There continues to be a need for an electromagnetic induction (EMI) system that can characterize unexploded ordnance (UXO) in the marine environment. There are many areas in the millions of acres that host underwater UXO. The US Army Corps of Engineers has identified 400 underwater formerly used defense sites. These sites all have munitions or UXO in less than 40m of water. A size-depth plot typical for these UXO from Di-Marco et al. (2010) was used as a design guide for the first MBUD system and is reproduced here as Figure 1. A compilation of polarizability calculations from a variety of targets for BUD and MBUD is shown by the dashed line in Figure 1. This line indicates that the MBUD system can effectively classify most targets to a depth of about 1.2 m.

Currently wide-area assessment is carried out using low and high-frequency acoustic and magnetometer arrays, both of which must be moved over the area at some distance above the sea bottom. The magnetic response of small targets decreases rapidly with increasing distance from the object. Further magnetometer surveys can be ineffective if the sea bottom has magnetic rock, sand, or gravel (as is common in Hawaii). Low-frequency acoustic scanners have low resolving power for small targets beneath the bottom and may not be effective in water depths of less than 3-5m.

Low-frequency acoustic scanning methods may eventually be able to characterize buried medium to large objects, but at present, they can only locate small objects as point scatterers. EMI systems must be operated on the bottom, and in many locations this is difficult or impossible. Quoting from a recent SERDP symposium, “EMI technology will probably be restricted to cued classification” and eventually will be deployed with a commercial midsized ROV. MBUD2 is well-suited for this role. The performance of MBUD2 was demonstrated on land and in San Francisco Bay. MBUD2 was operated in a cued mode – the system was stationery and targets were placed in various locations by an operator (on land) or a diver (in San Francisco Bay).

Figure 2a. Schematic of MBUD2 configuration.
Figure 2b. Assembled MBUD2 prototype system.

A simple open frame MBUD2 prototype system configuration consists of four, three-component, receiver cubes that are at the corners of a 1m2 area with the three orthogonal transmitters at the center (Figure 2) mounted on a simple platform. Differences in field at symmetrically positioned receivers cancel the response of the seawater and of the air-sea interface for shallow deployments. Note, MBUD2 is using existing components from MBUD1, which constrain possible changes that could be implemented. The pulser and electronics that are inside the pressure casing are at the other end of the platform. Next to it is a dedicated pressure casing, inside of which all the signal and power connections are redistributed to a bundle of long cables that reach the surface vessel where the data are processed and analyzed.

The orthogonal transmitter antenna system is completely encased in epoxy glass composite (an average thickness of 4 mm) to avoid any possibility of creating a contact between seawater and the three windings. The cable connections to the receiver windings are made through tubing fittings and the cables are surrounded with reinforced plastic tubing. To further prevent any seawater intrusion, the tubing enclosing the cables and any space around the receiver wiring are oil filled.

This work was done by Erika Gasperikova and Frank Morrison of Lawrence Berkeley National Laboratory, and Ugo Conti of Marine Advanced Research, Inc. for the Strategic Environmental Research and Development Program. For more information, download the Technical Support Package (free white paper) below. SERDP-0004

This Brief includes a Technical Support Package (TSP).
Development and Testing of a New Version of Mbud for Cued Classification of Marine UXO

(reference SERDP-0004) is currently available for download from the TSP library.

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