Estimating Overburden Velocity Changes from Pre-stack Time Shifts Using Linear Tomography
J.A. Edgar and T.D. Blanchard
Event name: 77th EAGE Conference and Exhibition - Workshops
Session: WS02 - Getting the Most out of 4D - From Reservoir to Surface
Publication date: 01 June 2015
Info: Extended abstract, PDF ( 1.25Mb )
Price: € 20
Production from oil and gas fields induces velocity changes and strains in the subsurface, which create time shifts between vintages of time-lapse seismic data. Recovering these time shifts can be useful for reservoir management, particularly through calibration of geomechanical models. Rickett et al. (2007) and Williamson et al. (2007) have shown that these shifts can be estimated using non-linear inversion of post-stack time-lapse seismic. These existing inversions of post-stack data assume that the seismic ray paths in all vintages are vertical. However, examining pre-stack time-lapse data shows that the recorded time shifts are offset dependent. To estimate the true subsurface changes we must invert pre-stack time-lapse seismic, which requires more accurate modelling of the seismic energy propagation. This abstract introduces a new tomographic inversion of pre-stack time-lapse seismic which aims at estimating the velocity changes that explain all pre-stack time shifts. We retain the assumption of the existing post-stack methods that the ray paths do not change between vintages, but discard the assumption that they are vertical. This allows a linear tomography system to be set up and solved for velocity change. We test this method on synthetic data and compare the results with an existing post-stack approach.