Acoustics 2000 1 Low Frequency Bottom Reflectivity from Reflection ,Alexander Kritski 1 and Chris Jenkins 2 1 School of Geosciences, University of Sydney, NSW, 2 Ocean Sciences Institute, University of Sydney, NSW. Abstract Ocean bottom reflectivity has been studied for a simple layered sedimentary model at shallow depth. Reflections from the bottom were determined from standard geophysical reflection data. The plane wave reflections have been obtained as bottom loss (-20log|R|) from the available reflection seismic data. Bottom loss estimated in the 10-250 Hz band are presented as functions of frequency and of angle of incidence (relative to horizontal) and are shown to depend on the properties of the layered ocean bottom. In the bottom loss results, a critical angle is observed which decreases from about 39 o at 40-10 Hz to about 20 o at 240 Hz. This critical angle is most likely associated with the reflection of compressional waves near the ocean-sediment interface. The frequency dependence indicates that the lower frequencies interact with a deeper sediment layer of greater sound speed than the near-surface sediments. At about 59 o there is an indication of another critical angle, possibly due to reflection from the substrate beneath the sediments. The significance of these results is that reflection seismic data can be used to derive inputs for naval sonar prediction models, especially in the frequency range of passive sonars. Indroduction Plane-wave reflection and refraction coefficients play an important role in the interpretation of acoustic data in modern marine seismology. These coefficients are generally based on the partitioning of energy at the interface between water and an elastic solid , and the result is the classical Rayleigh reflection and transmission coefficients that relate to the amplitudes of homogeneous incident waves and homogeneous reflected and refracted waves. For the case of an elastic solid, only two kinds of body wave can propagate and the particle motion is either parallel or perpendicular to the wave normal depending on whether a dilatational or shear wave is being considered. The angles of incidence and emergence are related by Snell's law and when one of the angles corresponding to a reflected or refracted wave increases to 90 0 , a "critical" angle of incidence is defined for the generating wave. When the angle of incidence of the generating wave exceeds a critical angle, the body wave which has become parallel to the interface no longer propagates and an interface wave is necessary to satisfy the boundary conditions. This wave decays exponentially away from the interface and its phase velocity is determined by the phase velocity of the generating wave projected onto the interface. A more realistic model of the sediments would include a viscous-elastic or porous viscous-elastic material rather then the elastic one. In this case there are fundamental differences in the response at the interface between water and sediment or/and between two different kinds of sediments. Reflected and refracted waves are rather inhomogeneous in the sense that the wave amplitudes vary in planes of constant phase and the trajectory of particle motion is elliptic in shape rather than parallel or perpendicular to the direction of the wave normal [8]. The reflectivity data used in the present work have been collected in experiments where receivers were located near the sea surface [1]. The geophysical information related to the all kinds of inhomogeneous waves including interface waves is not available at the receiving point as only compressional waves can propagate in the water column to reach the receivers. In this work the simple layered model has been suggested. The ocean is described by a constant sound speed half space, the bottom is described by a near surface sediment layer (200-300m depth) of constant sound speed gradient overlying another solid half space (low sediments, and then crust) of constant sound speed. The results of the reflectivity estimation, however, indicate that reflection coefficients become frequency dependent in a quite complex way. This effect might indicate viscous losses in the sediment layer or/and multilayering. Thus the bottom reflectivity measured from the interface for a simple model (homogeneous sediment layer with constant speed and attenuation) indicates that the properties of near-surface sediments fit a viscous-elastic model. Seismic reflection experiment Experimental setup The reflection seismic data used in these studies was collected from North West shelf area in the North Bonaparte Basin which situates between NW margin and the Timor Trough (Australian Geological Survey