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Site 1173

We drilled Holes 1173B and 1173C (Table T1) to obtain logging-while-drilling (LWD) data at a reference site on the seaward flank of the Nankai Trough and to install an ACORK long-term subseafloor hydrological monitoring experiment (Figs. F1, F2, F3, F4, F5). These holes complement Hole 1173A, which was cored from the surface to basement during Leg 190. This site provides a basis for comparison of physical and chemical properties between the incoming undeformed sediments and rocks of the Shikoku Basin with deformed materials of the accretionary prism and underthrust sediments cored at sites landward of the deformation front.

Log Quality

At Site 1173, the LWD tools measured resistivity at the bit (RAB), sonic velocity, density, porosity, natural gamma ray production, and photoelectric effect from the seafloor to basaltic basement. Additionally, the tools provided estimates of hole size and borehole resistivity images. A measurement-while-drilling (MWD) system supplied information on weight on bit (WOB), torque, heave, resistivity, density, and sonic velocity that was communicated to the surface and displayed instantaneously during drilling.

The overall quality of the LWD logs recorded in Holes 1173B and 1173C is excellent. The LWD logs generally agree with the more limited Hole 1173A wireline logs. In Holes 1173B and 1173C the drilling rate was maintained between 35 and 60 m/hr throughout the section, and all measurements were made within 1 hr of bit penetration. At least two depth points were measured in each 0.30-m interval. The caliper shows that the gap between the bit radius and the hole is <1 in throughout both holes, except for the uppermost 75 m of Hole 1173C where soft sediment washed out a gap of up to 2 in. Therefore, the density log over this shallowest 75-m interval is unreliable. This was the first use of the LWD sonic velocity (ISONIC) tool in such fine-grained unlithified sediment and its first use by ODP. Although the tool worked well, the processing of the waveforms was not straightforward and will have to be improved postcruise to yield reliable sonic data.

Logging Units and Lithology

Both visual and multivariate statistical analyses of the logs define five logging units that account for the lithologic variations observed in the cores.

A high variability in the differential caliper log and a large number of caliper values >1 in reflect bad borehole conditions during drilling of logging Unit 1 (0–122 meters below seafloor [mbsf]). This logging unit shows high neutron porosity and low density values with a high standard deviation. Some of these variations are real and reflect silt and sand turbidites of the outer trench wedge facies.

A significant decrease of resistivity, density, and gamma ray and increase of neutron porosity with depth show an abnormal compaction trend and define logging Unit 2 (122–340 mbsf). This logging unit correlates with lithologic Unit II (102–344 mbsf), which consists of hemipelagic mud with abundant interbeds of volcanic ash. The low density could be related to a cementation effect due to the formation of cristobalite. The logging Unit 2/3 boundary correlates with the diagenetic phase transition between cristobalite and quartz.

High gamma ray, density, and photoelectric effect log values that increase continuously with depth distinguish logging Unit 3 (340–698 mbsf). Resistivity and, less obviously, gamma ray logs show a cyclicity (480–700 mbsf) that reflects changes in lithology, which may in turn reflect an interbedding of coarser and finer grained sediments.

Logging Unit 4 (698–731 mbsf) is defined by broad variations in photoelectric effect, resistivity, neutron porosity, and gamma ray logs that correlate well with the presence of the volcaniclastic facies of lithologic Unit IV (688–724 mbsf).

Logging Unit 5 (731–735 mbsf) shows an abrupt increase of resistivity and decrease of gamma ray, which characterize the basaltic oceanic basement.

