The objective of this project was to develop and test an engineering prototype of a marine version of the Berkeley Unexploded Ordnance Discriminator (BUD). There is a pressing need to develop a geophysical system for detecting and characterizing unexploded ordnance (UXO) in the marine environment. To date, attempts to adapt land-based electromagnetic systems to the marine environment have demonstrated inferior performance compared to their relative success on land. The BUD system has demonstrated better than 98% identification of UXO targets at four prove-out sites in the United States. It has a fundamental transmitter-receiver configuration that inherently cancels the seawater response. The marine version is referred to as MBUD.
To optimize BUD for marine use, a three-component transmitter was mounted on a planar base and four, three-component receivers were mounted on the same base and on the corners of a square pattern centered on the transmitter. Differences in field at symmetrically-located receivers cancel the response of the seawater and of the air-sea interface above the system.
New ferrite-cored induction coils coupled with a feedback amplifier scheme provide high stability and a critically damped response. The coils were mounted in rigid blocks to provide three-component sensor modules. A first stage of amplification at the sensors provides a high level signal from a low impedance source to carry the signals, without added coupling noise, over cables to differencing amplifiers in the data acquisition module.
The current pulse in the transmitter coils is a bipolar half-sine of 5-ms duration with a repetition rate of 12.5 Hz. A new pulser, based on the BUD pulser, provides a peak moment of 2000 Amp.Turns.M2 with current pulses of 200 A and a net power consumption of 400 W.
Tests of the MBUD prototype on land and in 5 m of seawater showed that the system clearly detected a 6-inch (152.4-mm) steel test ball to over 57-cm depth. The test ball yielded identical transients in air and seawater. This shows that the effects of the seawater and the air-sea interface were canceled in the new configuration.
This project represents Phase I of a two-phase study and has shown the MBUD can operate at shallow depth and cancel the seawater response through receiver differencing. The marine prototype is expected to have the same discrimination abilities as BUD for objects larger than 60 mm on or beneath the ocean bottom, which will help support the detection and characterization of UXO in the marine environment.