Surface wave analysis for studying elastic properties of glacier bed sediments

To reveal if glacier bed sediments are unfrozen, we apply multi-channel analysis of surface waves MASW on seismic data acquired across a glacier at the southern central Spitsbergen in the Norwegian Arctic. The S-wave velocity varies significantly with the degree of freezing of the pore fluids. We clearly observe the offset of the dispersion curves at the transition of predominant modes. These higher modes may indicate a velocity reversal beneath the glacier or interface between glacier and sediment. Based on simulation studies, we find that the offset observed on the dispersion curve seems to be related to the glacier thickness when the velocity structure beneath the glacier is similar. Therefore, we can roughly estimate the geometry of the surface of the sediment sharp velocity contrast only from the transition of predominant mode on the dispersion curve. We then estimate the S-wave velocity distribution via Genetic Algorithm GA inversion, using information of ice thickness derived from Ground Penetrating Rader GPR data. The estimated S-wave velocity structure clearly represents the velocity reversal beneath the glacier.