Elastic versus acoustic 3-D Full Waveform Inversion at the East Pacific Rise 9°50'N
Résumé
In the field of controlled source seismology, the acoustic 3-D Full Waveform Inversion (FWI) technique has become a common tool for imaging geologically complex structures in land, as well as in marine settings. However, the Earth behaves elastically and, therefore, excluding the elastic effect could have a significant impact on the inversion results, especially for interfaces with large S-wave velocity contrasts. To examine the contribution of the elastic approach, we compare acoustic and elastic 3-D FWI applied to a 3-D seismic data set from the East Pacific Rise (EPR) 9°50' N, where the subsurface is represented by igneous basaltic rocks. To establish an efficient inversion strategy, we first conducted a number of tests, which suggest a simultaneous, multiparameter inversion as the most efficient approach when inverting signals with frequencies below 7 Hz. The reduction in the total misfit for the elastic case is 10-15 per cent lower than that for the acoustic one, suggesting that the elastic approach explains the observed data better than the acoustic approach. Furthermore, the compressional velocity images of the upper-oceanic crust obtained using the two approaches differ significantly, not only in velocity magnitude but also structurally. We argue that the results obtained from the acoustic modelling are geologically less plausible and suggest the elastic model as a more reliable representation of the upper-oceanic crust.
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