Abstract
The passage of the Sustainable Groundwater Management Act in California has highlighted a need for cost-effective ways to acquire the data used in building conceptual models of the aquifer systems in the Central Valley of California. One approach would be the regional implementation of the airborne electromagnetic (AEM) method. We acquired 104 line-kilometers of data in the Tulare Irrigation District, in the Central Valley, to determine the depth of investigation (DOI) of the AEM method, given the abundance of electrically conductive clays, and to assess the usefulness of the method for mapping the hydrostratigraphy. The data were high quality providing, through inversion of the data, models displaying the variation in electrical resistivity to a depth of approximately 500 m. In order to transform the resistivity models to interpreted sections displaying lithology, we established the relationship between resistivity and lithology using collocated lithology logs (from drillers' logs) and AEM data. We modeled the AEM response and employed a bootstrapping approach to solve for the range of values in the resistivity model corresponding to sand and gravel, mixed coarse and fine, and clay in the unsaturated and saturated regions. The comparison between the resulting interpretation and an existing cross section demonstrates that AEM can be an effective method for mapping the large-scale hydrostratigraphy of aquifer systems in the Central Valley. The methods employed and developed in this study have widespread application in the use of the AEM method for groundwater management in similar geologic settings.
Highlights
To ensure a secure food supply, many agricultural regions of the world require reliable sources of surface water and/or groundwater to meet irrigation needs
The key question is : How do we map the aquifer systems in the Central Valley in a cost-effective way? We propose regional implementation of the airborne electromagnetic (AEM) method to obtain information needed about the hydrostratigraphy of the aquifer systems
Let us consider the first question we addressed, related to the imaging capability of the method: How effective would the AEM method be in imaging beneath the shallow, electrically conductive clays commonly found in the Central Valley? Our concern had been that electrically conductive clays would limit the depth of investigation (DOI)
Summary
To ensure a secure food supply, many agricultural regions of the world require reliable sources of surface water and/or groundwater to meet irrigation needs. Answering these two questions, related to the ability to acquire and interpret high quality data, was an essential first step in determining whether the AEM method could be reliably used for mapping the aquifer systems in the Central Valley If so, this would be a new way of obtaining critical information needed for the implementation of SGMA, and could significantly transform the approach to groundwater management in California. NGWA.org at this depth in this area, becoming discontinuous in the northeast (the right side of Figure 8a) In addition to this presentation of the results we transformed the resistivity model to lithology in two other ways that communicate information about the lithologic variation in the subsurface mapped with the AEM method.
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