Building a resistivity atlas in the Central Valley of California

Electrical resistivity is a valuable geophysical parameter for hydrogeological studies due to its sensitivity to sediment texture, which controls fluid flow, water retention, and contaminant migration. Mapping subsurface resistivity in groundwater-dependent regions, such as the Central Valley of California can aid in the development of subsurface models, essential for the support of sustainable groundwater management. The challenge is developing the rock-physics transform required to link resistivity to sediment texture. Typically, these transforms are considered site-specific; however, considerable effort is required to obtain the data to build a transform at each site. Our goal is to develop a resistivity atlas of the Central Valley that compiles site-specific rock-physics transforms between resistivity and sediment texture. Such an atlas can serve as the starting point for acquisition of geophysical data in new study areas, for aiding in survey design and data interpretation, and for guiding decisions about ancillary data needs.

We present rock-physics transforms from eight sites around the Central Valley developed using resistivity models from ground-based time domain electromagnetic (TEM), airborne TEM, and direct current resistivity surveys, and direct sampling data including lithology data from wells, cone penetrometer testing data, and laboratory sediment texture analysis of cores. The assembled rock-physics transforms reflect the variations in geographic location, depth range, measurement scale, water content, and data source, i.e. the measurement workflow involved in data acquisition and analysis. In the future we will address a critical question: over what spatiotemporal scale are the rock-physics transforms applicable?