Velocity Model Building with Guided Wave Inversion

Guided wave inversion (GWI) estimates accurate P-wave velocity in the near surface by analyzing the dispersion curves of guided waves. However, the GWI problem is highly non-unique without proper treatment of both fundamental and higher-order modes. To reduce the non-uniqueness of the inversion, a two-stage inversion scheme is used. The water velocity and depth are inverted first using a picked dispersion curve of the fundamental mode. Secondly, higher modes are incorporated to invert for sub-waterbottom velocities with a fixed water column. In our example from the North Sea, near-surface P-wave velocity from the GWI is introduced to the initial model for Full Waveform Inversion (FWI) of towed steamer data. In addition to producing high resolution shallow velocity for interpretation, the GWI adds a long-wavelength change to shallow near-surface velocities which reduces cycle skipping for 4-6 Hz FWI. Consequently, 6 Hz FWI converges faster and produces more geologically plausible velocity perturbations. The example shows GWI improving the seismic image and gather flatness for both shallow and deep targets.