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doi:10.2204/iodp.proc.317.101.2011

Preliminary scientific assessment

Expedition 317 marked a new direction for IODP by incorporating (1) drilling in shallow water on the continental shelf and (2) unusually deep penetrations. Expedition 317 set records for the shallowest water depth for a site drilled for scientific purposes and the deepest hole drilled on a continental shelf by the JOIDES Resolution, as well as the deepest sediment site and deepest site drilled in a single expedition in the history of scientific ocean drilling.

Only Leg 174A (New Jersey margin) previously attempted shelf drilling using the JOIDES Resolution. Experience gained during Leg 174A, where unconsolidated sands caused hole collapse during coring, suggested that similar problems might be encountered during Expedition 317 and penetrations might fall far short of target depths.

However, penetration during Expedition 317 exceeded all expectations. Drilling of Canterbury Basin sediments went smoothly even at the shallowest water sites (including Site U1354 in only 85 m water depth) and reached 1030 m in Hole U1351B on the outer shelf (122 m water depth). The performance of the drillers and the level of cooperation and support from the operations staff were superb. There was no hesitation in taking on new challenges.

Recovery was highly variable and, indeed, poor within long intervals of unconsolidated sediment, particularly at shelf Sites U1351 and U1353 during XCB coring (Fig. F17). This may be unavoidable with existing IODP coring technology. It would be very useful to develop new coring methods, if possible, for future operations in such lithologies. Percussive or vibracoring tools could provide solutions, although it will be important to maintain a reasonable rate of penetration in order to core thick sections equivalent to those targeted during Expedition 317. On the other hand, XCB coring was effective in the uppermost 250 m at shelf site Site U1354. A different recovery problem was encountered at slope site Hole U1352C, where RCB cores of solid rock with very low recovery (1%–2%) were occasionally interspersed with cores having 100% recovery, for reasons not fully understood (it was speculated that heavy rock cores may have fallen out of the core barrels). A correlation between rate of recovery and degree of sediment lithification was noted, with XCB drilling failing to recover lithified sediments, and RCB drilling failing to recover unlithified sediments. However, we believe that recovery was sufficient to achieve most of the expedition's objectives, particularly in providing ground truth at the scale of seismic resolution (~5 m vertically).

Drilling in shallow water requires good weather (low heave and wind speeds that can be managed by the dynamic positioning system). In this we were extremely fortunate. Although it was often windy, only two delays were caused by weather. One was relatively minor because it occurred when the ship was about to move from slope Site U1352 to landward shelf Site U1353. The swell was considered too high for shelf operations, so we used the opportunity to duplicate the uppermost 127 m at Site U1352 by drilling Hole U1352D while we waited for conditions to improve. This occupied <24 h. The second delay was more serious and occurred at the final site (shelf Site U1354). The wind became too strong to maintain position over the hole in such shallow water (113 m), and operations in Hole U1354B were shut down after 73.6 m of APC coring. In this case we could only wait for weather conditions to improve, given the short time remaining for the expedition and the need to resume operations at Site U1354; however, once again the delay was <24 h. That the operational plan was impacted by <2 days in >6 weeks of operations, much of it in shallow water and in a region of unstable weather patterns, is remarkably fortunate and a tribute to the abilities of the dynamic positioning operators and drillers.

The upper sections (~500 m) of most of the boreholes contained coarse-grained, unlithified material that proved highly unstable. This made wireline logging challenging, even in one instance when a dedicated logging hole was drilled (Hole U1351C). However, the logging personnel worked hard to achieve success and were always prepared to commit the tools to these unstable holes. Strategies were refined to manage the difficult conditions, including using a special tool combination at the final site.

It was possible to keep the holes clear of collapsing sand and shell hash during drilling while rotation and circulation were maintained, but, when these were stopped during logging, unconsolidated material flowed into the hole, forming bridges. In Hole U1351C this situation trapped the triple combination (triple combo) tool string, which was rescued using the drill pipe in a 36 h operation. In addition, the Formation MicroScanner (FMS)-sonic tool string could not be pulled into the drill pipe in Hole U1353C because the centralizers were jammed by debris. The tool was retrieved by tripping the pipe as the hole collapsed around it.

In spite of these difficulties, valuable logs were obtained, particularly at Site U1351, where the triple combo tool string reached the bottom of Hole U1351B at 1030 m WSF during the first run; in Hole U1353C, where a triple combo run was also completed in a dedicated logging hole to ~530 m WSF; and at Site U1354, where a special tool string (gamma ray/sonic/resistivity) reached total depth at 380 m WSF. The caliper was at maximum extent over long intervals in all holes in which a caliper was run, implying significant wash-out.

The limited success at Site U1352, where logging was not possible below ~500 m WSF in the 1927 m deep Hole U1352C, was surprising given that the sediments at this slope site were more fine grained than at the shelf sites. However, the amount of time (2 weeks) that Hole U1352C was open prior to logging probably caused significant deterioration of the hole, which may have played a part. The varying success at different sites may also have been related to the use of varying amounts/weights of mud during logging at different sites. A degree of mud rationing was required because of the heavy use of mud while coring in unstable lithologies and in deep holes with enlarged diameters.

Sonic and vertical seismic profile (VSP) data were especially important to expedition objectives for seismic correlation. However, although complete runs with the triple combo tool string were made to total depth in Holes U1351C and U1353C, the holes did not stay open long enough to complete a run with the second FMS-sonic tool string, let alone the third run required for the VSP. Therefore, FMS-sonic runs were not completed to total depth (<500 m WSF in Holes U1351B and U1352B and ~230 m WSF in Hole U1353C). It was for this reason that a modified string was used, with success, in Hole U1354C to total depth at ~380 m WSF. No VSPs were conducted. Even if the holes had stayed open long enough for a VSP, wash-out made the holes too wide for the tool to make contact with the formation.

The sonic log from Hole U1351B requires postcruise processing, but the sonic log from Hole U1352B was used to generate a traveltime/depth conversion. The resultant velocities proved to be anomalously lower than the precruise-predicted function, which is itself evidently too low, at least for traveltimes corresponding to the deepest penetrations. Results from Hole U1353C may be more representative. The sonic log to ~230 m WSF yielded an excellent synthetic seismogram and estimated depths to seismic sequence boundaries within the logged interval (U13–U10) that were 5–10 m deeper than predicted precruise. The sonic log from Hole U1354C was acquired late in the expedition and was not assessed while at sea.

Although the logs obtained will be extremely valuable, the limited depths reached by sonic logs, the lack of VSPs, and poor hole conditions limit our ability to provide an improved traveltime/depth conversion for seismic correlation throughout the full extent of the drilled interval. In a sense, wireline logging was a victim of Expedition 317's success in achieving deep penetrations. However, the relative ease of drilling indicates that logging-while-drilling (LWD) would be ideal for Canterbury Basin sites. Use of LWD was discussed before the expedition. However, the level of drilling difficulty and the associated risks to LWD equipment were anticipated to be more severe than turned out to be the case. Coupled with the cost of LWD operations at a time of financial constraint, this led to the decision not to employ LWD during Expedition 317. We hope that we will be able to return with LWD in the future.

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