Monitoring Underground Heat Storages by Means of Borehole Electrical Resistivity Tomography. A Model Test.

Summary Underground heat storages are one out of several possibilities to store energy on a large scale. Using the subsoil as such a storage, it is necessary to monitor the expansion of the temperature field and to look out for possible leakages. In this study, geothermal experiments are carried out on an idealised meso-scale sand pit named "GeoModel". The sample is heated up and cooled down by circulating hot and cold water in a Borehole Heat Exchanger (BHE). Simultaneously, one half of the sand body is monitored by borehole electrical resistivity tomography including several cross-hole and in-hole configurations. Additionally, the temperature of the other half of the sample is measured by numerous sensors at different depths and distances to the BHE. According to theory, the results show a strong anti-correlation between electrical resistivity values and measured temperatures. Repeated heating and cooling cycles demonstrate reproducibility of the investigations. A leakage of the BHE, which occured during an active cooling phase, could be detected at an early stage. Inversion results are shown to be suitable for a qualitative observation of three-dimensional temperature fields in the sand pit.