Many active and former military installations have ordnance ranges and training areas that include adjacent water environments such as ponds, lakes, rivers, estuaries, and coastal ocean areas. Wartime activities, dumping, and accidents have also generated significant unexploded ordnance (UXO) contamination in the coastal and inland waters in the United States and abroad. Dredging projects frequently encounter UXO. Much of the U.S. underwater contamination has occurred near military practice and test ranges, which tend to be remotely located with minimal direct economic impact. Thus, clean up efforts at formerly used defense sites have typically ended at the water's edge. However, sites that were remote 50 years ago are often no longer so, and potential hazards to the public from encounters with underwater ordnance have emerged.
In 2002, Dillon Consulting Ltd (DCL) undertook a reconnaissance-level marine UXO geophysical survey of Wright's Cove near Halifax, Nova Scotia in water depths ranging from 2 to 70 ft. The objective of that effort was a statistical description of the distribution of metal on the seabed, based on approximately 5 percent coverage. The objective of this project was to determine whether the existing prototype equipment could be improved and adapted to facilitate more detailed mapping of UXO on the seafloor.
The research areas to be addressed were positioning and deployment of the receiver with respect to the transmitter and the seafloor; understanding the system response by detailed calibration in terms of target size, salinity, water depth, and system configuration; and fine-tuning of the system electronics based on calibration data.
The system consists of a large (5m by 8m) surface-floated transmitter and a 1m by 0.5m receiver mounted on a planing board towed so as to be located beneath the transmitter and close to the seafloor. Advantages of a surface transmitter include its electronic and operational simplicity, and the large "footprint" of its inducing field so that lateral positioning of the receiver is not as critical as with a local transmitter. Preliminary calibration of the system determined the size of objects (seafloor or just sub-seafloor) it could detect given the variables of water depth and receiver location. Many anomalies were mapped, and their destruction was consistent with historical facts and previous information.
The field tests were divided into four phases. The first, third and fourth were undertaken in freshwater and the second in a marine environment. An issue with electrical communication between the receiver and the transmitter proved bothersome. The source of the problem was identified and interim measures taken to minimize the noise.
Adaptations of a mining-based forward modeling program did not prove successful. The log-log decay curves of field data exhibited excellent fit to linear trends and consistent repeatability indicating a stable response. Background noise, typically less than 3 millivolts on all channels, provides a threshold for the response below which targets can not be detected. For example, with the receiver located 2 m above the bottom, a small projectile (50mm) could be detected to depths of 4 meters and objects the size of a 55 US gallon drum at 17 meters. Salinity was shown to have influence on the response and detection depth for smaller targets. The response of the system is fairly uniform over a broad area directly below the transmitter leaving the potential for multiple receiver coils. Receiver tilts of less than 20 degrees did not significantly compromise the ability to detect a given target. However misalignment of the receiver does influence the actual values observed which may cause difficulties for discrimination algorithms. Preliminary adaptations to the size and shape of the transmitter were shown to have minimal impact on detection depth.
The transmitter-receiver configuration shows promise as an inexpensive detection platform and has potential for adaptation to a variety of systems. It minimizes bottom contact, and has reasonable control of target location and medium water depth capabilities.