Reverse-time migration (RTM) is a powerful migration method provided that an accurate velocity model can be constructed. The full elastic wave equation is computationally expensive for modelling and migration. As a result it is preferred to propagate each body wave mode separately with approximate equations instead of propagating with the full elastic wave equation. In isotropic homogeneous media pressure waves satisfy an acoustic wave equation. In heterogeneous isotropic media this wave equation although not strictly valid can be used to approximate propagation of single body wave modes. This acoustic wave equation propagates with the correct phase velocity and ignors mode conversions. For anisotropic media the same procedure can be used to propagate quasi-P and quasi-S waves. The resulting equation is called a pseudo-acoustic wave equation. This equation contains generalizations of partial derivatives called pseudo-differential operators.
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