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UAVs help solve challenges at an Arizona open-pit mine

Photo: Synergy Geomatics

Photo: Synergy Geomatics

UAVs provided Synergy Geomatics with the safest and most effective way to survey and map a 2,400-acre open-pit mine, and collect about a gallon of water from the bottom of the pit.

The Phoenix, Arizona-based survey, mapping and inspection company took on these two tasks at the Sacaton Mine in Casa Grande, Arizona, which shut down in 1984.

With an old mine of that size, the topographic survey was a large undertaking that lasted several days, said Synergy Geomatics CEO Doug Andriuk. A JAVAD-1M receiver and Triumph-LS real-time kinematic land survey machine were used to set and collect about 80 ground control points.

“This is outstanding survey gear for a large project like this,” he said. “The batteries last 24 hours, the setup takes a couple of minutes, and the multitude of radio options keep us connected all over the project. The dataset was comprised of more than 6,000 images and took several days to process.”

One day of field work followed by 1.5 hours of image capture using a Cessna 172 equipped with the Syn-Cam was required to map the mine.

The company used a proprietary method to collect high aspect imagery of the mine pit, because it allowed for a greater level of accuracy on steep surfaces. Manual and algorithmic filtering removed all the vegetation and structures on the site, and then generated 1- and 5-feet contours.

Collecting water from the bottom of the mine pit presented Synergy Geomatics with several challenges. It had been 30 years since anyone had been to the bottom of the pit, and the roads that led there had washed away long ago.

The use of a manned helicopter was briefly considered, but was not going to work in the tight, 1,500-foot-deep pit.

“We had a better, safer and less expensive solution,” Andriuk said. “We proposed the use of a drone carrying a water-sampling bailer, which is a poly tube with only a one-way check valve on the end. You can dip it in the water, and it will just keep filling up.”

Not only did the drone have to descend 1,500 feet, it had to collect water, too. Testing the drone’s capabilities to collect and carry nearly a gallon of water helped ensure the company would complete the task successfully with a few modifications.

“Multicopters don’t like to descend straight down, as they hit their own propwash, so a spiraling descent has to be made without hitting the walls of the pit,” he explained. “Then the drone must dip the bailer and ensure that it collects the right amount of water.”

Holes were drilled into the bailer to limit the amount of water collected. That way, the drone was not overloaded and could travel safely back to its landing zone with the additional weight.

Determining how high the drone needed to be above the water surface also posed a challenge.
“We opted to use two drones, a large one for the water sample and a smaller drone with a high-definition camera to give the pilot of the first drone clear visibility of the bailer over the water,” he said. “It took four trips, each lasting about 10 minutes, to collect a full gallon of bright green highly alkaline mine pit water.”

Image: Synergy Geomatics

Image: Synergy Geomatics

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