120 Geophysics in Geothermal Exploration 3.2 Hydrogeological methods Conventionally, hydrogeological investigations concern hydraulic measurements such as flows and temperature. They also concern hydraulic testing such slug test and pumping test (Mari et Porel, 2024). A GFTC logger records logs which show the evolution of the Gamma radiation (G), the water velocity (F), the water temperature (T) and the electrical conductivity of the water (C) as a function of depth. Temperature logs are carried out in wells to detect any anomalies linked to water intakes in the borehole. Figure 3.3a shows temperature logs recorded in wells M8 and M13 of the HES (Figure 3.2a). For well M8, the temperature increases steadily with the depth. The increase is consistent with the regional geothermal gradient, which is about 2.5 degrees per 100 meters. For well M13, the temperature log shows abrupt variations about 60 and 85 meters deep. These variations are likely related to water intake. Recording the vertical velocity of the water makes it possible to determine the direction of flow circulation in a borehole (upward or downward flow). The type of experimentation can be carried out under static conditions or in dynamic conditions, either by pumping in the monitored well, or by pumping in a well offseted from the well being monitored. The experiment makes it possible to know precisely the depths of the producing levels. Figure 3.3b shows an example of flowmeter test performed in M07 well with pumping in M06. The flow log shows a downward flow between 35 and 88 meters, where the flow enters the formation, also visible both on temperature and conductivity logs. A slug test is a particular type of aquifer test where water is quickly added or removed from a groundwater well, and the change in hydraulic head is monitored through time, to determine the near-well aquifer characteristics. Figure 3.4 is an example of a slug test carried out on the well M19 (Figure 3.2a). The changes in hydraulic head versus time observed on the nearby wells (M16, P1, MP7, M22, MP6, M21) are displayed in Figure 3.4a. The slug test shows that wells MP7 and P1 are not directly connected to well M19. On the other hand, well MP6 shows oscillations due to the injection of water into M19. MP21 and MP22 seem to be strongly connected to M19. By repeating these operations on various wells of the experimental site, a map of connectivity between wells can be elaborated as shown in Figure 3.4b.
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