Subglacial bed properties from normal-incidence seismic reflection data

Recent applications of the normal-incidence seismic reflection technique to studying subglacial conditions are summarized. Some of the important aspects of the technique are discussed, including critical acquisition parameters and particular strengths and weaknesses. The main reason for deploying this technique, rather than other seismic methods, is that data acquisition is simpler and requires fewer resources. Current limitations of the technique include the inability to determine subglacial seismic velocities, uncertainty in the attenuation coefficient for ice, and assumptions of source repeatability. The reflection coefficient at the ice-bed interface is calculated from the energy reduction between primary and multiple reflections. From this, the acoustic impedance of the bed is derived and used to interpret the bed material. Beneath fast-flowing ice, dilatant, deforming sediment has been distinguished from a lodged sediment bed, using porosity as a proxy for sediment dilation. Subglacial water and permafrost have also been interpreted. Data from a number of locations can be used to develop a model of the basal conditions of a complete glacier. Results from sites on ice streams in West Antarctica show how the ice encounters a greater or lesser restraint to flow, from different basal conditions. Application to a glacier in the Arctic, suggests its most recent surge terminated when water escaped through discontinuous permafrost beneath the ice. Further glaciological questions that could be addressed using the technique are proposed.