Structural Geology

Structural data determined from RAB images of medium focused resistivity (penetration depth = 7.6 cm) indicate sparse deformation and predominantly subhorizontal bedding dips. Increases in bedding dips (5°–35°) at 50–200 mbsf and below ~370 mbsf agree with core data from Hole 1173A. Fractures are high angle (40°–80°), show normal displacement where measurable, and have random orientation. An increase in fracture intensity occurs at 380–520 mbsf, correlating with increased bedding dip. The upper limit of this zone corresponds to the projected stratigraphic equivalent of the décollement. At ~500 mbsf, bands of heterogeneous (mottled) high resistivity probably represent zones of intense deformation or brecciation. In general, deformation observed in Holes 1173B and 1173C is consistent with extensional faulting probably related to basinal compaction and burial and not propagating compressional deformation from the accretionary wedge.

Physical Properties

LWD density data in Holes 1173B and 1173C closely match core physical properties data from Hole 1173A, except for the uppermost 60 mbsf, where differential caliper exceeded 1 in. LWD densities are nearly constant in logging Subunit 1b (55–122 mbsf) and logging Unit 2 (122–340 mbsf), with the notable exceptions of two high-amplitude variations near the transition from logging Unit 2 (Upper Shikoku Basin) to lithologic Unit III (Lower Shikoku Basin). Logging Unit 3 (340–698 mbsf) is characterized by a steady increase in density consistent with a normal compaction trend. The LWD resistivity logs clearly respond to the lithologic boundaries identified in Hole 1173A. Within logging Unit 2 resistivity decreases with depth, whereas, in logging Unit 3, resistivity is nearly constant with depth, in contrast to the behavior of density in these two units. All LWD resistivity logs show similar overall resistivity trends, in good agreement with available wireline logs. Shallow button resistivities that are consistently higher than medium and deep resistivities are unusual and an unexplained feature of the Site 1173 LWD data.

Logs and Seismic Reflection Data

The velocities from the core and wireline data and densities from the core and LWD data from 0 to 350 mbsf were used to generate a synthetic seismogram in good agreement with the seismic reflection data. Good correlations exist between the synthetic seismogram and seismic reflections at ~80–100 (trench–basin transition facies), ~175, ~265–270, and ~300–350 mbsf (associated with the Upper/Lower Shikoku Basin unit boundary and the logging Unit 2/3 boundary). An increase in acoustic impedance associated with the phase transition from cristobalite to quartz may be, in part, responsible for this reflection. High reflectivity in the synthetic seismogram beneath ~350 mbsf does not match with the low reflectivity in the seismic data of the Lower Shikoku Basin unit (logging Unit 3) and may be due to a sampling bias toward more cohesive, higher velocity samples in the core velocity measurements.

ACORK Installation and Basement Coring

A four-packer, five-screen, 728-m-long ACORK string (Fig. F5) was deployed through the sediment section in Hole 1173B, configured 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, and a packer was placed immediately above the screen.
  2. Lower Shikoku Basin formation, well below the stratigraphic projection of the décollement, to assess the hydrological properties of a reference section of the Lower Shikoku formation 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 predécollement or stratigraphic projection of décollement at ~390–420 mbsf in the upper part of the Lower Shikoku formation seaward from Sites 1174 and 808. A symmetric array, ~100 m long, comprising three screens separated by two packers, was built into the ACORK string so that the screens were centered at 439, 396, and 353 mbsf. Objectives of this array include (a) documenting the variation of hydrogeological properties across and away from this zone as a reference for the state of the formation before the décollement actually develops closer to the trench axis and (b) detecting the possibility of fluid flow along the stratigraphic projection of the décollement. In addition, the central screen in this array (the screen that spans the predécollement) includes a second small-diameter line for eventual sampling of formation fluids from the well head.
After ACORK installation, the rotary core barrel (RCB) coring bottom-hole assembly (BHA) was successfully deployed through the ACORK casing to deepen the hole into basement to assure that the signal of basement hydrogeological processes will be transmitted to the deepest screen. A total of 19.5 m into basement was cored, with recovery of 5.2 m (27% recovery). The core comprises basaltic basement overlain by a thin veneer of volcaniclastics.

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, 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 suggests that the bridge plug is indeed set and there should be no broken pipe outside the ACORK head to inhibit future data recovery operations.

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