278 Geophysics in Geothermal Exploration demonstrated the theoretical detectability of the cold front, particularly identifying the frequencies at which the cold front response would be maximal. We use galvanic sources, injecting current between two electrodes (“poles”) placed in the ground. The placement of these sources is crucial for the success of such measurements. Specifically, it is essential to consider: 1. The distance between the source(s) and receiver(s), 2. The length of the injection dipole(s), or the distance between two electrodes, 3. The orientation of the injection dipole(s), 4. The injected current intensity, related to the grounding resistance (depending on the type of transmitter). Modeling conducted by Wawrzyniak (2019) and ITES (Strasbourg University) provided guidance on optimal source configurations. For the receivers, preliminary studies showed that a single receiver positioned at the bottom of the well significantly improves the likelihood of capturing the signal. Urban environments have high anthropogenic electromagnetic noise, which can mask the signal. An induction probe is used as the receiver, since measuring the electric field in a cased well must be complex. 10.2 Acquisition The geothermal doublet used for borehole measurements is the Dalkia-operated one in Evry. The injection well GEV4 is targeted, and six zones where injection sites could be established around the well’s shoe have been selected (see Figure 10.2). These sites were surveyed in October 2021. During the survey, factors likely to impact a CSEM survey includes: 1. Presence of power lines, fences, or pipelines. 2. Soil resistivity measurements in areas intended for electrode installation using electrical resistivity tomography (ERT, dipole-dipole) and TDEM with the TEMFAST device (aemr.net).The orientation of the injection dipole(s), 3. Site accessibility. 4. Safety aspects, vehicle traffic, and site activity. The reconnaissance campaign allowed us to visit each site, resulting in the following observations: • GEN 1: too small and with high vehicle traffic; low grounding resistance. • GEN 2: quiet, high electrical resistivity in parts, ample space. • GEN 3: limited security, low grounding resistance. • GEN 4: quiet, presence of underground pipes. • GEN 5: limited security, difficult access, no electrical resistivity measurements. • GEN 6: inaccessible (fenced), no electrical resistivity measurements.
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