Building starting models for full waveform inversion from wide‐aperture data by stereotomography

Abstract

ABSTRACTBuilding an accurate initial velocity model for full waveform inversion (FWI) is a key issue to guarantee convergence of full waveform inversion towards the global minimum of a misfit function. In this study, we assess joint refraction and reflection stereotomography as a tool to build a reliable starting model for frequency‐domain full waveform inversion from long‐offset (i.e., wide‐aperture) data. Stereotomography is a slope tomographic method that is based on the inversion of traveltimes and slopes of locally‐coherent events in a data cube. One advantage of stereotomography compared to conventional traveltime reflection tomography is the semi‐automatic picking procedure of locally‐coherent events, which is easier than the picking of continuous events, and can lead to a higher density of picks. While conventional applications of stereotomography only consider short‐offset reflected waves, we assess the benefits provided by the joint inversion of reflected and refracted arrivals. Introduction of the refracted waves allows the construction of a starting model that kinematically fits the first arrivals, a necessary requirement for full waveform inversion. In a similar way to frequency‐domain full waveform inversion, we design a multiscale approach of stereotomography, which proceeds hierarchically from the wide‐aperture to the short‐aperture components of the data, to reduce the non‐linearity of the stereotomographic inversion of long‐offset data. This workflow which combines stereotomography and full waveform inversion, is applied to synthetic and real data case studies for the Valhall oil‐field target. The synthetic results show that the joint refraction and reflection stereotomography for a 24‐km maximum offset data set provides a more reliable initial model for full waveform inversion than reflection stereotomography performed for a 4‐km maximum offset data set, in particular in low‐velocity gas layers and in the deep part of a structure below a reservoir. Application of joint stereotomography, full waveform inversion and reverse‐time migration to real data reveals that the FWI models and the reverse‐time migration images computed from the stereotomography model shares several features with FWI velocity models and migrated images computed from an anisotropic reflection‐traveltime tomography model, although stereotomography was performed in the isotropic approximation. Implementation of anisotropy in joint refraction and reflection stereotomography of long‐offset data is a key issue to further improve the accuracy of the method.