基于海底表层沉积物声速特征的南海地声模型

在由垂直声速梯度建立的地声模型基础上,通过引入沉积物与海水声速比和沉积物压缩波与切变波声速比两个表征沉积物声学特征参数得到更全面和有实际指导意义的地声模型。在沉积物声波传播FCMCM模型基础上,基于热作用和重力作用下沉积物两相介质的应力应变分析,建立TFCMCM和DFCFCM模型,运用模型校正表层沉积物声速特征来计算和解释地声模型。根据海底表层沉积物存在低声速和高声速两种类型,结合沉积物沿纵深孔隙度不变和变化两种类型,得到南海海底沉积物的两类四种典型地声模型:低声速孔隙度不变型、低声速孔隙度减小型、高声速不变型和高声速孔隙度减小型。运用这四种典型地声模型的组合解释了卢博提出的南海三种典型声速结构。认知声速结构将为南海声学探测海底、划分海底区域提供模型支持。

A geoacoustic model based on sound speed characteristic of seafloor surface sediments of the South China Sea

The sound speed ratio R1 of sediment to seawater and the sound speed ratio R2 of compressional wave to shear wave are added up to the vertical gradient of sound speed geoacoustic model(GM) so as to achieve a more comprehensive and instructional GM in practice.Considering stress-strain relations of two-phase medium of sediment under the effect of heat and gravity,TFCMCM and DFCMCM models are established on the basis of Men-Chen model by introducing a freedom factor and considering coupled-frame elastic modular(FCMCM).Both models are used to adjust the seafloor surface sediment sound speed characteristic into the in-situ state,calculate and interpret the GMs.According to analysis of the existence of high-speed and low-speed types in seafloor surface sediments and the two kinds of porosity-depth states of constant and descent beneath the seafloor sediment(PDC and PDD),four representative basic geoacoustic models of sediments in the South China Sea can be expressed as low-speed PDC,low-speed and porosity-PDD,high-speed PDC and high-speed PDD.With the usage of combination of the four basic models,the Lu Bo and Liang Yuanbo’s three sound-speed structures of the South China Sea can be rationally explained as an example of the actual sedimentary acoustic state.The understanding of basic geoacousic models will benefit the seafloor inspection and classification of sea bottom regions with acoustic methods in the South China Sea.