Long-term ambient noise interferometry in the NE Pacific deep ocean
Date:
Ambient noise interferometry is the method of using ambient sound to measure the time domain Greene's function (TDGF) between two nodes. Ambient noise interferometry is a well-developed technique in seismic imaging applications, allowing for the measurement of surface wave velocities using microseisms. Ambient noise interferometry has also been used in acoustic oceanography to measure oceanographic variables such as water temperature and flow velocity. Along with measuring the TDGF, ambient noise interferometry can also be used to characterize the directionality of coherent sound in the ocean. In this talk, we use six years of ambient sound data recorded by two bottom mounted hydrophones to investigate ambient noise interferometry in the deep ocean. The hydrophones are separated by 3.2 km and are both at a depth of 1500 m. We show that the TDGF between the two hydrophones can be resolved with averaging times around 200 hours. Additionally, the TDGF contains clear, multi-path arrivals. These multi-path arrivals have not been previously observed with ambient noise interferometry in the deep ocean. We also observe a chorus of fin whale vocalizations originating from near Alaska, that is not visible in data recorded by a single hydrophone. Lastly, the viability of using multi-path arrival times in the deep ocean for passive acoustic tomography will be explored. Multi-path arrival times provide a unique opportunity for ocean observation since they contain information about the entire, deep ocean, water column. We will present ocean acoustic simulations to estimate the expected seasonal fluctuations of the multi-path arrival times due to temperature changes and we will evaluate the viability of using the multi-path peaks measured with ambient noise interferometry for passive acoustic tomography. [work supported by ONR]