12-100 MHz depth and stratigraphic profiles of temperate glaciers

Current practice prescribes the long standing use of frequencies less than 10 MHz for GPR sounding of temperate glaciers to depths greater than about 100 m. We have obtained continuous profiles of ice depth over temperate ablation zones and have reached at least 278 m at 30 MHz on the Matanuska, 195 m at 100-MHz on the Muir and 295 m at 12 MHz on the Gulkana Glaciers of Alaska. Internal stratigraphy occurs in the Matanuska profile. Multiple bottom returns are ascribed to thrust planes and out-of-plane reflections from sloping bottoms. Inability to reach greater depths is ascribed to unfavorable bottom slopes, as verified try drilling on the Matanuska, and by beam steering studies on the Gulkana. Lateral wave clutter contaminates profiles at all frequencies and polarizations where crevasses are present. The strength of 12-MHz Gulkana bottom signals precluded the use of range gain and suggests that 500 m depth might be achieved at 12-30 MHz.