Inference of Geo-Acoustic Parameters from Bottom-Loss Data
In areas of thick sediments the received level of shallow-angle low-frequency bottom interacting signals tends to be dominated by a bottom-refracted rather than bottom-reflected path. The loss along this path, assuming a constant gradient, g, and attenuation, α, in the sediment is proportional to the ratio α/g. Assuming that α varies linearly with frequency, the shallow angle loss appears to increase linearly with both grazing angle and frequency. Hence bottom loss data appear to be incapable of separating α and g, providing only estimates of their ratio.
This paper describes a technique for estimating both α and g from classical bottom-loss versus grazing-angle data. In such data acquired in thick-sediment areas the lowest frequencies exhibit an abrupt Increase in loss at an apparent grazing angle, θ̂c, corresponding to the development of a minimum-range caustic in the bottom-refracted paths. The gradient may be estimated from θ̂c and the measurement geometry. The attenuation, α, is then estimated from the linearity of loss versus angle and frequency.
Examples using actual bottom-loss data and seismic time-domain data are shown. The Implications for depth-dependent gradients and attenuations are also discussed.
KeywordsGrazing Angle Terrigenous Sediment Refract Wave Constant Gradient Ocean Acoustics
Unable to display preview. Download preview PDF.
- 1.E. L. Hamilton, Prediction of deep-sea sediment properties: state-of-the-art, In “Deep-Sea Sediments, Physical and Mechanical Properties,” A. L. Inderbitzen, ed., Plenum Press, New York (1974).Google Scholar
- 2.J. S. Hanna, Some complications in the traditional measurements of bottom loss, Science Applications, Inc. report SAI-76–644-WA (1976).Google Scholar
- 3.G. V. Frisk, J. A. Doutt, and E. E. Hays, Bottom interaction of low-frequency acoustic signals at small grazing angles in the deep ocean, submitted to J. Acoust. Soc. Am. Google Scholar
- 4.R. L. Dicus, Preliminary investigations of the ocean bottom impulse response at low frequencies, U. S. Naval Oceanographic Office, TN 6130–4–76 (1976).Google Scholar
- 5.P. D. Herstein, NUSC New London, private communication.Google Scholar