Electrical imaging of fractures using ground-water salinity change

Cross-borehole Eletrical Resistivity Tomography (ERT) has been applied to map the spatial distribution of an electrically conductive zone within a hydrologically complex limestone environment. Advantage has been taken of a ''natural'' conductive tracer provided by salt-water migration resulting from long-term pumping at a ''well'' within the fieldsite. The resistivity distribution of 13 between-borehole planes has been reconstructed from borehole resistance measurements, and comparisons between resistivity images obtained prior to, and during, saline migration have been made. The distribution of resistivity obtained from the a priori measurements revealed the location of electrically conductive pathways between boreholes which correlate with cavity features obtained from Borehole Television (BHTV) logging. The continuity of these pathways between boreholes suggests the presence of a laterally extensive electrically conductive zone. The reconstruction of differences in resistivity caused by the salt-water migration confirmed the presence of this conductive zone and has shown that the saline water passed primarily through this zone. This preferential flow illustrates the hydrological importance of this zone and suggests that it is not only an electrically conductive, but also a highly hydraulically conductive feature.