A large munitions response site, coupled with heavy vegetation and hazardous wildlife, provided ideal conditions for Jacksonville District to use a helicopter magnetometer for initial fieldwork at the Avon Park Formerly Used Defense Site Sept. 28.
Jacksonville District’s project manager for the site, Bill Spence, said the helicopter is a great tool to use for large open acreage that needs to be investigated.
Flying at speeds of 30 to 40 mph at six feet above ground, the helicopter flies low enough to be effective, says Steve Morrissette, a project manager with Zapata, the Corps’ contractor for the fieldwork.
At a lower altitude, sensors are closer to the ground, which produces a better signal response from ferrous metallic debris at the site. As the helicopter flies higher due to buildings, trees and power lines, the magnetic signal to the sensors is lowered.
Because Avon Park is open prairie land and has minimal obstructions, use of the helicopter at this site was ideal, said Morrissette.
“Given the expected site conditions of high concentrations of practice bomb casings in localized areas, the helicopter system was ideally suited for the Avon Park site,” Morrissette said.
The helicopter works by detecting magnetic anomalies on the ground or in shallow subsurfaces. If magnetic metallic debris is present, it disrupts the normal flow of the earth’s magnetic field, which the sensors are able to detect. Morrissette said it’s an excellent way to look for ferrous metallic objects over large areas.
Prior to beginning the aerial work, landowners were briefed on the helicopter’s operating practices. Special conditions that may require landowner action, such as keeping ranch workers out of an area, are coordinated before the work begins. In the case of livestock, the helicopter is not flown in that area until the landowner relocates the animals.
Other precautions that are taken before work begins includes taking a reconnaissance flight over the site to determine conditions prior to flying close-to-ground transects. The pilot doesn’t fly over livestock that are cornered at a fence line or any other enclosure.
Spence says proper coordination with landowners before the aerial investigation begins, prevents the low-flying aircraft from scaring livestock. “We want to work with landowners to eliminate as much stress on the animals as possible,” he said.
“Typically the pilot observes livestock moving away from the area once they see and hear the helicopter. The pilot will also nudge cattle away from a transect area from a higher altitude prior to completing a transect,” Morrissette said.
Because the aircraft flies so low, it may seem as if the noise it produces would affect one’s hearing.
Morrissette says the helicopter creates a level of noise typical of a turbine powered helicopter. “My experience was that once you were about 100 feet or more away, hearing protection was not required. In operation over a munitions response site, unless a person was right under the helicopter, the noise level was not excessive.”
The use of the aircraft for metal detection at formerly used defense sites may never replace traditional methods of fieldwork. But Morrissette says he’s sold on its results.
“It may not be appropriate for every situation, but in less than a week, we had four 649-acre and two 20,000-acre munitions response sites flown and characterized. This would have taken several months on the ground with higher hazards and risks to workers. The work was completed at a fraction of the cost it would have taken to complete ground-based digital geophysical mapping over these large areas,” Morrissette said.