72 Geophysics in Geothermal Exploration works iteratively and stops when the updated distribution of resistivity allows to compute a set of apparent resistivities which fits, in a root mean square sense, the measured apparent resistivity distribution. Figure 2.13 shows an example of 2D electrical tomography, obtained on the rock glacier of Verbier in Switzerland. The example is a near surface example, with a depth investigation of 30 m. Figure 2.12 Electrical method. After D. Chapellier (2001b). (a) Equipotential and current flow lines for two sources of current. (b) Measurement of apparent resistivity. (c) Depth investigation of the electrical method versus the spread configuration. (d) Resistivity profiles. (e) Electrical apparent resistivity map. Document IGL. For deep target, specific field apparatus has been developed (Carrier et al., 2019). It consists of a set of 2-channel independent receiving nodes called V-Fullwavers, one current measurement unit called I-Fullwaver, an induced polarization transmitter (VIP). Current is injected through the induced polarization transmitter. The transmitter enables to inject current up to 10 Amps, 5000 W and 3000 V, with a frequency of 0.5 Hz. The receiving nodes (V-Fullwavers) continuously record the electrical field and the injection electrodes can be moved inside and outside the receiving nodes with any type of electrode array configuration. For the field (e) (b) (a) (c) (d)
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