Mapping Coastal Aquifers by Joint Inversion of DC and TEM Soundings‐Three Case Histories

Abstract : Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt walter-bearing formations. Interpretation models for these aquifers typically contain four layers : a highly resistive unsaturated zone ; a surficial fresh water aquifer of intermediate resistivity ; an underlying conductive, salt water saturated aquifer ; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated : direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thickness of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently.