Validity of acoustic early‐arrival waveform tomography for near‐surface imaging

Time domain early-arrival waveform tomography is an advanced approach to image the near-surface velocity structures. For the reason of computation efficiency, we solve an acoustic wave equation problem instead of elastic wave equation for forward modeling. Since we are fitting the amplitude and phase of the early arrivals, but the waveform includes converted waves, we should understand the validity of such acoustic waveform tomography for imaging the near-surface velocity structures in reality. We design numerical experiments to compute the waveforms by applying elastic wave equation modeling and then apply acoustic early-arrival waveform tomography to study the liability of the imaging solutions. The results suggest that the imaging solutions is sensitive to the window size. It should be reliable as long as the time window of the early arrivals is properly selected. Testing the foothill velocity model with elastic waveform input produces a reasonable near-surface solution, but does require more sophisticated processing to remove elastic effects.