Using tTEM for Subsurface Characterization at an AgMAR Site
The Place
Site 132 is a 56-hectare almond orchard located less than 10 kilometers west of Modesto in the Central Valley of California. The site overlies an incredibly heterogenous alluvial deposit, a common feature of agricultural groundwater basins. Land use in the surrounding properties consists of other almond and walnut orchards as well as some vineyards.
The Problem
Managed aquifer recharge (MAR) is being used to meet groundwater sustainability goals in California, and throughout the world. One approach to MAR is Ag-MAR, where agricultural lands are used for surface spreading recharge. There has been growing interest in this approach as these agricultural lands are abundant and often have existing infrastructure and motivated landowners. There are however several outstanding challenges with Ag-MAR implementation, including limited information about where to conduct Ag-MAR operations to maximize groundwater recharge and minimize potential adverse impacts to groundwater quality. An understanding of the subsurface sediment type is foundational information for any recharge operation. The traditional approach to getting this information, using wells, often fails to capture the spatial variability of these sediments.
Our Approach
At this site we used a towed-transient electromagnetic (tTEM) method to map resistivity. The system works by towing a transmitter loop and receiver behind a utility task vehicle (UTV) through a field. The transmitter loop generates a primary electromagnetic field that induces a secondary electromagnetic field in the subsurface detected by the receiver. Processing and inversion of the field data provided a detailed three-dimensional model of the subsurface resistivity as deep as 70 meters. Integrating the recovered resistivity model with well log information allowed us to link the geophysical properties to spatially varying sediment type.
Results to Date
tTEM data were collected at Site 132 in mid-February 2022.
Project Sponsors
United States Department of Agriculture
Project Leads / Contacts