Assume wave propagation is dominantly in the z -direction, and k 0 is taken in this direction. The SVEA is only applied to the second-order spatial derivatives in the z -direction and time. If Δ Δ ⊥ ⊥ ≡ ≡ ∂ ∂ 2 / ∂ ∂ x 2 + ∂ ∂ 2 / ∂ ∂ y 2 is the Laplace operator in the x × y plane, the result is: