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Use of Shear Waves to Measure Poisson's Ratio in Polar Firn

British Antarctic Survey, High Cross Madingley Road, Cambridge, CB3 0ET U.K.

We conducted a seismic experiment on polar firn on Adelaide Island, Antarctica. We measured the speed of compressional and shear waves at different depths in the firn by sampling waves that return to the surface following continuous refraction. To overcome the difficulty of picking shear wave arrivals in the presence of dispersed compressional waves, we made separate recordings using seismic sources applied to first one side of a pit, then the other, using first a hammer as the energy source and then small explosive charges. When the polarity of one of the records was reversed and then the two records summed together, the compressional wave arrivals were almost completely attenuated and the shear waves enhanced. The results provide the variation of P- and S-wave speeds with depth in the range 0 to 50 m, which were then used to derive the variation of Poisson's Ratio with depth. We find, for a location that experiences positive air temperatures during some summer days, that Poisson's ratio is around 0.2 at the surface, rises rapidly to around 0.3 over the first ten meters, and then rises more slowly to a maximum of 0.34 at 31.5 m depth. Our greatest depth of investigation was 50 m where Poisson's ratio was 0.326, close to the theoretical value for ice at –10° C.