Bistatic Acoustic Scattering from Munitions: Interactions with Nearby Clutter and the Water-Sediment Interface
Dr. Steven Kargl | Applied Physics Laboratory, University of Washington
The primary objectives of the project are: (1) the development of full bistatic scattering simulation capabilities; (2) an investigation into the interaction of an incident acoustic field with a target and nearby clutter; and, (3) the generation of simulated data in the development of classification algorithms for the Multi-Sensor Towbody (MuST) for Detection, Classification, and Geolocation of Underwater Munitions project ( MR18-5004). Objective (1) is required due to the relatively large spatial separation of the acoustic sources from individual elements in the receiving array of the MuST in comparison to the expected stand-off distances from ordnance near the water-sediment interface. When a target of interest is in close proximity to a clutter (e.g., losbster pot, boat anchor, etc), then the acoustic scattering from the target and clutter can interact, which can result in a complicate, composite target signature. Objective (2) adds the second, and possibly third, order multiple scattering ray paths to the simulations to account for the interaction of the target and clutter. The multiple scattering geometry is an inherently bistatic geometry, so progress on objective (1) is a requirement for objective (2). Currently, little to no experimental data are available for the MuST scattering geometry or frequency band. Objective (3) provides simulated data to aid in the development of detection and classification algorithms for MuST.
The target-in-the-environment-response (TIER) model is a high-speed and high-fidelity model that predicts the scattered acoustic signature of a target near a water-sediment interface. The recent Clutter Experiment 2017 (CLUTTEREX17) has shown that a target’s acoustic signature can be significantly affected by a clutter object in close proximity. To understand this interaction between target and clutter object and its potential impact on using simulated data for the training of classification algorithms, full bistatic acoustic scattering becomes important. In addition, if a target penetrates the water-sediment interface at an oblique angle (e.g., nose of a munition is in the sediment and its tail is in water), the interaction of the acoustic field with the water-sediment interface and target becomes a fully bistatic scattering scenario.
Full bistatic acoustic scattering capabilities provide several benefits. Training and testing of detection and classification algorithms require large sets of data for a number of diverse environments. Collection of these data can be both time-consuming and expensive with current sonar platforms. The TIER model can model acoustic scattering from isolated targets. However, a more sophisticated, bistatic, model is needed for conditions observed in CLUTTEREX17—namely, a target near a clutter object. Bistatic scattering is also required when the axis of symmetry of a cylindrical symmetric target is no longer parallel to the water-sediment interface or if the source and receiver are not co-located. Thus, development of a bistatic acoustic capabilities permits numerical simulation to augment available field data under more general conditions.
In addition, for new sonar platforms currently under development to address the munitions remediation problem, simulation capabilities will aid in assessing design decisions. Furthermore, model-data comparisons, as systems come on-line, will allow identification of deficiencies and/or improvements to the systems. Finally, the current TIER model and a model with full bistatic capabilities can be used to investigate the differences in acoustic signatures of an intact munition and a munition that has been altered (e.g., missing its nose).