Effects of Upper Crustal Geoacoustic Parameters on Low Frequency Sound

Abstract

A seismo-acoustic paradigm of the upper crust is proposed, based on analysis of low-frequency reflectivity versus angle data at a 0.5 million-year old sediment-free site, and on spatially well sampled seismic refraction measurements at sediment-free and thinly sedimented sites in the Pacific Ocean. Crustal velocity measurements at thick sediment covered sites are evidently substantially higher, and are excluded from the analysis presented here. Interfacial compressional and shear speeds of thinly sedimented, geologically young upper crust (between 0.5 and 5 million years) at this site are estimated to be about 2800 m/s and 800 m/s respectively. At such sites sub-basement gradients are approximately 4 s-1. The rms roughness of the basement, which is Fresnel zone size dependent, has significant effects at frequencies as low as 10 Hz. Low shear speeds imply no shear critical angle. Hence, at low shear speed sites, low-frequency energy incident on the bottom at small grazing angles is in large part transmitted into the rock, refracted by the gradient, and reradiated into the water. Scattering loss occurs at both the initial incidence and the interaction of the refracted energy at the boundary. Low interfacial shear speeds lead to large grazing angles at the boundary for the transmitted shear waves, large wavenumbers and, hence large boundary scattering losses; small changes in interfacial shear speed produce large changes in subsurface boundary-scattering loss, and hence in the reflection coefficient. Sediment-free/thinly sediment-covered crustal shear speeds are projected to increase with age, eventually (at an unknown age) becoming faster than the speed of sound in water, resulting in critical angle reflection.