Experiments

2006-2007 experiment

  Four OBSs were deployed in September 1-4, 2006 using the NTU vessel Ocean-Researcher 1 (OR1) (Fig. 1) (Video 1). We lost acoustic communication with OBS-S3 during its descent. Among the three successfully deployed, two (S2 and S4) were recovered in July 24-25 (OR2) and August 15-17 (OR1), 2007 (Video 2). The locations of OBSs are accurate to within about 200 m, and the clock drift-rate is on the order of 1 s a year. At S1, the OBS was alive and kicking acoustically in the later recovery attempts in August and September, 2007, but failed to pop up. A volcanic eruption was reported to have occurred in the vicinity of S1 in early September 2006 (Lin et al., 2008), and may have either buried the instrument or jammed its release mechanism. A rescue operation using a deep-water acoustic transducer (rescue beacon) in the September 2008 visit proved to no avail. The deep-tow camera operation in mid-2008 collected visual information about this site.

Photo Gallery

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2007-2008 experiment

  In September 20-23, 2007, 2 OBSs, S5 and S6, were deployed in the OT (Fig. 3),  following our strategy to pair our OBSs with land stations, this time being the F-net IGK at Ishigaki. Each was located on either side of the spreading axis of the back-arc basin. They were scheduled to be recovered in September, 2008.

  The 2008 TowCam operation. Intrigued by the suggestion that submarine volcanic activity may have hindered the recovery of the OBS at S1, we arranged a towed camera operation to survey the area from June 29 to July 1. The TowCam was operated by a team led by Saulwood Lin of the IO-NTU. Although the odds of catching sight of the instrument was predictably low, the experience was good for both teams and learning of the status of the instrument (if lucky) as well as the seafloor condition was especially valuable for us. The TowCam engineers usually “fly” the system ~5 m above the ocean bottom with a cruise speed ~1.5 knots, and pictures of the seafloor ~5 m x 6 m were snapped every 10 s. Over the short 3-day cruise, two and a half tows, with one tow typically lasting ~6 hours, were launched to photograph the 400 m x 400 m area centered at S1. Although the target was never spotted, the seafloor, however, seemed undisturbed by any severe underwater volcanic eruptions thus achieving a secondary goal of further exploring the ocean bed for activity.

  2008 Recovery. OBSs S5 and S6 were recovered during a cruise from September 18-20, about 2 days after typhoon Sinlaku had devastated Taiwan and ultimately bound for Japan. The operation at S6 required several cycles of “release 2,” the burn-wire command issued every 15 seconds successively for 10 minutes. The recovery at S5 was relatively quick and smooth. Both operations were conducted at night under good sea conditions (video 1). Because of the high political sensitivity of the localities, i.e., S5 is 30 nm south of 釣魚台 and S6 is 30 nm north of Ishigaki, the follow-up multi-channel seismic experiment along the line between the two islands was cut short by the interference of the Japan coast guard. The MCS was forced to move to the westernmost OT in the remaining days.