Subbottom swath mapping with HAWAII MR1

The HIGP Acoustic Wide Angle Imaging Instrument Mapping Researcher 1 (HAWAII MR1) swath mapping sonar system was used to conduct an experiment applying synthetic aperture sonar (SAS) techniques to imaging objects beneath the seafloor. Potential applications of this technology include locating buried toxic wastes and removal of unexploded ordnance from weapons test ranges. The prime contractor Alliant Techsystems (ATK) subcontracted to the Hawaii Mapping Research Group at HIGP to supply the sonar system used in the experiment. The objective was to combine the bottom-penetrating 12 kHz frequency and differential phase technique of HAWAII MR1 with SAS signal processing to enhance resolution and enable the detection and identification of buried objects over a swath of seafloor. HAWAII MR1 was originally designed for measuring acoustic backscatter and bathymetry in deep ocean across swaths up to 25 km wide. A number of modifications were necessary to adapt the system to this experiment. ATK provided a linear hydrophone array and two projectors, which were installed in the tow vehicle. The control and data acquisition electronics were modified to increase pulse repetition rates and decrease transmitted pulse widths while maintaining a constant phase with respect to the carrier frequency. The HAWAII MR1 attitude and depth sensors were supplemented with an Inertial Measurement Unit (IMU) and Conductivity/Temperature/Depth sensors (CTD) supplied by ATK. HAWAII MR1 was towed at altitudes of 5-15 meters over a target field having a variety of targets proud and buried at depths of 1-4 feet. SAS processing carried out by ATK revealed distinct returns from cylinders beneath up to two feet of sand