288 Geophysics in Geothermal Exploration confirms that the cold front could be detected in this configuration, which is a major result of this study. In the other two configurations, 2 and 3, the recorded signal is too weak to allow detection. In configuration 2, the receiver is too far from the source (>5 km), and the response is of the same order of magnitude (10–⁸ nT) as the noise. In configuration 3, the cold front is too far from the receiver, and the secondary field generated and measured at the receiver is too weak (10–¹² nT), well below the noise level, making it undetectable. The main result of this analysis is that in the configuration where the receiver is closest to the anomaly and the source-receiver distance is approximately 3 to 4 km, the anomaly caused by the cold front is detectable. Conclusions The objective of this project was to establish an initial geo-electric state of a geothermal doublet and determine whether a cold front could be detected under ambient noise conditions and using the CSEM sources employed. These objectives were partially achieved. Indeed, the detectability of the cold front was established thanks to the short signal recorded inside the casing. Using the experimental calibrations, which calibrated the variation of resistivity as a function of temperature, the medium’s response with and without the cold front was calculated and compared to the noise level extracted from the downhole recordings made by the probe. The modeling shows that the bubble can be detected in specific RX-TX configurations. It was confirmed that the receiver must be as close as possible to the cold front, and the source (transmitter) must be within 4 km of both the bubble and the receiver. These results validate the project’s central idea: that a cold front can be detected using the CSEM method (surface-to-well). During the project’s execution, we observed the complexity of installing current sources in a highly urbanized environment. We were fortunate to have the proximity of the Sénart forest to set up the sources and test the method. For this project, we used only one injection site, which would not suffice for imaging purposes. In such cases, several sites at different azimuths would be necessary to accurately locate the cold front in space. During the campaign preparation, six injection sites were identified; however, only three were deemed viable. The others were too close to power lines, pipelines, or in areas where the safety of personnel and equipment could not be ensured. Injection poles require large spaces and the absence of conductive structures (high-voltage lines, pipes, etc.). Therefore, the applicability of the method seems limited to areas with sufficient nearby space to install sources/transmitters unless research efforts can reduce the footprint of these sources. The receiver may be another project’s weak point. It is clear that to further develop this method, work on the probe will be necessary, either internally or through a partnership with specialized manufacturers. For example, integrating three components instead of one could improve result quality. If the above issues are addressed, the most critical challenge remains: access to the well. Obtaining permission to lower
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