Gravity Survey of a Deep Buried Valley

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A gravity survey was conducted as part of a ground-water investigation covering 400 mi2 (1,000 km2) in Outagamie County, northeastern Wisconsin. The objective of the gravity survey was to supplement bedrock topography data obtained from well logs and seismic refraction profiles. The gravity method was chosen as it is a rapid and low-cost reconnaissance technique, and conditions in the study area were favorable for a successful gravimetric survey. The simple Bouguer gravity values were referenced to a local base station as only relative gravity values were required. The data were interpolated to a regular grid by an inverse-distance weighted average algorithm. The gravity residual was derived using a five-ring inverse weighted filtering routine. The residual map accurately reflects the major topographic features of the bedrock surface. A two-dimensional geologic model fitted to the residual data agrees well with the simple flat plate approximation used to obtain quantitative estimates of glacial drift thickness. Estimates of drift thickness from the gravity data also agree well with bedrock depths obtained from well logs. The most productive drift aquifers are restricted to the major bedrock valleys. Gravity data were coupled with seismic refraction and well data to produce a bedrock elevation map, from which it is possible to predict the extent and distribution of the major drift aquifers.

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Groundwater, v. 18, issue 1, p. 24-30