Approximating moments of acoustic fields in random media

Starting from a path integral representation of the fourth moment of an acoustic field, two types of approximations are considered in detail: a 'zero-kink' approximation and a 'single-kink' approximation. The expressions 'zero-kink' and 'single-kink' describe families of paths which are used to approximate path integrals over all continuous paths. The single-kink approximation reduces the propagation problem to a phase screen problem with fluctuations on the phase screen containing information about fluctuations along entire paths from source to receiver.