Another helicopter-borne technology, used this time to survey subsurface conditions of the levees, is called geophysical electromagnetic (EM) surveys. This method is capable of gathering a large amount of information from beneath the earth's surface in a relatively short period of time.

Suspended from a helicopter about 100 feet above the levees, an EM survey sensor-which resembles an airborne torpedo-senses variations in the ground's electrical conductivity to depths of more than 100 feet underground. The goal is to map important changes in soil types and ground conditions, identifying zones where permeable soils are present or excessive water penetration is taking place.

These surveys were conducted in September 2007 along more than 300 miles of urban levees, as well as 150 miles of other non-project-related levees (in an effort to assist local agencies and reclamation districts). From Oroville in the north to Lathrop in the south, the flights are taking place over levees on the Feather River, Bear River, American River, Sutter Bypass, Yolo Bypass, Sacramento River, Stanislaus River, San Joaquin River, and their tributaries.

Because homes, businesses, and other sensitive land uses are in close proximity to parts of the levee system, DWR coordinated with community leaders, residents, law enforcement and the media to ensure that local communities are aware of the purpose and timing of the flyovers.

For more information . . .
For more details about the EM survey process and the technology being used to conduct the surveys, please view the following informational materials:

• Fact Sheet & Frequently Asked Questions   En Español

EM Survey Maps:

• Overview
• North
• Central
• South