Swath bathymetry and the requirement for handling multiple same-time arrivals

Swath bathymetry sonar is used to obtain profiles of the bottom by estimating the arrival angle of the backscatter for a given range. Simple interferometric sonars estimate the angle by measuring the phase difference between two receive elements. This procedure works well if the backscatter is from a single area of the bottom and there are no competing signals. If, however, two or more signals from different directions arrive at the elements at the same time, the resulting phase difference will be in error, and hence, the estimated bottom location will be in error. This paper describes typical survey situations where multiple same-time signals are likely and provides examples from survey data. The situations discussed include shallow water applications where multipath signals and surface scattering are present, and the situation where two bottoms are present within the profile. Both towfish and pole mount deployment configurations are considered. It is concluded that a swath bathymetry system must be able to operate in the presence of at least two same-time signals, and preferably three or four. Mitigation techniques are useful for eliminating multipath and surface scatter, but multiple bottoms must be estimated simultaneously requiring the swath bathymetry system to estimate multiple angles simultaneously.