281 10. Feasibility of monitoring cold fronts of geothermal doublets the voltage-regulated transmitter, lower grounding resistance allows for higher injected current. A strong current induces stronger primary and secondary fields, making the signal more detectable. The measured resistances for the four dipoles were as follows: 1. TX2-S – TX2-W1: resistance 50 Ω·m, length 1180 m, 2. TX2-S – TX2-W3: resistance 50 Ω·m, length 610 m, 3. TX2-S – TX2-W4: resistance 50 Ω·m, length 360 m, 4. TX2-S – TX2-N1: resistance 26 Ω·m, length 730 m. These grounding resistances are acceptable and would allow for a minimum current injection of around 10 A (with 550 V voltage). A follow-up inspection in February 2022 confirmed that the electrodes remained intact and well concealed under the branches and leaves placed over them. 10.3 Receiver conception The receiver used to measure the magnetic field at the bottom of the well was developed by LBNL (US) and loaned to BRGM for one year. It is an induction probe (BF4) housed in a fiberglass and epoxy protective casing (see Figure 10.4, left). The BF4 probe is connected to an electronic circuit that provides power and amplifies the measured signal. The probe measures 2.4 meters in length and weighs approximately 20 kg. Its pressure resistance was tested by BRGM at the SDP logging company’s logistics base. This test indicated that the probe remained watertight (no internal pressure increase) at a pressure of 220 bars, which is sufficient for the maximum depth of about 1700 meters where it will be deployed in the well. A GO7 head provides the electrical power supply to the probe and transmits the signal back to the surface, while also ensuring the attachment of the probe to the logging cable. With the GO7 head used and due to the design of the probe, it is not possible to connect additional logging instruments to measure other physical properties or to precisely determine the probe’s position in the borehole during the measurement campaign. Consequently, when the probe is in the borehole, its location can only be determined by the length of the deployed cable, which is not an exact measurement. Weight bars and centralizers were added below the probe to facilitate its descent and ensure proper positioning at the center of the borehole (see Figure 10.4, right).
RkJQdWJsaXNoZXIy NjA3NzQ=