High-Resolution Acoustic Seafloor Mapping

ABSTRACT SeaMARC/S is a 150 kHz towed sonar mapping system that produces synoptic acoustic backscatter images and hydrographic-quality swath bathymetry charts in water depths to 1400 meters. The system is versatile, operating in a range of swath widths from 1000 meters for reconnaissance surveys to 50 meters for maximum resolution. It is also economical to operate because of its high rate of survey coverage and its ability to be easily transported between ships-of-opportunity. High resolution seafloor maps are becoming increasingly important as marine exploration and engineering efforts become more complex and expand into deeper water. INTRODUCTION Side-scan sonar systems have been used with considerable success for many years to provide images of seafloor features and as a search tool to identify objects on the seafloor. The best of these conventional systems digitally record the side-scan data for later processing and for corrections for survey ship and sonar array navigation. Some also apply simple approximations to correct for ray path variations and to calculate horizontal range from slant range. All these systems are limited because they can only measure the range of echoes, so they approximate the reflector position based on the range and the limits of the beam pattern. Thus conventional side-scan sonar systems yield only distorted images, not geometrically accurate charts, because they do not measure the true positions of sonar reflectors on the seafloor. As seafloor exploration and engineering projects become more complex, however, the need has arisen for mapping tools that can provide marine data which approximate the precision of maps on land. The SeaMARC/S (Sea Mapping and Remote Characterization/Shallow) has been developed to meet this need. SeaMARC/S is a 150 kHz towed sonar system that produces geometrically correct side-scan maps of the acoustic character of the seafloor. To produce these precise acoustic maps the three dimensional position of all seafloor reflectors is measured. This position is accurately determined in three dimensions, resulting in simultaneously-generated, hydrographic quality, swath bathymetry maps of the same ensonified swath of seafloor covered in the acoustic backscatter side-scan swath. These co-registered SeaMARC/S data sets are thus the marine acoustic analog of co-registered aerial photographs and topographic maps that are merged to accurately depict land areas. Just as accurate aerial photos and topographic maps are the fundamental background data for most large-scale engineering efforts on land, SeaMARC/S data provide the essential base maps for similar efforts at sea. SeaMARC/S DESCRIPTION The side-scan imaging portion of the SeaMARC/S is based on a SeaMARC CL developed by International Submarine Technology, Ltd. (1ST), of Redmond, Washington. In cooperation with IST, Seafloor Surveys International, Inc. (SSI), of Honolulu, Hawaii, has expanded the sides-can system by using multiple rows of transducers to measure the phase delay (and thus the angle of incidence) at the towed array from all echoes from the seafloor. The principles used in this measurement are similar to those employed in the University of Hawaii's SeaMARC II system, described in some detail by Blackinton and others (1983).