Equivalent Medium Parameters for Numerical Modeling in Media with Near-Surface Low Velocities

We have developed a methodology to discretize an isotropic velocity model with low velocities near the free surface for full waveform numerical modeling. The method modifies the near-surface minimum velocity in a given (original) model by replacing parts of the model with equivalent medium parameters (EMP). The discretized model (with EMP) has a higher minimum velocity and minimizes the difference between the seismograms evaluated for the original model and the model with EMP. The method is suitable for studies requiring full waveform numerical modeling with a limited frequency range (such as a finite-difference full waveform modeling in a sedimentary basin). The discretized model with EMP is set to match locally surface-wave velocities evaluated in the original model over the frequency range of interest. The difference in group velocity calculated for the original vertical profile and the vertical profile with EMP provides an estimate of the error due to the modification of the original model. Online material: source code for the discussed algorithm.