Geoacoustic inversion for acoustic parameters of sediment layer with low sound speed

Abstract

Acoustic inversion of sediment parameters in muddy bottom environment has received much attention in the field of underwater acoustics. In shallow water, when there is a low-speed layer of unconsolidated sediment, such as mud in which the sound speed is lower than that of the sea water, on the top of a high-speed bottom, the transmission losses at different frequencies will increase periodically under the condition of small grazing angles. Based on this phenomenon, an acoustic inversion method of seabed parameters for low speed sediments is proposed. Firstly, the analytical expressions between the frequency interval of the transmission loss (TL) periodical increasing and geoacoustic parameters, including the sound speed and the thickness of sediment layer and the sound speed of seawater near the bottom, are derived under the condition of small grazing angles. Secondly, using the broadband sound propagation signals received under the thermocline in the 2002 summer acoustic experiment conducted in the Yellow Sea, the TL at small grazing angles increases periodically with the frequency, and it is determined that the sediment of this sea area is a low-speed sediment. Then, taking the analytical expression as the constraint condition and combining with Hamilton's empirical formula, the sound speed, density, thickness of sediment layer and the sound speed and density of the seabed are inverted by matched field processing. Finally, the bottom attenuation coefficients at different frequencies are inverted by using the long-range TL, and the linear relationship between the attenuation coefficients and the frequencies is obtained. The equivalence between the two different bottom models is discussed in the end. The inversion results can provide seabed parameters for the study and application of the sound propagation law in shallow water with a low-speed sediment.