Comprehensive Study of Inversion Methods for Sound Speed Profiles in the South China Sea

Traditional acquisition method of sound speed profiles using hydro-acoustic instruments is accurate but time-consuming and costly. To overcome this problem, some inversion methods have been developed over the last few decades. In this study, a comprehensive comparison of two inversion methods — the acoustic inversion method (AIM) and the satellite observation reconstruction method (SOR) — is presented. For AIM, the sound speed profile is first parameterized by the empirical orthogonal function (EOF) and the optimal parameters are searched by simulated annealing algorithm with respect to the cross-correlation function of the receiving signal and the simulation signal. For SOR, remotely sensed data are used to construct sound speed profiles. An experiment was conducted in the northeast of the South China Sea to verify the two methods. Both methods can obtain sound speed profiles quickly and cheaply. Compared with the sound speed profiles obtained by a conductivity-temperature-depth (CTD) instrument, the root-mean-square-error (RMSE) of AIM is 0.55 m s⁻¹ and that of SOR is 1.71 m s⁻¹. It is clear that AIM provides better inversion performance than SOR. Another primary benefit of AIM is that this method has no limitation to the inversion depth. The simulation results of sound propagation in regard to the inversed sound speed profiles show that the transmission losses of AIM and CTD are consistent and that of SOR is adversely affected by the inversion error of the sound speed and the inversion depth. But SOR has particular advantages in the inversion coverage. Together, all of these advantages make the AIM particularly valuable in practice.