NEAR-SURFACE STRUCTURE CHARACTERIZATION USING SURFACE WAVES FOR THE ROCK VALLEY DIRECT COMPARISON (RV/DC) TESTBED

Improving the subsurface structure characterization of the Rock Valley Direct Comparison (RV/DC) Testbed is important for validating and improving the techniques of earthquake-explosion discrimination, a focus of the Source Physics Experiment (SPE) Phase III: RV/DC. A parallel study based on an active-source seismic survey using accelerated weight drop sources show that there exist complex structures caused by major and small-scale faults in this region. However, because the low signal-to-noise ratio of the active-source data and the dominance of surface waves in the shot gathers, it is challenging to perform a reliable S-wave velocity tomography using the data. In this study, we intend to specifically use enhanced surface waves to derive a high-resolution near-surface S-wave velocity model to support the subsurface characterization task. We use a super-virtual interferometry technique to denoise and extract surface waves from the gathers. The resulting enhanced surface waves extend to large offset with high signal-to-noise ratio and show evident dispersion characteristics. Then we adopt a global-correlation-based elastic full-waveform inversion (EFWI) to obtain a high-resolution near-surface S-wave velocity model. We find that our surface-wave EFWI reveals a concentrated low-velocity zone around the intersection region of the active-source seismic survey lines, indicating a possible fault zone at this location. In addition, the lateral variations of the inverted S-wave velocity are consistent with the locations of major faults in the geological framework model (GFM) built for this region. Our subsurface characterization results refine the understanding of the near-surface geology in this region and provide valuable velocity and structural information for the modeling & simulation effort for underground explosions monitoring.