Scattering from sub-critically insonified buried elastic shells

Abstract

Buried mines have been so far generally considered undetectable by conventional high frequency sonars mostly due to the low levels of energy penetrating into the sediment at high frequencies. Furthermore, in a shallow water environment a high frequency high grazing angle mine-hunting sonar approach is vastly limited by the coverage rate, making the detection and classification of buried objects using subcritical grazing incidence an attractive alternative. In mine countermeasurements (MCM) applications resonance of man-made elastic targets is a pivotal concept that distinguishes them from rocks or other clutter that may have a similar mine-like shape. Nevertheless, burial of an elastic target in the seabed results in a variety of modifications to the scattered response caused by different physical mechanisms, geometric constrains, and intrinsic properties of the sediment. The goal of this thesis was to identify, analyze and explain the fundamental effects of the outer sediment and the proximity of the seabed interface on the scattering of sound from elastic shells insonified using low frequencies at subcritical incident angles.

(cont.) The approach to scattering from buried elastic shells consisted of the development and the evaluation of ways of computing the scattered field, and the interpretation of the physical events taking place. A class of numerical models called virtual source models was developed and used, which take into account and are able to represent all of the, to our problem, significant physical mechanisms comprising the scattering effect observable in an at-sea experiment. Time-frequency and array processing methods for extracting properties of buried target signatures that can be used to classify the targets based on their re-radiated returns, have been developed. A theoretical time-of-arrival tool was created and implemented to obtain the expected times of arrival of specular and elastic responses of buried elastic targets and therefore identify the target elastic waveforms. In addition, a novel focused beamforming approach that was formulated and implemented, was used to determine the elevation angles at which specular and elastic returns emerge from the seabed into the water-column.

(cont.) Through the analysis of GOATS98 experimental data, validation of target scattering models, and hypothesis validation, the frequency and the amplitude content as well as the times of arrival of target elastic response have been examined. For the first time in literature, a difference in the frequency content of clockwise and counter-clockwise Lamb wave components under subcritical insonification was observed and explained. Using focused beamforming, specular and elastic arrivals were identified, demonstrating the different elevation angles at which they emerge from the seabed into the water-column. As a result, a hypothesis about the physics of propagation of elastic waves under evanescent insonification was validated, thus confirming that the nature of the physical processes taking place can not be described using the traditional wave-tracing arguments, rendering them inadequate under these circumstances. Furthermore, related to the detection of fully buried targets, which was so far considered a challenging problem, the shown specifics of the structural waves radiation and propagation process provide a distinct way of determining the presence of these otherwise hardly detectable targets.