An unusual flying object will visit Silverton in the next couple of weeks, but residents have no reason to fear a secret government experiment or an encounter with creatures from another planet.

Using a helicopter carrying an electromagnetic mapping device, the U.S. Geological Survey is trying to understand the geological makeup of the San Juan Mountains, and hopefully, its groundwater workings. The aim, project managers said, is to inform decision-making for the Bonita Peak Mining District Superfund site.

“We’re going to provide a better picture of the whole study area,” said Douglas Yager, a geologist with the USGS in Denver. “Hopefully, we’ll be able to provide some useful information for land managers making decisions.”

The mountains around Silverton sit on a caldera – a large, collapsed magma crater formed by an explosion of a volcano 27 million years ago.

The volcanic activity unloaded the rich deposits of mineralized rock that drew droves of miners to Silverton in the late 1800s. And it left behind a complex geological puzzle that challenges agencies attempting to clean up the miners’ mess.

“This is a really complex area,” Yager said. “But since we have new technology advancements ... I think we do have an opportunity to learn new things.”

The Bonita Peak Mining District Superfund site was declared in fall 2017, about a year after the Environmental Protection Agency caused a blowout at the Gold King Mine, leaking wastewater into the Animas and San Juan rivers.

Although minor cleanup projects have occurred in the past 1½ years, the EPA has spent much of its time trying to understand the upper Animas River basin and surrounding mountains in an attempt to form a comprehensive remediation plan.

The last USGS attempt to perform a geological survey of the San Juan Mountains around Silverton was in 1999. Now, technology allows geologists to peer deeper into the Earth and produce much more precise models of an area’s geological makeup.

The USGS hopes to map groundwater flow paths, which would allow researchers to determine whether water is entering into areas of heavy metals.

During the survey, a low-flying helicopter tows a large wire loop that looks like a Hula-Hoop.

Adrian Sarmasag, a senior project manager for Geotech Ltd., said the loop is about 60 feet in diameter.

“Flying heli-borne geophysical surveys in the mountains requires specialized flying techniques: Mountain flying experience combined with long-line slinging operations and low-level flying techniques with the ability to perform both while maintaining an accurate line direction and height,” he said. “This is the reason we brought our helicopter provider from British Columbia. They are familiar with our equipment, the type of terrain and provide a level of safety and flying consistency that are needed for a successful survey.”

Yager said the helicopter carries an electromagnetic system that measures tiny voltages used to map the Earth’s subsurface. During the next two weeks, nearly all the 48 mining sites included in the Bonita Peak Superfund site will be surveyed.

“Each Superfund site has its own unique geologic setting,” he said. “With this data, we’ll be able to better interpret, on a larger and finer scale, what’s happening underground.”

The EPA expects the findings will help its cleanup efforts, an agency spokeswoman said.

“It will provide information that will improve our knowledge of the subsurface and potentially provide interpretations of how groundwater flows through geologic structures, bedrock, mine tunnels and surface deposits,” EPA spokeswoman Cynthia Peterson wrote in an email.

Ben Martinez with the U.S. Forest Service said the data will help inform decisions about projects to improve water quality in the headwaters of the Animas River.

“Anything that helps inform us what is happening naturally in the environment, helps inform us with our reclamation efforts,” he said. “It’ll be interesting to see what comes out of it.”

The USGS helicopter survey is part of a three-year project to build a comprehensive 3D model of the Silverton caldera. Last year, the USGS used technology to survey more than 6 miles into the Earth, far deeper than the helicopter survey, which will all be compiled in the 3D map.

The USGS hopes to release the maps in 2020, Yager said.

jromeo@durangoherald.com