USING GROUND PENETRATING RADAR TO MAP THE INTERNAL STRUCTURE OF GLACIAL OUTWASH DEPOSITS: GRAND MESA, CO, USA

The origin of deposits on the southern slopes of Grand Mesa have been problematic. Recent research suggests that these deposits are a combination of glacial outwash deposits and debris flow deposits. We used GPR to map the internal structure and boundaries of these series of deposits. These deposits are alternating layers of basalt cobbles and boulders with finer materials. GPR data were collected along a 407-meter transect along the top of an exposure and three transects, each 46 m long, perpendicular to the transect. The local relief along the transect is 6 m. Collection of data was complicated, as some of the area is covered with residential development. As a result of man-made alterations to the terrain, several sections along the long transect path were avoided, yielding a 394 m data transect. The perpendicular line data were collected to facilitate construction of a 3D GPR model.

The GPR data were collected using a PulseEKKO ^TM 100A subsurface imaging radar with 25 and 50 MHz antennas.

The cobble and boulder layers outcropping on the surface were identified during surficial mapping. These data helped in more complete processing where we were able to eliminate “mirror images” of these surface features. The 50 MHz antennae allowed us to penetrate to a depth of 40 m. We are able to distinguish the uppermost crude structure, generally found within the upper 7 m. Detailed imaging of lower layers will require the use of 100 MHz or higher.