Structural Acoustic UXO Detection and Identification in Marine Environments
Many active and former military installations have ordnance ranges and training areas with adjacent water environments in which unexploded ordnance (UXO) now exist due to wartime activities, dumping, and accidents. Innovative technologies are needed to characterize and separate UXO from false targets and to discriminate among individual UXO targets themselves. In SERDP project MR-1513, researchers successfully demonstrated that typical underwater UXO scatter appreciable levels of sound over what is called the structural acoustics (SA) domain; that these signals would be detectable at modest ranges in most environments of interest; and that, in the free-field case, SA features can be used to separate UXO from other targets.
The objective of this project is to address the scientific and technical issues whose resolution would permit the realization of an efficient, high-performance structural acoustic, feature-based underwater sonar technology that can detect and localize proud or buried targets and separate the detections into UXO versus non-UXO without the need for high-resolution imaging. This will be realized by employing a marine-based measurement sonar--one that can look both downward and horizontally, conducting a series of measurement exercises in suitable marine environments, combining these databases with numerical models to develop a physics-based understanding of the structural acoustics, developing robust identification algorithms based on the SA features, and demonstrating the ability of the SA technique to detect and classify proud and buried UXO in the presence of natural and man-made clutter.
Conventional sonar approaches that form images must operate at relatively high frequencies since the image resolution cell size is proportional to the acoustic wavelength. In this regime, acoustic wavelengths are short compared to the target dimensions; the waves are scattered for the most part from the external boundary of the target; and sediment absorption precludes prosecution of most buried targets. In contrast, in the SA regime, acoustic wavelengths are comparable to, or longer than, the target dimensions. Sound readily penetrates the target and the sediment, and the acoustic scattering is now related to the vibrational dynamics of the object, both whole body and internal structure. The time-frequency features in the scattered echoes can then be used to "fingerprint" the target without the need to form an image. In this project, an active sonar designed for the UXO SA frequency band will be implemented on the bottom (or side) of an underwater platform or vehicle. This sonar system will utilize broadband, low frequency, compact acoustic source technology and a broadband receiver array. The sonar will operate in the "pitch/catch" mode as the vehicle moves forward, and the collected data will be post-processed in a synthetic array fashion to provide bi-static broadband target scattering data over small patches of the sea floor at various target aspects for submission to the SA feature-based target identification algorithm.
The inherent advantages of the structural acoustics approach include diverse "fingerprints," leading to high probability of detection and low false alarm rates; longer range operation for wide area coverage; and low frequency sediment penetration and propagation, leading to buried target prosecution. (Anticipated Project Completion - 2014)
Points of Contact
Dr. Brian Houston
Naval Research Laboratory (NRL)
SERDP and ESTCP
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