132 Geophysics in Geothermal Exploration Figure 3.12 Lithology and VSP logs: velocity and attenuation logs (after Mari et al., 2024). In detail, the lithology record by the Geological Survey shows, after the 2 rather homogeneously marly units lo4 and lo3, a gradually increasing content in organic matter, observed in the lo2 and lo1 units (70–126 m), also showing a thin lamination, culminating in the lo1a unit below (126–139 m), which is more silty, sandy and contains bituminous horizons. Below, the lm3 unit appears to have an even higher sand/silt content but is also richer in limestone nodules and beds (140–210 m). The following lm2 unit, the sand and silt content gradually decreases again until the depth of 230 m and the basis of this unit (at 340 m) is homogeneously marly. We note a significant decrease in shear velocity in the 140–200 m depth interval corresponding to the lo1a and lm3 units, richer in sand/silt and organic matter (lo1a) or limestone (lm3). Figure 3.12 (on the right) shows the attenuation logs computed from the downgoing P and Stoneley waves. The results obtained (decrease of both energy and velocity of the Stoneley wave) are consistent with the results which could be obtained by a Biot-Rosenbaum model (Rosenbaum, 1974) used to access to permeability from the evolution of Stoneley’s phase velocity and attenuation (Mari, 1989). The velocity and attenuation VSP logs show a very good correspondence with respect to the lithological variations observed in the borehole (Figure 3.12). The well was equipped with a hybrid cable, comprising 2 optical fibers and 2 electrical conductors, suitable for geothermal applications. Fiber optic temperature measurement enables optimal monitoring of temperature distribution and thermal conductivity in the subsurface as a function of depth. Temperature measurements are made before and after heat injection phases, which are carried out by sending an electric current through the electrical conductors of the hybrid cable. Before heat injection, the temperature increases linearly from 12 oC at 20 m to 23 oC at 320 m (Figure 3.13). Once the heat injection phase has begun, temperature profiles, recorded after different heating time intervals, show the evolution of the subsurface temperature after respectively 1 h (cyan curve), 3 h (yellow curve) and 108 h (red curve) of thermal
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