Combined application of dc and TEM to sea‐water intrusion mapping

A geoelectric survey using a dc resistivity method and a transient electromagnetic induction (TEM) method was carried out from 1986 to 1988. It was used to help map the lateral and vertical distributions of the freshwater/salt‐water interface in the Pei‐kang area on the west coast of Taiwan. The dc and TEM soundings were performed at 79 localities over an area of 240 km 2 of Quaternary alluvium. Significant changes in pore‐water conductivity at some places were detected by these two methods. A low resistivity value (<1.5 ohm-m) implies salt‐water contamination of groundwater. The results of spatial distribution of apparent resistivity indicate the salt‐water‐affected aquifers are confined to the southwest part of the study area, and the affected aquifers are confined to the top of two aquifers. A geoelectric model with three to four layers is inferred from the joint inversion of dc and TEM data. Based on a modified Archie’s law suggested by the authors, an empirical relation between pore‐water resistivity of the stratum and formation resistivity can be obtained. It can be used to convert the computed resistivity of each geoelectric layer (aquifer) to the resistivity of the water contained in these layer, thus leading to the assessment of groundwater contamination. Repeated dc resistivity measurements have been carried out at selected locations once every year from 1986 to 1988. These data were used to monitor the temporal variation and a possible spreading of the salt‐water invasion. During 1995, a dc survey was conducted in the same area to compare with the dc sounding results obtained in 1988. The final results provide an evaluation of the efficiency of groundwater management in the study area.