ACOUSTIC REFLECTION FROM QUASI-PERIODIC SEDIMENTARY SEQUENCES

A useful theoretical measure of acoustic interaction with the seafloor is the plane wave reflection coefficient (R), which is commonly expressed as bottom loss (- 20 log Absolute value of R). Predictions indicate that when the seafloor is modeled as a refracting layer over a basement half-space, the bottom loss is proportional to frequency. Bottom loss measurements in the 50-1600-Hz band, however, frequently show a loss that is inversely proportional to frequency. For example, roughly one-third of a set of measured bottom loss data in the North Atlantic exhibit this anomalous frequency dependence. It is concluded that the anomalous frequency dependence is due to sedimentary fine-scale layering arising from turbidity currents. The evidence presented consists of ( 1 ) a high correlation between the anomalous bottom loss stations and the bounds of the abyssal plains and (2) favorable predictions between a model accounting for the layering and the measured data. The study area was the western North Atlantic including the Sohm, Hatteras, and Nares abyssal plains. A simple, approximate stochastic model of reflection from a quasiperiodic sedimentary sequence was developed that appears to account for the dominant physical mechanisms important in the reflection process in this type of physiographic province.