COMBINED GEOELECTRICAL AND DRILL-HOLE INVESTIGATIONS FOR DETECTING FRESHWATER AQUIFERS IN NORTHWESTERN MISSOURI

A geoelectrical investigation was conducted in an area of northwestern Missouri which was previously explored by drill holes. The purpose of the survey was to evaluate the effectiveness of the method in locating potable groundwater. It was found that resistivity depth soundings using the Schlumberger arrangement can partially localize and determine the thickness and depth of both near-surface and basal fresh-water-bearing gravel bodies in glacial deposits. The basal gravel is usually confined to preglacial stream channels and has a particular importance as a fresh-water aquifer. Only in a few cases does the resistivity increase at the depth of the bedrock. More often the depth soundings show an appreciable increase of resistivity at a depth well within the bedrock. High permeability and porosity at a lower water conductivity in the gravel is compensated by lower permeability and porosity at a higher water conductivity in the bedrock. This can explain why the formation resistivities found from depth soundings are basically the same. An increase of resistivity in the deeper bedrock occurs due to a tightening of fissures containing salt water. In one particular well, which penetrates a basal gravel, it can be shown that the salt content of the well water originates from salt water encroachment, most likely from the bedrock. © 1974 Society of Exploration Geophysicists.