ACORK INSTALLATION

The LWD data collected in Hole 1173B essentially reconfirmed the scientific rationale for the planned distribution of four packers and five screens in the ACORK. These were configured in a 728-m-long ACORK string (Fig. F1; Table T7), to emphasize long-term observations of pressures in three principal zones, as follows:

1. Oceanic basement below 731 mbsf, to determine permeability and pressures in the young oceanic crust being subducted, and thereby assess the role of oceanic crust in the overall hydrogeology at Nankai Trough. A screen was installed immediately above the ACORK shoe, centered at 722 mbsf, with a packer immediately above. As described in "Operations" the hole opening run successfully penetrated several meters into basement, and coring through the ACORK after its installation penetrated another 19.5 m. This assured that the signals of basement hydrogeologic processes would be transmitted to this deepest screen.
2. Lower Shikoku Basin deposits, well below the stratigraphic projection of the décollement zone, to assess the hydrological properties of a reference section of the lower Shikoku Basin deposits and test for fluid pressure propagation from basement or possibly higher in the section. A packer was centered at 495 mbsf to isolate a screen centered at 563 mbsf.
3. The stratigraphic projection of the décollement zone in the upper part of the lower Shikoku Basin deposits seaward from Sites 1174 and 808. The Leg 190 cores and wireline logs and Leg 196 LWD data showed only the slightest physical properties variations across this zone at ~390 to 420 mbsf, ~50-80 m below the boundary between the upper and lower Shikoku Basin deposits. A symmetric array ~100 m long, comprising three screens separated by two packers, was built into the ACORK string such that the three screens were centered at 439, 396, and 353 mbsf. Objectives of this array include (1) documenting the variation of hydrogeologic properties across and away from this zone as a reference for the state of the formation before the décollement zone actually develops closer to the trench axis and (2) detecting the possibility of fluid flow along the stratigraphic projection of the décollement zone. In addition, the central screen in this array (i.e., the screen that spans the stratigraphic equivalent of the décollement zone) includes a second small-diameter line for eventual sampling of formation fluids from the wellhead.

After installation of the ACORK casing string, the RCB coring BHA was successfully deployed through it (Fig. F34), with the principal objective of deepening the hole into basement to assure that the signal of basement hydrogeologic processes will be transmitted to the deepest screen. A total of 19.5 m into basement was cored, with a recovery of 5.2 m (27% recovery). The core comprises basaltic basement overlain by a thin veneer of volcaniclastics. These cores were left on board for description by members of the Leg 197 Shipboard Scientific Party (see "Igneous Petrology and Volcanology").

Following the basement coring, the final step in the ACORK installation at Hole 1173B was deployment of a bridge plug to seal the bore of the casing and isolate the basement section to be monitored by the deepest screen. We intended to set the bridge plug very near the bottom of the ACORK string, allowing future deployment of other sensor strings within the central bore. However, as is described in "Operations" the bridge plug apparently set prematurely at 466 mbsf; this was not sensed at the rig floor and ensuing operations resulted in breaking the pipe off at the ACORK head. Nevertheless, detailed analysis suggested that the bridge plug was indeed set and there should be no broken pipe outside the ACORK head to inhibit future data recovery operations. Hence, we proceeded directly to Site 808 for the second planned ACORK installation; once that was completed we returned to Hole 1173B for an inspection that confirmed that the ACORK was undamaged and that the bridge plug was set (Fig. F35).