Detection and classification of unexploded ordnance (UXO) in a shallow underwater environment presents special problems that render some of the conventional geophysical methods normally used on land unusable or less effective. However, the deployment of galvanic resistivity methods is aided in such environments because the water provides a medium in which electrodes make continuous galvanic contact. The Model RX-1 Beach Mine Detector developed by the Office of Naval Research during the latter part of World War II represents an early effort to use the induced polarization (IP) method to detect mines. Though the RX-1 was deployed in Okinawa in 1945, the war ended before it was widely used and, as far as can be determined, there has been no further development of IP as a method for mine or other metal object detection. IP was, however, successfully developed and applied for mineral exploration.
The objective of this proof-of-principle project was to investigate the detection of UXO in underwater sites using a towed-array resistivity/IP system.
On land, some experiments and tests have demonstrated that IP is associated with UXO. Resistivity and IP surveys also have been conducted in shallow water using a towed electrode array. In this project, IP responses associated with a few UXO types (e.g., grenade, mortar rounds, rockets, etc.) were measured in the laboratory, and a full-scale experiment was conducted using an instrument system previously assembled by Zonge Engineering & Research Organization, Inc. and deployed in shallow sub-bottom resistivity profiling for environmental applications. The effort sought to confirm that there is a measurable IP response associated with isolated metal objects that are either laying on the bottom in shallow water or buried immediately beneath the sub-bottom.
Researchers conducted laboratory-scale model experiments and numerical model experiments in an effort to establish the scale of the IP response. The laboratory experiments and numerical modeling demonstrated that a measurable IP response is associated with metallic objects submerged in an aqueous electrolyte. Targets in a saltwater solution generate a significantly higher IP response than equivalent targets in fresh water. Based on results of the model, researchers designed a simple electrode array and tested the resulting system at the Mare Island Naval Shipyard (MINS) in San Francisco Bay. It is clear from the results that UXO generate a measurable IP response. However, targets seeded in the MINS Reference Area were not easily detected with the deployed surface electrode array partially due to inadequacies in the experimental apparatus. Small targets were not detected. As predicted, the MINS field test indicates that both target orientation and target/receiver dipole separation are significant detection factors.
A relatively low-cost IP method useful for UXO detection in shallow-water environments was identified and has the potential to be deployed either for wide area assessment or, alternatively, for the detection of individual UXO targets. (Project Completed – 2004)