Quantifying acoustic–elastic AVO consistency for seismic monitoring of CO₂ storage

Amplitude-versus-offset (AVO) analysis is widely used for monitoring sequestered CO₂ in geological formation, because fluid changes affect reservoir elastic properties and seismic amplitudes. However, the ability of acoustic modeling to reproduce elastic PP (AVO) behavior remains unclear. This study examines a CO₂–brine sand interface using analytical reflectivity, 2D finite-difference modeling, and Gassmann fluid substitution. The two- and three-term Shuey approximations were first compared with the Zoeppritz solution over 0–30°, followed by an acoustic–elastic comparison using the two-term formulation. Acoustic and elastic simulations were then conducted under identical modeling, acquisition, and processing conditions. Analytically, the intercepts were identical (0.059874), while the gradients differed by only 0.022%, indicating near-equivalent reflectivity. In contrast, numerical intercepts of 0.850 and 0.756 and gradients of − 2.210 and − 3.310 yielded differences of 12% and 33%, respectively. The divergence increased with incidence angle and may reflect a combination of physical, numerical, and processing effects. Gassmann modeling over 0–100% CO₂ saturation showed pronounced P-wave velocity sensitivity, whereas the AVO curves remained closely spaced, indicating limited sensitivity to partial saturation. Overall, acoustic modeling provides a useful first-order approximation of elastic PP reflectivity over 0–30°, with the numerical simulations showing qualitative consistency in AVO trends despite quantitative amplitude differences. This study extends analytical comparisons by testing whether acoustic–elastic agreement persists under controlled numerical wave propagation and by evaluating the extent to which modeling effects may influence amplitude changes relevant to CO₂ monitoring.