Passive acoustic characterization of sub-seasonal sound speed variations in a coastal ocean

Acoustic noise interferometry is applied to retrieve empirical Green's functions (EGFs) from the ambient and shipping noise data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Despite strong internal wave-induced perturbations of the sound speed in water, EGFs are found on 31 acoustic paths by cross-correlating the noise recorded on a single hydrophone with noise on the hydrophones of a horizontal linear array about 3.6 km away. Datasets from two non-overlapping 15-day observation periods are considered. Dispersion curves of three low-order normal modes at frequencies below 110 Hz are extracted from the EGFs with the time-warping technique. The dispersion curves from the first dataset were previously employed to estimate the seabed properties. Here, using this seabed model, we invert the differences between the dispersion curves obtained from the two datasets for the variation of the time-averaged sound speed profile (SSP) in water between the two observation periods. Results of the passive SSP inversion of the second dataset are compared with the ground truth derived from in situ temperature measurements. The effect of temporal variability of the water column during noise-averaging time on EGF retrieval is discussed and quantified.