Hazardous ordnance items are present along coastlines and in rivers and lakes in waters shallow enough to cause concerns for human recreational and industrial activities. The presence of water makes it difficult to detect and remove these hazardous legacies induced from wars, military training and deliberate disposal. Various techniques have been proposed to detect and characterize Unexploded Ordnances (UXO) and discarded military munitions (DMM) in the underwater environment including acoustic waves, magnetometery, and electromagnetic induction (EMI).

Frame built for the underwater tests, comprising two receive cubes and a 2 m x 1 m transmitter coil.

In recent years, terrestrial munitions response has seen significant improvements in our capability to discriminate Munitions and Explosives of Concern (MEC) from benign metallic clutter. These advances have been primarily driven by the development of next-generation EMI sensors designed to interrogate small, near-surface targets. This research concerns underwater sensing using EMI which is distinct from the terrestrial setting in several respects including positioning requirements and techniques, noise environment, and practical constraints on deployment of sensor systems. In terrestrial settings, conduction currents can be ignored in most soil types (conductivity 0.01 S/m). The measured magnetic fields from a subsurface metallic object in the low-frequency EMI regime can be modelled as if the object were in free-space. In contrast, marine environments are generally highly conductive with an average seawater conductivity of around 4 - 5 S/m.

For the numerical studies, an integral equation approach for a layered medium was developed that could account for the changing conductivity of the air, marine and sea-bottom. The model was extended so that it was possible to compute both the background and scattered field response from a highly conducting and permeable sphere for dipole and loop transmitters and receivers. A series of synthetic experiments was carried out by considering various factors that might influence EMI signals, including current channeling effects, sea depth, the size of a loop, lateral offset of the receiver, host conductivity, excitation waveform and antenna insulation.

For the marine measurements, a 2m x 1m x 1m fiberglass frame was built to encase two receiver cubes in epoxy to make them waterproof. The transmitter loop comprised 12 turns of wire arranged in a 2m by 1m rectangle. A 24V power-supply was used to provide a maximum current of 11.4A using a 25 Hz base-frequency with a 50% duty cycle waveform. A series of measurements were conducted in sea-water depths of between 2 and 14m.

The response from a metallic body immersed in a conductive medium is a combination of the eddy current response (ECR) due to currents generated in the target and the galvanic coupling of currents through the body (the current channeling response, CCR). In terrestrial environments only the ECR is important. Simulations showed that the CCR from a highly conducting and permeable sphere embedded in the air-sea-sediment is far smaller than the ECR and decays much faster than the ECR at a rate of t −3. For the time range of 0.1 ms - 25 ms, the CCR contributes little to the target signals and thus can be ignored. At times beyond several hundred microseconds, the ECR response approaches the value for the same object embedded in free space. These numerical observations were confirmed by measurements of an insulated and non-insulated 105 mm projectile at a range of different receiver, transmitter and object offsets.

When considering a survey close to seafloor, it was found that the decay rate of the background response is affected by the sea depth, or equivalently by the distance of the sensor from the air-sea interface. Results showed that the background responses in shallow water decay faster than in deeper water. In deeper water where the sensor is far away from the air-sea interface, the corresponding background responses asymptotically approach the response of a half-space. Simulations demonstrated that sea depths don't impact the scattered field response from a buried metallic object. Measurements conducted in water depths between 2 and 14m showed the response in shallower water (2m) falling off faster than the measurements at 14m. Observed decay rates were between t−5/2 and t −3.

This work was done by Stephen D. Billings of Black Tusk Geophysics Inc. and Lin-Ping Song of the University of British Columbia for the Department of Defense Strategic Environmental Research and Development Program (SERDP). SERDP-0001

This Brief includes a Technical Support Package (TSP).
Determining Detection and Classification Potential of Munitions using Advanced EMI Sensors in the Underwater Environment

(reference SERDP-0001) is currently available for download from the TSP library.

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