Modeling backscatter of marine mammal biomimetic sonar signals from bubble clouds

Bubbles are powerful and complex sound scatterers in water because of the impedance mismatch at each water-air/gas interface. Bubbles are formed by natural processes that include rainfall, gas emission from the sea bed, boat wakes, living or decomposing organisms, and wave breaking- the latter being the dominant cause of bubble entrainment in the surf zone. Despite the complications of sound propagation in bubble-populated water, some species of cetaceans have been observed to hunt efficiently in shallow coastal waters and in biologically active rivers where bubbles persist. In contrast, the performance of man-made sonar systems has always been greatly handicapped by this phenomenon and improvement is necessary. How cetaceans manage to overcome the problem of echolocating through bubbles clouds is still largely unknown. However, if we can gain a more complete understanding of the ways in which bubbles respond to echolocation signals transmitted by cetaceans, this might provide insight into the strategies they use for detecting prey under such conditions, or, indeed, how they manipulate the ´bubble screen´ effect to catch fish. New insights could then contribute to the development of improved, bio-mimetic sonar systems for use in difficult environments such as the littoral zone. This paper presents a model describing the backscatter of marine mammal bio-mimetic sonar signals from bubble clouds. Echolocation signals from a dolphin and a porpoise were modeled, and we simulated the backscatter response of single bubbles from a discretized bubble population distribution when driven by each type of echolocation click. Responses from individual bubbles with various radii were then used to find the total bubble cloud backscatter response. This paper explores the backscatter from bubble clouds using two types of bio-mimetic echolocation pulses that have different signal characteristics.