HR: 08:30h
AN: C21A-03 INVITED     [PDF]
TI: Subglacial Conditions From the Seismic Reflection Technique
AU: * Smith, A M
EM: amsm@bas.ac.uk
AF: British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET United Kingdom
AB: For many decades the use of seismic methods in glaciology was overshadowed by radar. Radar acquisition is quicker, easier and can be done from an aircraft. However, recent years have seen a resurgence in seismic techniques, enabled largely by improved instrumentation. Seismic data have been particularly effective at determining basal conditions and here I discuss some of our seismic reflection results. Seismic acquisition parameters can be tailored to many different targets of interest. Here I concentrate on near-vertical incidence seismic reflection data, where source-receiver offsets are small relative to the ice thickness. Energy loss can then be attributed to geometrical spreading, attenuation and transmission/reflection coefficients without any influence from amplitude- (or phase-) vs-offset effects. This makes the analysis of field records particularly straight-forward. Quantitative interpretation of the data to determine basal conditions relies on the ability to calibrate the reflection coefficient at the ice-bed interface. Hence, the seismic data need to include multiple reflections from the bed, either all the way along a profile or at a number of specific locations. The reflection coefficient is controlled by the acoustic impedance contrast at the interface, ie the change in the product of seismic velocity and density in the two materials. In most situations, the impedance of the ice is known accurately enough for that of the sub-glacial material to then be calculated and the nature of the material determined, eg. deforming sediments, non-deforming sediments, permafrost or bedrock. I present results from Antarctic ice streams. Variations in the basal conditions can be identified, which in some cases can show major transitions over short distances. Data from a number of different sites can be used to quantify the way that basal conditions control the ice flow.
DE: 0935 Seismic methods (3025)
DE: 1827 Glaciology (1863)
DE: 9310 Antarctica
SC: Cryosphere [C]
MN: 2003 Fall Meeting