Geothermal Areas

The Puga area in eastern Ladakh, Jammu and Kashmir, is known to be the most promising geothermal field in India, as pointed out by geo-scientific studies in the area. However, lack of deep subsurface information put constraints to the geothermal resource evaluation in Puga. The recent magnetotelluric study in Puga revealed anomalous conductive (~5 Ohm.m) structure at a depth of 2 km in the area of geothermal manifestation. Analysis of temperature logs indicated a high temperature (~260°C) associated with the anomalous conductive structure and signifies potential geothermal resource in the area. We discuss the results in the context of geothermal resource utilization for power generation in the area.

Identification of hot geothermal reservoir at great depths (1-5 Km.) is an important task in decision making to establish and use the heat source for power generation etc. as sustainable source for a long time is a necessary requirement. Telluric and magnetotelluric studies were carried out in and around Tatapani hot spring zone. From these studies, the presence of a deep crustal conductor related to geothermal reservoir system at a depth of about 2 km extending in E-W direction is detected. (Ref: NGRI Report No: NGRI-2000-EXP-282)

As in the case of previous study, the northern part of the Konkan geothermal province was investigated jointly by using both telluric and magnetotelluric techniques and identified a deep reservoir (2-3 km) below the volcanic rocks. It is interpreted that meteoric water from the western ghats percolated through deep fractures through normal gravitational force and has emerged as hot springs due to differences in hydrostatic pressures. (Ref: Harinarayana, Ph.D Thesis, 1988, Harinarayana and Sarma, 1999)

Hydrocarbon Exploration Studies

Encouraged by the delineation of the Mesozoic sediments in the northwestern quadrant of the Saurashtra peninsular region, ONGC Ltd. supported a major project for the delineation of sediments and mapping of the basement structure for oil exploration. Five magnetotelluric traverses across the Kutch region with 50 magnetotelluric sites have not only delineated yet another location, but also indicated sharp variation in the basement. It is observed from 1D and 2-D modeling of MT studies and also from gravity and Deep resistivity soundings, that the region west of Mundra village seems to possess large thickness of sediments. Interestingly, this region falls in the vicinity of the northwestern quadrant of Saurashtra region near Jamnagar (Ref: NGRI Report No: NGRI-2000-EXP-296).

Large part of this region is covered by volcanic rock and acts as a mask on the subsurface structures and makes it difficult for conventional geophysical techniques. Magnetotelluric studies using state-of-the art equipment covering most of the Saurashtra region with 690 stations showed the presence of thick (3-4 km) sediments below the volcanic rock towards north-western part of Saurashtra peninsula around Jamnagar . The presence of thick sediments towards the north western part of Saurashtra peninsula, which was FIRST indicated by MAGNETOTELLURICS and also decrease in resistivity of the sediments indicating the association of the marine environment of the sediments have opened up this region potential for oil exploration. This work was carried out as a sponsored project by ONGC Ltd. and is of classified in nature as the results are directly related to the oil potential of our country. (Ref: NGRI Report No: NGRI-98-EXP-237).

As in the case of Saurashtra, this region also covered by volcanic rocks and became a difficult terrain for conventional geophysical techniques. Magnetotelluric field studies using state-of-the art equipment near Nagpur-Wardha region with 138 stations along radial profiles and the strategic modeling have shown very thick sediments, of the order of 3 to 4 km. Apart from the estimations of the thickness of sediments, several structural features, basement undulations and also gave a new evidence for the possible extension of PRANHITA-GODAVARI Graben underneath the Deccan trap cover. This new evidence paved a way to the oil industry for exploration for coal and coal bed methane in Katol and it's surrounding regions. (Ref: NGRI Report No. NGRI-98-Expl.222)

Latur Earthquake has occurred in aseismic region and it is important to know the deep crustal signature of the region. MT results in this region has brought out anomalous conductivity anomaly at a depth of about 7-10 km and indicated the presence of FLUIDS. Seismic and gravity data also supported for the presence of fluids that might have triggered the Latur earthquake. The new evidence from this study paved a way for possible identification of such zones in other seismic regions of the world as indicated recently from Kobe seismic epicentral zone (Ref: H.K.Gupta et al , Geophysical Research Letters, 23, No.13, 1569 – 1572,1996).

Unlike above, this area is seismically active and has a series of E-W trending faults. However, it is important to know and distinguish between active and relatively passive faults. Magnetotelluric studies across the NSL zone cross both the Narmada South and North faults. Modeling results of MT data have clearly brought out the ACTIVE nature of the NSL SOUTH FAULT as compared to NORTH fault with a distinct conductivity contrast. This study thus gave evidence and a new way that it is possible to identify the active and relatively passive faults, which in turn helps to identify the hazardous zones as active faults that are associated with seismic activity. These results are presented in two international workshops (Ref: Harinarayana et al, Chapman conf., NGRI, and EM workshop, Romania, 1998).

MT studies were taken up during May-June 1999 in Chamoli epicentral region, Garwal Himalayas and during March 2001 in Kutch region. The studies were carried out by establishing 23 stations in Chamoli region and 36 (21+15) stations in Bhuj region. The stations occupied are oriented mainly along three different profiles in each region. Three profiles in Chamoli region are northeast-southwest (NE-SW) oriented profile from Gaucher towards southwest to Joshimath towards northeast direction, two east-west oriented profiles from Nijmula towards east and Mandal towards west and Chamtoli towards east and Appala towards west. The profiles occupied in Bhuj region are – a northeast-southwest profile from Mundra towards southwest to Rapar towards northeast, from Mundra towards south and Kavda towards north and from Kodki towards west and Manaba towards east. The data have been processed using robust processing techniques and modeling exercises have been carried out using 1D and 2D modeling schemes to derive the subsurface geoelectric structure. (Ref: Gupta, H.K, Harinarayana, T and others, 2001, Geo. Soc. Of India , 57, 3,275-278.; Begum S.K., Ph.D Thesis, O.U. 2002; .)