Operations plan and drilling strategy

Expedition 346 aims to achieve an ambitious coring program that prioritizes eight primary sites and two alternate sites in 316–3435 m of water depth (Tables T1, T2). Eight of the sites are in Japanese and Korean territorial waters in the Japan Sea and two are in Japanese waters in the East China Sea. The final operations plan and number of sites to be cored is contingent upon the JOIDES Resolution operations schedule, operational risks (see below), and the outcome of requests for territorial permission to occupy these sites. Of particular relevance is the planned ~13–14 day transit from Valdez, Alaska, prior to beginning coring operations in the Japan Sea. Should ship speed be less than the estimated average of 10.5 kt, the drilling schedule could be significantly impacted.

Coring strategy will consist of APC coring using nonmagnetic core barrels in three holes (A, B, and C) at each site to ~200 meters below seafloor (mbsf) or APC refusal, with the exception of proposed Site JB-1, which has slightly shallower depth objectives (150 mbsf). As described below and pending further discussions, at this point only cores in Hole A at each site will be oriented.

For planning purposes, the APC refusal depth is estimated at 200 mbsf, although we anticipate that this may be exceeded at some of the more mud-rich sites with target depths greater than 200 mbsf. APC refusal is conventionally defined in two ways: (1) a complete stroke (as determined from the standpipe pressure after the shot) is not achieved because the formation is too hard and (2) excess force (>100,000 lb) is required to pull the core barrel out of the formation because the sediment is too cohesive or “sticky.” In cases where a significant stroke can be achieved but excessive force cannot retrieve the barrel, the core barrel can be “drilled over” (i.e., after the inner core barrel is successfully shot into the formation, the bit is advanced to some depth to free the APC barrel). When APC refusal occurs in a hole before the target depth is reached, the XCB technique may be used to advance the hole.

The target depth at the five primary proposed sites (YR-1 [450 mbsf], YB-1 [500 mbsf], YB-2 [400 mbsf], UB-1 [285 mbsf], and ECS-1B [800 mbsf]) is greater than the APC refusal depth. The deeper sections will be advanced by XCB coring on one or two of the holes, time permitting. If the target depth at proposed Site ECS-1B cannot be reached by XCB coring, RCB coring may be employed, time permitting, to reach 800 mbsf. Rotary coring will allow penetration through a significant portion of the Pliocene and possibly into the upper Miocene at proposed Site YR-1 (see Tables T2 and T3 for operations details per site). All target depths in the current operations plan have been approved by the IODP Environmental Protection and Safety Panel (EPSP) and the Texas A&M University Safety Panel, but a request to extend the penetration depths at all sites (except proposed Site ECS-1B) by 50 m will be submitted after publication of this Scientific Prospectus. Triple APC holes will allow us to build a composite stratigraphic section at each site for the upper ~200 mbsf.

According to the current operations plan, Expedition 346 will core ~6820 m of sediment and potentially recover ~6060 m of core. Considering the significant transit time at the beginning of the expedition (~2 weeks), this coring schedule within the remaining 6 weeks of the expedition is indeed ambitious and will require tight operational planning and flexibility. The estimate of the amount of core recovered is based on 100% recovery with the APC system and 65% recovery with the XCB system.