There are two components that make up this project. The first component involves detection of surrogate unexploded ordnance (UXO) in surf-zone or near surf-zone conditions with an unmanned surface vehicle (USV) based interferometric sidescan sonar, which is a proven technology in calm water. By quantifying sonar performance relative to sea state conditions with a relatively simple sonar system, a basis for performance of more complex sonar systems capable of detecting buried targets can be formed. In addition to UXO detection, the performance of the system for bathymetric mapping will be quantified as a function of sea conditions in and near the surf-zone. The second component involves demonstration of modelling capabilities for UXO mobility and burial in sandy surf zone environments. The project team will focus on low density munitions as they are most likely to migrate. The performance of the models will be evaluated by measuring the migration of surrogate UXO (sUXO) munitions deployed in a cross-shore array spanning locations from the inner surf zone to well outside the surf zone.
A cross shore array of sUXO spanning the inner surf zone to well offshore of the break point, with a depth range from 2 to 8 meters, will be deployed in calm to moderate wave conditions. The sUXO will have internal sensors to measure waves and depth via pressure and burial via a light sensor, and tethered active pingers. A wave buoy will measure incoming waves and sensors will be deployed at several locations across the surf zone to measure current and local wave conditions. Immediately afterward, deployment surveys will be conducted using a recently developed surf capable USV with an interferometric sidescan sonar. The surveys will attempt to detect unburied sUXO using the backscatter data and will map bathymetry in different regions of the surf zone which have different wave conditions due to bathymetry and wave shoaling. Additional surveys will be conducted in slightly more energetic conditions to determine the statistics of detection as a function of wave conditions. The deployments will take place in fall, winter, and spring on the south shore of Martha’s Vineyard, MA, which has intermittent energetic wave events that can cause mobility of low density sUXO. After an energetic wave event, the sUXO array will be resurveyed with a passive acoustic system on the USV that can localize the tethered pingers with 100% detection probability even if the sUXO are buried. This will allow measurement of sUXO migration and burial during the energetic wave event. Based on previous data (two data sets with different wave conditions but the same initial deployment depth of 3 to 4 meters) the project team has developed preliminary models for predicting UXO mobility and burial by coupling existing wave resolving nearshore hydrodynamic numerical models with a parameterized munition mobility post processing module. Initially the project team will run these models in a predictive mode by forcing the model with offshore wave conditions and by using parametrizations based on the previous data. This will allow the project team to determine if the parameterizations developed from a limited data set can predict mobility over a wider range of conditions and initial deployment locations. If the models are not successful in predictive mode, additional mobility parametrizations, hydrodynamic and morphodynamic forcing models will be investigated.
The results of the detection measurements with a relatively simple acoustic system will be useful to inform developers of more complex systems on the potential for acoustic measurements in surf zone conditions and will provide a useful unmanned system for wide area surf zone surveys, bathymetric, and UXO in areas where targets are expected to be unburied and detectable by sidescan sonar. The deterministic models that the project team plans to validate with in situ data on sUXO mobility will represent some of the first models that have been tested with a wide range of measured data. They can be used for input to probabilistic models of UXO mobility and burial such as Underwater Munitions Expert System by running with a large variety of initial conditions for ensemble modelling, or used directly for short term deterministic modeling to answer specific questions such as will a specified density UXO migrate toward the beach in response to specified wave conditions, with known initial conditions?