Advanced SAS Imaging Methods for Resonating Underwater Acoustic Targets
MR-2322
Objective
The U.S. Army Corps of Engineers and the U.S. Navy have identified more than 400 underwater sites that are potentially contaminated with munitions, and most are in shallow water posing a threat to human safety and the environment. Similar to dry land unexploded ordnance (UXO), successful and economical remediation of these sites requires a robust combination of sensor technologies and effective signal and image processing algorithms that are able to detect munitions with high probability, but also reject clutter with minimal false alarm rate. In pursuit of this goal, this project aims to develop a new holographic imaging capability designed to highlight and spatially localize key features, namely acoustic resonances expected in hollow targets, both buried and proud, which strongly discriminate between UXO targets and clutter. An advantage of the method is that it will make use of existing data collection technologies, such as the Buried Object Scanning Sonar (BOSS), greatly reducing the cost of implementing the new capability in support of underwater UXO remediation efforts.
Technical Approach
Conventional synthetic aperture sonar (SAS) uses data from a single side-looking transmitter-receiver pair that transmits and detects reflected pings as it propagates along a one-dimensional trajectory. By coherently combining signals from multiple positions, high resolution two-dimensional (2D) images of the sea bottom can be created. Conventional analysis of data collected by a multi-receiver bottom-scanning array, such as the BOSS platform, can improve this by using the 2D extended aperture swept out by the line array as it moves along its trajectory to create three-dimensional (3D) images of the bottom surface and shallow buried scattering features, but assumes that the scattering response is spectrally flat. In fact, hollow UXO-like targets are uniquely expected to display acoustic resonances in the frequency range used by BOSS. This can strongly degrade conventional images by defocusing return energy across many image range pixels, reducing the return intensity and hence decreasing target resolution. The innovation of this project is to focus on the expected sharp frequency responses by creating instead 3D holographic images, in which the third dimension maps out the frequency response of each bottom or near-bottom scatterer. Acoustic resonances will appear as hot spots in the 3D image space, in particular highlighting their position on the bottom. Although direct depth resolution has been traded for frequency resolution, the project team will restore target depth localization of an identified hot spot through a set of image processing tools that optimally focuses its intensity for the correctly chosen depth. It is the innovative use of frequency-domain coherent processing over the full 2D aperture of the available array that enables the identification and extraction of these key new features. The signal and image processing tools will be developed and initially tested on synthetic data, and then on data collected using the BOSS platform under progressively more realistic scenarios.
Benefits
The benefit of this work to the DoD user community will be a new holographic image creation and processing toolbox, thoroughly tested and validated through end-to-end simulations and with real data. Its unique ability to simultaneously exhibit and spatially localize key UXO features will benefit future underwater UXO remediation efforts through increased detection probability and reduced false alarm rates. As the 3D data cube is developed using standard back projection techniques, the toolbox can be designed for easy transition to any existing (e.g., the several generations of the BOSS platform currently in use) or planned sonar array platforms. Other applications that could benefit from the combined detection and classification capabilities of the processing scheme include mine hunting in the littoral environment against seafloor and sea surface clutter, and active sonar antisubmarine warfare applications against quiet targets. (Anticipated Project Completion - 2015)
Points of Contact
Principal Investigator
Mr. Ira Ekhaus
BAE Systems
Phone: 781-262-4271
Fax: 781-273-9345
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
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- Workshop Report - Summary of workshop discussion and findings.
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