AVO analysis is an important basic technique in exploration geophysics for lithology and fluid estimation. Currently, the AVO equation of Aki and Richards (ARE), which is approximation of the Zoeppritz equation (ZE) is widely used for AVO analysis and AVO inversion. However, ARE assumes small an elastic property contrast between media. This study investigated a condition where the approximated AVO relation can be deviated from the actual equation, by using the regional rock physics model in the Browse Basin and calculating angle-dependent reflectivity. The intercept, gradient (and curvature) term originally defined by ARE are calculated on ZE as if ZE equation is approximated with these two and three terms. The calculation is performed between two media, seal as the upper medium, and sandstone and shale as the lower medium. These approximated two and three terms are compared with the original parameters of ARE. The study revealed that the 3-term ARE has more aligned with ZE derived angle-dependent reflectivity than that of the 2-term. Deviation of the angle-dependent reflectivity from ZE occurs around 20 degree. Comparison of two approximation methods (2-term vs 3-term) indicates, the 3-term equation has smaller gradient errors with ARE than the 2-term equation at target depth. In the 2-term approximation, gradient errors get larger with the larger angle utilized for the approximation. This implies that, for the Browse Basin, the 3-term ARE approximation is preferable to correctly estimate angle-dependent reflectivity. For use of the 2-term approximation, narrower angle range (around 20 degrees) is suggested otherwise error correction is required (at far angle range). These results emphasize the requirement of careful investigation of seismic quality specifically at far angle, and analysis of angle-dependent reflectivity of expected lithology for AVO analysis.

Open-Access Online Publication: May 29, 2023