Coherent Backscatter Enhancement from Finite Sized Aggregations of Scatterers

Abstract

When sound is projected into the ocean, the backscattered signal may provide information about the object(s) from which the sound has scattered. When the backscattered sound comes from an aggregation of strong scatterers, such as a school of fish at their swim bladder resonance frequency, a phenomenon known as Coherent Backscatter Enhancement (CBE) may occur and this phenomenon could aid in discriminating fish schools from other scatterers in the ocean water column. When CBE occurs, the addition of the in-phase path pairs enhances the scattered field intensity by as much as a factor of two in the direction opposite to that of the incident wave. This thesis describes the results from simulating aggregations of randomly placed scatterers using the Foldy (1945) equations. The simulations are verified and validated by checking conservation of acoustic energy and by replicating results from near-field optics and acoustics CBE experiments. Simulations of far field CBE from aggregations of scatterers are presented and compared with far-field backscattering from ideal spherical scatterers. Parametric scalings for the height of the CBE peak are proposed for both near- and far-field geometries to relate the extent of multiple scattering effects to aggregation properties. Possible means are presented for distinguishing when backscattered returns come from a school of fish or from a single large scatterer.