oVerView of dwr’s statewide aeM surVeys Project

by Chris Petersen1, Julián Consoli2, Timothy Parker3, Max Halkjaer4, Ahmad Behroozmand5, Katherine Dlubac6, Steven Springhorn7, Benjamin Brezing8, and Timothy Godwin9

Overview of DWR’s Statewide AEM Surveys Project

This article is the first in a series of four HydroVisions articles that provides basic information, answers general questions, and connects the reader to more detailed information online about the California Department of Water Resources’ (DWR’s) Airborne Electromagnetic Surveys (AEM Project) This work was initiated in 2021 and will be completed in 2024 and is being funded through Proposition 68 Funding. Visit the AEM Project webpage for more information: https://water.ca.gov/ Programs/SGMA/AEM.

This first article provides a high-level overview of the AEM Project and subsequent articles include:

• Spring 2023: How Does AEM Work and What are the Applications? Getting into the weeds on the technical details

• Summer 2023: How Does AEM Improve Management of Groundwater? Case Studies from Denmark and other States in the US.

• Fall 2023: How can AEM Improve Management of Groundwater in California and the US? A Case Study in California

So with that, let’s dive in!

What is AEM?

AEM is a geophysical method that measures the electrical resistivity of subsurface materials from either airplane or helicopter-mounted equipment. In this article, AEM refers to a helicopter-mounted system named SkyTEM, which has been used in the statewide project. The AEM sensor is housed in a large frame that is hung beneath the helicopter (Figure 1). A strong electric current, generated in the transmitter loop, is abruptly turned off to induce an eddy current in the ground, which generates a secondary electromagnetic field. The earth response is measured in a receiver coil mounted on the frame. The measured response is related to the electrical resistivity of the subsurface materials. Typically, materials that are electrically conductive are interpreted as fine-grained materials, like silts and clays, or can be interpreted as water with high salinity. Materials that are electrically resistive are interpreted as coarse-grained materials, like sands and gravels, and rocks as shown in Figures 2, 3, and 4. The helicopter flies at a groundspeed of 80-100 kilometers per hour (50-60 mph) carrying the equipment approximately 30 meters (100 feet) above the ground surface and collects data along a pre-defined flight path. Flight paths are developed in coordination with local Groundwater Sustainability Agencies (GSAs) and are designed to collect data over open spaces in a coarsely-spaced grid, with a line spacing of approximately 3 by 13 kilometers (2 by 8 miles). The AEM data are interpreted to show the distribution of coarse-grained and fine-grained materials in the subsurface, which improves the understanding of aquifer structures.

You will find all article figures and endnotes at the end of the article.

Why are we doing this?

DWR is required by the California Water Code to investigate the groundwater basins in the State and has a long history of basin characterization and reporting this information through California’s Groundwater Bulletin 118. This project builds on that tradition by utilizing new and innovative technology that is providing state and federal agencies, GSAs, interested entities, and the public with basin-specific and cross-basin geophysical data, tools, and analysis aligned to the technical requirements of SGMA and regulations for groundwater sustainability plans (GSPs). The resulting information will provide a standardized, statewide dataset that will improve the understanding of large-scale aquifer structures, which aids in the development or refinement of a hydrogeologic conceptual model and identification of possible recharge areas. AEM data will support the development and refinement of groundwater models, improve the potential for the successful implementation of GSPs, and reduce uncertainty in identifying locations of structural features such as the Corcoran Clay or areas of high salinity. The coarse-grid AEM data also serve as the basis for the collection of fine-grid AEM data by local, state, and federal agencies in the future.

Where is the data being collected and over what time frame?

DWR is collecting AEM data in all of California’s high- and medium-priority groundwater basins, where data collection is feasible. AEM data collection started in 2021 and will continue through 2023. Table 1 provides the AEM Survey Schedule and Figure 5 is a Survey Location Map. Figure 6 shows the survey lines of AEM data collected to-date.

How is the data and information being made available to the public?

DWR publishes all project related data reports and datasets on the California Natural Resources Agency Open Data Portal (https://data.cnra.ca.gov/dataset/aem). Data reports describe how AEM data were collected, processed, and inverted and provide an interpretation of each lithology. Datasets include AEM data at each stage of the process (raw, processed, inverted, and interpreted) and supporting data (newly digitized lithology and geophysical logs). Reports and datasets are published on a rolling-basis for each Survey Area as tasks are completed by the contracting team.

In addition, DWR develops tools and maps to help the general public view the data. DWR’s most recent data visualization tool is the AEM Data Viewer. This online, GIS-based tool displays AEM data in a three-dimensional space and allows the user to view the data from various angles and to zoom in/ out. The tool can be accessed on a computer or mobile phone and does not require data to be downloaded or the use of specialized software. The AEM Data Viewer can be accessed here: https://data.cnra.ca.gov/dataset/aem/resource/29c4478dfc34-44ab-a373-7d484afa38e8.

Next HydroVisions Article

The next article, coming in Spring 2023, will provide details on the AEM method and will answer the following questions: how are data collected? How are data processed? Once processed, what is involved in correlating the AEM models to a large number of lithologic and geophysical logs? What is the uncertainty of the interpretation of subsurface conditions from AEM data and how is uncertainty determined? What actions can local or regional agencies take to utilize AEM data to improve basin understanding?

Endnotes:

1 California professional geologist (PG) and certified hydrogeologist (CHg) with GEI Consultants and task order manager for AEM Project

2 PG with GEI Consultants and technical lead on compiling supporting data for AEM Project

3 PG and CHg with Ramboll and lead reviewer for AEM Project

4 Geophysicist with Ramboll, Lead Geophysicist and Project Director for AEM Project

5 PhD and Senior Geophysicist with Ramboll

6 PhD, PG and Statewide AEM Survey Project Manager, DWR Sustainable Groundwater Management Act (SGMA) Office

7 PG and Technical Assistance Section Manager, DWR SGMA Office

8 PE and Data Support Section Manager, DWR SGMA Office

9 PG, CHg, and Policy and Technical Advisor to the Deputy Director, DWR SGMA Office

Figure 1 – AEM Array Suspended from Helicopter

Figure 2 – General correlation between resistivity, type of sediments and rocks, and water of varying quality.

Table 1 Survey Area Area/Basin Completed #1 Salinas Valley, Cuyama Valley August 2021 #2 Lassen and Modoc Counties November 2021 #3 Napa and Sonoma Counties November 2021 #4 Southern San Joaquin Valley March 2022 #5

Middle San Joaquin Valley April 2022 #6

Northern San Joaquin Valley and Southern Sacramento Valley April 2022 #7 Northern Sacramento Valley, Redding Area, Eel River May 2022 #8 Gilroy, Santa Cruz, and Salinas Areas November 2022

Figure 3 – An example of AEM resistivity cross section from the Cuyama Valley. The location map of the section within the survey area is provided in the top panel and the resistivity profile along the section line is the bottom panel. Well lithologic logs near the section are projected on the section. The warm and cool colors show high and low resistivity values, respectively. All AEM data collected within the Cuyama Valley can be viewed on the online AEM Data Viewer.

Figure 4 Texture Model Fence Diagram

The AEM resistivity models are compared with lithologic descriptions from wells to determine the coarse materials (sand and gravel) where water is stored and can easily flow, and fine materials (silt and clay) that inhibit groundwater flow.

aeM surVeys Project

Figure 5 – Survey Location Map as of November 2022. Map is updated periodically be DWR and posted on website.

Figure 6 – AEM Flight Line Map as of October 2022. Map is updated periodically by DWR and posted on website.