TRANSFORMING RESISTIVITY DATA TO LITHOLOGY FOR LARGE SCALE AIRBORNE ELECTROMAGNETIC (AEM) SURVEYS

Airborne electromagnetic (AEM) surveys provide a measurement of the electrical resistivity of the different geological strata up to 300 meters in depth. Given that, in general terms, resistivity decreases as clay content increases, these surveys provide information on the distribution of coarse-grained and fined-grained materials in the subsurface. This is useful in mapping out geometry and geologic properties of the underlying aquifer systems.

California’s Department of Water Resources conducted a state-wide AEM survey, collecting over 25,000 km of resistivity data in the state’s high and medium priority groundwater basins. As part of this project, a resistivity to lithology transform method, specifically developed to handle large AEM datasets, was used. This method combined the detailed high-quality well lithologic data with information on the spatial heterogeneity from the resistivity to provide an interpretation of lithology. All well lithology information within 800 meters of the AEM data was collected and digitized. To ensure a robust transform, it was essential to confirm that well placement was within 50 meters of the registered coordinates. The lithology data descriptions were then aggregated into either coarse (sand or gravel) or fine (clay and silt). Computer-based calculations using an iterative inversion algorithm were then preformed, numerically comparing the resistivity with the simplified (binary) well lithology log data from the nearby wells. This produced a model of the coarse fraction thickness consistent with the lithology log coarse fraction in 2D sections along the AEM flight lines. However, since resistivity is also influenced by salinity, the lithology transform is limited to areas where groundwater salinity is under 3,000 mg/l.

The coarse fraction calculated from the AEM survey data has provided valuable information to better understand the spatial distribution of coarse and fine sediments. The results have been used in the basin characterization efforts, including identifying potential recharge areas, interconnectivity of aquifers and thickness and extent of confining clay layers.