Techniques for estimating sediment properties from chirp sonar data

Abstract

The chirp sonar is a towed wideband FM reflection profiler developed to collect normal incidence reflection data suitable for sediment property inversions; the sonar also generates detailed reflection profiles of the seabed. The chirp sonar system transmits FM pulses with high time-bandwidth products to attain high pulse energy; consequently, the acoustic data have a high signal-to-ambient-noise ratio after correlation processing. The vertical resolution of the images is about 1 cm when transducer bandwidth is approximately 40 kHz. Data processing methods have been developed to automatically map the locations of sediment layer interfaces, and to estimate compressional wave attenuation, acoustic impedance, and compressional wave velocity of sediment layers using normal incidence FM reflection data. The height of sand ripples and the phase dispersion of compressional waves can also be measured from the acoustic imagery. Field data collected by other investigators during the High Frequency Acoustics DRI experiments off Fort Walton Beach were used to verify the acoustic property estimates. Comparisons show that chirp sonar estimates of acoustic properties agree with direct measurements of the properties based on in situ acoustic probe data and sediment core data collected by investigators from Naval Research Laboratory, Stennis Space Center, and APL, Washington State.