Airborne Electromagnetic Method (AEM) Project

AEM Report and Data Available!

Airborne Electromagnetic Method (AEM) HelicopterFor a project two years or more in the making, we are excited to make the report and data set available for the Airborne Electromagnetic Method (AEM) survey that was conducted via helicopter in late 2018. The AEM survey essentially took a “Magnetic Resonance Imaging (MRI)” of the ground using a geophysical method to collect data to better characterize the aquifer structure of the groundwater system in a portion of the Northern Sacramento Valley in Butte and Glenn Counties. This dataset helps paint a better picture of the layering and connectivity of sands, gravels, and clays that make up the aquifers of our regional groundwater system.

Download the Report

The report includes maps of aquifer materials (lithology), maps of stratigraphic units, and maps of estimated potential recharge areas.

Project Goal

To acquire data using AEM to characterize the stratigraphy and aquifer architecture (hydrogeologic framework) and map out the distribution of clays, silts, sands, and gravels to a depth of approximately 1500 feet, with vertical resolution on the order of 3 feet near the surface to tens of feet at depth, utilizing existing well data for comparison.

What Are We Hoping to Learn?

  • Delineate major aquifer and aquitard units to improve hydrogeologic conceptual model
  • Assess spatial distribution of clay-rich layers. How extensive are they?
  • Examine level of connectivity between upper and lower portions of the Tehama/Tuscan aquifer systems
  • Identify hydrostatigraphic layers with similar aquifer characteristics (transmissivity, specific yield, boundaries, surface water-groundwater relationships) for use in groundwater model development

Where Did We Obtain Data?

Airborne Electromagnetic Method (AEM) Survey AreaThe map shows the flight lines for which geophysical data was collected in the study area. The AEM survey utilized two different systems. The SkyTEM304M system provides higher-resolution data at shallower depths, and the SkyTEM312 system provides better resolution at greater depths. Approximately 361 line-miles were acquired by the SkyTEM312 over the Butte-Glenn counties AEM survey area west of Chico on November 30 through December 2, 2018. Then on December 3, 2018, approximately 138 line-miles were acquired by the SkyTEM304M southeast of Chico. The Haigh Field at the Orland airport was used for landing and refueling between production flights.

Diving Into the Data

For each flight line shown on the map, a cross-section of inverted resistivity data, interpreted lithology (i.e. data categorized as clay, sandy clay, sand and gravel, or lahar/course sediments), and interpreted geologic units (i.e. Tehama, Tuscan formation) is available. The 2014 Department of Water Resources report on the geology of the northern Sacramento Valley and its cross sections was a primary reference for the geologic interpretation. 

You can explore the details of this data set for each flight line. Download the Read Me (PDF) document to find out how.

Airborne Electromagnetic Method (AEM) Flight Paths and Map

How Does It Work?

Geophysical instruments attached to a low-flying helicopter (50-100 feet above the ground surface) towing a large hoop with instruments transmit a weak electromagnetic field. This field interacts with the ground, and the response from the ground is measured using a set of receiver coils attached to the hoop. The helicopter flies back and forth over a series of regularly spaced lines, similar to mowing a really big lawn. Under some conditions, it can collect data to a depth of about 1500 feet below ground resulting in 2D slices of detailed variation in electrical resistivity of the subsurface. Once combined and calibrated with well data and knowledge of the geology, this dataset can be used to map out the layering of “course” (sand and gravel) and “fine” (silts and clay) grained materials that make up the aquifer system. 

This study was designed to help better understand the structure and vertical connectivity of the aquifer materials and confining beds (i.e. clay layers, how extensive are they?) and to gain additional insight on the transition from fresh to saline water at depth. This enhanced scientific understanding of the groundwater system will improve coordination across and between sub-basins and perhaps also help guide the placement of new monitoring wells or identification of recharge project locations. The project has two distinct areas as shown in the map. 

Western Area

The Western Area includes a region where the transition between the Tuscan and Tehama formation occurs, and coincides with an area where pumping tests determined that some regions of the Lower Tuscan/Tehama system behaves as a confined aquifer system, and other areas where it behaves as a “leaky” or partially confined aquifer system. Establishing the geometry and nature of the confining aquitard (clay layer(s)) will greatly enhance the ability to model and predictively manage the aquifer system. 

Eastern Area

The Eastern portion of the study area includes a likely recharge region, approaching the eastern edge of the alluvial basin near the foothills. Understanding the lithology of this area will have a substantial impact on modeling the movement and effect of surface water infiltration on aquifers in the basin. Basin geometry is poorly understood in this area, and there should be sufficient contrast between basin sediments and bedrock to characterize the shape and structure of the bottom of the basin in this area with AEM. This area is a transition zone from alluvial to fractured rock groundwater systems. 

Next Steps

Analysis of this incredibly rich data continues by county staff, local California State University (CSU) Chico geology professor Todd Greene, and by researchers at Stanford University. Interpretations and conclusions in the report in some cases may change as additional knowledge and existing data is incorporated into the analysis. This work will inform the Hydrogeologic Conceptual Model developed for the Groundwater Sustainability Plans for the sub-basins in the study area (Vina, Colusa, and Corning sub-basins). A public workshop with a presentation and discussion of these results will occur in the Fall. Stay tuned!