NOAA’s use of U.S. Navy fixed hydrophone arrays for monitoring the Juan de Fuca Ridge

Abstract

NOAA’s VENTS Program studies the impact of seafloor hydrothermal emissions on the chemical and thermal budgets of the global ocean. The research focus has been on seafloor spreading centers, in particular the Juan de Fuca Ridge in the northeast Pacific Ocean, and early results indicated the episodic nature of activity. Since no method existed for long-range continuous monitoring, NOAA obtained permission to collect acoustic data from U.S. Navy’s sound surveillance system (SOSUS) in the north Pacific Ocean. Two NOAA systems have been installed to process the signals independently and in parallel with the Navy’s operational systems. One system uses outputs from two end hydrophones from five arrays to analyze seismicity through the north Pacific. The second system collects outputs from formed beams directed at the Juan de Fuca Ridge for real-time monitoring. An acoustic beacon (260 Hz), installed on a seamount, allows continuous monitoring of travel times for location correction. Use of the SOSUS system produces an increase in the number of earthquake detections from ∼350 events/year to ∼7500 events/year for the northeast Pacific. In 6 months of operation, the real-time monitoring system has detected two episodes of volcanic activity, later documented by field observations.