Recent years have seen the emergence of full-waveform inversion (FWI) approaches for generating detailed velocity models for imaging. For land seismic data, especially in areas of complex near-surface geology, a proper near-surface model is crucial for imaging. These two observations naturally lead to the question whether FWI approaches can also be used for near-surface model building purposes. As most wave propagation effects in the near surface are inherently elastic, this requires elastic FWI. The combination of small scales involved, and typical large sizes of modern land surveys, may make an approach based on finite differences or finite elements computationally very demanding. We propose a fast approximation to elastic FWI, modal elastic inversion, where wave propagation in the near surface is modeled by means of a (small) number of horizontally propagating waves: ground roll, its higher modes and guided waves. These waves comprise the vast majority of energy in land data and are sensitive to near-surface velocities. We apply modal elastic inversion to a 2D test line and show that both the shallow P-wave and S-wave velocity models can be recovered well.


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