SCIENCE AND MAPPING IN VERY SHALLOW COASTAL WATERS WITH A PHASE-MEASURING SIDESCAN SONAR

Globally some of the most energetic environments are open-ocean beaches and tidal inlets. Bathymetric data in these areas have been difficult and at times, dangerous to collect. Vessel-based acoustic surveys have typically avoided very shallow waters when swath bathymetry was needed. The swath width of the best multi-beam echosounders was usually cost-prohibitive or too time-consuming in very shallow waters. These limitations have been lessened considerably with the advent of Interferometric Sidescan Sonar and more recently with Phase-Measuring (or Phase-Differencing) Sidescan Sonar.

Currently, a second field season is underway with a Phase-Measuring Sidescan Sonar collecting data as part of a 3-year effort to map benthic habitats in shallow waters in Cape Cod National Seashore, Massachusetts. The first full field season was completed in the fall of 2014. Over 570 km of vessel-based acoustic data were collected within two shallow embayments (~24 km²) with a mean depth of approximately 3 m. The acoustic data is supported by hundreds of bottom grab samples, and underwater video, to ground truth acoustic data as well as to identify and catalog organisms for benthic habitat maps. The instrument collects coincident, dual-frequency, backscatter imagery (op. freq. 550/1600 kHz) and swath bathymetry (op. freq. 550 kHz). This yields three distinct, yet co-located data sets. The backscatter resolution for the 550 and 1600 kHz frequencies is 0.01 m and 0.006 m respectively. The bathymetric resolution is approximately 3 cm vertically and horizontally. This instrument has also been used in conjunction with a sub-bottom-profiler and a reconnaissance coring effort to detail shallow stratigraphy.