Page 33 - Geothermal Energy Systems Exploration, Development, and Utilization

1.1 Expressions of Earth’s Heat Sources 9
differences in subsurface temperatures may be negligible. These subtle effects
are illustrated in Figure 1.4, where two scenarios of heat refraction effects are
illustrated. The objective here is to show that high surface heat flow variations
do not necessarily correlate with large subsurface temperature differences since
geometrical effects have to be accounted for. Indeed, above a large aspect-ratio
insulating body, isotherms are uplifted so that surface heat flow above the anomaly
center corresponds to the equilibrium one. On the contrary, isotherms cannot be
distorted for a small aspect-ratio conducting body but the resulting surface heat
flow is enhanced.
In sedimentary basins, presence of salt may also induce heat refraction effects
since thermal conductivity of halite may be four times greater than surrounding
sediments (e.g., 1.5 W m −1 K −1 for sediments and around 6–7 W m −1 K −1 for rock
salt and halite, according to Clauser, 2006). Consequently, temperature gradient
within a thick evaporitic layer may thus be decreased by a factor of 4, leading to a
cooling effect of several tens of degrees centigrade for a 2–3-km-thick layer.

A B
0
100 °C
4
8 200
Depth (km) 400 High heat producing −3
300
granite: A = 10–20 µW m
500
600 °C
35
(a) Mantle heat flow = 25 mW m −2
700
A, Q = 10 µW m −3
600
B, Q = 10 µW m −3
500
Temperature (°C) 400 B, Q = 20 µW m −3

300

200
∆T = 42 °C ∆T = 90 °C
100
0
0 5 10 15 20 25 30 35
(b) Depth (km)
Figure 1.5 Two-dimensional effect of a high heat producing
granite on temperature field (a) and geotherms (b). Here, a
fixed mantle heat flow of 25 mW m −2 is imposed, as well as
an averaged thermal conductivity of 3 W m −1 K −1 and a bulk
−3
crustal heat production of 1 µWm .