OUTCROP-SCALE IMAGING OF LACUSTRINE DELTA SEDIMENTS USING GROUND PENETRATING RADAR AND SEISMIC METHODS

Outcrop-scale ground penetrating radar (GPR) and seismic reflection imaging in complex structural and stratigraphical geologic environments is difficult due to extreme contrasts in physical property values and poor resolution. We imaged the complex stratigraphy and structure of poorly consolidated unsaturated fluvial and lacustrine sediments that are exposed in roadcuts along the margins of the Western Snake River Plain in southwest Idaho. One outcrop contains a 10 m exposure of a delta sequence with topset beds tectonically tilted 10 degrees, underlain by 30 degrees dipping foreset sand beds with mud drapes, and cut by near-vertical faults. Beneath the foresets are lacustrine clay sediments over an irregular rhyolite bedrock surface.

We collected 50 and 100 MHz frequency GPR and high-resolution seismic profiles along this roadcut to image the upper 10 m below land surface. The 100 MHz GPR data show the observed dip of both the foreset and topset beds and also document the contact with the underlying bedrock. The seismic results show reflections from the bedrock surface at 6 m depth, but due to wavelengths that are similar to the 50 MHz GPR data, reflections from the upper section were not clearly imaged. Seismic velocities range from 400-1000 m/s with observed frequencies of more than 250 Hz. The imaging studies will be extended to additional outcrop locations, 3D GPR coverage, higher frequency (shorter wavelength) data collection, and more sophisticated processing routines to improve the quality and utility of GPR and seismic surveys to image sub-meter scale geologic stratigraphy and structures.