents, LATEA and the Orosirian Stripe. We deployed two magnetotelluric (MT) profiles consisting of 33 broad-band MT stations and combined these with aeromagnetic data, aiming to define the crustal structure in detail. The resistivity cross-sections obtained from the 3-D inversion of full impedance tensor and tipper data from stations along the profiles, confirm the main Precambrian faults, some of which are covered by Quaternary sediments and hence, have not yet been deciphered. The cross-sections also highlight the Cretaceous–Quaternary sedimentary basins represented by low resistivities. The upper crust is typically cratonic with a high electrical resistivity. On the contrary, the lower crust shows a drastic drop in resistivity (&amp;lt;10 Ωm). The most plausible hypothesis is that the study area corresponds to a Cretaceous rifting zone. The Cretaceous magmatic event and its related fluids and mineralization as well as the recent fluids associated with Cenozoic volcanism, are plausible causes of a very conductive lower crust. However, we cannot exclude other reasons such as: (i) a high-temperature and strongly sheared mobile belt or (ii) a contribution of inheritance involving Pan-African events that affected this former suture area.

Magnetotelluric investigations in India during last five years

Magnetotelluric investigations in India during last five years

Magnetotelluric investigation for imaging the subsurface geoelectrical feature of the prospective Sembalun-Propok geothermal zone, Indonesia

This paper discussed the investigation of the Sembalun-Propok geothermal system, West Nusa Tenggara Province, Indonesia. We analyzed the magnetotelluric data and its implication to the subsurface structure related to the system of Sembalun-Propok geothermal reservoir. In this study, information on the dimensionality of regional conductivity structure and its regional strike were analyzed by applying the magnetotelluric phase tensor. By using induction arrows analysis, the influences of the sea toward the magnetotelluric data were also observed. The results show that it could be performing the 2-D modeling for the higher frequency (≥ 1 Hz) magnetotelluric data. The 2-D inversion modeling suggests that the potential geothermal zone is situated in the western-southern part of the study area. In addition, it also can reveal the depth of the unaltered layer (ρ ≥ 500 Ωm). It is probably related to the volcanic rocks in the study area, and its depth is about 0–500 m from the surface. Then, underlying the unaltered layer possibly corresponds to the cap rock layer (ρ = 1–10 Ωm) of the system of the geothermal reservoir.

Two-Dimensional Inversion Magnetotelluric Investigation of the Pariangan Geothermal Field, Indonesia

Pariangan geothermal field is located in Tanah Datar District, West Sumatera Province, Indonesia. The structure developed in this region is influenced by a large fault that extends through Sumatera from Banda Aceh to the Gulf Semangko in Lampung and also the pattern of radial faults that follow the development of Complex Marapi Volcanism. This study is aimed at delineating the shallow as well as deeper conductive anomalies related to the geothermal system in the Pariangan area. Dimensionality analysis showed a two-dimensional (2D) behavior for the data, and therefore 2D inversion was performed. The result of magnetotelluric (MT) data processing indicates a low resistivity (<15 Ω.m) distribution at depths of 1300-3000 meter, which is interpreted as the caprock. Reservoir rock is indicated by the medium resistivity (15-93 Ω.m) at depths of 1200-3200 m. Below the reservoir the deep reaching conductive anomaly is interpreted as circulating hot fluids which heat up the fluids in the reservoir due to the subduction process. Type of alteration developed in research area are dominated by argillic type characterized by the presence of illite, kaolinite, montmorillonite, dickite, and halloysite; propylitic type characterized by the presence of chlorite, epidote and, quartz; and phyllic type characterized by the presence of sericite, quartz, and chlorite. Basement rock zone is indicated by the high resistivity (93-1000 Ω.m) at the depths of 1500-6000 m. MT in this study shows a powerful geothermal exploration tools because of its sensitivity to image hot saline fluids as zones of high conductivity.

Magnetotelluric investigations in the southern end of the Cambay basin (near coast), Gujarat, India

Cambay basin is one of the major onshore oil-bearing sedimentary basins of India situated at its western margin in Gujarat state. The region is seismically active and experiences three devastating earthquakes in the past. The region comes under the seismic zone IV and V and necessitates the extensive geophysical studies in for subsurface characteristics and identification of hidden faults in the region for seismic hazard estimation. A magnetotelluric (MT) survey has carried out in the southwestern part of the Cambay basin to identify the sedimentary thickness, the depth of Deccan trap and for identification of West Cambay margin that passes through the region. The MT data, at 13 stations, in the broad frequency range is acquired along an east-west profile with an inter-station spacing of 2–3 km. After processing and analysis of the data, transfer functions in the period band of 0.001–300 s are used for two-dimensional joint inversion. The resultant structure from the 2-D modeling infers the sedimentary thickness of ~0.5 km. A high conductive zone dipping towards east observed in the eastern part of the profile might be due to the West Cambay fault. Basement fracturing at a depth of 2.5–3.0 km at the central portion of the profile might act as a heat exchanger between the deeper geothermal source and nearby hot water spring.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.