Passive acoustic thermometry of the deep water sound channel using ambient noise

Abstract

Cross-correlation processing of ocean ambient noise has been proposed as a totally passive alternative to existing active methods for sensing the ocean environment such as acoustic tomography or acoustic thermometry. To this end, we investigated the spatial coherence of low frequency (f < 40 Hz) ocean noise recorded in the deep sound (SOFAR) channel to demonstrate the feasibility of passive acoustic thermometry. Continuous recordings of ambient noise obtained from hydroacoustic stations of the International Monitoring System were processed between the years 2006 and 2012. Each hydroacoustic station uses two triangular horizontal arrays separated by approximately 100 km, and each array has three hydrophones. Coherent arrivals were extracted from time-averaged cross-correlations between the two spatially separated triangular arrays. A beamforming procedure, using data-derived adaptive weights, was used to track the seasonal fluctuations of the arrival time of the coherent wavefronts throughout the 6 years. These measured travel-time fluctuations were estimated to primarily result from seasonal temperature fluctuations on the order of 0.1°C in the SOFAR channel, which is consistent with independent temperature measurements made by the Argo float global array in the vicinity of the hydroacoustic stations over the same time period.