Direct inference of first-year sea ice thickness using broadband acoustic backscattering

Abstract

Accurate measurements of sea ice thickness are critical to better understand climate change, to provide situational awareness in ice-covered waters, and to reduce risks for communities that rely on sea ice. Nonetheless, remotely measuring the thickness of sea ice is difficult. The only regularly employed technique that accurately measures the full ice thickness involves drilling a hole through the ice. Other presently used methods are either embedded in or through the ice (e.g., ice mass balance buoys) or calculate thickness from indirect measurements (e.g., ice freeboard from altimetry; ice draft using sonars; total snow and ice thickness using electromagnetic techniques). Acoustic techniques, however, may provide an alternative approach to measure the total ice thickness. Here laboratory-grown sea ice thicknesses, estimated by inverting the time delay between echoes from the water-ice and ice-air interfaces, are compared to those measured using ice cores. A time-domain model capturing the dominant scattering mechanisms is developed to explore the viability of broadband acoustic techniques for measuring sea ice thickness, to compare with experimental measurements, and to investigate optimal frequencies for in situ applications. This approach decouples ice thickness estimates from water column properties and does not preclude ice draft measurements using the same data.

Towards basin-scale in-situ characterization of sea-ice using an Autonomous Underwater Glider

“…Although the transmitted wave from seawater to sea-ice will reflect off of the air and partially transmit back into seawater, this pathway yields a weaker acoustic intensity compared to the direct seawater → air reflection. This analysis is confirmed in [Bassett et al, 2020]. Therefore, based purely off of the bulk properties of the three mediums, it can shown that the free surface yields a higher acoustic return than the thin sea-ice cover.…”

“…This process leaves behind hollow columnar sections known as brine channels. These brine channels are known to contain bubbles which then act as scatterers when the sea-ice is ensonified by an acoustic signal [Bassett et al, 2020]. In addition to brine channel structures, another important sea-ice structural element is the vertically aligned congelation ice crystals that Seawater is assumed to have a temperature of −2 ∘ C and a salinity of 32 g kg −1 .…”

“…form at the bottom of ice floes [Bassett et al, 2020]. These ice crystals lead to a complex ridging structure found at the bottom of sea-ice floes and are referred to as skeletal ice.…”

“…These bulk properties are crucial for understanding acoustic scatter intensities resulting from the interface of two mediums. Since the speed of sound difference between sea-ice and seawater is much greater than the density difference between the two mediums, the acoustic return intensity models are less sensitive to precise values for seawater and sea-ice density [Bassett et al, 2020]. Other notable properties of sea-ice, including mechanical properties, thermal properties, and electromagnetic properties, are discussed in [Schwarz and Weeks, 1977].…”

“…In the Arctic context, it has been shown that broadband acoustic backscatter techniques may be capable of detecting oil layers directly underneath the sea-ice cover when the sonar is operated with normal incidence with respect to the oil and sea-ice layer [Bassett et al, 2016]. Additionally, methods exist for estimating sea-ice thickness solely from acoustic backscatter, rather than considering freeboard altitude or keel depth [Bassett et al, 2020]. Thus, methods of characterization based on acoustic signature can be used in combination with sonar mapping techniques to establish a more detailed understanding of the environment.…”