MAPPING THE ESTUARINE SEAFLOOR WITH VESSEL-BASED ACOUSTIC INSTRUMENTS: THE SHALLOWEST WATER SURVEY

Estuaries are among the world’s most productive ecosystems and mapping the estuarine seafloor can provide useful information with regards to benthic habitats, ecosystem state, sediment transport and other biological and physical characteristics and processes. Turbid waters in many estuaries prevent optical methods, such as lidar, from being used. In addition, lidar, as of yet, provides only elevation information, though experimental data layers similar to acoustic backscatter imagery are in the early stages of development.

A series of vessel-based acoustic surveys onboard a custom-built, shallow draft pontoon boat using a Phase-Measuring Sidescan Sonar in a very shallow, tidally-restricted estuary were conducted in June 2016. 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. Underwater video and grab samples were collected to ground-truth the backscatter imagery and improve our understanding of the seafloor. Water level loggers we installed to determine the tidal symmetry and range to refine survey planning and data collection efficiency. Salinity measurements were made throughout the estuary.

Three vessel-based acoustic surveys were conducted between 24-26 June, 2016 in the Herring River Estuary Wellfleet, Massachusetts. Salinities ranged from 32-33 ppt at the first tidal restriction to 0 ppt at a small box culvert upriver. Over 9.2 km of acoustic data were collected along 45 survey lines, yielding over 10.7 ha of estuarine seafloor mapped. The average water depth was approximately 1 m. The sonar regularly collected 30-40 meter swaths of backscatter imagery in <1m of water. Swath bathymetry was collected and the typical swath width to depth ratio was 6:1 – 7:1. The instrument was able to map in high salinities, fresh water and the transition zone with little to no loss in data quality. This instrument/platform combination makes vessel-based acoustic mapping in these types of systems more feasible, efficient, and desirable.