Airborne Electromagnetic (AEM) Surveys
The Department of Water Resources is conducting airborne electromagnetic (AEM) surveys in California’s high- and medium-priority groundwater basins, where data collection is feasible, to assist local water managers as they implement the Sustainable Groundwater Management Act (SGMA) to manage groundwater for long term sustainability.
The AEM project provides state and federal agencies, groundwater sustainability agencies (GSAs), stakeholders, and the public with basin-specific and cross-basin geophysical data, tools, and analyses.
The surveys are funded by voter-approved Proposition 68, Senate Bill 5, and from the general fund. More information can be found in the AEM Proposition 68 Fact Sheet.
During an AEM survey, a helicopter tows electronic equipment that sends signals into the ground which bounce back. The process has been compared to taking an MRI of the ground’s subsurface. The data collected is used to create continuous images that are interpreted for underground geology.
The resulting information will provide a standardized, statewide dataset that improves the understanding of aquifer structures. It can also help with the development or refinement of hydrogeologic conceptual models and can help identify areas for recharging groundwater.
- AEM measures the electromagnetic response of the subsurface.
- AEM data are collected using geophysical instruments on a hoop that is towed beneath a helicopter.
- A current is generated in the hoop, which sends an electromagnetic signal into the subsurface. The response of the subsurface materials is measured in a receiver mounted on the helicopter.
- The measured response is related to the electromagnetic properties of the subsurface materials. Typically, materials that are electrically conductive are interpreted for fine-grained materials, like silts and clays, or can be interpreted for water with high salinity. Materials that are electrically resistive are interpreted for coarse-grained materials, like sands and gravels.
- The interpretation of AEM data is supported by information that describes the physical characteristics of the rocks. Existing information from well completion reports, electromagnetic geophysical (resistivity) logs, water quality reports and water level reports are used to do obtain this information.
- The AEM method can image the subsurface to depths of up to about 1,000 feet, depending on the electrical properties of the subsurface materials.
A conceptual diagram for how the electromagnetic method works
- AEM data will not be collected over residential areas and structures containing people or confined livestock.
- The helicopter is flown by an experienced and licensed pilot who follows all Federal Aviation Administration, state, county, and city laws and regulations.
- The helicopter flies at about 200 feet above ground and carries the instruments about 100 feet above the ground.
- The noise generated by the helicopter will be less than the noise generated from operating a leaf blower or lawn mower.
- The signal generated by the AEM equipment is comparable to the signal generated one foot away from a standard kitchen standing mixer.
AEM equipment setup and pre-flight preparation.
AEM data will be collected in a grid and the amount of data collected in each basin will be proportional to the total basin area. DWR will determine the flight lines by considering local hydrogeology and important areas within each basin, including areas with known data gaps, areas adjacent to critical infrastructure, or areas where recharge is being considered. DWR will contact GSAs prior to flight line development to gather information about important areas within each basin.
GSAs are encouraged to provide relevant, existing data to DWR to support the interpretation of the AEM data. The following types of existing data are requested:
- Lithology logs
- Electrical resistivity logs
- Hydrogeologic conceptual models
- Groundwater models and texture data
More information about the data type, quality, and format requested by DWR can be found in the Existing Data Fact Sheet.
Example of AEM data displayed as a fence diagram
(Stanford University, AEM Pilot Study.)
A technical advisory committee (TAC) will be formed in 2021 to support the statewide AEM program’s continued development.
- The TAC will provide recommendations on data collection and interpretation, data management, guidance documents, and data use.
- The TAC will consist of experts and practitioners from federal, state, local, and academic agencies, and a California Registered Professional Geophysicist.
Three pilot studies were conducted in California to support the development of the AEM project in the Sacramento Valley in Colusa and Butte county groundwater basins; the Salinas Valley in Paso Robles groundwater basin; and in the Indian Wells Valley groundwater basin. Data collected provided foundational information about the aquifer systems in these areas which has been used to develop or refine the hydrogeological conceptual models for each area.
All pilot study reports and data are available on the California Natural Resources Agency Open Data Portal.
AEM pilot survey flight lines in Butte and Glenn counties
Aqua Geo Frameworks, AEM Pilot Study
DWR's Public Outreach Materials
DWR Public Events
Public AEM Webinar June 8th, 2021
The AEM method has been used for decades to map aquifer structures and support groundwater management, both nationally and internationally. The links below highlight programs that demonstrate both the state and international approach for collecting and utilizing AEM data.
- Stanford Groundwater Architecture Project (GAP)/California AEM Pilot Studies
- United States Geological Survey, Mississippi Alluvial Plain (MAP) Regional Water Availability Study
- University of Nebraska - Lincoln, Nebraska GeoCloud
- State of Wisconsin, Department of Agriculture, Trade and Consumer Protection, AEM Survey
- GeoScience Australia Airborne Electromagnetic programs:
- Australia, New South Wales Government, MinEx Cooperative Research Center
- Hydrogeophysics Group, Aarhus University, Denmark
North Central Region
South Central Region
AEM Project Manager