155 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben model was built in a geomodelling tool (Renard and Courrioux, 1994). After drilling operations, various geophysical logging tools were used in the geothermal wells mainly at SsF. They aimed to characterize both the petrography of the crystalline basement (bulk density, sonic velocity), the hydrothermal alteration due to severe geochemical interactions with natural fluids (spectral gamma ray with U, K and Th, resistivity logs), and temperature and flow logs for identifying discrete permeable fractures. For example, the most striking observation derived from spectral gamma ray, was the significant increase of K related to argillic alteration within fractured zones (Traineau et al., 1991). It corresponds to the precipitation of clay minerals bearing potassium like illite related to fluid circulation. Acoustic and electrical resistivity image logs were also extensively used for mapping in situ, the orientation of natural fractures as well as orientation of the principal horizontal stress field based on the observations of drilling induced tensile fractures or borehole breakouts (Figure 4.3). Caliper logs were also used systematically during the technical phases for cementing operations. By using various geophysical logs acquired in the Soultz wells, many research attempts have been done by using statistical tools like Principal Component Analysis, Hierarchical Ascending Classification or neuronal network for mapping clay-rich zones in the basement. Figure 4.3 Example of borehole imagery logs acquired in the Lower Triassic sandstone (left) or crystalline basement (right) of the geothermal well GRT-2. Natural fractures appear as sinusoidal traces on both electrical and acoustic borehole image logs. Nearly horizontal stratification, and vertical induced drilling fractures are also visible on the electrical image log (left).
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