284 Geophysics in Geothermal Exploration 10.5 Data processing The data processing is based on the PROCATS processing software developed at BRGM (Bourgeois and Girard, 2010). Transfer functions between the magnetic signal at the borehole bottom, at the surface, and the source are calculated for the different emission frequencies. These transfer functions consist of a real part and an imaginary part, corresponding to the in-phase and quadrature components of the subsurface response resulting from the current injection. The processing involves extracting, for each station, each transmitter polarization, and each injection frequency, the spectral content of the signals recorded at the stations and normalizing them by the dipole moment emitted at the source (the product of the dipole length and the injected current intensity). The result is a measured magnetic induction field in nT/(A·m). This processing allows the magnetic field measured along the borehole axis (or in three spatial directions for the surface station) to be obtained, along with an estimate of the noise for each component. To provide a reference and compare the surface signal with the borehole signal, a magnetic field measurement station was also installed at the wellhead on the boiler plant site. Figure 10.6 shows the respective positions of the source and the receivers Table 10.1 Sampling depths for the magnetic field recording. Table 10.2 Injection sequences. Measurement point Depth below the end of casing in m Emitted frequency (Hz) Duration 1 105 0.5 5 min 2 85 2 2 min 3 65 8 30 s 4 55 16 30 s 5 45 24 30 s 6 20 32 30 s 7 –10 (test inside the casing) 64 30 s 128 30 s Figure 10.7 displays the calibrated time series of the signal emitted by the transmitter (TX), the signal received by the surface sensor (RX1), and the signal received by the borehole sensor (RX2). It is clear that the signal emitted by the transmitter is a square wave. A very similar low-frequency behavior is observed between RX1 and RX2. However, the borehole sensor (RX2) shows a significant reduction in highfrequency noise caused by anthropogenic activities. This effect is widely expected as the ground acts as a low-pass filter, removing the high-frequency content from the electromagnetic signal.
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