The Saurashtra Peninsula and its adjoining regions covered by Deccan Traps (DT) are one of the important parts of the Indian continental lithosphere with interesting geophysical anomalies, tectono-thermal evolution since the Mesozoic times. Knowledge on the deep structure beneath these formations is important for understanding the seismo-tectonics of the region. This region has gained importance after the occurrence of a major earthquake (7.9 Mw) north of Saurashtra, namely Bhuj earthquake during 2001. It is also observed that Saurashtra region has experienced several earthquake swarms limited to small regions. Accordingly, it is important to investigate the deep structure of the Saurashtra region from seismotectonics point of view. In our study, magnetotelluric results of the deep crustal structure along five NS oriented traverses are presented. The five traverses are—Halvad-Rohisa (HR), Sapar-Iswaria (SI), Mota Dahinsara-Bamagadh (MB), Jodiya-Jamkhandorna (JJ) and VavBeraja-Devda (VD). The total length of these 5 traverses is about 670 km. The derived deep geoelectric structure is also compared and correlated with gravity data to get more confidence on the derived results. The 2-D geoelectric section has delineated anomalous high conductivity structure at places extending from 20 km to about 40 km. From the spatial correlation, anomalous high conductive structure derived from MT data with intense localized seismic activity is an interesting observation. In the present study, the results of magnetotelluric studies along with other geophysical results are presented.

Maitri Station (70.76 degrees S; 11.73 degrees E) is located in Schirmacher Oasis, a coastal nunatak in north-central Dronning Maud Land covering an area of 35 km(2). Here, we report results from the first magnetotelluric experiments and delineate the deep electrical conductivity structure under Schirmacher Oasis using the data acquired during the 24th Indian Antarctic Scientific Expedition. The magnetotelluric method has the advantage of shallow to deeper level coverage as the data acquisition covers a wide frequency band of 10(-3)-10(3) Hz, permitting different penetration depths depending on the frequency and conductivity of the layer under investigation. The modelling results indicate the presence of a highly resistive (8000-10 000 ohm m) upper crust, which shows a lateral variation in thickness from 20 km (below site 6) in the east to 10 km (between sites 1 and 2) in the west. It is underlain by a less resistive (500-600 ohm m) lower crust. The highly resistive upper crustal structure supports the existing notion that western Dronning Maud Land is a stable, cratonic platform. Results of free-air gravity, seismic, geomagnetic and surface wave dispersion investigations in East Antarctica also indicate a cratonic-type crust. The results of our study allow us to identify a westward thinning of the upper crust with a marked boundary between sites 1 and 2. We also find evidence for the continuity of the Mozambique mobile belt in East Antarctica on the western side of Schirmacher Oasis.

Two-dimensional geoelectric section derived along the Malkapur-Mandhata magnetotelluric (MT) profile, cutting across the Narmada-Son lineament (NSL) zone, brought out a conductive (<50 Ohm m) middle and lower crust under the NSL region. This region underlying the resistive (300-3000 Ohm m) Archaean basement shows shallow depth underneath the weak tectonic zones identified in the area. The constrained density modelling of the Bouguer gravity values suggests high density (2.8 g/cm(3)) material corresponding to the conductive middle lower crust zone. As the region has experienced widespread volcanism during the Cretaceous Tertiary period, the crystallized mafic magmas that were poured into the crust during the volcanism and the fluids entrapped (expelled during magma cooling process) could be the possible explanation for the conductive and high-density characteristics beneath the NSL zone.

