**2. The study area: Bakreswar geothermal province**

A cluster of seven<sup>1</sup> hot springs is scattered over Bakreswar geothermal area within a confined zone of the surface area of about 3350 sq. m [19, 28]. The area, which is a geologically complex, heterogeneous, and extensively faulted region, is situated at the eastern end of the SONATA (Son–Narmada–Tapi) geothermal province (**Figure 2**, window a) [19, 28]. The area lies in the West Bengal Basin (WBB), the extension of the Chotanagpur Gneissic Complex [29]. Furthermore, it is linked with a 1.2-km-long shear zone, which is characteristic by 50 m wide breccia/cherty quartzite aligned through the almost north–south trend-line [26] (**Figure 2**, window b). The springs here are connected with the extinct Rajmahal volcanic activity (115 Ma), and hence, are associated with the Precambrian granitic rocks [30] (**Figure 2**, window b). The highly permeable and porous subsurface of the site is facilitated due to the presence of the brecciated, highly sheared, and mylonitized rock here [26]. The association of the study area with the eastern edges of two major fault systems (the ONGC fault and the SONATA fault) made the region to be in a stressed state [31, 32]. This region is characterized by a very high geothermal gradient (90 °C/km) and a high heat flow rate ( 230 mW/m<sup>2</sup> ) [19, 33]. The

#### **Figure 2.**

*(a) Regional and (b) Local geological maps of the study area (modified after [19, 26]).*

<sup>1</sup> He emanated from other six hot springs and through the soil (soil gas) of the geothermal area are not included in the estimation due to the lack of other necessary information.

*Quantitative Approximation of Geothermal Potential of Bakreswar Geothermal Area… DOI: http://dx.doi.org/10.5772/intechopen.96367*

presence of a high heat-conducting zone in this area is confirmed by electrical resistivity studies. This conducting zone, which starts at a depth of around 2.8 km and goes down up to a depth of 4 km [34], is supposed to act as the heat feeder to the fault system linked with the Bakreswar hot springs. It is to be noted that the crustal thickness at the study area is only 24 km, whereas the average of the same throughout the country is 38 km [29, 35]. Besides, the average density of the crustal substance here is relatively low. Therefore, inert volatiles like He and 222Rn gases can easily transmit to permeate through crustal constraints due to the presence of the thinner lithospheric overburden here. As a result, the spring and the soil gases here are dominated by the presence of high 222Rn and He flux [29]. High 222Rn and He gases are continuously conveyed and dispersed into the atmosphere via molecular diffusion and the formation of micro-bubbles at the hot spring vents. Here, temperature and He emanation profile of some sites of Bakreswar geothermal provinces are also tabulated in **Table 1** for a reference to attain the brief geophysical properties of the study area. Moreover, the reservoir temperature of the geothermal system underneath Bakreswar was predicted to be 100 5 °C (at 1 km depth) by [38]. The same was estimated to be in the range of 130 °C to 175 °C (by Na/K ratio) and 110 °C to 124 °C (by TSiO2) by [39]. Furthermore, the range of the reservoir temperature was also evaluated as 212 °C to 124 °C, 118 °C to 120 °C, and 126 °C to 130 °C by means of silica geothermometry by [32, 38, 40] respectively. The audiomagnetotelluric (AMT) studies of the sub-surface beneath the Bakreswar geothermal area were conducted by [41] (**Figure 3**). The rapid relaxation inversion (RRI) for both transverse-electric (TE) and transverse-magnetic (TM) modes was carried out to figure out the resistivity profile of the subsurface of the site. Here, the suitable locations for drilling for the installation of a future geothermal power plant


*Note: HSG = Hot spring gas; SG = Soil gas; AA = Ambient air; BG = Borehole gas; Conc. = Concentration; RWA = Recent work by the authors.*

#### **Table 1.**

*Temperature and He emanation profile of some sites at Bakreswar geothermal province.*

**Figure 3.**

*(A) AEW traverses (AMT sites) on the map of the study area; (B) 2D RRI (rapid relaxation inversion) along traverse AEW1; (C) 2D RRI along traverse AEW2; (D) 2D RRI along traverse AEW4 (modified after [41] [personal communication] and [42]).*

at the study area were identified by the authors using the result of that AMT survey [personal communication] and the same is discussed later.
