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doi:10.2204/iodp.proc.332.103.2011

Discussion

The discussion about Site C0010 reviews VIV data from the SmartPlug during the recovery run, examines the performance of the drill string with ropes attached to it, and compares the data to earlier data from the dummy run with the first-generation instrument carrier at Site C0010 during Expedition 319 in 2009. We also look at the performance and overall value of using a temporary mini-CORK firmly mounted near the screened casing section of the megasplay fault for 15 months.

VIV

As briefly outlined in Expedition 319 Scientists (2010), a dummy run of the long-term borehole monitoring system sensor carrier with three instruments failed during Expedition 319. VIV was very strong (see data example from the initial phase of drifting toward Site C0010; Fig. F14), particularly during the later part of the record. As a result, instruments were lost or damaged, and countermeasures had to be taken thereafter. Foremost, these included the attachment of four lines of ropes to the drill string in the uppermost 500 m of water column to break the turbulent flow in the wake of the drill string. This strategy was tested on land in a wave tank at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in early 2010, at sea during a limited test run aboard the Chikyu, and then adopted as a standard operation on the Chikyu. When comparing the Expedition 332 three-component acceleration data at Site C0010 (see Fig. F13) to the older record, it can be seen that with the ropes attached during 2010 operations, acceleration was well below ±1 g despite the fact that the absolute speed (i.e., ship speed–current speed) exceeded 1 kt. By comparison, conditions at Site C0002 a few weeks later enabled a more careful drifting operation with lower absolute drifting velocities ~0.5 kt (see “Vortex-induced vibration measurements” in the “Site C0002” chapter [Expedition 332 Scientists, 2011b]). Figure F20 in the “Site C0002” chapter (Expedition 332 Scientists, 2011b) illustrates that VIV in both x- and y-directions were even lower than those encountered at Site C0010.

SmartPlug

The SmartPlug placement was designed to monitor pressure and temperature at a screened interval in the formation where the megasplay intersects, as well as to monitor hydrostatic pressure to be used as a reference. Time series data during deployment, and particularly during instrument recovery, provide additional confirmation that the packer had successfully isolated the formation (Fig. F10). A cursory review of the data identified multiple pressure disturbances potentially related to far-field seismic events, although postcruise detrending and processing of the data are required to better resolve and quantify pressure anomalies. Still, given the rather low sampling interval of 60 s, these signals are textbook examples of seismic arrivals and tsunami arrivals (e.g., Fig. F9). Similarly, the tidal forcing record shows dampening of signals in the tight, somewhat overpressured formation. Results have to be treated with caution because it looks as if the borehole has not yet equilibrated 15 months postdrilling. The temperature record is a less powerful tool for drawing conclusions; however, good resolution was achieved and individual events causing temperature excursions were identified (Fig. F12).

In summary, it can be said that having had a chance to successfully install the SmartPlug was a great achievement for the following reasons. First, the instrument turned out to be sufficiently robust to withstand the strong (>5 kt) Kuroshio Current and remained undamaged at conditions where the first-generation instrument carrier and components attached to it were destroyed and damaged (Expedition 319 Scientists, 2010). Second, the data quality of the simple SmartPlug is astonishing and is mainly hampered by the fact that borehole equilibration was not reached during the deployment period. Third, having been able to establish the first long-term monitoring system in the NanTroSEIZE program is a significant step forward toward the more sophisticated permanent observatories planned within the complex drilling project.

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