VANCOUVER, Wash. — A helicopter, trailing an instrument that a scientist said resembles "a big orange Hula Hoop," is opening a new research chapter on Mount St. Helens.

The U.S. Geological Survey is using a low-flying helicopter to collect water-saturation data that will help identify potential hazards.

Carol Finn, a USGS geophysicist based in Denver, said the project will help scientists better understand landslide potential on the volcano.

The actual testing began Friday after Thursday's calibration flights.

The USGS issued a news release a few days ago so climbers and hikers would not be alarmed by the low-flying helicopter trailing equipment on a 100-foot-long cable. Finn said the survey measures the rocks' resistance to electricity, because "electricity goes through water easier," telling scientists the amount of water in the volcano.

The measurements are made with a time-domain electromagnetic and magnetic survey instrument.

"It's like a big orange Hula Hoop," Finn said.

It does its job about 100 feet over the ground — midway between the terrain and the helicopter, which flies at about 200 feet.

"The current goes through the hoop, creating a magnetic field," she said. "The magnetic field creates a secondary electrical field that goes into the earth and comes back and gets recovered in the hoop. The timing of the return tells you about the resistivity."

The helicopter flies at a ground speed of 50 to 60 mph, back and forth across the entire volcano along equally spaced lines.

Finn said the Mount St. Helens survey was inspired by similar research on Mount Rainier, Mount Adams and Mount Baker.

On the morning of May 18, 1980, a 5.1-magnitude earthquake under the volcano triggered the biggest landslide in recorded history.