Magnetotelluric data at 45 locations along the Mahan–Khajuria Kalan profile in the central India tectonic zone are analysed. This 290km long profile yields data in the period range 0.001–1000s across the tectonic elements of the study region bounded by Purna fault, Gavligarh fault, Tapti fault, Narmada South fault and Narmada North fault. Multi-site, multi-frequency analysis suggests N70°E as the geo-electric strike direction. Data rotated into the N70°E strike direction are modelled using a non-linear conjugate gradient scheme with error floors of 10% for both apparent resistivity and phase components. Two-dimensional magnetotelluric model yields conductors that correlate with known faults in the study region and regional seismicity. Presence of a −30 mgal gravity high together with the observed conductive bodies (less than 20ohmm) in the deep crust beneath the Purna graben and Tapti valley is explained by the process of magmatic underplating. The conductive bodies beneath the Mahakoshal rift belt and Vindhyans accompanied by regional gravity lows of the order −70mgal are attributed to the presence of deep crustal fluids. Following the re-activation model proposed for the entire region, the conductors (20ohmm) at various depth levels correspond to mafic magmatic and/or fluid intrusions controlled by deep-seated faults that seem to tap reservoirs beyond the crust–mantle boundary. The shallow depth localized faults also seem to have facilitated further upward movement of these underplated material and fluids release during this process.

Complete text of publication follows. Indian subcontinent is collage of cratons like Dharwar, Bundelkhand etc. and mobile belts like Delhi-Aravalli, Satpura etc. The Bundelkhand and Dharwar cratons were sutured through WSW-ENE oriented Satpura mobile belt. The drainage in this area is westward, which is reverse to the general (eastward) pattern over other parts of the Indian subcontinent. This region most of the area is covered by Deccan flood basalts which might have been erupted due to interaction between Reunion mantle plume and the overlying continental lithosphere at {approx}65 Ma during the northward movement of the Indian plate. Various geophysical data sets (Magnetotelluric, Deep seismic, Gravity etc.) have been utilized to unravel the crustal structure of this region in addition to geological and tectonic studies. Magnetotelluric data acquired along three N-S profiles and one E-W profile cutting across Narmada-Son region have been modeled to derive the electrical structure at crustal and upper mantle depths. These results have been integrated with deep seismic and gravity sections. Presence of high conductive layers at crustal depths within Narmada-Tapi and Cambay regions supports the concept of remobilization within these zones as compared to the adjacent cratonic regions. In addition to this, presence of high conducting layer at subcrustal levels in the NW part of this area has been discussed in light of pre-outburst phase of Reunion plume during 70-65 Ma. The possible relation of the high conductivity anomalies in the Satpura, Vindhyan and Malwa uplifts, may be due to outburst of mantle plume, and also to the mineralized belt of Archaean age is discussed.

Complete text of publication follows. A magnetotelluric survey has been conducted along a traverse of 220 km oriented in SW-NE direction, covering three major geological formations covering 18 broad band MT sites in Saurashtra peninsula. The traverse passes over Tertiary sediments, Deccan traps on the SW side and Mesozoic sediments on the NE side. The Porbandar formations and alluvium are well reflected close to the coast with a thickness 1 Km. The plutonic masses towards SW of Rajkot are well reflected in 2D subsurface geo-electric section. A significant change in geological structure (from about 3 km. onwards) has been observed at sites close to NE side of the profile. Undulating basement topography has been observed throughout the traverse with as shallow as 1 km on NE side to as deep as more than 5 km. at other sites. The most interesting feature in this traverse is that the deeper structure in the SW direction is highly resistive. Another interesting feature observed is that two deep conducting features separated by a highly resistive structure is prominent in NE part of the profile at some sites indicating deep tectonic activity, probably during the Cretaceous period. Study of gravity anomalies along the traverse are correlatable with the geo-electric section obtained through 2-D modeling. Mesozoic sediments in SW part is either thin or absent, but is well reflected upto depths of at least 500 m. towards the NE part. Extension of exposed sediments beneath the Deccan traps ({approx}65 Ma) is seen at sites close to NE side with a thickness of about 500 m. A highly resistive feature has also been observed upto upper mantle levels at a site close to NE side. In the present study the deep electric structure of Saurashtra peninsula is discussed in relation to its earlier tectonic activity of upliftment, erosion of volcanic plugs etc.

