Bakreswar Geothermal Province (BGP) lies over the Archaean gneisses and schists formations. The heat flow and geothermal gradient of BGP are $$\sim $$
 230 mW/m
 $$^{2}$$
 and $$95{^{\circ }}\hbox {C}/\hbox {km}$$
 , respectively. The Bakreswar hot spring (BHS) (
 $$23{^{\circ }}52^\prime 30^{\prime \prime }\hbox {N}$$
 ; $$87{^{\circ }} 22^{\prime }30^{\prime \prime }\hbox {E}$$
 ) is located over NNW–SSE trending fault. It lies in the eastern flank of BGP and is connected by Sone–Narmada–Tapti (SONATA) and deep-seated ONGC fault systems. Earlier studies indicated that the geothermal fluid flowed towards the same path as that of deep ONGC fault. In view of this, 24 audio-magnetotelluric soundings in the frequency range 10 kHz–10 Hz were conducted at one end of the deep ONGC fault, which is about 20 km northwest of BHS. The data analysis indicated that the data are internally consistent. Analysis of dimensionality and strike are indicative of complex conductivity structures. The 3D inversion of full impedance was performed to obtain a conductivity model of the study area and resulted in root mean square (RMS) for sites close to unity. The results have been discussed by extracting cross-section along 3 to 7 profiles (four of the profiles are perpendicular to strike, whereas three profiles were oblique to the strike direction and are perpendicular to east–west trending ONGC faults). Several low conducting zones are observed in the profiles drawn parallel and perpendicular to the strike. These common conductors show consistent model features but modified resistivity and dimension. Depth slices from 200 to 1400 m suggest that the conductors in the northern part of the study area are deeper than the conductors in the south. The study area is dominated by several shallow conductors which coincide with several fractures present in the study area. The conductors are surrounded by a resistive feature that can act as an impermeable bed and probably restricts the flow of water within the zone. The four major conductors along the oblique profiles (profiles perpendicular to deep ONGC fault) coincide with deep ONGC fault, through which probably the geothermal fluid flows.

/pdf/audio-magnetotelluric-investigation-of-bakreswar-geothermal-zfk73bs7wn.pdf

Audio-magnetotelluric investigation of Bakreswar Geothermal Province, Eastern India

Geothermal parameters and thermal energy potential of bakreshwar tantloi group of hotsprings in birbhum district, west bengal and dumka district, biha r

Electromagnetic (EM) induction technique is one of the most important geophysical techniques in understanding the subsurface structure. The theory of magnetotelluric (MT) method, the main branch of the EM technique, was introduced during 50’s by Tickonov (1950) and Cagniard (1953) with the natural variation of electromagnetic fields, as its source. In India, National Geophysical Research Institute (NGRI) initiated this technique during 70’s with the establishment of stationary geoelectric observatory at Choutuppal, Hyderabad. Application of this technique has grown up steadily over the years for various geological problems in India. Indian geology is quite complex with it’s vast cover of basalt occupied in most parts of western India, thrust regions of north and northeast Himalayan terrains with collision tectonics, mosaic of cratons in peninsular India and large sedimentary basins. MT method has brought out deep crustal structure in these terrains. It is also being used extensively for oil and geothermal explorations, and geo-engineering problems. Apart from this, continuous monitoring of MT signals to study the earthquake precursor phenomena near Koyna and also exploration of the Antarctic continent are new initiatives. Very recently, NGRI has conducted MT measurements for the first time in our country in marine environment i.e. marine magnetotellurics (MMT). NGRI, along with various other institutions such as Indian Institute of Geomagnetism, Mumbai, Indian Institute of Technology at Kanpur, Kharagpur and Roorkee and Indian School of Mines, Dhanbad, Osmania University, Hyderabad have contributed significantly for deep crustal studies in different parts of India. In the present paper, an overview of the contributions of the technique, applied to various geological problems in India for the last three decades is presented.

