The mean and variance of the acoustic field forward propagated through an ocean waveguide containing random internal waves is modelled using a normal mode formulation [1] that takes into account the 3-D interaction of the acoustic field with medium inhomogeneities. The effects of dispersion, attenuation and redistribution of modal energy due to random multiple scattering in the forward azimuth are treated analytically. The inhomogeneous medium's scatter function density is modelled using the Rayleigh-Born approximation to Green's theorem to account for random fluctuations in density and compressibility caused by the internal waves. The generalized waveguide extinction theorem [2] is applied to determine the attenuation due to scattering from the internal wave inhomogeneities. Simulations for typical continental shelf environments quantitatively show how coherent and incoherent acoustic field intensities vary with range, depth and internal wave field properties