Electrical resistivity tomography monitoring studies at Balcova (Turkey) geothermal site

This investigation examines the electrical resistivity tomography (ERT) monitoring performed at geothermal sites. It includes two phases of time-lapse ERT studies of the shallow geothermal liquid transportation that results from the circulation of injected and re-injected liquids: synthetic modelling and field surveys. The synthetic modelling stage indicated that the injection process in the horizontal layered environment could cause temporal variations. The field investigation was performed between March 2010 and February 2011 at the Balcova geothermal site, Izmir-Turkey, which has supplied in-house heating since the 1990s. The time-lapse ERT investigations were performed on a shallow aquifer system near two boreholes used as injection and re-injection. In this study, the percentage changes in the synthetic models were determined and compared to the observed inverted time-lapse monitoring data. We propose that time-lapse inversion results are more reliable than the standard inversion approximation. However, the resistivity artefacts were significant, and were dependent on the selection of the comparison model and the inversion parameters. Also the selection of the initial model was shown to be the most important step in constraining the later temporal datasets. During the monitoring period, we observed very substantial decreases in the resistivity of time-lapse results. We conclude that these observed decreases are a result of subsurface temperature or salinity changes associated with the hydrothermal system and fault zone. As a result, this study indicates that time-lapse studies could be important in the characterisation of the subsurface changes that are connected with geothermal operations and climatic fluctuations.