Low-frequency, long-offset elastic waveform inversion in the context of velocity model building

Classic imaging approaches consist of splitting the earth into background and reflectivity models. When justified, this separation of scale is quite powerful, although this approach relies on some smoothness and weak contrast assumptions. This approach allows for the imaging methods to be based on acoustic wave propagation after having identified the compressional waves through picking or signal processing. Over the past years, wave-equation tomography and waveform inversion approaches have become routine, complementing the classic approaches to derive background models. They do not rely on high-frequency picks, unlike ray-based traveltime tomography, but on low-frequency cross-correlation to define time shifts and on waveform matching. In the presence of large earth parameter contrasts, time shifts and waveforms of compressional waves may depend on elastic parameters when interferences occur within the Fresnel zones. This challenges the recovery of the background model under an acoustic assumption with low-frequency data. Accounting for an elastic propagation in waveform inversion, even in the context of model building, could help to reduce the artifacts seen in acoustic results. A synthetic and a real data example are presented to illustrate the potential benefit of using an elastic waveform inversion approach when inverting long-offset, low-frequency seismic data.