Olivieri, Marc P.

Buckingham et al. (Nature Vol. 356, p 327) first introduced the concept of acoustic imaging using ambient noise as a method for passively detecting objects in the ocean. Several analytical studies followed, and it was shown that a two dimensional acoustic image could be obtained based on this approach, and that at least 900 pixels are necessary to restitute the details of spherical objects placed in an underwater sound channel. The alternative approach described in this paper is based on a signal processing which uses the broadband nature of the ambient noise in the ocean, and therefore, optimizes the use of available sound energy scattered by the object. Images with thousands of pixels can be obtained using a relatively small number of transducers. This method has been validated using simple experiments in air, scaled to represent an ocean application, and results showing images of various objects will be presented.

Ocean ambient noise has been studied extensively in the past, and several models have been developed to predict its level and to characterize it in different ocean environments. However, most of these studies have been motivated by the need to reduce the negative effect of the ambient noise on measurements. Recently, several studies have used the ambient noise to obtain information about the ocean floor and to achieve passive target detection. This dissertation describes the development of an Ambient Noise Sonar (ANS) which can be used to measure ambient noise directivity to obtain information about the environment. The system was designed for use on an Autonomous Underwater Vehicle (AUV) and has been used to study the ambient noise in the near shore region by investigating different types of noise in shallow water, such as boat traffic, biological noise and naturally occurring noise up to 20 kHz. There are two advantages to the ANS. First, when mounted on an AUV its mobility will allow the ambient noise to be mapped over an extended area. Second, the very small number of transducers (6 total) used in the array, makes this system suitable for AUV operations because it uses a limited amount of space and power. This dissertation presents the theory of the array processing and preliminary results including examples of boat noise, and biological noise. It was found that the noise from biological sources such as snapping shrimp contributed to the anisotropic component of the acoustic field. These biological sources were found to be concentrated around artificial structures such as piers and groins, and also around natural reefs. Finally noise maps of coastal areas are presented to illustrate the potential of this system to measure the noise field in the coastal region and to evaluate acoustic propagation using passive sources such as the biological noise clusters found along the coast. In the future the ANS will be integrated onto the AUV 'Ocean Explorer II'.