/pdf/applications-of-magnetotelluric-studies-in-india-2q97ypemmu.pdf

Applications of Magnetotelluric Studies in India

Changes in the temperature dependence of the electrical conductivity associated with thermal decomposition of several phenolic materials have been measured to 600/sup 0/C in nitrogen and air environments. The materials were phenolic resins reinforced with chopped glass fabric. Thermal decomposition product data and mass loss data were obtained from mass spectroscopy and thermal gravimetric analysis. Results showed that peaks in the conductivity temperature dependence were associated with material decomposition and/or outgassing, and that the presence of oxygen accelerates decomposition above approx. 450/sup 0/C. In the region of ohmic conduction (electric fields less than or equal to 4 x 10/sup 3/ V/cm), there was excellent correlation between thermal decomposition characteristics and the temperature dependent electrical properties. This correlation suggests that electrical conductivity can be used as a thermal analysis tool in characterizing phenolic materials.

Correlation of electrical conductivity and thermal decomposition of phenolic materials

The extensive volcanic activity of Rajmahal occurred during early Cretaceous (~117 Ma). Potentially perspective Gondawana sediments containing coal seam is partially covered by these basalts. Basalts as well as coal are already known to be of immense economic importance over Rajmahal traps. The present study mainly deals with delineation of basaltic lava flows and associated coal layers using the Audio-magneto-telluric (AMT) study in the south central region of Rajmahal traps near Suri, West Bengal, India. Initially, the field situations have been simulated using forward model with top alluvium cover followed by basaltic lava flows, a wedge shaped inter-trappean sedimentary formation with second basaltic lava flows and basement at the bottom. The model was constrained using available litholog. The model study reveals the presence of moderately resistive basalts, relatively conductive inter-trappeans and the basement, except the top alluvium layer and thin inter-trappeans. Subsequently, threeAMT sounding have been carried out over Cholaguria, Hatgacha and Pachami, Suri, West Bengal. The field AMT study reveals three basaltic lava flows with varying thickness from ~40m to ~350m and composite thickness of ~620m to ~640m, which match well with the published borehole litholog. The inter-trappeans have been identified at 500m to 620m, 310m to 400m and 500m to 640m depths over Cholaguria, Hatgacha and Pachami, respectively. It is interesting to mention that the identified inter-trappeans are inferred to be coal/shaly-coal which are matching very well with the published borehole lithologs.

Delineation of Rajmahal basaltic flows, inter-trappeans and associated coal layers using Audio-magneto-telluric method

Summary Audio magnetotelluric (AMT) and magnetotelluric (MT) studies were carried out in the western part of Bakreswar geothermal region covering an area of 330 square km. Apparent resistivity maps obtained from the AMT results delineated a well defined conducting zone at lower frequencies. AMT survey in the vicinity of Bakreswar hot spring (BHS) shows that there is no deep geoelectrical structure near the hot spring which can act as the source of Bakreswar hydrothermal system. Both AMT and MT results confirm the location of a major electrically conducting zone of considerable importance.

https://www.eageseg.org/data/egm2007/Sessione%20B/Oral%20papers/B_OP_19.pdf

Study of a Confined Hydrothermal Systemover the Geothermal Province of Bakreswar, Eastern India

A remote reference audio-magnetotelluric (AMT) study has been carried out in Bakreswar Geothermal Province to map the possible geothermal reservoir of the area. The heat flow of BGP is two and half times of average global heat flow and is nearly equal to that of young oceanic ridge. A NS (~16 km) profile delineated a geothermal reservoir of resistivity less than 10 ohm-m at a depth of about 3 km of large horizontal extent(>2 km). The sensitivity analysis suggests that the delineated reservoir is less sensitive. BGP belongs to high temperature volcanic region which is dominated by meteoric fluid. These fluids are heated at greater depth and rises by convection to near-surface depths, where it is entrained into the overall pattern of groundwater flow. BGP thus supports the conceptual model of magmatic geothermal plays volcanic field rather than hydrothermal alteration.

R. K. Sinharay

Papers

Study of a Confined Hydrothermal Systemover the Geothermal Province of Bakreswar, Eastern India

Bakreswar Geothermal Reservoir Revealed From Audio Magnetotelluric (AMT) Results