Constraints on the variable subglacial structure of Whillans Ice Stream from ambient noise Rayleigh wave H/V ratios

Horizontal-to-vertical (H/V) ellipticity ratios of Rayleigh waves have been used to determine shallow (<10 km) velocity structure on regional scales using passive, broadband seismometers. Here we adapt the H/V method of Lin et al. (2014) to a 35-station, 50-day deployment on the Whillans Ice Stream (WIS) in West Antarctica. Drilling has shown that the WIS is known to be underlain with saturated, dilatant till, with reflection imaging showing a deeper sedimentary package that extends to an unknown depth. It is also known that the frictional properties of the WIS ice-bed interface at 700 m depth are highly heterogeneous, including stick-spots of high friction, possibly as a result of compacted sediment or bedrock, and active subglacial lakes where frictional coefficients are effectively zero. Ambient noise cross-correlations are calculated between all station pairs, restricting the minimum interstation distance to 20 km, as well as constraining valid H/V ratios of radial and vertical sources between the same station pair to < 20% difference. We are able to collect fundamental mode Rayleigh wave energy with good signal-to-noise between 6 s and 20 s that are sensitive to the shear velocity of the shallowest sedimentary layers beneath the ice stream and is combined with average phase and group velocity of the area to help constrain the inversion. H/V ratio modeling results suggest that ratios are highly susceptible to sedimentary layer thickness. Ratios also increase over the observed frequency band with the presence of a shallow, saturated sedimentary layer with high Vp/Vs. In preliminary results, we observe an increase in H/V ratio towards the grounding line as well as at stations where hydro-potential surface is high. These higher ratios can be attributed to higher water content within sediments, or an increase in the sedimentary layer thickness.