168 Geophysics in Geothermal Exploration Figure 5.1 Typical configuration of a recording unit used in this study. Three electrodes (P1, P2 and P3) are arranged for setting up two dipoles (P1-P2 and P2-P3). These dipoles could be either in the same direction (electrodes set up in-line as displayed on this figure) or perpendicular. Transmitter device. A high power current transmitter 3, specially designed for deep resistivity investigations, was used. It allows of handling a power up to 6 kW and injecting up to 16 A and 3000 V into the ground. The transmitter was powered by a large dedicated three-phase 15 kVA motor generator (see Figure 5.2). The “automatic range” mode allows the optimal injection level to be automatically selected according to the contact resistances at the TX-A and TX-B electrodes. The current is injected into the ground according to a 2 sec IAB+, 2 sec OFF, 2 sec IAB-, 2 sec OFF scheme and is recorded every 10 ms by means of an current recorded box. Thanks to its internal GPS, the current is also timestamped. This allows, as already mentioned, the retrospective synchronization of all potential signals (Vmn) recorded together with the injected electric current (Iab). Figure 5.2 On the left, high power transmitter of 6 kW. On the right, the three-phase 15 kVA motor generator which powers the transmitter. 3. TIP6000 from IRIS Instruments.
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