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

Abstract

Ambient noise can be used to produce images of submerged objects using the mean intensity of the backscattered energy, a technique coined 'acoustic daylight' because of its direct analogy to vision. It is suggested that there may be substantial additional information in higher moments of the data. At high frequencies (>10 kHz), absorption suppresses long-range propagation so that a received signal is largely dependent on the local geometry, source characteristics, and the scattering properties of interceding objects. It is shown that for snapping shrimp (Cragnon, Alpheus, and Synalpheus) illumination (the primary sources in warm shallow water above a few kHz), significant information is embodied in the second temporal moments of intensity. There is no visual analog to this concept, which suggests a broader imaging approach which may be termed ambient noise imaging (ANI). Another ANI technique explored is the use of spatial cross correlation, which works well and also has no visual analogy. A model-based processor (Kalman filter) is also applied to track targets subject to highly variable illumination such as provided by snapping shrimp. Examples are presented using data provided by Scripps Institution of Oceanography from the initial deployment of the Acoustic Daylight Ocean Noise Imaging System (ADONIS) in San Diego.