The Effect of Internal Wave-Related Features on Synthetic Aperture Sonar

In October 2012, the Centre for Maritime Research and Experimentation (CMRE, La Spezia, Italy) conducted trials from the NATO research vessel (RV) Alliance, off Elba Island, Italy. During this trial, data were collected by the Norwegian Defence Research Establishment (FFI, Kjeller, Norway) using a HUGIN autonomous underwater vehicle (AUV) with interferometric synthetic aperture sonar (SAS) in repeated passes. Large linear structures (tens of meters by several meters) observed in both the SAS images and SAS bathymetry during the initial pass were absent in data taken on a repeated pass the following day. We suggest that these phenomena were not true seafloor features, but were caused by features in the water column, known as boluses, which can form after breaking internal wave events. The changes observed in acoustical intensity and phase appear to be caused by the interaction of the acoustical field with the lower average sound-speed structure of the bolus, constructing features in both SAS imagery and SAS bathymetry that looked like seabed topography. In this paper, we present examples and give an interpretation of the results based on an acoustical ray model. We discuss different techniques for recognizing these phenomena: repeat pass imaging and interferometry, multilook and multiaperture processing, and moving target analysis.