Detection and identification of underwater Unexploded Ordnance (UXO) is a significant problem in remediation efforts at formerly used defense sites. Presently there is no system available that can accurately survey and map the location of UXO at underwater sites and reliably discriminate UXO from debris. The UXO survey problem is complicated by a wide range of targets as well as clutter both natural and man-made. The objective of this work was to improve the ability to remediate underwater sites by developing a system for the detection and classification of proud, partially buried, and buried ordnance in shallow (< 5 m) and moderate depth (< 100 m) water. The focus of this research was to develop algorithms and design an acoustic array that will image buried UXO with sufficient detail to measure their length, diameter, and shape. These metrics can be used as features for a standalone UXO classifier or as an input constraining the solution of a classifier using acoustic color map features. This improved Buried Object Scanning Sonar (BOSS) technology, which is the EdgeTech BOSS (eBOSS), will increase the probability of detecting and classifying objects that have settled or have been buried in shallow water sites.
The BOSS systems are a family of low frequency penetrating imaging sonars that employ a wing that is populated with an array of hydrophone sensors, which uses synthetic aperture sonar beamform processing to create a 3D image of buried objects. The eBOSS is the successor to the original BOSS systems and is free from the International Traffic in Arms Regulations restrictions that limited the application of the original BOSS systems. The eBOSS consists of newly developed hardware and software created using internal funding.
Low frequency sonars such as eBOSS and BOSS cannot produce target imagery suitable for recognizing/differentiating UXO from other buried clutter because those single source systems generate incomplete target imagery, a result of the limited target views. At certain target aspects, the sonars will measure echoes off only a target end providing no target shape or dimensional information. Low-frequency sonars can provide the raw inputs to produce acoustic color maps or other structural acoustics for target classifiers.
This effort defines a strategy to improve the imaging performance of a future eBOSS-type system, which is targeted for UXO remediation. The key improvement over existing systems is the utilization of multiple sources to illuminate all sides of each target and to allow generation of imagery for measuring size and shape metrics as well as acoustic color maps. Multi-source ensonification of a target field will create full 360-degree viewing angles of buried objects, which is valuable because most targets have specular returns related to their shape and orientation.
Additional improvements over previous versions of the BOSS have also been identified. Image resolution will be improved via higher bandwidth sources and basic enhancements to beamforming algorithms. The research team showed that refraction corrections and real-time measurements of sediment sound velocity are necessary for producing usable images of buried UXO. They also present a method for measuring the critical angle which yields the sediment sound velocity. Shallow water operation will be improved by suppressing surface reflections and multipath interference by employing baffling and directional sources.
To overstate the obvious, an all aspect angle ensonification strategy requires that transmit sources be placed on all sides of targets in a coverage swath. This means that a future system will require a longer wing aperture with multiple transmit sources spaced across this aperture.
The across-track wing aperture of this future system will be populated with multiple rows of hydrophones. The multiple rows are desirable to supplement the along-track Synthetic Aperture Sonar aperture with a physical aperture to maintain ½ wavelength sample spacing along track. As the minimum velocity increases and/or the number of transmit sources (which are operated round robin) increases, the number of hydrophone rows required must also increase to maintain ½ wavelength spacing along track because there is a maximum ping rate. The maximum ping rate is constrained by the required slant range recorded per each ping.
The future eBOSS system will have higher data collection rates and related higher channel count hydrophone arrays. A straight forward strategy will be proposed to achieve this higher channel count system using segmented/modular panels and etched polyvinylidene fluoride. The manufacturing process for such an array will need to be validated and stress tested for reliability.
As a next step system, the research team proposes to enhance the SERDP funded Applied Physics Lab / University of Washington Focus vehicle, which already contains an eBOSS system, with next generation imaging features so that the concepts described here can be tested with real-world field data.
This study was guided by the belief that superior classification can be achieved by a system that guarantees the sensing of all specular scattering from UXO. It proposes a hardware architecture that can meet this objective. What is the combination of hardware, signal processing data products (e.g., images and acoustic color), and operational strategies needed to meet the metrics defined by the stake holders associated with SERDP/ESTCP? The final answer to this question will undoubtedly require an iterative process relying on in-the-field lessons learned as the current generation of eBOSS systems is tested. The research team believes this report details methods that will remain valuable throughout this process.