The influence of sediment thickness on vertical coherence of ambient noise in shallow water

Abstract

The ambient noise field in shallow water is contributed to by many noise sources and it is also influenced by environment boundaries. The ambient noise field strongly depends on the properties of the seabed, so it is often used for undertaking geoacoustic inversion. Geoacoustic inversion is performed using either vertical coherence or directionality, as these are relatively stable features of noise and are largely determined by the seabed. Certain parameters of the seabed, such as the sound speed, density and attenuation are usually the focus in this field of research. However, an additional parameter, namely the thickness of the sediment, also has a significant effect on the ambient noise field. The purpose of this paper is to determine the effect of sediment thickness on the ambient noise field in shallow water. In the first step, a model is constructed, with the shallow water divided into three layers - water, sediment and elastic seabed. The vertical coherence is obtained from the ambient noise model which is based on normal mode theory. Once the model is completed, simulations are performed on two key aspects in order to determine how sediment thickness influences vertical coherence. In the first simulation, all parameters except for the sediment thickness are kept constant. The result from the first simulation shows that the coherence curve becomes increasingly flat with the increase of sediment thickness. In the second simulation, four thicknesses are chosen, 0.5m, 1m, 2m, 10m respectively. For each thickness, four vertical coherence curves for different sound speeds and attenuation are plotted in the same figure. By comparing these four figures, it is seen that the vertical coherence becomes less sensitive to the parameters in the seabed with the increase of sediment thickness. Once the sediment thickness exceeds 1m, the curve remains nearly unchanged with changes in the shear wave speed and attenuation in the seabed. However, the vertical coherence is still sensitive to the compressional wave speed. Therefore, the thickness of sediment plays an important role in geoacoustic inversion. When the sediment is deep enough, the vertical coherence can only be used to undertake inversion on compressional wave speed rather than on other parameters.