Improved Imaging of the Large-Scale Structure of a Groundwater System With Airborne Electromagnetic Data

Working with airborne electromagnetic (AEM) data acquired in the Kaweah Subbasin in the Central Valley of California, USA, we developed a new approach for imaging the bedrock surface and the confining Corcoran Clay layer. Our approach, which incorporated the prior knowledge of the targets for the improved imaging, included multiple L-2-norm and L-p-norm inversions as well as an interpolation process. The major improvement in imaging the targets was made in the L-p-norm inversion step by incorporating prior knowledge. For the Corcoran Clay, pairs of resistivity and driller's logs at two wells guided the selection of the primary resistivity model and were used to increase the accuracy of the estimated Corcoran Clay thickness. The bedrock surface was poorly constrained by well data in the existing groundwater model, appearing as a flat surface. Acquired AEM data covered most of the area, so we had higher confidence in the obtained map of the bedrock surface at depths ranging from 15 to 160 m. There was relatively good agreement between the location of the Corcoran Clay in the AEM data (depth ranging from 50 to 130 m and thickness ranging from 3 to 25 m) and the existing groundwater model, with both depth and thickness showing similar to 15% relative difference. The AEM data provided information about the continuity of the Corcoran Clay that is challenging to capture in the well data. The locations of the bedrock and Corcoran Clay were used in a structurally constrained inversion to improve the imaging of the smaller-scale resistivity structure.