Application of Frequency-dependent Traveltime Tomography and Full Waveform Inversion to Realistic Near-surface Seismic Refraction Data

Abstract

We present a synthetic test that uses a workflow consisting of a new frequency-dependent traveltime tomography (FDTT) method to provide a starting model for full waveform inversion (FWI) for near-surface seismic velocity estimation from refraction data. Commonly used ray-theory-based traveltime tomography methods may not be valid in the near surface given the likelihood of relatively large seismic wavelengths compared to the length scales of heterogeneities that are possible in the near surface. FDTT makes use of the frequency content in the seismic waves in both the forward and inverse modeling steps. In this application to a near-surface benchmark model, the results show that FDTT can better recover the magnitude of velocity anomalies than infinite frequency (ray-theory) traveltime tomography (IFTT). FWI can fail by converging to a local minimum when there is an absence of sufficiently low frequency data and an accurate starting model, either of which, if present, can provide long-wavelength constraints on the inverted velocity model. Both IFTT and FDTT models can serve as adequate starting models for FWI. However, FWI produces significantly better results starting from the FDTT model as compared to the IFTT model when low frequency data are not available. The final FWI models provide wavelength-scale structures allowing for direct geologic interpretation from the velocity model itself, demonstrating the effectiveness of FDTT and FWI in near-surface studies given the modest experiment and data requirements of refraction surveys.