Imaging in the ocean with ambient noise: the ORB experiments

Acoustic daylight imaging is a new technique that has been proposed for cre ating pictorial images of objects in the ocean from the ensonification prov ided. by the incident ambient noise field. To investigate the feasibility o f the technique, a series of experiments was performed from the research pl atform ORE, moored in San Diego Bay, Southern California. Central to these experiments was an acoustic receiver known as ADONIS (acoustic daylight oce an noise imaging system), which consists of a spherical reflector, 3 m in d iameter, with an elliptical array of 130 hydrophones at the focal surface. This system, which is broadband, operating between 8 and 80 kHz, forms a to tal of 126 receive-only beams spanning the vertical and horizontal. The amb ient noise power in each beam is mapped into a pixel on a VDU, Various type s of targets-were used in the experiments, including planar panels and cyli ndrical, polyethylene drums containing wet sand, seawater or syntactic foam (essentially air), and most of the experiments were conducted with the tar gets at ranges between 20 and 4b m, At the time of the experiments the nois e field in the area was created primarily by snapping shrimp, Moving, color images of the object space were successfully created with ADONIS. Some rep resentative static images from the:moving sequences are presented and discu ssed in the paper. (C) 1999 Acoustical Society of America: [S0001-4966(99)0 4611-1].