Mapping Buried Bedrock Topography with Gravity

In the formerly glaciated Midwest, bedrock formations commonly are denser than overlying glacial sediments. Thus, gravity anomalies have a direct relationship to bedrock topography. Where the density contrast between the glacial sediments and the bedrock is known, the anomalies can be used to map the bedrock configuration if they can be isolated from gravity anomalies originating elsewhere in the geologic section. A method of isolating bedrock topography gravity anomalies is suggested based upon known bedrock elevations obtained from drillhole data. A regional gravity anomaly map is prepared by subtracting the excess gravitational effect of the known bedrock relief above a datum from the observed gravity anomalies at all drillhole sites. The regional gravity anomaly map is subtracted from the observed anomaly map to obtain a residual anomaly map reflecting the bedrock topography. The bedrock configuration can be calculated directly from this map. This method of isolating bedrock topography anomalies was found to be superior to the use of cross‐profiling and least‐squares polynomial approximation procedures in a study of Kalamazoo County, Michigan. The bedrock topography map of this County determined by the gravity‐geologic method shows a complex bedrock topography and drainage system.