285 10. Feasibility of monitoring cold fronts of geothermal doublets Figure 10.6 Positioning map of the source dipole in Sénart Forest (TX2 or TX), the borehole probe at the bottom (RX2), and the surface MT station (RX1). Figure 10.8 shows the amplitude spectrum of the three-time series presented in Figure 10.7. In this example, the signal emitted by the transmitter is a square wave at 0.5 Hz, which appears clearly in the amplitude spectrum’s magnitude along with its odd harmonics (1.5, 2.5, 3.5, …) Hz. Thanks to the noise reduction induced by the ground, the emission peaks from the TX are visible in the borehole magnetometer data (RX2), whereas they are not visible in the surface magnetometer data (RX1). The first clearly visible peak is at 1.5 Hz, which is very distinct on RX2 but completely absent on RX1. These observations clearly show that the signal recorded by the probe corresponds to the signal emitted by the source. The advantage of recording at the borehole bottom to eliminate anthropogenic noise is well validated here. The remaining task is to confirm the order of magnitude of the measured magnetic field value by comparing it with numerical modeling.
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