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Improving pre-salt image with reflection full-waveform inversion in Santos BasinNormal access

Authors: W. Kang, M. Ortin, J. Monteiro, M. Fagundes, O. Zdraveva, D. Vigh and M. Shadrina
Event name: First EAGE Workshop on Pre-Salt Reservoir: from Exploration to Production
Session: Session III
Publication date: 05 December 2019
DOI: 10.3997/2214-4609.201903443
Organisations: EAGE
Language: English
Info: Extended abstract, PDF ( 555.26Kb )
Price: € 20

Over the last ten years, full-waveform inversion (FWI) has become established as an integral part of earth model building (EMB) workflows. Recently, the industry witnessed the introduction of many types of FWI, differing either by the portion of the wavefield used in the inversion or by the nature of the objective function. Among these methods, reflection full-waveform inversion (RFWI) can utilize reflection data to build smooth velocity updates that reach beyond the penetration depth of refractions that are used in conventional FWI. Due to the limited opening angle of subsalt illumination, RFWI becomes more and more popular to update velocity models in the subsalt zone. In this case study in the Santos Basin, we discuss RFWI strategies and results, in which the pre-salt target level image has a better quality and continuity after being migrated with a moredetailed model created using a comprehensive EMB workflow that includes common image point tomography and conventional least-squares FWI for the post-salt, CIP tomography and RFWI for the salt and pre-salt. From the various existing RFWI strategies, we adopted Born modelling to generate reflections and a traveltime-based adjustive objective function, and then form the gradient with a combination of crosscorrelation between forward or backward and source or scattered wavefield. This approach will be validated with quality controls in both the shot and image domains. The updated model along with its updated image generate modelled shots that are more consistent with observed data, and the updated image is more geologically plausible with stronger focusing and continuous events.

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