A study of geoacoustic model and acoustic reflection properties of fluid mud layer in Changjiang Estuary and Hangzhou Bay

A generalized geo-acoustic model of fluid mud layer in Changjiang Estuary and Hangzhou Bay has been derived through a large amount of in-situ measurements of the bulk density (ρ) of the layers and lab measurements of the acoustic velocity (c) and attenuation coefficient (β) of the fluid mud samples with different values of ρ for four frequencies of 100,150,500,1500 kHz.The maill features of the geoacoustic model can be expressed as follows:in the range of ρ=10.0~15.0,the bulk density of the fluid mud ρ increases linearly with depth z,however,there is a gradient change (knee) when ρ is about 12.5.After ρ more than 15.0,the fluid mud quickly transform into the ooze layer.In the fluid mud layer,the sound velocity c can be regarded as constant since its variation with z less than 1.5%,and a minimum value exists when ρ is about 13.5.The variations of β with ρ and with frequency f are linear.Based on the geo-acoustic model and the ray theory,simulations of sound reflection properties of the fluid mud layers have been made,and some significant results obtained.