PRELIMINARY GEOPHYSICAL INVESTIGATIONS AT THE PHASE II SITE OF THE BALL STATE UNIVERSITY GEOTHERMAL CONVERSION

In May 2009, Ball State University (Muncie, IN) began drilling the nation’s largest ground-source geothermal project to replace aging coal-fired boilers. This project is divided into two phases. In Phase 1, approximately 1800 geothermal wells were drilled to a depth of 400 ft in a 15-ft grid pattern in the northern part of the campus. This portion of the project is now complete and online. The construction of Phase 2 in the southwest corner of campus began in November 2011. It will have similar characteristics to Phase 1, except wells will extend to a depth of 500 feet and contain a single closed loop of heat-exchange pipe.

During December of 2011, electrical resistivity transects in the Phase 2 project area were collected in a cooperative agreement between Mundell & Associates, Inc. and Ball State University faculty and students. The surveys were conducted using an AGI Super String R8 with 56 electrodes configured in a dipole-dipole array. Seven transects were completed over a two day period. Transects 1-4 and 6 had an electrode spacing of five meters. Transect 5 had a spacing of 6 meters and transect 7 had a spacing of 4.5 meters. Transects 1 and 2 overlapped as well as transects 3 and 4. Earthimager 2D was used to compute inversions of the raw data.

The results from this study achieve similar findings to those found in the geophysical surveys in the Phase 1 field. The inversions reveal a complex bedrock surface indicating paleokarst. As during Phase 1, we expect drilling in Phase 2 will encounter solution cavities. The paleokarst is overlain by glacial till. The till is comprised of clay, silt, sand, and gravel and averages 30 m in thickness. In the inverted data, a distinct layer of gravel shows up as a band of higher modeled resistivity. The lithology and depth of this gravel as well as the average depth to bedrock have been confirmed by on-site observations from drill logging. The data further suggest that this gravel layer is less distinct toward the west.