**1. Introduction**

The origin of the geothermal energy is connected with the internal structure of the planet and the physiochemical processes occurring therein. According to the current knowledge, geothermal energy is unevenly distributed throughout the globe near the surface to the deep interior of the Earth [1, 2]. Depending upon the accessibility as well as the opportunities for the utilization of modern technology, many nations in the world are exploiting this natural energy resources for the commercial production of electric power [3, 4]. Geothermal energy hence, geothermal areas are generally defined through the parameter, geothermal gradient, which is the rate of the increment of the temperature profile of underneath bedrock of the Earth. The average (global) value of the geothermal gradient is

typically 30 °C/km in the continental crust and 100 °C/km in the oceanic crust [1, 5]. However, in geothermal areas, its values are well above (>40 °C/km) the global average value [6]. It is so because of the magmatic intrusion. This intrusion is nothing but the molten magma, trapped within the Earth's crust at a depth of 5–10 km beneath the surface. This may still in a fluid state or the process of solidification and releasing heat constantly [2, 7, 8]. According to the origin of geothermal energy, it is categorized into two. One was from a relic of the Earth's accretion process, in which huge energy was trapped within the Earth's interior (�4.5 billion years ago) [7]. This one is named as the primordial heat source. Another one is the radiogenic heat source, which is produced by the natural decay process of long-lived radioisotopes such as 238U, 235U, 232Th, and 40K. These nuclei, of which the half-life (T1*=*2 Þ are comparable to the age of our planet, are found with significant abundance within the crust of the geothermal areas [9, 10]. A considerable amount of heat is contributed from the natural radioactive decay process. Eq. (1) to Eq. (3) represent the physicochemical processes and the heat energy released from the naturally occurring radioactive disintegration in each of the complete decay chain [11–14]. Moreover, it shows the produced crustal He (4 He) atoms and neutrinos during each decay process.

$$^{232}\_{90}\text{Th} \rightarrow ^{208}\_{82}\text{Pb} + 6\,^{4}\_{2}\text{He} + 4\,^{0}\_{-1}\text{e} + 42.60 \text{ MeV/atom} \tag{1}$$

$$\mathbf{t}\_{92}^{238}\mathbf{U} \rightarrow \mathbf{t}\_{82}^{206}\mathbf{Pb} + \mathbf{8}\_{2}^{4}\mathbf{He} + \mathbf{6}\_{-1}^{0}\mathbf{e} + \mathbf{51.70}\text{ MeV/atom} \tag{2}$$

$$\mathbf{^{235}U} \rightarrow \mathbf{^{207}Pb} + \mathbf{7}\,^{4}\_{2}\text{He} + \mathbf{4}\,^{0}\_{-1}\mathbf{e} + \mathbf{46.40}\,\text{MeV/atom} \tag{3}$$

Moreover, within the deep Earth, the production rate of He from 232Th and 238U [and 235U] radio-nuclei are encountered to be 2*:*<sup>43</sup> � <sup>10</sup>100 *atoms=m*<sup>3</sup>*=<sup>s</sup>* and <sup>1</sup>*:*<sup>03</sup> � <sup>10</sup><sup>8</sup> *atoms=m*<sup>3</sup>*=s*, respectively [10]. The fact of characteristics heat–helium coherence at any geothermal system (under the deep reservoir) is interpreted by such physicochemical processes [15]. This radiogenic heat, which is one of the main sources of the Earth's internal heat, powers all geodynamic processes underneath [16]. Generally, geothermal heat is transferred from the aquifer (reservoirs) to the Earth's surface by the conduction and convection process. Here, the geothermal fluid (meteoric water) acts as the carrier [1] and the radiogenic He, being highly diffusive gas, generated from the host mineral and mixes by diffusion with the fluid that circulates into the deep Earth [17]. The reservoir, which is nothing but a volume consisting of hot permeable rocks, is usually sandwiched by capping of impermeable rocks. And it is favourably connected to a recharge (surficial) area [18], from which geothermal fluids percolated to recharge the aquifer cyclically [18, 19]. The circulating fluids, to which the heat is transferred from the reservoir, escape through fracture and features from the deep reservoir and manifest through geysers, fumaroles, hot springs, etc. [8, 19]. Moreover, through diffusion and advection process, the radioactive inert gases (like 222Rn & 220Rn) including the stable and inert gases (such as He, Ar) are spontaneously migrating upward from the deep Earth to the superimposing atmosphere [20–22]. This process, known as 'Earth degassing', is non-uniform over space & time [23, 24]. The prominent signature of this degassing is generally noticed along active faults, fractures, oceanic ridges, geothermal fields, and even deep wells [25–27].

It is notable that geothermal energy sources are still overlooked in India for power generation even after the existence of a lot of potential resources, which are seen in twelve geothermal zones of the country [19]. However, several of them could be well utilized for the generation of power by means of developing geothermal power plants. For the sake of investigation, the hot spring site at Bakreswar in

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

West Bengal, India, was selected as shown in **Figure 1**. Now, knowing the amount of by-product, He gas which is ultimately reaching the surface through the fracture, fissure and hot springs vents, etc., the associated heat energy (radiogenic) produced inside the reservoir can be estimated. The energy released per unit time from underneath bedrock at the study area was calculated by means of measuring the average amount of He emanated from Agni Kunda hot spring at Bakreswar. Here mainly the decay series of 238U, 235U, and 232Th were considered, and the amount of heat energy contributed due to each series was evaluated. Here the question may arise that each decay series [Eq. (1) to (3)] takes a long period (in geological time scale) to complete its disintegration process and release a certain amount of heat and He discretely. But, heat and He generated due to each series were utilized to calculate the amount of heat production at the said reservoir at a certain instant of time. However, He emanation at the study area shows stable activities for a longtime-interval (5 years), as established by [19]. Therefore, He generation is also stabilized for a long period, i.e., He generation due to every radioactive decay series

**Figure 1.** *Location of the study area Bakreswar in the map of India (modified after [19]).*

and emanation of the said gas is in an equilibrium condition. Therefore, no He is being stored at the reservoir at the instant, and, therefore, the He emanation could be considered to be equal to the generation of the same due to the radioactive disintegration process.
