Using passive acoustic remote sensing to measure the current speed in the Florida Straits

Noise interferometry is a low-cost passive acoustic remote sensing method to approximate the acoustic Green's function between two locations with the cross-correlation function (CF) of the ambient noise at these two locations. Due to acoustic reciprocity, the ambient noise CF is symmetric in time when the ocean is stationary. However, this symmetric property breaks when ocean current exists. Using three bottom-moored sound recording systems at 5 km, 10 km, and 15 km separations in a 100-meter-deep water of the Florida Straits, we have obtained the ambient noise CFs, which are asymmetric in time due to strong currents. To estimate the current speed, we first solve for two independent clock corrections by minimizing the coherence loss between all three clock pairs. The resulting CFs at positive and negative lags describe propagation with and against the current, with timing shifts that allow the current speed to be estimated. The depth-averaged current estimates based on all three instrument pairs are shown to be consistent with each other, and with ship-based ADCP measurements.