A GRAVITY SURVEY OVER THE BURIED MAHOMET BEDROCK VALLEY, PIATT COUNTY, ILLINOIS

In some situations, gravity surveys have been shown to be effective and efficient for finding buried bedrock valleys. Seismic reflection is also a well known and established method for exploring the subsurface structure including buried bedrock valleys. However, gravity may hold some advantages over seismic reflection. Traditional seismic exploration requires an eight-person crew, while gravity data acquisition needs a crew of only two or three.. Also, with the traditional seismic method, a crew can acquire 1 mile/day of 48-channel data, but a gravity crew can acquire as much as 4-5 miles/day of data. In addition, while the seismic method is a quick and valuable way to acquire data for 2-D analysis, gravity data acquisition is more rapid for a 3-D survey. Gravity methods also have their limitations. To obtain meaningful gravity measurements, a distinct density contrast between subsurface layers is needed as well as precise elevation recordings for each measurement location. Both methods are nevertheless complementary.

We collected gravity data in an approximately 6 by 12 mile area in east-central Illinois over the Mahomet Bedrock Valley. We expect the small density contrast between the shale bedrock and glacial fill material to produce a small, but measurable negative gravity anomaly over the valley. The acquired data was corrected and analyzed in 2-D along a road which runs north-south through the center of the study area. We then compared this corrected data to seismic reflection data previously acquired along this same road. This seismic reflection data shows the buried Mahomet Bedrock Valley running east-west beneath the study area. A negative gravity anomaly covers an area very similar to the cross-sectional area of the buried bedrock valley. We also discovered a small increase in gravity over the center of the buried valley. This small increase may be caused by a wedge of high-density Glasford Formation till that is known to fill the valley.