LOCATING GROUND WATER WITH ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) IN THE EOCENE AND OLIGOCENE WHITE RIVER FORMATION, CONVERSE COUNTY, WYOMING

The resistivity method of shallow geophysical analysis is an inexpensive and effective method of locating shallow reservoir rocks (aquifers) that may contain significant groundwater in terrestrial fluvial systems. The White River Formation studied contains 90-95% very conductive clay-rich mudstones and siltstones representing over-bank floodplain deposits along ancient river systems. The formation also contains highly resistive lenses of coarse grained quartzofeldspathic sandstones and minor conglomerates, representing ancient stream channels often compose less than 5% of the formation and hence are very difficult to locate by random drilling methods.

For this application, the team at Casper College used the Advance Geoscience, Inc. (AGI) SuperSting R8 with a 56-passive electrode array equidistantly placed at five meter intervals. In addition to the resistivity equipment, geolocating was performed with Garmin GPS and ArcMap by ESRI. Changes in elevational grade along the transects were measured with a Brunton hand transit, Jacob’s staff and tape measure. The acquired data was downloaded and processed using AGI SuperSting Administrator and AGI EarthImager software.

The initial processing of data from 2016 was not accurate as to depth caused by non-unique solutions. Shallow extremely resistive sandstone bodies represent air filled pores and are dry reservoirs. Close proximity of multiple anomalies within the target area processed together as one anomaly. By reprocessing our data with previous and post drilling measurements, we were able to improve the geophysical imagery and location of the aquifer. This formation, covering hundreds of square miles of dry surface lands in Wyoming, Colorado, Nebraska and South Dakota often contains unrecognized and undiscovered ground water resources.