The effect of near‐surface layering on the reflectivity of the ocean bottom

Abstract

The effect of near‐surface layering in marine sediments on the plane‐wave reflection coefficient R of the ocean bottom is examined using a numerical model. A comparison of bottom reflection loss, BRL = −20 log10 (‖R‖), from a 500‐m‐thick sediment with and without realistic near‐surface layering shows that additional reflections from the layering can reduce 1/3‐ octave averaged BRL (BRLA) by more than 10 dB at 1600 Hz for grazing angles above 20°. This results in a frequency inversion since, at lower frequencies, the effect of the layering is much smaller; at 50 Hz there is almost no effect. An important feature of this dramatic decrease in BRLA is its relative independ‐ ence from the details of the layering structure. However, the exact nature of the layering does have an important effect on BRL and the phase of R at a single frequency, and on BRLA at about 800 Hz. Studies of an artificial layered structure made up of equally spaced, identical layers show that the physical process producing the decrease in BRL is a collective effect of the layered structure and is related to the constructive interference of reflections from each of the layers.