THE USE OF MARINE ELECTROMAGNETIC CONDUCTIVITY AS A TOOL IN HYDROGEOLOGIC INVESTIGATIONS

Field procedures for collecting marine electromagnetic (EM) conductivity data were developed by the U. S. Geological Survey as part of a hydrogeologic investigation of the Delaware River shipping channel. Eighty miles of marine EM data collected along the border between Pennsylvania and New Jersey were used to help interpret results from high-resolution, continuous seismic-reflection surveys done in the summer of 1984. Comparison of EM data with lithologic logs from river borings showed a direct relationship. EM conductivity of sand or bedrock ranged from 4 to 10 millimhos per meter (mmhos/m). Extensive clay beneath the shipping channel generally gave readings exceeding 20 mmhos/m. Conductivity readings over silts were almost always between these values (11 to 20 mmhos/m). This relation was used to help interpret the continuous seismic-reflection data.