GEOPHYSICAL GEOMORPHOLOGY OR “GEOPHANTASY”: DOES GROUND-PENETRATING RADAR (GPR) SHOW DEPOSITIONAL REGIME BOUNDARIES AND STRUCTURES IN ARMORED DEPOSITS?

Ground-penetrating radar (GPR) is a tool used by geomorphologists to reveal locations of buried forms and to illustrate structure, bedding and other characteristics in these buried features. Standard practice assumes that GPR profiles acquired are accepted as representative of the real features -- almost without question. In the past, features that have been mapped have not been compared with actual cross-sections of the deposits. But, without an actual one-to-one comparison, one has to question if GPR accurately reveals shallow subsurface structures and features. We think this accepted assumption needs to be explored. We hypothesized that GPR accurately represents subtle structures in the features.

To answer this question, we used GPR to examine a series of basalt-armored deposits on the southern flanks of Grand Mesa in western Colorado. These deposits are a combination of debris flows, mud flows and various fluvial events enhanced by late Pleistocene climatic warming. The exposure parallels CO 65 near Cory Grade. The road has cut the deposit, exposing a complete cross-section for observation.  Distinct layers showing different depositional regimes have been identified in the outcrop. Thus, we “shot” a GPR profile along the top of the outcrop and parallel to its strike as well as numerous profiles orthogonal to the long GPR profile to create a 3-D view.

Fieldwork was conducted Summer 2009. The GPR data were collected on top of the deposit, adjacent to the road cut. After processing and interpretation, the data were compared to a mapped sequence complied along the road cut to evaluate the ability of GPR to distinguish subtle changes in layering and composition within a geomorphological feature. The GPR data were collected using a PulsEKKO^TM 100A subsurface imaging radar with 25 and 50 MHz antennas. We were able to distinguish major contacts between large depositional regimes, but we were not able to distinguish minor structure and subtle changes observed in the outcrop with the GPR. In the future, we will revisit the question using various antennas as well as attempting to develop new post-processing software to enhance the GPR signature.