Investigation of Geoacoustic Parameters of Fine-Grained Sediment in the South China Sea Using Sequential Inversion

Broadband signals interacting with deep ocean fine-grained sediment are crucial in shaping the acoustic field of the geometric shadow zone. These signals travel through both the seabed reflected path and the refracted path. In this article, a sequential inversion scheme is employed to estimate the geoacoustic parameters in abyssal clay sediments. This inversion is based on seabed reflection loss data at different frequencies, as well as travel time difference data between refractions and reflections obtained from the South China Sea Experiment in 2018. Depth-dependent profiles of geoacoustic parameters are formulated using Bernstein polynomials. The polynomial coefficients and their posterior probability density functions are efficiently estimated using the adaptive simplex simulated annealing method and an approximate variational inference technique known as Variational Bayesian Monte Carlo. This technique demonstrates superior efficiency and comparable accuracy to Markov Chain Monte Carlo sampling. The inversion results indicate that the abyssal clay sediments in this area exhibit a positive sound speed gradient and relatively low attenuation, both with high probabilities. The deduced seabed model accurately predicts the transmission loss, aligning well with the experimental data.