Modelling sound propagation under ice using the Ocean Acoustics Library's Acoustic Toolbox

• Main Authors: Alexander, P, Duncan, A, Bose, N
• Format: Conference Object
• Language: English
• Published: Australian Acoustical Society 2012
• Subjects: Engineering; Maritime Engineering; Ocean Engineering; Antarctic; Arctic; Antarc*; Sea ice
• Online Access: http://www.acoustics.asn.au/conference_proceedings/AAS2012/index.htm; http://ecite.utas.edu.au/82700

Acoustic propagation in the Arctic and Antarctic is largely characterised by the presence of a highly variable ice canopy.To model sound in these environments requires both a way of effectively representing the ice layer and modelling its effecton signal transmission. The Ocean Acoustics Library has a powerful open source Acoustics Toolbox that contains Fortrancode for running Ray, Normal Mode, and Wavenumber Integration models. There are two parts to modelling a sea iceenvironment: modelling the ice as an elastic acoustic medium, and modelling the roughness of the ridging characteristicsof the ice. This work considers the scenario of an Autonomous Underwater Vehicle (AUV) producing a survey underridged sea ice. This specifies a range of interest of 10km and a frequency band of interest of 3kHz-13kHz. An overviewof methods for modelling ice as an acoustic medium and as a ridged surface is provided, and the applicability of differentpropagation and ice models for this scenario is discussed. The scenario is then implemented as a specific test case for twoexample ice canopy profiles. The ice canopy profiles used are sea ice draft measurements recorded in the Arctic using anupward looking SONAR on a nuclear submarine. Beam and ray methods are the only computationally fast propagationcodes for this frequency range and are included in the BELLHOP module of the Acoustics Toolbox. With these methodsthe options for including the elastic properties of the ice are limited and only include reduction in the coherent field onreflection. Two methods for including the ridging of the ice canopy are implemented, one statistically based and one usingdirect input of measured ice canopy data. The statistically based method uses Twersky boss scattering, and the directmethod inputs the draft data as an altimetry file. Gaussian beam tracing using BELLHOP is run to generate ray trace andcoherent transmission loss estimates of this environment. The advantages and limitations of these implementations arediscussed with suggestions for future improvements to the Acoustics Toolbox to better model the ice scenarios outlined.The improvements identified from this review and test case are: the capability to include specific ice condition data whereavailable, better consideration of the elastic properties of the ice in BELLHOP; and new statistical methods for modellingunknown variable surface boundaries that provide statistical distribution information as well as mean field values.