Characteristics of the SeaMARC II phase data

Abstract

SeaMARC II is an interferometric, shallow-tow, long-range side-scan sonar system that is capable of producing bathymetry as well as side-scan images of the bottom in real time. To produce the bathymetry, phase differences between the echoes received by two linear acoustic arrays are measured. The phase difference data, sampled in travel time, show a slightly skewed, near-Gaussian distribution. The ambient noise and signal interference from the sea surface seem to cause increase of the phase data spreading. For SeaMARC II, the following are true: (1) Both the surface reverberation and the indirect echo path via water surface cause significant phase dispersion throughout the echo return time; (2) ambient acoustic noise is the main limiting factor for the angular resolution in a deep-water survey; and (3) the phase sample distribution is near-Gaussian and skewed in time. The relationships of these factors to the induced spread of the phase data are discussed. The current scheme for producing bathymetry from the phase difference data is discussed in the light of signal interference and phase sample distribution. Based on the Gaussian and isotropic ambient noise assumption, a simple analytical equation for the SeaMARC II phase data spreading is derived as a function of sonar parameters including the bottom backscattering coefficient. [Work supported by ONR.]