173 5. DEEP ERT/IP for geothermal exploration and de-risking Figure 5.8 Close up view of Figure 5.5 showing transmissions performed in the zone of interest. Pole A was alternatively placed at TX9, TX13 and TX15, while pole B was moving on TX24 to TX50. 5.3.5 Quality control With regard to chargeability data (induced polarization), which the receiving system and injection protocol enable to acquire in an auxiliary manner, measurements are of good quality at the first 350 to 400 meters. For deeper measurements, characterized by low-intensity electrical potential signals, the potential discharge curves do not allow this information to be extracted. Figure 5.9 shows the details of the signal recorded at the first channel of the RX3 receiver at the transmission event between the TX1 and TX2 electrodes, the smallest dipole of the main transmission axe. This demonstrates how the receiver effectively detects the subtle signal from the electric field generated by the transmission dipole, even over a significant distance between the injection point and the receiver. The presence of about 40 measuring stacks, recorded during almost 3 minutes of current injection, allows us to identify a very good average signal (graph at the bottom left in Figure 5.9), with an amplitude of the order of 0.04 mV. The chargeability measurements (graph at the bottom right) are of mediocre quality, with potential discharge curves that are difficult to identify. Figure 5.10 displays the same type of graphs, in the more favorable situation of the TX15-13 transmission dipole recorded by the first channel of receiver no. 16. Note that in this case, by virtue of the higher V signal (about 12 mV), the discharge curve and the consequent derivation of the chargeability measurement is more robust.
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