AIRBORNE GEOPHYSICAL SURVEY AND GROUND-BASED GEOELECTRICAL SOUNDINGS NEAR BERRIEN SPRINGS, MICHIGAN

In October, 2000, a geophysical survey was flown for the Central Great Lakes Geologic Mapping Coalition of a 1:24,000 quadrangle-sized area centered on Berrien Springs, MI. The contract helicopter carried sensors 100 ft above ground level along lines spaced 0.2 miles apart. The resulting maps show total magnetic field; Uranium, Thorium, Potassium, and Total Count radiometrics; and electrical resistivity at 5 frequencies. The magnetic and radiometric maps largely reflect moraines and streams, whereas the resistivity maps reflect geoelectrical features at successive depths down to ~ 50 m below the surface. Resistivities of these features range from ~3 to ~300 ohm-m. The St Joseph River is a relative conductor on all the resistivity maps, suggesting that its paleovalley contains conductive material to substantial depths. Surface time-domain electromagnetic soundings in the area show shallow (0 - 40 m) resistivities of ~200-400 ohm-m (above water table?); deeper resistivities of either ~30 ohm-m (diamicton?) or ~300 ohm-m (sand?); and deep (40-120 m) resistivities of <10 ohm-m (bedrock shales?). We intend to use surface soundings from locations near logged wells to better 0correlate resistivities with types of lithologies. Such correlations will then be used to infer 3-D lithologic variations throughout the area using the airborne resistivity maps.