Acoustic Scattering from Elastic Ice: A Finite Difference Solution

• Main Author: Fricke, J. Robert
• Other Authors: WOODS HOLE OCEANOGRAPHIC INSTITUTION MA
• Format: Text
• Language: English
• Published: 1991
• Subjects: Snow; Ice and Permafrost; Acoustics; *ARCTIC OCEAN; *ACOUSTIC SCATTERING; *FINITE DIFFERENCE THEORY; *ICE; SCALE MODELS; MODELS; AIR; WATER; MATERIALS; EDGES; MASS; EXCITATION; REASONING; DISPLACEMENT; STRUCTURES; CURVATURE; ULTRASONICS; PLATES; SCALE; BROADBAND; PRESSURE; SLOPE; PARTIAL DIFFERENTIAL EQUATIONS; DIFFERENTIAL EQUATIONS; FLUIDS; POWER; WORK; EQUATIONS; ROUGHNESS; WAVES; RIDGES; CONTRACTS; SCATTERING; ULTRASONIC FREQUENCIES; FREQUENCY; PROPAGATION; Arctic; Arctic Ocean; Ice; permafrost
• Online Access: http://www.dtic.mil/docs/citations/ADA248281; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA248281

In this thesis I consider acoustic scattering from Arctic ice. No analytic scattering theories are able to explain the observed low frequency (10- 100 Hz) loss measured in long-range propagation experiments. Reasoning that a full-wave solution holds the key, I use the finite difference method to solve the elastodynamic equations. This technique allows arbitrary roughness, unrestricted in slope, displacement, or radius of curvature and provides direct, physical insight into the rough ice scattering mechanism. The underlying partial differential equations treat the air-ice-water complex as a heterogeneous continuum in three dimensions. A plane strain approximation reduces the formation to two dimensions, which limits the computational burden, while permitting the salient features of the scattering process to be studied. Broadband numerical scattering experiments are conducted using two classes of roughness features: ice edges and pressure ridges. Specular loss due to an ice edge is found to be too low to explain the observed scattering loss in the central Arctic. The loss due to a ridge is affected by three phenomena: mass loading, excitation of plate waves, and a material dependent power law. The first two affect the magnitude of the loss, while the last affects the frequency dependence. The observed loss is best explained by scatter from relatively young, large pressure ridges, which are modeled as fluid structures. The fluid model is appropriate because young ridges are loose aggregations of ice blocks and hence cannot support shear strain. Multi-year ridges, in contract, are completely frozen and are better modeled as elastic structures.