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Treading delicate waters

by Virginia Heffernan on AUGUST 8, 2016 Technology

Tetra Tech's hovercraft-based electromagnetic system was used to detect likely munitions in the Caribbean, where shallow coral reefs had been excluded from a broader geophysical survey because of the potential for damaging the coral.

Tetra Tech's hovercraft-based electromagnetic system was used to detect likely munitions in the Caribbean, where shallow coral reefs had been excluded from a broader geophysical survey because of the potential for damaging the coral.

An example of the data collected with the TEMA-Lite (Tetra Tech towed electromagnetic array) system used on the hovercraft.

An example of the data collected with the TEMA-Lite (Tetra Tech towed electromagnetic array) system used on the hovercraft.

Tetra Tech uses hovercraft-based EM system to detect munitions around coral reefs

A hovercraft-based electromagnetic (EM) system has detected likely munitions at a location in the Caribbean previously used for target practice by the U.S. Navy. The area covers more than 200 acres of shallow coral reefs that had been excluded from a broader geophysical survey because of the potential for damaging the coral.

By using a hovercraft and a near zero draft sensor array, Tetra Tech was able to survey the vulnerable areas previously inaccessible except by more expensive and less sensitive helicopter based geophysical systems.

“We designed the system so it could be used in very shallow water where you can’t touch the bottom,” says Richard Funk, principal geophysicist for Tetra Tech’s Marine Mapping Group. “We can hover just a few inches over the coral and not damage it.”

The survey represents Phase 2 of a remedial investigation and feasibility study on one of several former defense sites in the Caribbean. The entire 1,030-acre site served as an impact range for aerial bombs, rockets, missiles, mortars and naval projectiles from 1903 until 1975.

Despite the challenges of wind, waves, hypersalinity and heat, Tetra Tech’s crew was able to collect accurate data because of the maneuverability of the hovercraft designed by Indiana-based Neoteric Hovercraft. Neoteric’s vehicle is the only model on the market with a reverse thrust system that allows the pilot to finely control the hovercraft while the sensors do their job.

“We had to be flexible, changing our plans based on wind direction and we were constantly having to maintain the system to keep everything running properly in a marine environment,” says Funk. “But it worked because the hovercraft was able to stand still on cushion.”

The team managed electromagnetic noise from the engine by placing the EM sensors at the front of the hovercraft. Other customizations included mounts for GPS antennas, detector batteries, and data collection consoles, computers and displays.

Following the successful deployment in the Caribbean, the team has other projects pending that will use the hovercraft-based system, not only for unexploded ordnance (UXO) detection, but for other purposes such as bathymetry, side-scan sonar, and sediment sampling, as well as laser mapping in wetlands and other delicate upland areas.

The Tetra Tech towed electromagnetic array (TEMA) system is an in-house design based on decades of Munitions and Explosives of Concern (MEC/UXO) detection and remediation experience. The system has two deployment configurations, each covering a 3-metre swath: the TEMA MK3, which is dynamically flown through the water column in depths from 3 to 100 metres; and TEMA-Lite (the system used on the hovercraft), which floats and can be pushed in extremely shallow water or towed on a sled mount to depths of about 40 metres.

The EM system is more effective than a magnetometer in developed areas because it is less susceptible to interference and signal masking from nearby piers and other large metallic structures. It also can detect both ferrous and non-ferrous metals, unlike a magnetometer.

Both TEMA configurations use the high power variant of Geonics EM61MK2 EM sensor, increasing detection ranges by 45-80% over the standard EM61MK2. On the TEMA-MK3 a fibre optic multiplexer streams high definition video live to the surface, allowing the crew to differentiate between visible debris and munitions. The multiplexor also sends data from the EM units, altimeters and heading and tilt sensors and still photos from a custom digital single-lens reflex (DSLR) underwater camera housing.

Funk and his team process the auxiliary sensor measurement data such as GPS locations using hydrographic survey software, and merge it with the EM data using Tetra Tech-developed software. The merged data are then imported into Geosoft Oasis montaj for processing, target detection and further analysis with the montaj UXO software extensions.

“Geosoft is the industry standard, so it’s easy for all the geophysicists in the company to share our data,” says Funk.


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