Audiomagnetotelluric Studies To

Case History Audiomagnetotelluric studies to trace the hydrological system of thermal fluid flow of Bakreswar Hot Spring, Eastern India: A case history Rajib K. Sinharay 1 , Shalivahan Srivas tava 1 , and Bimalendu B. Bhattacharya 2 ABSTRACT An audiomagnetotelluric AMT study has been carried out in the Bakreswar Hot Spring BHS area of eastern India to loca te the geother malsourcein the vici nityof BHS . Pha se- tensor analysis of the AMT data shows that the region is broadly 2D. Rapid relaxation inversion RRI for both trans- verse-electric TE and transve rse-ma gnetic TM modes has been carried out to obtain resistivity images of the subsurface. AMT results show that the north-south fault close to Bakreswar is a shallow featur e, not deeper than 300 m, and thuscannot act as a heatsource.Thesubsur faceforma tionbe- lowthe faultzone is hi ghl y resi st iveup to a gr eatdept h, indi- cating the absence of a heat source and geothermal reservoir in thevici nity of theBHS.AMTres ult s ind ica te that theloca- tion of the geothermal reservoir is deep and lies beyond the profiles of mea sureme nt in the northwestern side of the Bakreswar Hot Spring. INTRODUCTION The Bakreswar geothermal region is located in Birbhum district, West Bengal, a state in eastern India . The main hot spring is located at Bakreswar 23°5248 N; 87°2240 E Figure 1. The heat flow of the area is very high, with values varying from 145 mW / m 2 to 180 mW/ m 2 Shanker et al., 1991. Chandrasekharam 2000 shows tha t itis morethantwic e theaverageglobalheat flowand si m- ilar to the young oceanic ridges. The geothermal gradient near the hot spring is 90°C/ km. An important feature of this group of hot springs is high helium content in water with values up to 1.2% Na- gar et al., 1996, and upto 2%in ai r at1 m above t he gr ound, w hi ch is far above the atmospheric background level, and the gas is re- leas ed in per iodi c burstsGhose et al. , 2002. Hel ium isextr act edat a fewhot spr ingsfor commer cialpurpos es. Alth ough Bakr eswar Hot Spr ing BHS is loc ate d over the Chotana gpur Gneiss ic Complex CGC, the mineral compos ition of BHS water does not resemble the water of granitic terrain. In con- tras t, the composition of Bakreswar spr ing wat er is consider ed to be of vol cani c ori gin.Chowdhury et al. 1964 show that, geochemical- ly , Bakr eswar spr ing wate r dif ferssharpl y from the loca l groundwa- ter and other water sources. Furthermore, they suggest that the high concent rat ion of nitr ogen and heli um is deri ved from the pri mor dial sources. Murty and Sinha 1991 carried out helium and neon iso- tope studiesin the Bakres warHot Spr ingand found thatthe presence of helium in the BHS is caused by radiogeni c sources. Another important observation is that the chemical composition of the spring water collected in premonsoon and postmonsoon periods is fairly similar, suggesting very little seasonal variation Murty and Sinha, 1991. Shankeret al . 1991 also repo rt the mar ginal seas ona l var ia- tionof the mineralconsti tuents in ther malwater . In addition to geologic surveys Mukhopadhyay, 1996; Nagar et al., 1996; Mukherjee and Majumdar, 1999, various geophysical studies, viz., self-potential SP, gravity, magnetic, resistivity, and audiomagnetotelluric Roy et al., 1985; Mukhopadhy ay et al., 1986; Bha ttac har ya et al., 1992; Maju mda r et al. , 2000; Bha ttac har ya et al. 2002 surveys, and geochemical Chowdhury et al., 1964; Murty and Sinha, 1991; Shanker et al., 1991 and isotropic studies Murty and Sinha, 1991; Shanker et al., 1991 have been carried out over a very limited area close to the BHS. These studies cou ld not identify anygeother malreservoiror heatsourceassoci atedwith the sys tem. Providing electrical resistivity images of subsurface structures that control the geothermal fluid flow is a critical component in un- derstan ding the hydrot hermal propert ies of complexgeothermal res- ervoirs. The resistivity in thermal areas is affected by the vertically ascending, hot mineralized waters or vapors that originate from the 1 India n Schoo l of Mine s Univ ersit y, Depar tmentof Appl ied Geop hysi cs, Dhan bad,India. E-mai l: rsris m@ya hoo. com;svismagp @yah oo.co .in. 2 S. N. BoseNationalCentrefor Basi c Scien ces, SaltLake, Kolk ata,India. E-mai l: bimal endu@bose. res.i n.

Audiomagnetotelluric Studies To

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