Modelling acoustic transmission loss due to sea ice cover

• Main Authors: Alexander, PM, Duncan, A, Bose, N, Smith, D
• Format: Article in Journal/Newspaper
• Language: English
• Published: Australian Acoustical Society 2013
• Subjects: Engineering; Maritime Engineering; Ocean Engineering; Sea ice
• Online Access: http://www.acoustics.asn.au/joomla/journal.html; http://ecite.utas.edu.au/84567

The propagation of underwater acoustic signals in polar regions is dominated by an upward refracting sound speedenvironment and the presence of a dynamic highly variable ice canopy. This paper provides an overview of the acousticproperties of sea ice and assesses the influence of ice canopy and water column properties on acoustic transmission loss forpropagation within 20 km of a sound source at 20 m depth. The influence of the ice canopy is assessed first as a perfectlyflat surface, and then as a statistically rough surface. A Monte Carlo method is used for the inclusion of ice deformation androughness. This involves the creation of sets of synthetic ice profiles based on a given sea ice thickness distribution, followedby statistical methods for combining the output of individually evaluated ice realisations. The experimental situation beingconsidered in the framing of this problem is that of an Autonomous Underwater Vehicle (AUV) operating within 50 m of thesurface. This scenario is associated with a frequency band of interest of 9-12 kHz and a horizontal range of interest up to20 km. The situation has been evaluated for a set of typical ice statistics using Ray and Beam acoustic propagation techniques.The sound speed profile (based on real data) results in a strong defocussing of direct path signals at ranges from 9-20 km anddepths shallower than 50 m. This reduction in the signal strength of the direct path creates areas where the influence of surfacereflected paths becomes significant. The inclusion of a perfectly flat ice layer reduces the transmission loss between 9-20 kmby 15-50 dB. When the ice layer is included as a rough surface layer the results show a boost to signal strength of up to 6 dBin the small areas of maximum defocussing. Sea ice is a strongly time and space varying sea surface and exists in areas wheredefocussing of the direct path due to the sound speed profile reduces the range of direct path dominated transmission. Thiswork presents methods for including a statistically relevant rough surface through a technique for generation of sets of surfacesbased on ice deformation statistics. It outlines methods for including ice in acoustic modelling tools and demonstrates theinfluence of one set of ice statistics on transmission loss.