Sediment Parameter Inversions in the East China Sea

Abstract

Acoustic propagation in shallow water is influenced by the characteristics of the ocean bottom sediments and substrata. Hence accurate estimation of bottom properties is vital for accurate modeling of the acoustic field. The acoustic properties of the sediments (compressional and shear wave speeds and attenuation) have been estimated either by measuring the physical properties (porosity, grain size, bulk density etc.), using in-situ sound velocity probes, or via geoacoustic inversions. The sediment physical properties are usually measured by the analysis of sediment cores (gravity, piston or vibro cores). Geoacoustic models such as Grain Shearing (GS) model1 can provide estimates of the geo-acoustic parameters (compressional and shear speeds and attenuations) using the sediment parameters as inputs. Sediment disturbance is possible during the coring process and the extent of this disturbance will depend on the type of sediment (silt, sand or clay/mud). The in situ probes uses acoustic sources and receivers in the coring devises to measure acoustic speeds and attenuation in situ. Another popular approach to characterize the sediments is via geoacoustic inversions. Acoustic field measured at a receiver (or an array of receivers) or other observables derived from the field (reflection coefficient, mode or ray arrival times, modal phase, etc.) is used to estimate the bottom properties (unknown model parameters) via an inversion scheme. In this study, sound speeds of sediments are estimated through an inversion scheme using acoustic normal mode dispersion data. Broadband data collected during the Asian Seas International Acoustics Experiment (ASIAEX) were used for the inversions. Prior inversions and core data from the location were used to constrain the parameter search bounds. The geo-acoustic parameters are also calculated from the physical parameters using viscous grain shearing model for comparison with the inversions.