Operations plan/Drilling strategy

The Expedition 353 coring program prioritizes six primary sites and seven alternate sites in 1091–2925 m water depth (Tables T1, T2). This includes two primary sites in the Andaman Sea, three primary sites in the Mahanadi Basin, and one primary site in the southern Bay of Bengal.

The order in which the sites are planned for drilling has been chosen to minimize transit times between locations and maximize the potential utility of alternate sites, should they be necessary. The Andaman Sea sites will be cored first, followed by the Mahanadi Basin sites on the Indian margin, and then the southern Bay of Bengal site (Fig. AF1).

The first two sites cored will be Andaman Sea Site AA-4B followed by Site AA-2B. Site AA-4B will have three holes (A, B, and C). Each hole will be cored first using the advanced piston corer (APC) to refusal and then cored to a target depth of 422 meters below seafloor (mbsf) using the extended core barrel (XCB). Site AA-2B will utilize a similar strategy; however, only two holes (A and B) will be cored and Hole B will be wireline logged. Target depth for Site AA-2B is 738 mbsf.

Mahanadi Basin Site BB-7 will be drilled first; it will consist of three holes APC cored to refusal followed by XCB coring to a total depth of 184 mbsf. It is possible that the APC can be pushed to the total depth at this site by using the half-length APC. Site BB-5 will be drilled next and will consist of two holes APC cored to refusal then XCB cored to a total depth of 680 mbsf. Following the comparison of the age models and carbonate preservation between Sites BB-7 and BB-5, the decision will be made to either continue with additional deep holes at Site BB-5 (and not drill Site BB-2B at all) or to terminate Site BB-5 and drill Site BB-2B. This decision tree is designed to account for the observation that Site BB-2B (a shallow ridge-top site offering good carbonate preservation and reduced turbidite input) does not have the same seismic units as found at Sites BB-1B, BB-4, BB-7, and BB-8B. Compared to these sites, Site BB-2B seismics indicate two possibilities. One interpretation is that part of the Pleistocene may be missing from the top of Site BB-2B but it has a section below the light reflectors (i.e., 2.0–2.1 s two-way traveltime at Site BB-7) that is not present in the other sites (Fig. AF12). The other interpretation is that Site BB-2B has an expanded upper section but is missing the light reflector interval.

The final site (N90E-2C) will consist of three holes APC cored to refusal then advanced to the target depth using the XCB. Hole C will be logged using the triple combination and Formation MicroScanner (FMS)-sonic tool strings.

For planning purposes, APC refusal depth is estimated at 100 mbsf, although we anticipate that this may be exceeded at some sites. APC refusal is defined in two ways: (1) a complete stroke is not achieved because the formation is too hard or (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.” When APC refusal occurs in a hole before target depth is reached, the half-length APC can be employed to advance the hole. When refusal occurs with this system, the XCB technique may be used to advance the hole.

According to the current operations plan, Expedition 353 will core ~6190 m of sediment. Core recovery is often 100% with the APC system but is typically more variable with the XCB system, depending on lithology.