Two-interval high-accuracy NMO approximation

摘要

This paper discusses reducing computation costs for traveltime calculations in multi-layered anisotropic models. Fomel and Stovas suggested a five-parameter approximation that they named "generalized" because it reduces to several known three-parameter forms. Model tests, demonstrated by the authors, showed that this generalized approximation provides very high accuracy, implying that it can be used in place of the exact moveout function in modeling, migration and traveltime inversion. However, detailed model studies show that for some models, with high-velocity layers, this approximation leads to significant errors. I developed a new three-ray approximation that provides higher accuracy and can replace the exact traveltime function that requires numerical ray calculations for each receiver. I. call it "two-interval approximation" because it uses two different equations for two offset intervals. Model tests show this two-interval approximation can bring much higher accuracy compared to the generalized approximation because of the usage of an additional reference ray. The new two-interval approximation can be used instead of exact traveltimes for many practical purposes.