Low Frequency Bottom Reflectivity from Reflection

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 at 40-10 Hz to about 20 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 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.