Bubble acoustics: what can we learn from cetaceans about contrast enhancement?

Abstract

The profound effect of bubbles on the propagation of sound and ultrasound through liquids and tissue has meant that understanding of this process is key to a wealth of applications. These range from cases where that interaction is exploited (such as in the use of biomedical contrast agents) to circumstances where the potency of the effect massively hinders our capabilities (for example, the operation of sonar in coastal waters). The two diagnostic examples given above are revealing. The fact that in biomedicine the bubbles are exploited, whilst in the oceanic case they are problematic, stems from the readiness with which the biomedical field has embraced the concept of bubble nonlinearity, compared to the response of the sonar community, which relies upon linear propagation models. This is not because of differences in the abilities of the workers in the two fields, but rather for two more subtle reasons: first, the bubble size distribution for contrast agents is so well-known and well- constrained that researchers in the field need rely on little more than single-bubble models. This compares to the oceanic case, where the distribution of bubble radii will often span four orders of magnitude, will change dramatically over the course of a single measurement, and is often unknown. Indeed, the usual course in ocean acoustics is to appeal to historical datasets (often taken in vastly different environments, such as surf zone and deep water, with a wide range of windspeeds, fetch and air/sea temperatures etc.). These data provide some sort of estimate against which, for any given bubble size, it is hoped that the actual bubble number density does not vary by more than one order of magnitude. Second, the task in ocean acoustics would be to minimize the contribution of the bubbles to the detected signal and enhance the scatter from some other target. This undertaking is vastly more complicated than the task with biomedical ultrasonic contrast agents, which is to maximize the scatter from the bubble as opposed to the tissue. However there are intelligent creatures with a lifetime of experience of working in ocean acoustics, and generations in which to evolve techniques for coping with bubbly ocean water. This paper addresses the question of what physics would allow the cetaceans to do in bubbly ocean water in order to exploit the peculiar propagation conditions there. This question is particularly apt given that there are indeed instances where cetaceans generate bubbles in the water in order to facilitate their hunting. The question of whether cetaceans do indeed exploit the available physics is beyond the scope of this paper. However in discovering what techniques the physics would allow them to exploit, the opportunity opens up for humans to exploit the same techniques in order to enhance sonar in bubbly ocean water, and to enhance the exploitation of ultrasonic contrast agents.