167 5. DEEP ERT/IP for geothermal exploration and de-risking The geology of the area under investigation bears witness to two distinct stratigraphic cycles. The first, of early-middle Pliocene age, is characterized by marine facies deposits. The second is marked by a phase of intense alteration, manifested in the form of brackish facies of age early Pleistocene. An extensive debris layer has also been recognized, deriving from the intense weathering of oldest geological formations. This referred to the Riss-Wurm interglacial about 100 thousand years ago. 5.3.1 Unconventional ERT data acquisition The survey is characterized by its significant extension and the required high investigation depth for ERT method. Moreover, the area is characterized by a typical hilly environment, with forests, roads and isolated houses or dispersed hamlets. Given both these geographical constraints and the high depth of investigation, we considered that a traditional multi-electrode approach, using a conventional resistivity meter connected to multicore cables, would be unrealistic. Investigating at great depth requires an acquisition system that covers a very large area, which is incompatible with the geographical constraints mentioned above. The acquisition system consists of a set of independent, stand-alone units, which record the variation in electrical potential over time on two measurement channels. The set of recorders is independent of the high-power underground current injection device. The current transmitter is powered by a generator and connected to a current recorder over time. The only cables running through the study area are those connecting the current electrodes to the transmitter. These are two small-section cables, far different from the multiwire cables used for traditional resistivimeter. In particular, for this project, the configuration described below was used. Receiver devices. The receiver system consists of 34 small and lightweight receiver boxes 2 that were easily deployed on the ground. Each box is independent and autonomous, powered by an internal battery, and allows electrical potential measurements to be performed at two dipoles: P1-P2 and P2-P3 (see Figure 5.1). Each unit is therefore able to work independently from the others and this allows an “unconventional” 3D acquisition design in such a complex morphologically and logistically context. The receiver is able to continuously measure the electrical potential at the two dipoles with a frequency of 100 measurements per second. Its internal GPS device allows its localization and the timestamp for each measurement. The potential measurements are synchronized a posteriori with the current measurements recorded at the transmitter. 2. V-FullWaver from IRIS Instruments.
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