Full Waveform Inversion Imaging of the Critical Zone: Unlocking Near-Surface Seismic Data with Open-Source Technology

For over a decade, near-surface seismic methods have been used to catalyze high-impact research in the critical zone. Most commonly, these techniques include ray-based travel time tomography and surface wave dispersion inversion. Despite the usefulness of these methods, there exists significant potential for improvement with the advent of full waveform inversion (FWI). For example, typical near surface seismic methods use highly linearized inversions of first arrival travel times, and less commonly, dispersion measurements of fundamental mode surface waves. In contrast, FWI fits the actual waveforms of first arrivals and fundamental mode surface waves, as well as scattered p-wave coda and higher-mode surface waves. Thus, FWI leverages vastly more of the seismic wavefield, creating commensurately high-resolution wave speed models, and elucidating previously unseen features. Nevertheless, various technical and practical aspects of FWI need to be studied before the method can be easily and repeatedly applied to the critical zone. These aspects include initial model building, selecting a robust objective function, estimating source-time functions, devising a hierarchical multiparameter inversion strategy, and utilizing an accurate wave solver. At the Fall Meeting, we will present our findings on how these aspects relate to applying FWI to seismic data from the Blair-Wallis catchment in the Laramie Range, Wyoming. We will ground-truth FWI models with borehole logs, and appraise our inversion strategy by inverting synthetic data. These results will all be derived by adapting existing open source technologies, namely the wave modeler, SPECFEM, and the seismic inversion workflow manager, Seisflows. Thus, the methods we develop will be reproducible, and applicable to other critical zone data sets.