Low-frequency negative bottom loss: An effect of the ocean-bottom acting as a focusing mechanism

Abstract

Ocean bottom reflectivity is generally described by a single function, the reflection coefficient (R), defined as a ratio of reflected-to-incident intensities; −10 log(R) is bottom-reflection loss. Measuring bottom-reflection loss within the constraints of the definition is virtually impossible. In practice it is estimated by (1) measuring propagation loss of acoustic pulses transmitted via the path which interacted once with only the bottom, (2) calculating the water-region propagation loss only for an idealized bottom-reflection path by assuming a flat, single-interface bottom having a reflection coefficient of one, and then (3) comparing (measured minus calculated) values. This estimate is called bottom loss. The above assumption inherently ignores subbottom refraction and reflection of sound; important effects at low frequencies (<500 Hz). Sound returning from the subbottom can constructively interact with sound reflected from the water-sediment interface, yielding “negative bottom loss” results. This paper presents such results (obtained using a self-calibrating bottom-loss measurement technique) and suggests that the ocean bottom, by refracting 2nd reflecting sound, can partially focus energy over a considerable volume within the ocean. Three following companion papers discuss aspects of this premise. [This work was sponsored by Naval Sea Systems Command, SEA 06H1-4, A.P. Franceschetti.]