A vertical hydrophone array coupled via inductive modem for detecting deep-ocean seismic and volcanic sources

A vertical autonomous hydrophone (VAUH) array useful for a long-term low-frequency underwater acoustic propagation study was developed at Oregon State University (OSU), North Carolina State University and the National Oceanic and Atmospheric Administration's (NOAA) Pacific Environmental Lab (PMEL). To analyze the arrival structure of the hydroacoustic signals in deep water, we needed a multichannel vertical hydrophone array with relative timing accuracy of as good as 10 ms/year where no GPS or Network Time Protocol (NTP) is available. A new scheme takes advantage of Inductive Modem Modules (IMM® from Sea-Bird Electronics) and a low-power accurate clock (QT2001® from Q-Tech Corporation). With the master unit sending an accurate 1-PPS pulse train once a day to slave instruments over a single wire inductive modem/mooring cable, it synchronizes the other slaves' clocks and keeps the timing errors among the instruments less than 10msec. As compared to the timing synchronization methods based on three-wire serial or NTP network interface, it only requires an insulated single wire mooring cable using seawater as a return. It is robust, low power and useful for longterm time synchronization of multiple instruments serially connected. As a trial, an array consisting of three vertical autonomous hydrophones (VAUH) was deployed in the Lau Basin from December 2009 to April 2010 at 21° 25'12.60"S, 176° 12'45.50"W. Each unit was fastened on a 1000-m long 5/16" jacketed cable with a 500 m of separation. All three VAUHs recorded continuously the low frequency acoustic signal at 250-Hz sampling rate and maintained a relative timing accuracy of less than 10 ms. The acoustic record shows that the entire region is active with seismicity and submarine eruptions. The results of the four-month long monitoring and comparison with other single hydrophone moorings in the area are discussed.