Ambient noise imaging in warm shallow seas; second-order moment and model-based imaging algorithms

Ambient noise can be used to produce images of submerged-objects using the mean intensity of the backscattered energy, a technique coined "acoustic da ylight" because of its direct analogy to vision. It is suggested that there may be substantial additional information in higher moments of the data. A t high frequencies (>10 kHz), absorption suppresses long-range propagation so that a received signal is largely dependent on the local geometry,! sour ce characteristics, and the scattering properties of interceding objects. I t is shown that for snapping shrimp (Cragnon, Alpheus, and Synalpheus) illu mination (the primary sources in warm shallow water above a few kHz), signi ficant information is embodied in the second temporal moments of intensity. There is no visual analog to this-concept, which suggests a broader imagin g approach which may be termed ambient noise imaging (ANI). Another ANI tec hnique explored is the use of spatial cross correlation, which works well a nd also has no visual analogy. A model-based processor (Kalman filter) is a lso applied to track targets subject to highly variable illumination such a s:provided by snapping shrimp. Examples are presented using data provided b y Scripps Institution of Oceanography from the initial deployment of the Ac oustic Daylight Ocean Noise Imaging System: (ADONIS) in San Diego. (C) 1999 Acoustical Society of America. [S0001-4966(99)01711-7].