The directional dependence of the Ground Penetrating Radar response on the accumulation zones of temperate Alpine glaciers

Recent work has shown that ground penetrating radar (GPR) is an effective tool for mapping the thickness and internal structure of temperate glaciers (e.g. Nobes et al. 1994; Hochstein et al. 1995) and of polar glaciers (Jacobel et al. 1990; Jacobel & Hodge 1995). GPR, in turn, is a logical successor to radio echo sounding (RES), as it has been called in the glaciological literature (e.g. Bjornsson 1986; Brown et al. 1986; DeLisle et al. 1989). The distinction between RES and GPR is discussed briefly by Arcone et al. (1995). Some preliminary results from recent work on the temperature glaciers of New Zealand show that the radar reflectivity appears to depend on the orientation of the radar antennas relative to the direction of flow of the glacier (Nobes & Owens 1995). This is not a new observation; van Overmeeren (1994), for example, noted a difference in the radar response across a shallow sedimentary sequence. However, although a systematic test was carried out to determine the nature and extent of the directional dependence in the radar response on glaciers, because of weather and time constraints, the test could only be carried out on one glacier, the nevé or accumulation zone of the Franz Josef Glacier. Nonetheless, the results clearly suggest that the directional dependence is present and should, at the very least, be tested at each site before deciding upon the radar antenna orientation to be used for acquisition of the radar profile.