Bottom scattering by a moving, narrowband source

Abstract

During the Target and Reverberation Experiment in 2013 (TREX13), two vertical line arrays were deployed and a mid-water source was towed past these arrays while transmitting 15-s long, mid-frequency CW tones. While the received signals exhibited the expected positive and negative Doppler shifts as the source moved toward and away from the arrays, a significant amount of energy was always present in the band between these two frequencies. This counterintuitive result is due to sound scattering from the seafloor in the vicinity of the source. This sound will experience a different Doppler shift and although it may be incident above the critical angle, it can scatter into propagating modes that can be received on the arrays. As opposed to wide-band reverberation where the energy received at a given time can be associated with an elliptical scattering patch, in this case the energy received at a given frequency can be associated with scattering from a hyperbolic scattering patch. A normal-mode reverberation model for this effect has been developed and is used to examine whether this data can be used to invert for the seafloor scattering strength at the TREX13 site. [Work supported by ONR.]