Airborne electromagnetic imaging of stratigraphy at the Hanford Site, Washington

Abstract:

At legacy waste sites, it is critical to have a detailed understanding of subsurface stratigraphy and flow pathways that influence contaminant transport to minimize risk to human health and the environment. Airborne electromagnetics (AEM) can non-invasively provide information on kilometer-scale or larger subsurface stratigraphic features to fill-in informational gaps from sparsely located boreholes. We present inversion results of a 412 line-km frequency-domain AEM survey to delineate subsurface stratigraphy at the Hanford Site, located in southcentral Washington State (USA). The inversion is performed using a massively parallel 3D electromagnetic modeling and inversion code, where the forward modeling is based on solving Maxwell’s equations using unstructured-mesh finite-element method, and the inversion employs a Gauss-Newton optimization scheme. The results are compared to an underlying geologic framework model (GFM), built by interpolating contact depths of stratigraphic units interpreted from site borehole datasets. In areas with a good borehole coverage, the inversion results show a consistent match with the GFM to a depth of about 60 m. Where borehole coverage is sparse, the results show some inconsistency from the assumptions made to create the GFM, demonstrating that the AEM survey results can be used to improve the understanding of the geological conceptual model. Delineation of such stratigraphic units can help in understanding transmissive pathways, which in turn, will allow us for more effective monitoring and remediation planning of subsurface contaminants at legacy waste sites.