Low‐frequency backscatter from near‐surface bubble plumes

Abstract

Anomalous low‐frequency backscatter from the sea surface is governed by a variety of near‐surface effects. Principle among these are microbubble distributions and their subsequent effects on the local sound speed structure. Although a rigorous explanation of these scattering results (including the presence of spikey events often observed in both deep water scattering experiments and sonar trials) would likely require consideration of a host of physical acoustic mechanisms, Occum’s razor would dictate that a simple explanation can be both effective and illuminating. This paper examines the role of monopole radiators, the simplest form of radiation, beneath a rough pressure‐release surface to explain the presence of low‐frequency false targets. It is shown that for certain sea state conditions such false targets can result from transient bubble plumes acting as acoustically compact scatterers. An attempt will also be made to estimate, from the combined probabilities of the presence of white caps and the likelihood that a given bubble plume possesses the requisite volume fraction to produce such an event, the number of false targets to be expected as a function of sea state. [Work supported by ONR and ARPA.]