Cost-efficient imaging and monitoring of saltwater in a shallow aquifer by using long electrode ERT

Abstract Electrical resistivity tomography (ERT) is an important method for imaging and monitoring saltwater intrusions into aquifers. Limitations regarding investigation depth can be overcome by using metal-cased boreholes as electrodes for characterising and monitoring groundwater salinity. We develop a scale-independent approach for imaging aquifers with long electrode ERT (LE-ERT). A synthetic study for a given electrode distribution is conducted to investigate model resolution properties for a complete and reduced data set. Results show that resistivity anomalies due to salinity variations can be imaged well, particularly if a-priori structural information is incorporated. The reduced data set exhibits a similar resolution of the given model compared with the complete data set. Repeated field measurements were conducted on a 500 × 300 m test site using 12 metal-cased boreholes. Additionally, seven surface electrodes were installed in order to increase model resolution. Data were inverted using a smoothness-constrained Gauss–Newton approach on a triangular prism mesh. Inversion results of a single survey of field data showed good agreement with geology known from boreholes and with results from standard surface ERT profiles. At the bottom of the first aquifer (about 40 m depth) a saltwater body was observed, predominantly in the eastern part. This finding is supported by in-situ fluid conductivity measurements. LE-ERT monitoring over a period of two years shows only little changes in the upper aquifer. Resistivity ratios between subsequent time steps indicate a slow dilution of the saltwater body which correlates with borehole fluid conductivity monitoring.