Why is a helicopter hoisting a giant hexagon over Paso?

In the next few weeks, a large hexagon will soar through the sky, dangling from a low-flying helicopter over the rural towns and farms east of Paso Robles.

It isn’t the latest trend in skydiving. Rather, it’s the frame for an aerial mapping technology that California is borrowing from Denmark to study how water moves underground.

That’s important information to have.

For years, farmers in the area have pumped water out of the ground faster than the aquifer could refill.

Now people in the Paso Robles Groundwater Basin have to plan to better manage groundwater resources, and that requires a better understanding of where water flows and pools underground.

San Luis Obispo County is one of three areas selected to participate in a pilot study using the technology, called an aerial electromagnetic method, that Supervisor Debbie Arnold likened to an x-ray.

This hexagonal frame will be used to survey the groundwater aquifer in the North County by using a weak electromagnetic field to map the structure of the subsurface basin. The pilot project is lead by Stanford University, the California Department of Water Resources and the Kingdom of Denmark. Courtesy of the County of San Luis Obispo

“I’m really excited about that,” said Arnold, whose district includes part of the Paso Robles Groundwater Basin. “We won’t have to guess anymore. Otherwise, the only tool we use is just test wells. Having an x-ray is so cool.”

The state in 2017 signed an agreement with the Kingdom of Denmark — which relies exclusively on groundwater — to share the latest water technology, research and management techniques, including this method.

The project is being led by Stanford University, the state Department of Water Resources and the Kingdom of Denmark. Other pilot projects are in Butte County and Indian Wells Valley.

How aerial groundwater mapping works

Here’s how it works, according to an explainer written by the county Public Works department:

Instruments attached to a helicopter flying about 100 feet above the ground transmit a weak electromagnetic field.

Courtesy of County of San Luis Obispo

This field interacts with the ground, and the response is measured with receiver coils attached to the hexagon. Data can be collected up to a depth of about 1,400 feet below the surface.

The dataset can then be used with existing information about the basin to map out whether areas are sand, gravel, silt, clay or bedrock to show the structure of the underground geology.

The helicopter flies back and forth along a grid pattern, covering a distance about 497 miles.

The technology was already used in California, including as part of a project to learn more about saltwater intrusion of fresh water aquifers beneath the town of Marina.

The study near Paso Robles will take place for three to five days in early to mid-November in areas by Highway 46 near Whitley Gardens, Highway 229 near Creston and Highway 41 near Shandon, according to a county news release.

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County of San Luis Obispo
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Monica Vaughan reports on health, Cal Poly, San Luis Obispo County, oil and wildlife at The Tribune. She previously covered crime and justice in the Sacramento Valley, is a graduate of the University of Oregon journalism school and is a sixth-generation Californian. Have an idea for a story? Email: mvaughan@thetribunenews.com