GEOPHYSICAL INVESTIGATION OF SUBSURFACE CHARACTERISTICS OF ICY DEBRIS FANS WITH GROUND PENETRATING RADAR IN THE WRANGELL MOUNTAINS, ALASKA

Icy debris fans have recently been described as deglaciation features on Earth and similar features have been observed on Mars, however, the subsurface characteristics remain unknown. We used ground penetrating radar (GPR) to non-invasively investigate the subsurface characteristics of icy debris fans near McCarthy, Alaska, USA. The three fans investigated in Alaska are the East, West, and Middle fans which are between the Nabesna ice cap and the McCarthy Glacier. Icy debris fans in general are a largely unexplored suite of paraglacial landforms and processes in alpine regions. The results showed that each fan’s composition is primarily influenced by the type and frequency of mass wasting processes that supply the fan. Photographic studies show that the East fan receives far more ice and snow avalanches whereas the Middle and West fan receive fewer mass wasting events but more clastic debris is deposited on the Middle and West fan from rock falls and icy debris flows.

GPR profiles and WARR surveys consisting of both, common mid-point (CMP), and common shot-point (CSP) surveys investigated the subsurface geometry of the fans and the McCarthy Glacier. All GPR surveys were collected in 2013 with 100MHz bi-static antennas. Four axial profiles and three cross-fan profiles were done on the West and Middle fans as well as the McCarthy Glacier in order to investigate the relationship between the three features. Terrestrial laser surveying of the surface and real-time kinematic GPS provided the surface elevation used to correct the GPR data for topographic changes. GPR profiles yielded reflectors that were continuous for 10+ m and hyperbolic reflections in the subsurface. The WARR surveys provided the GPR signal velocity through the subsurface material and allowed transformation of two-way traveltimes (TWTT) in GPR profiles to be converted to depth.

The profiles on the west fan contain sub-horizontal reflections that extend laterally down and across the fan while the middle fan and parts of the glacier have larger areas of chaotic reflections that transition into areas with less energy. The difference in reflection boundaries of the west and middle fan is interpreted as discrete interfaces and diffuse boundaries respectively between layers of differing electrical properties.