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 expre ssed as bottom loss (- 20 log Absolute value of R). Predictions indica te that when the seafloor is modeled as a refracting layer over a base ment half-space, the bottom loss is proportional to frequency. Bottom loss measurements in the 50-1600-Hz band, however, frequently show a l oss 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 layeri ng arising from turbidity currents. The evidence presented consists of ( 1 ) a high correlation between the anomalous bottom loss stations a nd the bounds of the abyssal plains and (2) favorable predictions betw een a model accounting for the layering and the measured data. The stu dy area was the western North Atlantic including the Sohm, Hatteras, a nd Nares abyssal plains. A simple, approximate stochastic model of ref lection from a quasiperiodic sedimentary sequence was developed that a ppears to account for the dominant physical mechanisms important in th e reflection process in this type of physiographic province.