Assessing Velocity and Impedance Changes Due to CO2 Saturation Using Interferometry on Repeated Seismic Sources

The injection of CO2 into saline aquifers produces small changes in seismic properties. The aim of time lapse monitoring is to look for these small variations of the seismic attributes through time, such as the velocity and the impedance, and to relate them to perturbations in the medium properties. However, as injection proceeds, at higher levels of saturation, relative changes in the data become very small. Assuming that a time-lapse seismic survey is a doublet, we can determine the time delay between repeat survey differences, using wave interferometry. These are then relates to velocity changes. Additionally, we can do the same for amplitude variations and determine impedance modifications. We investigate the feasibility of this technique on 1) numerical data computed from a simple layered porous medium; and 2) for synthetic data calculated in a more realistic 3D reservoir using a poro-elastic finite-difference solution. In both cases, the reconstructed values are in good agreement with the true modifications.