Copter's 'torpedo' surveys levees

In coming days, helicopters dangling 30-foot sensors should be skimming the tops of San Joaquin County levees.

The effort is part of a strength analysis of 250 miles of urban levee by the California Department of Water Resources. The sensors pick up electrical conductivity in the soil and give scientists an idea of how strong the levees are.

The department started work early this year by drilling holes in the tops of levees every 1,000 feet. Those 4-inch-diameter borings showed the exact composition of sand, soil and gravel, which indicates how well it should hold up in a flood.

The huge torpedo-shaped sensor hanging from the helicopter picks up the same kind of data along all parts of the levee, said Claudio Avila, engineering geologist for the department.

"It's not as exact as a soil boring, but it gives us an idea of where we may see anomalies and where we may need to do additional borings," he said.

Scientists matched information from physical samples in Sutter County against data picked up by a sensor in a test run along the same part of levee last week and found it was a close enough match to proceed with the rest of the levees. Avila said the helicopters will head south, but it wasn't clear Thursday when the helicopters would be in San Joaquin County.

The helicopters will fly at about 200 feet and the sensor is suspended by cables about 100 feet below that. The sensors are powerful enough to read as far as 150 feet below the surface. Each type of sediment, such as clay or gravel, carries a different electrical conductivity so scientists can determine how much of each the levee is made of and their location.

More information about the sensors, pictures and a video are at www.levees.water.ca.gov/evaluation.

Bee staff writer Inga Miller can be reached at imiller@modbee.com or 599-8760.