2D surface wave inversion on the DAS data at CaMI Field Research Station: Multi-offset MASW vs FWI
Luping Qu, Kristopher A. Innanen, Jan Dettmer, Wenyong Pan
Two major methods for 2D near-surface investigation are the Multi-offset MASW (MO-MASW) and surface-wave full waveform inversion. We test these two methods and analyze their limitations and advantages. MO-MASW can roughly detect and locate the abrupt lateral variation. However, it has difficulties in resolving irregular anomalous bodies or complex subsurface velocity structure. Full waveform inversion method overcomes these problems by providing a detailed subsurface shear-wave velocity structure, but easy to fall into local minimum since surface wave has a shorter wavelength. We test the surface wave full waveform inversion method on synthetic models containing high-velocity and low-velocity anomalous bodies. To avoid the cycle skipping, misfit function, combining waveform-difference and envelop-difference, is adopted, and a frequency-decreasing multi-scale approach is incorporated. From our synthetic tests, we find besides the negative and positive velocity anomalies, the nested velocity anomalies can be well recovered as well. At last, a preliminary test on surface-trenched DAS data is conducted using this multi-scale full waveform inversion process.