Laboratory measurements of high-frequency broadband acoustic backscatter from sea ice, oil under sea ice, and oil encapsulated in sea ice

To investigate the potential for detection of crude oil under sea ice using active acoustics techniques, measurements of high-frequency broadband backscattering (75–590 kHz) from crude oil of different thicknesses (0.7–8 cm) under, and frozen within, laboratory sea ice have been performed at the Cold Regions Research and Environmental Laboratory (Hannover, NH). Backscattering measurements were performed at normal and 20 degrees from normal incidence. The data have been analyzed in both the temporal domain and in the frequency domain, allowing scattered spectra from the oil and ice to be measured. The results show structure consistent with scattering from multiple interfaces following the injection of oil under the ice and during the subsequent encapsulation the oil layer. The acoustic estimates of oil thickness are in general agreement with ancillary measurements. The sound speed of the crude oil was separately measured over a range of relevant temperatures, both to inform a scattering model and to accurately infer the oil thickness. Predictions based on a simple scattering model for the frequency-dependent reflection coefficient of oil under ice agree well with the normal incidence measurements prior to oil encapsulation. At angles off normal incidence, volume inhomogeneities appear to dominate the scattering.