Time-frequency sonar detection of elastic wave reradiation

Abstract

This work examines the effectiveness of quadratic time-frequency methods in exposing the presence of elastic wave reradiation in active sonar returns. Elastic wave reradiation here means the coupling of elastic surface waves from an object under sonar interrogation into the surrounding water, where it is recorded in the sonar return—reradiation, because the original excitation generating the elastic surface waves is assumed to be the active sonar transmission. A brief survey of quadratic time-frequency representations and their underlying theory is provided, and a novel representation is proposed. The suitability and performance of the novel and existing representations are analyzed and compared in view of this application. The analysis is supported by simulation and by processing of data collected in a sonar experiment. In this experiment, air-filled cylinders of steel, aluminum, and PVC with similar geometries were interrogated in a controlled acoustic tank by a series of sonar waveforms which together span 5 kHz to 150 kHz in frequency at a variety of incident angles. A summary of current acoustic literature guides the simulation design and informs conclusions. The accumulation of evidence and observations identifies two distributions with strong advantages in relation to the elastic wave reradiation application: the Born-Jordan and the proposed distributions. These demonstrably facilitate acoustic discrimination based on elastic surface waves.