Mapping coastal aquifers by joint inversion of DC and TEM soundings - Three case histories

Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-be aring and salt water-bearing formations. Interpretation models for these aq uifers 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. Additio nal layers may be added to allow for variations in lithology within the fre sh water and salt water layers. Two methods are evaluated: direct current r esistivity and time domain electromagnetic soundings. Use of each method al one produces nonunique solutions for resistivities and/or thicknesses of th e different layers. We show that joint inversion of vertical electric and t ime domain electromagnetic soundings produces a more tightly constrained in terpretation model at three test Sites than is produced by inversion method s applied to each data set independently.