Subsea Karst Detection and Imaging Improvement Using Full Waveform Inversion

This paper describes the construction of a high-resolution anisotropic FWI model that is used to help image a karstified carbonate layer (Faumai) and also to help define the underlying clastic reservoirs. The karst zones show extreme velocity rugosity. Conventional reflection tomography cannot resolve these anomalous velocities, and the seismic images below these karstified carbonates are distorted and are poor for interpretation. In 2009 an isotropic FWI algorithm, up to 7Hz, was applied to this OBC survey by BP. Image quality was significantly improved. However, in and below the karst formation, the FWI velocity resolution improved less. In 2015, anisotropic FWI, up to 12Hz including Q attenuation, was applied to this survey using the latest algorithms and methodologies. This paper presents the improved results and discusses the best strategies for determining the appropriate input data for FWI (diving-wave, reflection, or both), and depth limitation. The complete modelling required five components: diving-wave velocity updates to depths of 2200m; reflection waves included from 1600m onwards; derived anisotropic values; derived Q-model; and residual curvature tomography.