Complete text of publication follows. A magnetotelluric study has been carried out along a 270 km long N-S trending traverse extending from Akola (in the south) to Sehore (in the north) with a station spacing of 4-7 km. This traverse cuts across several major E-W trending faults viz., Purna, Gavilgarh, Tapti and Narmada faults. The data are analyzed and rotated to N70E. 2D inversion has been carried out by using NLCG (Non Linear Conjugate Gradient) scheme. The upper crust has shown high resistivity values (approx 10 000 ohm-m) towards the north compared to south of Narmada south fault. Mid lower crust is less resistive (1 000 - 2 000 ohm-m). The present MT model is compared with Ujjain-Dorwa-Mahan and Khajuriakalan-Multan-Pulgaon deep seismic sections and also with bouguer gravity anomaly. Our results have identified underplated material associated with deep seated faults in the mid lower crustal depths. An attempt also been made to identify the boundary (and its nature) between Bundelkhand Craton and Dharwar Craton.

Understanding deep continental structure and the seismotectonics of Deccan trap covered region has attained greater importance in recent years. For imaging the deep crustal structure, magnetotelluric (MT) investigations have been carried out along three long profiles viz. Guhagarh–Sangole (GS), Sangole–Partur (SP), Edlabad–Khandwa (EK) and one short profile along Nanasi–Mokhad (NM). The results of GS, SP and NM profiles show that the traps lie directly over high resistive basement with thin inter-trappean sediments, where large thickness of sediments, of the order of 1.5–2.0 km, has been delineated along EK profile across Narmada–Son–Lineament zone. The basement is intersected by faults/fractures, which are clearly delineated as narrow steep conducting features at a few locations. The conducting features delineated along SP profile are also seen from the results of aeromagnetic anomalies. Towards the southern part of the profile, these features are spatially correlated with Kurduwadi rift proposed earlier from gravity studies. Apart from the Kurduwadi rift extending to deep crustal levels, the present study indicates additional conductive features in the basement. The variation in the resistivity along GS profile can be attributed to crustal block structure in Koyna region. Similar block structure is also seen along NM profile.

To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.

Puga geothermal field in NW Himalaya, Ladakh district, Jammu and Kashmir, India, is situated near the junction of the Indian and Asian plates. The thermal activity is attributed to the widespread igneous activity during Upper Cretaceous to late Tertiary age. The study area located at an altitude of 4600 m above mean sea level is characterized by springs with temperatures up to 84 °C that correspond to the boiling point of water at this altitude. In order to image the shallow and deeper parts of the area, wideband (1000–0.001 Hz) magnetotelluric (MT) measurements have been carried out. Five-component MT data were acquired from the E–W-trending, 15-km-long and 1-km-wide Puga valley at 35 locations. The data have been subjected to one- (1D) and two-dimensional (2D) modelling. The results confirm the presence of a shallow conductive region in the area and also indicate the presence of a deep conductive region (5–15 Ω m) commencing from a depth of about 1.5 to 2.0 km and related to the presence of a geothermal reservoir.

A difficult task for geo-engineers involved in tunnel construction in Himalayas is to know the possible interference of hot water regimes that may pose a problem during the construction of the tunnel. One possible solution for such a problem is to estimate the subsurface parameters using geophysical and geothermal investigations. Magnetotelluric (MT) studies have been carried out over a 35 km long North-South profile in Loharinag Pala area of lesser Himalayas in Uttaranchal. Rishikund, Sunagarh and Bhukki are prominent hotsprings in this region. A total of 16 MT stations could be occupied along the profile, many of them being close to the proposed tunnel alignment; four of these being situated away from the profile towards north as well as South. Electrical structure of the subsurface has been derived from 2-D modeling. To determine the strike direction we have used Groom-Bailey (GB) decomposition and Parkinson Induction vectors. North- West (-550) direction coinciding with the regional strike direction is derived. Geothermal investigation has also been carried out to estimate the temperature at different depths from bore holes. Modeling study showed conductive feature with 5-40 ohm-m resistivity and predominantly appears in TE component as compared to TM. The southern part of the profile showed high resistive zone with a resistivity, of the order of 500-1000 ohm-m. The resistivity structure along the tunnel alignment showed anomalous conductive zone towards north probably due to fractured rocks filled with water and minerals. The results obtained from the present study are useful to the geo-engineers in order to devise safety measures suitable during construction of the tunnel for hydroelectric power project.