Among varieties of waveform inversion techniques, acoustic waveform inversion has been a popular choice because of its simple formulation and modest computational costs. However, the earth consists of elastic materials, and thus there remain concerns about reliability, since behaviors of elastic wavefields, such as P-S convergence, are not properly accounted for. We demonstrate the practical validity of acoustic waveform tomography in marine settings using real data and synthetic studies. Ocean Bottom Seismograph (OBS) data in the seismogenic Nankai subduction zone were inverted with the acoustic implementation. We clearly delineated major geological features including the mega splay fault, and thrusts in the accretionary prisms. The mega splay fault accompanies a low velocity layer, which indicates fluid migration or a lithology change. The fault structure underneath the ridge could be debatable, due to the similarity to the topography. Synthetic waveforms kinematically well coincides with observed waveforms, but there remain discrepancies in amplitudes. The results validate the applicability of waveform tomography to elastic wavefields, but the elastic and attenuation effects need to be investigated further. In order to validate the real data results, preliminary 1D evaluation of the inverted results was conducted with synthetic elastic wavefields. The recovery of major structures was verified, but degradation was admitted in vertical velocity contrasts. 2D synthetic results will be computed to further investigate the ability of the acoustic implementation to retrieve spatial velocity contrasts, and the contamination by topography effects.


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