Building starting model for full waveform inversion from wide-aperture data by stereotomography

Building a reliable starting model remains one of the most topical issues for successful application of full waveform inversion (FWI). In this study, we assess stereotomography as a tool to build a reliable starting model for frequency-domain FWI from long offset (i.e., wide-aperture) data. Stereotomography is a slope tomography method based on the use of traveltimes and slopes of locally-coherent events in the data cube. A key feature of stereotomography is that it can be coupled efficiently with semi-automatic picking, which partially frees one from the tedious and difficult interpretive traveltime picking. We assessed a tomographic workflow based on stereotomography and frequency-domain FWI with the 2D acoustic synthetic Valhall case study. The Valhall model is<br>mainly characterized by a large-scale low velocity zone associated with gas layers above the reservoir level. We first computed an acoustic full-wavefield dataset using a finite-difference time-domain modeling engine for a wide-aperture survey with a maximum offset of 16 km. The source bandwidth is between 10 and 45 Hz. Compared to the conventional application of stereotomography, we assess in this study the benefits provided by the joint inversion of refraction and reflection traveltimes from long-offset data. Use of refraction traveltimes is expected to stabilize and improve the reconstruction of the shallow part of the model. In a similar manner for frequency-domain FWI, we design a multiscale approach which proceeds hierarchically from the wide-aperture to the short-aperture angles to mitigate the non-linearity of the inversion. Starting models for FWI were built by stereotomography using two sets of picked events. For the first data set, the picking is limited to reflection traveltimes with a maximum offset of 4 km, while both refracted and reflected events were picked in the second case using the full range of offsets (± 16 km). We highlight the improvements of the FWI results obtained from the starting stereotomographic model built from the long-offset data set. The improvements are observed at the reservoir level below the gas layers but also in the upper part of the model where the joint use of refraction and reflection traveltimes is helpful to improve the ray illumination.