32 Geophysics in Geothermal Exploration naturally flows from areas of higher temperature to areas of lower temperature. These processes create a transfer of heat from the Earth’s hot interior to its cooler surface where we live, resulting in what’s known as a geothermal gradient, which is a measure of how temperature increases with depth (Figure 1.1). In general, the deeper we go into the Earth’s crust, the hotter it gets. On average, temperatures rise by roughly 25–30 °C for every kilometer below the surface, but this geothermal gradient varies significantly depending on location due to the underlying geology. Figure 1.1 From Kolawole and Evenick (2021). (a) Typical averaged (and simplified) temperature profile of the Earth, showing the variation of temperature with depth (modified after Mckenzie and Bickle, 1988; Boehler, 1996). Inset in panel (a) shows a zoom-in of the non-zero curvature of slope of temperature(T)- depth(y) profile, in which the local slope is defined by Fourier’s law, and for a constant thermal conductivity (k), heat flow (q) is a function of depth, y, q = q(y) (modified after Turcotte and Schubert, 2002). (b) Schematic representation of variation in geothermal gradient with depth (Z) as a function of k in sedimentary sequences (after Chapman et al., 1984). (c) Cartoon showing the crustal and lithospheric structures of the Earth with the primary sources of geogenic heat (after Evenick, 2019). (a) (b) (c)
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