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Reconnaissance hydrogeologic mapping and ground-water-flow modeling in the glaciated Northeastern United States require data on the hydrogeologic boundaries and grain-size characteristics of sand and gravel aquifers. The combined use of surface-geophysical methods such as seismic-refraction, direct-current-resistivity, very-low-frequency terrain-resistivity, and inductive terrain-conductivity can provide these data. Forward-modeling studies of 10 hypothetical aquifer systems indicated that very-low-frequency terrain-resistivity and inductive terrainconductivity methods can be used to detect horizontal and vertical changes in the electrical properties of typical sand and gravel aquifers. Surface-geophysical surveys at eight sites in Connecticut, New York, and Maine, have shown that the combined use of four geophysical methods can define the hydrogeologic boundaries and can distinguish between fine-grained and coarse-grained stratified drift. The seismicrefraction method could be used to determine depths to the water table and to bedrock at all the field sites where it was used, but could not be used to detect lithologic changes within the aquifer. Direct-currentresistivity methods could be used to detect large-scale resistivity changes that were related to either specific conductance of ground water or major lithologic changes in the aquifers. In some hydrogeologic settings, this method could be used to identify the water table and bedrock surface. Very-low-frequency terrain-resistivity and inductive terrain-conductivity methods used in conjunction with the other surface-geophysical methods can be used to identify anomalies caused by small-scale changes in aquifer lithology or specific conductance of ground water. Comparisons of the interpreted electrical resistivity of the aquifer with the logs of test holes and wells indicate that for a given specific conductance of ground water, the bulk electrical resistivity of the stratified drift increases with increasing grain size. This relation is useful for distinguishing fine-grained drift deposits from coarse sand and gravel deposits.

Application of surface-geophysical methods to investigations of sand and gravel aquifers in the glaciated Northeastern United States