Variations in basal conditions on Rutford ice stream, west Antarctica

Seismic reflection data from two lines on Rutford Ice Stream are presented and are compared with data already published from a third line on the ice stream. The amplitude and phase of the reflections have been used to investigate the properties of the sub-ice material. Multiple reflections on long record-length data allowed calibration of the reflection coefficient at the ice-bed interface and determination of the acoustic impedance of the bed material. The characteristics of the bed material vary both along and across the ice stream. The average acoustic impedance of the bed material across the glacier at the upstream line is 3.88 x 10(6) kg m(-2) s(-1). This decreases to 3.19 x 10(6) kg m(-2) s(-1) 52 km further downstream. These values are within the range which is typical of soft sediments. Using acoustic impedance as an indicator of subglacial porosity, some areas of the ice-stream bed are interpreted as dilatant water-saturated sediments undergoing pervasive deformation. In other areas, the bed is not deforming and basal sliding-may be a more important process. The proportion of the ice-stream width over which bed deformation occurs increases downstream.