Low-frequency Acoustics for Munitions Response

Over the past ten years, SERDP has invested heavily in determining the potential applicability of low-frequency acoustic systems to munitions response. These systems operate in the 1 to 20 or 30 kHz region. In general, the wavelengths at these frequencies are too long to be used for target imaging (although development of analysis techniques to obtain 2D images has made progress); rather, the data are used to obtain so-called acoustic color plots exhibiting the frequency angle response of an object. This is often referred to as structural acoustics. An added benefit of these frequencies is that they afford significant penetration into the sediment and thus can interrogate fully buried objects. A good discussion of the state-of-the-art in this area can be found in a SERDP Workshop report from last year.

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Following on from this SERDP work, ESTCP has initiated two demonstration projects focusing on structural acoustics. Brian Houston and colleagues at the Naval Research Lab are conducting a “Demonstration of AUV-Based Structural Acoustic Look-Down and Side-Look Sonars for Underwater Buried UXO Detection and Classification.” An illustration of the down-looking variant is shown at the right as it approaches a buried target. For this system, the acoustic source is located on the AUV body and the receiver arrays are on the extended wings. This variant provides a shorter range, high classification performance capability against fully buried (and proud) UXO. It is projected to cover about 0.5 sq nautical miles a day (non-SI units being allowed in a blog post). The side looking variant is intended for wide area, high coverage rates (~7 sq nautical miles per day) against proud and partially buried UXO. Both systems have been tested at a seeded test area in the approaches to Boston Harbor. Data are being analyzed as I write this and we expect an initial report at the upcoming MR IPR and a fuller report at this year’s SERDP and ESTCP Symposium.

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Kevin Williams and his team at the University of Washington Applied Physics Lab are taking a slightly different tack in the project “Multi-Sensor Towbody (MuST) for Detection, Classification, and Geolocation of Underwater Munitions.” This system is deployed on a tow-body rather than an AUV (illustration at left). It will have a similar low-frequency sonar to the NRL system but will have the ability to deploy other sensors such as magnetometers on the tow body (hence the Multi-Sensor in the name although I like to think of the name as an homage to MTADS). Initial testing of this system starts soon and we hope to see results by the time of the Symposium.

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