Abstract:
The seismic signals designated P[inf o] and S[inf o] are oceanic crustal phases that can be recorded by hydrophones and ocean bottom seismometers. They are characterized by long high-frequency codas and by their efficiency of propagation. Many features of their propagation can be accounted for by considering water-sediment reverberation and fine random layering on the oceanic lithosphere. However, it appears that some range-dependent mechanism, perhaps forward scattering from rough surfaces, is necessary to completely describe the observed signals. An energy diffusion theory based on the coupled mode representation for the solution of the wave equation is described. The theory treats the diffusion of energy in spectral space in the strongly forward scattering limit, and may provide a realistic physical model for the development of the long P[inf o]/S[inf o] coda. The energy diffusivity is derived directly from the continuum limit of the mode coupling matrix; it is proportional to the spatial autospectrum of the coupling matrix. The applicability of the theory requires that the mean free path for multiple scattering must be large compared to the size of the medium fluctuations, and the spatial scale of the heterogeneities must be large compared to the wavelength. [Work supported by ONR.]