133 3. Borehole geophysical methods dissipation (Figure 3.13). Different variations, similar on each of the curves, can be identified during the heat injection phase. The main anomaly, located between 160 and 180 m deep, results in a smaller increase in temperature compared to the surrounding depths. Based on the lithologic description the occurrence of a higher sand/silt, organic matter or limestone content observed in the units lo1a and lm3 can be identified at the depths corresponding to these anomalies with lower temperature increases. These can therefore be interpreted as a due to a higher groundwater flow rate in the facies having a slightly higher permeability, causing a leaching of the thermal plume. The heat supplied is more efficiently dissipated thanks to this flow, resulting in a smaller rise in temperature. The presence of flows is confirmed by Stoneley wave velocity decrease (Figure 3.12), Stoneley wave and P-wave attenuation increase (Figure 3.14) and the presence of a fluid wave (Figure 3.10) in the 140–180 m depth interval. We also note a good correspondence between the thermal conductivity profile (Figure 3.13) and the attenuation VSP logs (Figure 3.14). The conventionnal processing sequence of a VSP includes amplitude recovery, picking of the arrival times of downgoing wave fields, wave separation using both f-k filters and SVD (singular value decomposition) filters (Mari, 2015), deconvolution of upgoing P-wave fields by the associated downgoing P-wave fields, design of stacking corridor on flattened deconvolved upgoing P-wave section and computation of corridor stacked traces in time. Figures 3.15 and 3.16 ilustrate the processing sequence of a near surface VSP recorded in borehole B1 of the experimental site located in the Cher region (Figure 3.5a). The borehole sensor is an anchored vertical geophone. The source is a weight drop (Figure 3.5a). The VSP is acquired in the 25 to 90 m depth interval, with a depth sampling interval of 5 m (Figure 3.15a). The listening time is 250 ms. The time sampling interval is 0.25 ms. Figure 3.13 Geothermal tests. Service géologique du Luxembourg document.
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