169 5. DEEP ERT/IP for geothermal exploration and de-risking Topography – For an ERT survey, accurate location of the receiving and injection electrodes is essential. The position of each electrode was measured by a DGPS Leica GS18 system with an accuracy of less than 3 cm. A digital Lidar terrain model with a 2 m×2 m mesh was used to build a detailed 3D finite element mesh for the ultimate phases of data processing. 5.3.2 Acquisition methodology The implementation of a DERT project on the ground can be divided into 4 fundamental phases: survey design, field preparation, data acquisition and equipment recovery. The survey design is a step done at the office that must not be neglected. Backed in part by logistical issues, it contributes to the optimization of human resources, intervention time and data quality. A detailed study of the area based on aerial photos, topographic map and direct inspections allowed to identify the access routes, the ideal position for the receiving units and transmissions electrodes. Using our proprietary ERTdesign© software (GEG Experts), we generated an initial theoretical layout of receivers and transmissions over the study area (see Figure 5.3). We then optimized the results by accurately moving each receiver or transmission point based on consideration as access or visibility of satellite constellations to ensure a good GPS connection. Figure 5.3 Acquisition design performed before the survey with ERTdesign©. The software provides the total number of receiver units (displayed on the figure) and transmission points.
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