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.