High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Wideband Chirp Reflection Profiling

Steven Schock
Dept. of Ocean Engineering
Florida Atlantic University
Boca Raton, FL 33431
(561) 297-3442, 347-1431 (fax), schock@oe.fau.edu

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The main objective of the research is to generate predictions of the acoustic properties of the top meter of the seabed and to measure the wideband acoustic response of buried targets. The measurements and predictions will be compared with the outputs of acoustic models developed by other investigators. A secondary task is to generate high resolution images of the top meter of the seabed at the proposed site of the October 1999 DRI experiment for purposes of finding a site for which the top meter of the seabed is homogeneous.

Multi-Band Chirp Sonar

A chirp sonar with three transmission channels and two receiving channels will be towed approximately 5 meters above the seabed to collect the normal acoustic response of the seabed over the band of 1 to 40 kHz. The wideband transmissions will generate images approaching 1 cm in vertical resolution. Three piston sources will be driven simultaneously to generate an outgoing pulse with an approximately constant power spectrum over the range of 1 to 40 kHz. Using three sources of different sizes and operating bands helps minimize the effects of changing beamwidth with frequency. Two planar arrays are used as receivers. The larger array will generate images with reduced surface and volume scattering interference while the smaller array will generate data that is less sensitive to vehicle motion and attitude.

Measured Parameters

Since the sonar will have an absolute calibration across its frequency band of 1–40 kHz before the experiment, the data can be used to provide absolute reflection coefficient and target strength measurements. The pressure reflection coefficient of the seabed will be reported over the range of 1 to 40 kHz. The reflection coefficient frequency function can be used to check the accuracy of sediment-acoustic models. The target strength of buried targets will be reported over the range of 1–40 kHz.

Predicted Parameters

The normal incidence data will be used to predict the impedance and attenuation of subsurface sediment layers. The post processed data will yield predicted vertical profiles of the attenuation coefficient and impedance. The change in impedance between adjacent sediment layers is calculated from the amplitude and the phase of the interlayer reflection. The attenuation coefficient is calculated from the spectral ratio of interlayer reflections.

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