Abstract:
The propagation of low frequency seismo-acoustic waves in the Arctic Ocean ice
canopy is examined through the analysis of hydrophone and geophone data sets collected
in 1987 at an ice camp designated PRUDEX in the Beaufort Sea.
Study of the geophone time series generated by under-ice explosive detonations reveals
not only the expected longitudinal and flexural waves in the ice plate, but also an unexpected
horizontally-polarized transverse (SH) wave arriving at a higher amplitude than
the other wave types. The travel paths of all three observed wave types are found to be
refracted in the horizontal plane along a line coincident with a known ridge separating
the ice canopy locally into two distinct half-plates, the first of thin first year ice and the
second of thicker multi-year ice. The origin of the SH wave appears to be near the detonation
and not associated with the interaction of longitudinal, flexural or waterborne waves
with the ridge line. The need to determine the exact location of each detonation from the
received time series highlights the dramatic superiority of geophones over hydrophones in
this application, as does the ability to detect the anomalous SH waves and the refracted
ray paths, neither of which are visible in the hydrophone data.
Inversion of the geophone data sets for the low frequency elastic parameters of the ice
is conducted initially by treating the ice as a single homogeneous isotropic plate to demonstrate
the power of SAFARI numerical modeling in this application. A modified stationary
phase approach is then used to extend SAFARI modeling to invert the data sets for the
elastic parameters of the two ice half-plates simultaneously. The compressional/shear
bulk wave speeds estimated in the half-plates, 3500/1750 m/s in the multi-year ice and
3000/1590 m/ s in the new ice, are comparable to previously obtained values; however, the
compressional/shear attenuation values in the two half-plates, 1.0/2.99 d/Bλ. and 1.0/2.67
dB/λ, respectively, are somewhat greater than previously measured values and four times
greater than estimates extrapolated from high frequency data.
