In environments with water depth variations, one-way modal solutions involve mode coupling. Higham and Tindle developed an accurate and fast approach using perturbation theory to locally determine the change in mode functions at steps. The method of Higham and Tindle is limited to low frequency(≤250 Hz). We extend the coupled perturbation method, thus it can be applied to higher frequencies. The approach is described and some examples are given. In environments with water depth variations, one-way modal solutions involve mode coupling. Higham and Tindle developed an accurate and fast approach using perturbation theory to locally determine the change in mode functions at steps. The method of Higham and Tindle is limited to low frequency We extend the coupled perturbation method, thus it can be applied to higher frequencies. The approach is described and some examples are given.