Muhumuza, K., Jakobsen, M., Luostari, T. and Lahivaara’ T., 2018. Seismic monitoring of CO injection using a distorted Born T-matrix approach in acoustic approximation. Journal of Seismic Exploration, 27: 403-431. Monitoring the injected CO: distribution is at the core of any carbon capture and storage project. Waveform inversion methods can be used to obtain high-resolution images for monitoring the injected CO in the subsurface, but remains computationally challenging. Efficient modelling approximations are desirable for solving time-lapse inversion problems and to test the settings in which they give accurate predictions. We employed the distorted Born approximation (based on scattering integral equation) to simulate time-lapse synthetic data for a CO injection scenario with a single injector, and benchmarked it against the finite element and exact T-matrix approach. The distorted Born approximation presented, considers a general heterogeneous reference medium; and provides a framework for imaging of regions of time-lapse variation using the baseline survey as a reference and the monitor survey as perturbed to directly estimate the perturbation. Based on our simplified velocity model of CO injection, synthetic testing demonstrated that the new distorted Born approximation provides accurate predictions of the difference data seismograms. We tested the distorted Born iterative T-matrix (DBIT) inversion method on a synthetic dataset generated using T-matrix forward modelling, and 0963-065 1/18/$5.00 © 2018 Geophysical Press Ltd. 404 we investigated three inversion approaches. The inversion results showed that the DBIT method sufficiently retrieves the time-lapse velocity changes even in the cases of relatively low signal-to-noise ratio. The inversion approach that focused on the time- lapse data variation (perturbation only) gave improved results in noisy and noise free environments. We also applied DBIT inversion to a synthetic dataset generated using the finite element method, in order to avoid inverse crimes. The inversion recovered the trend of the velocity models, but with some inaccuracies in the estimates for the time- lapse velocity changes. The DBIT method which considers a dynamic background media and T-matrix approach may be a potential tool in seismic characterisation of subsurface reservoirs and efficient for monitoring of CO2 sequestration.