ELECTRICAL-RESISTIVITY MEASUREMENTS TO ESTIMATE TRAVEL-TIMES THROUGH UNSATURATED GROUND-WATER PROTECTIVE LAYERS

Evolving environmental regulations have increased the need for defining the protective properties of naturally occurring geologic layers. Of particular significance is the need for defining vertical times-of-travel (TOTs) through protective layers located above municipal water supply wellfields in order to prevent contamination from cultural activities. Surface geoelectrics provide a relatively fast and inexpensive field measurement method that can be used to estimate the protective properties of geologic layers. In particular, the TOT through unsaturated geologic layers is theoretically linearly related to the longitudinal unit conductance (S) of the layer. Using a limited number of DC soundings and laboratory analysis, TOTs through a protective layer above a municipal wellfield in Lincoln, Nebraska were predicted from the longitudinal unit conductance of the layer with an estimated standard deviation of 2.9 yr.