SERDP’s Munitions Response program is interested in identifying reliable techniques for detailed survey of unexploded ordnance (UXO) at underwater remediation sites that span a range of water depths. As emphasized during the SERDP Workshop on Acoustic Detection and Classification of UXO in the Underwater Environment, the UXO detection and localization problem is particularly important to solve in very shallow water (depth less than 5 meters) because of the increased chance for human interaction. While these water depths are critically important, they present unique challenges which make detailed surveys difficult. Many sensor systems used for underwater survey are either towed by boats or are carried by unmanned underwater vehicles. In very shallow water, it can be difficult to safely deploy these types of platforms. Shallow water environments also pose a particular challenge for acoustic survey sensors – multipath reverberation, which is similar to the background noise you might hear having a conversation in a stairwell, can mask the intended returns from objects on or in the seabed.
A research team at the Applied Research Laboratory at the Pennsylvania State University is leading an effort to address these problems in the SERDP project titled Sediment Volume Search Sonar Development. This research is conducted in collaboration with the Applied Physics Laboratory at the University of Washington and the U.S. Naval Research Laboratory at the Stennis Space Center. This team has developed a prototype sonar system that can conduct detailed ordnance surveys in water as shallow as a meter by mounting the sensor to a pontoon boat. This sonar system is downward looking and utilizes a technique called synthetic aperture sonar to create high-resolution, three-dimensional imagery of the seabed. Additionally, this sensor’s imagery localizes (latitude and longitude) suspected ordnance with an accuracy of less than 5 cm by using a real-time kinematic GPS. Pairing high-resolution imagery with high-accuracy localization provides SERDP with a sensor that can be used to conduct detailed surveys for ordnance in a very shallow water environment.
In early March 2020, the Sediment Volume Search Sonar prototype was tested at the Aberdeen Test Center. Surveys were conducted in the Littoral Warfare Environment and a neighboring river suspected to contain man-made clutter. The Littoral Warfare Environment test area consists of a sandy beach that was prepared by the Penn State team with a number of targets including inert ordnance and manmade clutter items. The river testing, conducted in an unprepared site, afforded the SVSS an opportunity to collect data in a real-world environment with a soft mud river bed and a range of natural and manmade clutter objects. An early result from this test is provided in the pair of figures below. These two figures show a pair of slices through one of the three-dimensional images created by the sonar system. The depth slice is taken 40 cm into the sediment, and a pair of clutter objects are easily visible. The corresponding cross-track slice also passes through these two objects. In both figures the targets are easily detectable above the background. Using this type of imagery it will be possible to accurately detect deeply buried objects in very shallow water depths.