The Development of Chirp Sonar Technology and Its Applications

Abstract

The Geology and Geophysics Program at the Office of Naval Research sponsored the development of chirp sonar technology during the late 1980s and early 1990s and the development of remote sediment property estimation techniques during the remainder of the 1990s. The chirp sonar is a quantitative, wideband FM reflection profiler developed to collect normal incidence reflection data suitable for sediment property inversions; it also happens to generate detailed reflection profiles of the seabed. The vertical resolution of the images can approach 1 cm for bandwidths exceeding 20 kHz providing detail about the sediment structure near the sediment-water interface. The chirp sonar system transmits FM acoustic pulses to attain high pulse energy; consequently, the imagery has a high signal to ambient noise ratio. The images are normally scattering limited; that is, the subsurface depth of usable imagery is limited by acoustic noise scattered from inhomogeneities within the sediment structure. The chirp sonar technology was transitioned to industry during the early 1990s. The chirp sonar has become the industry standard for generating high resolution images of the uppermost 10 to 100 meters of sediments in the seabed. It is widely used by geologists, archeologists, companies conducting hazard survey and cable route studies, and river, port and ocean engineers requiring knowledge of the seabed structure. In commercial applications, the sonar is mounted on AUVs, ROVs, towed vehicles and ship hulls. Development of a technique for remotely measuring sediment properties using chirp sonar data is nearing completion. Currently, vertical profiles of attenuation and impedance are calculated from the chirp sonar data. Pattern recognition is used to automatically detect the locations of reflectors within the subsurface images. The amplitude and phase of the reflector echoes are used to recursively calculate impedance for each layer starting at the sediment-water interface. The spectrum of the reflectors is used to calculate the attenuation of the sound. The acoustic attenuation of sound in sediments causes the higher frequency portion of the pulse to decay more rapidly than the lower frequency portion of the pulse; consequently, the amplitude spectrum of subsurface reflections rolls off with increasing frequency. The slope of the rolloff is proportional to the attenuation coefficient. Several spinoffs technologies from this Geology and Geophysics ONR program include 3D subsurface imaging of buried objects and chirp sidescan. A 32 channel system that generates images of buried objects in real time allows the user to measure the location and size of buried features in three dimensions. The sonar has generated images of objects has small as 1.5 inch diameter buried cables and 6 inch diameter pipes buried in sand. In addition, chirp processing enhances the range of sidescans allowing sidescans to operate at higher frequencies without sacrificing range. Recently, in the August 2000 Fleet Battle Experiment, a 850kHz chirp sidescan mounted on an AUV, demonstrated that the bottom mines can automatically be detected in real time and the location data can be sent back to the surface ship via an acoustic modem.

Department

Center for Coastal and Ocean Mapping

Publication Date

2000

Volume

81, Issue 48

Journal Title

Fall Meeting, American Geophysical Union (AGU)

Series

Fall Meeting, Supplement

Conference Date

Dec 15 - Dec 19, 2000

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Poster

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