THE EFFICACY OF INTERFEROMETRIC SONAR TO MAP THE SEAFLOOR IN VERY SHALLOW WATERS: CAPE COD NATIONAL SEASHORE AND VICINITY

The development and improvement of interferometric sonar has allowed investigators to more efficiently map the seafloor in very shallow waters (<10m). A 3 year pilot project funded by the state of Massachusetts is underway to determine the efficacy of using this technology to develop nearshore resource characterization maps in portions of Cape Cod Bay in and around Cape Cod National Seashore.

The interferometric sonar system for this project collects coincident, high resolution imagery and bathymetry at 200 kHz. The unit is pole-mounted on to the bow of the research vessel. A real time kinematic Global Positioning System (GPS) is used for positioning and tide corrections. A dual antenna, differential GPS provides vessel heading and a motion reference unit gathers information primarily on vessel pitch and roll. A sound velocity probe collects data during surveys to increase the accuracy of the two-way travel time of sound through sea water.

Initial reconnaissance surveys conducted in the fall of 2009 in 1-5 m of water have been encouraging. Over 100 km of survey lines have been collected. Swath width to depth ratios have reached 10:1 in these shallow waters. Bedforms spaced at approximately 20-30 cm are readily discernible in the sidescan imagery. Uncertainty analyses are ongoing but initial results are similar to those attributed to this system for across track resolution: 4-5 cm in the sidescan imagery; and 5-6 cm in the bathymetry. Similar results are expected for the vertical resolution of the bathymetric data.

In the 2010 field season data will be collected to produce a seamless, tide-coincident, onshore-offshore map. Using terrestrial lidar the beach-dune system will be mapped at low tide followed by a vessel-based survey at high tide. The area to be mapped in Cape Cod Bay has a 3 m tidal range which will facilitate the spatial overlap of the lidar survey and the vessel-based survey throughout most of the study area.