Detection of a submerged object insonified by surface noise in an ocean waveguide

Abstract

A theory is developed and applied to determine whether an object submerged in an ocean waveguide and insonified only by surface-generated noise can be detected with conventional sensing arrays. An expression for the total noise-field covariance of a stratified waveguide with a submerged object present is derived using full-field wave theory. This is evaluated by numerical wave-number integration for a spherical object in a shallow water waveguide. The Cramer–Rao lower bound on detection error is computed for several realistic shallow water scenarios at both low and high frequency. The results indicate that cross-range localization is possible when the array aperture is sufficient to resolve the object scale. This conclusion is supported by beamforming simulations. Range localization is possible at greater distances. However, this requires high correlation between direct and scattered noise fields at the sensor, which is difficult to replicate via matched field processing. In addition, wave theory indicates that high resolution imaging of reflected ambient noise is generally most effective within the deep shadow range of the object. Beyond the deep shadow range, diffractive interference from the total forward field may overwhelm reflections, depending upon the incident noise directional spectrum and measurement range. Overall, present analysis indicates that the proposed detection scheme presses the limits of current technology.