Negative apparent resistivity and chargeability in crosshole electrical surveys

As part of a project aimed at testing various geophysical methods for environmental applications at a site with very high-salinity groundwater, we tested crosshole resistivity and IP tomography using many electrode configurations. Tests have shown that both negative apparent resistivity and chargeability are possible and are related to subsurface geology. We also measured other effects, which might be caused by the high–electrical conductivity fluid. At this site the high resistivity contrast between unsaturated and saturated sediments seems to be responsible for the negative apparent resistivity, as proved by numerical modelling.