As a result of historical training and disposal activity, munitions and explosives of concern (MEC) can be found in the marine environment in a variety of underwater topographies ranging from open waters to bays to port areas to lakes and ponds. Although the geophysical sensors used for MEC detection in the marine environment—magnetometers and pulsed electromagnetic (EM)—are the same as those used in the terrestrial environment, there is not a one-size-fits-all solution because of differences in sea state and water depth.
The Underwater Simultaneous Electromagnetic Induction (EMI) and Magnetometer System (USEMS) is designed to survey shallow (1 to 3 meter) water such as lakes, ponds, rivers, streams, coastlines, and obstructed areas where a larger cable-towed array is not able to navigate. USEMS consists of a 17 ft boat towing a towfish that houses an EM61 submersible coil and a total-field magnetometer. The towfish is attached to the transom of the boat with a rigid carbon fiber boom whose rotational degrees of freedom are instrumented with encoders to directly measure its yaw, pitch, and roll relative to the back of the boat. The magnetometer and EM61 are operated concurrently via the interleaving technique developed and demonstrated under ESTCP projects MR-200208 and MR-200414.
The objective of this project was to demonstrate and validate USEMS. The project team evaluated USEMS’ ability to collect concurrent EM61 and magnetometer data in an actual MEC shallow water environment. Metrics evaluated included the hydrodynamic stability of USEMS’ submerged towfish, the ability of USEMS to maintain a constant height above bottom, the accuracy of the geodetically combined sensor and positioning data, the ability of USEMS to cover an area with data tracks, and the system’s general ease of operation.
USEMS was demonstrated at Plum Tree Island, Virginia, where it surveyed a shallow water test site and a deeper water test site and acquired traverse data. The objective of surveying at a planned height above bottom was met, with a standard deviation of approximately 19 cm from the planned height. In shallow water (approximately 1 m), the system had a vertical oscillation of approximately 50 cm. However, this oscillation vanished when the system was deployed in deeper (>2 m) water and thus is likely due to interaction with the boat motor’s propeller wash. Objectives for the geodetic accuracy of located targets were met, with an average location error <37 cm and standard deviation <19 cm. Noise objectives were met for the magnetometer data, but not for the EM61 data. The magnetometer was effective at detecting objects at standoff distances of 0.5 m, 1.0 m, and 1.5 m off the bottom. The EM61 was effective at detecting objects at a standoff distance of 0.5 m off the bottom. The ability of the system to cover an area with planned data tracks improved over the course of the demonstration, but fell short of the 95% coverage objective. After the demonstration, changes were made to the boat wiring, lessening the coupling of noise into the EM61, and modifications were made to facilitate line following for the next survey.
USEMS provides the Department of Defense a method for surveying underwater sites for MEC. The system is capable of filling most data gaps caused by lack of maneuvering room using large systems or systems with long tow distances. Furthermore, since the system simultaneously collects magnetometer and electromagnetic data, USEMS provides both for the same cost as that of any other single sensor system currently available.