Solution of constant values of φ for WKB approximation of a fast wave generated on lower boundary for x₀ = 0, −1 ≤ y₀ ≤ 0, z₀ = 0.2 and its resultant propagation in the yz-plane at various times. Displayed times have been chosen to best illustrate evolution so time between frames is not necessarily uniform. The wavefront consists of crosses from the WKB wave solution, to better illustrate the evolution. The blue line indicates the location of the X-line.