Using ocean ambient noise cross-correlations for noise-based acoustic tomography

Recent theoretical and experimental studies have demonstrated that an estimate of the Green’s function between two hydrophones can be extracted passively from the cross-correlation of ambient noise recorded at these two points. Hence monitoring the temporal evolution of these estimated Green’s functions can provide a means for noise-based acoustic tomography using a distributed sensor network. However, obtaining unbiased Green’s function estimates requires a sufficiently spatially and temporally diffuse ambient noise field. Broadband ambient noise ([200 Hz–20 kHz]) was recorded continuously for 3 days during the SWAMSI09 experiment (next to Panama City, FL) using two moored vertical line arrays (VLAs) spanning the 13-m water column and separated by 150 m. The feasibility of noise-based acoustic tomography was assessed in this dynamic coastal environment over the whole recording period. Furthermore, coherent array processing of the computed ocean noise cross-correlations between all pairwise combinations of hydrophones was used to separate acoustic variations between the VLAs caused by genuine environmental fluctuations–such as internal waves–from the apparent variations in the same coherent arrivals caused when the ambient noise field becomes strongly directional, e.g., due to an isolated ship passing in the vicinity of the VLAs.