Estimating ocean variables using ambient noise interferometry

Abstract

Ambient noise interferometry is a method of estimating the time domain Greene's function (TDGF) by correlating and averaging sound from adjacent hydrophones. These averaged cross correlations are called noise cross correlation functions (NCCFs). Noise interferometry has been used in ocean acoustics to estimate ocean variables such as water temperature by measuring propagation times. In this talk, a 6-year NCCF stack between two Ocean Observatories Initiative cabled array hydrophones is investigated. The hydrophones are separated by 3.2 km, and are bottom mounted at a depth of 1500 meters. They are located in the caldera of the Axial Seamount volcano approximately 470 km off the Oregon coast. The TDGF estimate from the NCCF stack contains clear, multipath arrivals. Fluctuations in the arrival time over the six years are on the order of a single time bin. However, if the signal to noise ratio is high enough, it is possible to measure the arrival times with more accuracy than a single time bin. In this presentation, signal processing methods to accurately estimate the propagation time will be explored, as well as the viability of inverting these multipath arrival times to estimate ocean variables such as water temperature. [Work supported by ONR.]