Monitoring the Elastic Properties of Ice with Resonant Ultrasound Spectroscopy

Abstract:

The elastic properties of ice are of interest in understanding (the
evolution of) sea ice, glaciers and ice sheets, in general. Such data
are crucial if we are to use elastic (ultrasonic to seismic) data to
constrain the internal structure and fabric of ice bodies and their
environmental conditions. Fabric (crystallographic preferred
orientation) and temperature are two key factors that control the
rheology of ice sheets. Fabric and temperature data at depth are
limited to the very small number of ice drill holes in Antarctica and
Greenland, mostly at ice divides. Thus, there is a need to develop our
understanding of elastic properties and wave propagation in ice to
extract better ice information from seismic data sets.

Resonant Ultrasound Spectroscopy (RUS) is used to measure the resonant
modes of samples, from which we can invert for the full elastic
tensor, and estimate the attenuation quality factor. Compared to more
traditional time-of-flight ultrasound measurements, there a few
obvious advantages. First, RUS is typically done at an order of
magnitude lower in frequency, which brings it closer to seismic
frequencies. This is important when attempting to map out the
dispersive nature of these elastic properties. Second, it is often far
from trivial to pick the shear wave arrivals in ultrasound in
heterogeneous media. RUS does not rely on this distinction
between primary and shear wave.

After having developed and applied RUS successfully to rock
samples, an extension of RUS to ice cores in the Physical Acoustics
Laboraty shows great promise. For example, we successfully inverted
for the isotropic parameters (bulk and shear modulus) of crystalline
man-made ice, and estimated the attenuation quality factor Q. By
controlling the freezer settings in the set-up, we were able to
monitor changes in these properties as a function of temperature. The
resultant data are consistent with published results from other
approaches in the laboratory and the field.

RUS is sufficiently fast and portable that it could be applied in
remote field sites (in Antarctica for example), negating the need to
send large sample volumes back to the laboratory. Application in the
field is particularly important for complex systems like sea ice,
where the physical properties are likely to change as a function of
time and temperature.