QUANTIFYING POSITIONAL ACCURACY OF SIDESCAN SONAR IMAGERY FOR NEARSHORE SHALLOW WATER SURVEYS

The 2-dimensional positioning of targets within a sonogram were shown to range from ~0.10 – 0.45 m. The use of a dual-frequency (550 and 1600 kHz) phase-measuring sidescan sonar and ancillary instruments have improved the localization of targets within these data sets. These types of acoustic instruments collect bathymetric data in addition to sidescan imagery and as such have highly accurate data streams including RTK-GPS position and vessel heading information as well as heave, pitch and roll data. Data presented here were collected in Cape Cod Bay, Massachusetts, USA, on a sandy intertidal flat with ~3 m of tidal range. At low tide, the intertidal area, ~1,500 m², was seeded with targets, positional information on the targets was collected using an RTK-GPS instrument and Uncrewed Aerial System surveys of the intertidal flat were also completed. At the next high-tide, a vessel-based acoustic survey was conducted, and the targets were identified in the sonograms. Targets were repeatedly surveyed in multiple directions and distances from nadir. Approximately 3 hours of acoustic surveys were completed resulting in 24.25 km of sidescan data. Survey depths ranged between 1-2.5 m. At the subsequent low tide the targets were re-surveyed with the RTK-GPS instrument to determine if the targets shifted. Individual targets were identified multiple times in both frequencies, n = 895 in 550 kHz, n = 1,122 in 1600 kHz. Localization was optimal when distance from nadir was between 1-11 m. Overall, positioning accuracy had a median value of 0.31 m ± 0.25 m for both the 550 and 1600 kHz frequencies. The use of intertidal surveys, RTK-GPS instruments and precise motion data allowed us to create guidelines and workflows to maximize our confidence in target localization in sidescan sonograms.