Many approximations to different order of the one-way scalar wave equation have been suggested in seismic imaging or modeling. Of these approximations, the first-order approximation, usually called the 15-degree equation, is most commonly used in industry because of its high efficiency. However, one common constraint of all these approximations is that they cannot handle large-angle events exactly.
Through a linear transformation of the wave equation, the LInearly Transformed Wave EQuation (LITWEQ) is obtainable, without approximation. The LITWEQ has the form of the 15-degree equation. The solution to the LITWEQ is still a two-way wave solution. By imposing the boundary condition for upcoming (or downgoing) waves, the LITWEQ can be applied to seismic imaging (or modeling). Implementing the LITWEQ with a finite-differencing algorithm gives a 180-degree, or all-dip, finite-difference wave-extrapolation operator, which solves the angle limitation problem of conventional finite-difference methods.