Acoustic Surface Backscatter vs Incidence Angle from Glacial Ice

Abstract

Surface acoustic backscatter angular response is commonly used for seafloor sediment characterization, however composite models of this type do not yet exist for the backscatter angular response of sea ice. For seafloor measurements, the response near normal incidence is often dominated by a specular return and influence from returns from the volume beneath the water-sediment interface, which is highly dependent on substrate grain size, acoustic impedance, and porosity. At oblique angles beyond the water-sediment critical angle, seafloor roughness due to grain size and bedforms dominate the response. The combined angular response can therefore be highly suggestive of the composition of the seafloor. For sea ice, at angles beyond the critical angle, roughness models similar to those used for the seafloor are applicable; however, near normal incidence angles, models remain largely undeveloped. With the development of appropriate models, acoustic measurements may allow discrimination of ice characteristics such as ice air content, interstitial fresh-water content, sea-ice age and ice origins (i.e., terrestrial vs. marine). Similar to seafloor measurements, the combined angular response of sea ice may provide a remote sensing method for widespread mapping and characterization of ice from surface and submersible vessels. In 2011, as part of an effort to gather information on the precise shape of large tabular icebergs to calibrate the Canadian Ice Service ice hazard drift and deterioration model, portions of the periphery of several pieces of the Petermann Ice Island, were surveyed with a Kongsberg EM3002 multibeam sonar. In addition to the geometric data, the collected measurements provide a means to measure the surface acoustic backscatter response of the ice surface as a function of angle of incidence, providing a preliminary characterization data set of tabular ice islands. While it is difficult to draw conclusions at this early stage, models for the seafloor provide some insight to the data. For example, acoustic models at oblique angles compare with those whose bedforms and grain size correspond to coarse sand. This prediction may result because bedforms supportable by sand may be comparable in size and shape to the scalloped surface of the ice observed visually near the surface. Acoustic remote sensing adopted to sea ice in this way, may provide a method for wide spread mapping of the processes with which ice forms, melts and evolves in the ocean.

Department

Center for Coastal and Ocean Mapping

Publication Date

6-2013

Journal Title

FEMME-2013

Conference Date

Jun 16 - Jun 19, 2013

Publisher Place

Boston, MA, USA

Document Type

Conference Proceeding

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