This project sought to demonstrate a capability to conduct wide area mapping of the distribution of munitions at shallow underwater sites (≤ 20 m depth), using commercial-off-the-shelf (COTS) high-frequency sonar technology and advanced signal processing techniques. The goal was to quantify and improve the probability of detection of unexploded ordnance that may be found on the bottom while providing the geomorphological context necessary to estimate the extent of impacted sites based on predictions of munitions mobility and burial. The long-term goal was to produce distribution maps of munitions for site managers to inform remediation decisions.

The technical approach of this study was centered on (1) data acquisition performed with an existing commercial state-of-the-art high-frequency multibeam echosounder owned and operated by the U.S. Naval Research Laboratory and (2) data processing using a series of algorithms that culminate in the production of maps of the distributions of munitions over the study area. The objective was to apply an in situ beam pattern estimation technique that achieved range and cross-range normalization while preserving the acoustic backscatter intensity measurement. To do this, the project team calibrated the backscatter measurements using standard reference targets (e.g., tungsten carbide sphere). The resulting acoustic backscatter intensity image was used for target identification as well as sediment characterization, the latter being key to providing the necessary context for munitions distribution.

A field experiment in the northern Gulf of America along the Mississippi coast just offshore of Pascagoula was executed. The field experiment encompassed an approximately 4 km² region in 10 m water just south of Horn Island, MS. An acoustic target string composed of inert munitions was deployed in the study area. Additionally, four small instrument moorings were placed around the survey region to collect contextual wave, current, and environmental parameters (e.g. sound speed) critical to signal processing. A series of surficial sediment push cores were collected around the mooring and target string by divers and transported back to the lab for analysis. Due to technical issues with the selected sonar system, the project team was unable to proceed beyond field experimentation to the advanced processing task nor to demonstration and transition.

Despite technical issues, novel methods and present recommendations to improve multibeam sonar survey data quality were demonstrated. Specifically, the ability to (1) correct for vessel squat using an easy-to-implement approach, (2) recognize and address systematic sound speed and heave errors, and (3) conduct repeatable patch test corrections over non-ideal (i.e. low relief) seabeds were demonstrated. Further, the importance of COTS high-frequency sonar systems as useful tools for site characterization was demonstrated. The ability to normalize backscatter and link the backscatter to surficial and shallow subsurface geotechnical properties improves the ability to predict burial depth, monitor for site changes that may result in transport or unburial, and inform detection, classification, and localization surveys. (Project Completion - 2025)