Carbon dioxide has been injected into the Utsira sand at Sleipner since 1996, with more than 8 mmt currently in the reservoir. Seismic monitoring surveys to follow the migration of the CO₂ in the reservoir have been conducted in 1999, 2001, 2002, 2004, and 2006. The CO₂ plume is imaged on the seismic data as a prominent multitier feature, comprising several bright subhorizontal reflections, growing with time and interpreted as arising from as many as nine discrete layers of high-saturation CO₂, each up to a few meters thick. A quantitative seismic interpretation of the time-lapse data has included synthetic seismic modeling to derive CO₂ distributions in the reservoir. Convolution-based modeling has shown that seismic reflection amplitudes are broadly related to layer thickness via a tuning relationship. However, acquisition geometry, lateral velocity changes, mode conversions, and intrinsic attenuation are all likely to affect amplitudes and need to be incorporated within a rigorous quantitative analysis. A first attempt to incorporate some of these effects, through more realistic prestack elastic modeling and processing, is presented here. Both the acquisition geometry and the processing sequence of the synthetic data are comparable to the real field data. Results support the basic amplitude-thickness relationship but with some important additional effects.