Gravimetry Of A Buried Bedrock Valley Near Sulphur Springs In East-Central Indiana

To trace the New Castle bedrock valley or its tributaries, guide drilling to characterize<br>unconsolidated valley fill deposits, and facilitate groundwater exploration and protection, gravity was<br>measured at 270 stations along 45 km of profiles in Henry County in east-central Indiana. A “locally<br>derived” regional gravity field estimate was obtained as a quadratic polynomial using the control of<br>bedrock elevations in eight wells and an assumption that the residual Bouguer at these wells could be<br>approximated by the “infinite-slab” model. The bedrock surface derived from the residual gravity<br>supplements the interpretation of that surface using sparse well control and reversed seismic refraction<br>profiles. Within the 11 km x 16 km map area the bedrock elevations in wells vary from 595 to 966 feet<br>(181 to 294 m) above sea level and the depths to bedrock vary from 54 to 497 feet (16 to 151 m). The<br>gravity contours confirm the existence of a bedrock high defined by reversed refraction seismic profiles<br>a few hundred meters to the west of the lowest bedrock elevation penetrated by a well. These contours<br>trace this high as a bedrock ridge that trends to the north-northwest. Additional gravimetry is planned to<br>guide the drilling of deep test wells.