We drilled Holes 1173B and 1173C to obtain logging-while-drilling (LWD) data at a reference site on the seaward flank of the Nankai Trough and to install an Advanced CORK (ACORK) long-term subseafloor hydrogeologic monitoring experiment. 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 to the northwest.
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 (Fig. F1). Additionally, the tools provided estimates of hole size and borehole resistivity images. A measurement-while-drilling (MWD) system provided information on weight on bit, 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 0.3-m interval. The caliper shows that the gap between the bit radius and the hole is <1 in throughout both holes, with the exception of the uppermost 75 m of Hole 1173C, where soft sediment washed out a gap up to 2 in long. Therefore, the density log over this shallowest 75-m interval is unreliable. This was the first ever use of the LWD Anadrill Integrated Drilling and Logging (IDEAL) sonic-while-drilling (ISONIC) velocity tool in such fine-grained, unlithified sediment and the first use ever by the Ocean Drilling Program (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 velocity data.
Both visual and multivariate statistical analyses of the logs define five log 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 the upper 75 m of log Unit 1 (0-122 meters below seafloor [mbsf]). This log unit shows high neutron porosity and low density values with a high standard deviation. These variations might be in part real and reflect the presence of silt and sand turbidites of the outer trench-wedge facies. A significant decrease in resistivity, density, and gamma ray and an increase in neutron porosity with depth show an abnormal compaction trend and define log Unit 2 (122-340 mbsf). This log 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 caused by the formation of cristobalite. The log 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 characterize log Unit 3 (340-698 mbsf). Resistivity and, less obviously, gamma ray logs show a cyclicity of values (480-700 mbsf) that reflects changes in lithology, which may in turn reflect an interbedding of coarser and finer grained sediments. Log 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). Log Unit 5 (731-735 mbsf) shows an abrupt increase in resistivity and a decrease in gamma ray log values, which characterize the basaltic oceanic basement.
Structural data determined from RAB images of medium-focused resistivity (penetration depth 7.6 cm beyond standard borehole radius) indicate sparse deformation and predominantly subhorizontal bedding dips. Increases in bedding dips (5°-35°) at 50-200 mbsf and below ~370 mbsf are in agreement with core data from Leg 190 Hole 1173A. Fractures are oriented at high angles (40°-80°), show normal displacement where measurable, and have variable strike orientation. Resistive fractures dominate and might reflect nonconductive clay gouge, mineralization, or porosity collapse due to compaction. An increase in fracture intensity occurs at 380-520 mbsf, correlating with an increase in bedding dip. The upper limit of this zone corresponds to the projected stratigraphic equivalent of the décollement zone. At ~500 mbsf, bands of heterogeneous (mottled) high resistivity are thought to represent zones of intense deformation or brecciation. In general, the deformation observed in Holes 1173B and 1173C is consistent with extensional faulting probably related to basinal compaction and burial and not to propagating compressional deformation from an accretionary wedge.
LWD density data in Holes 1173B and 1173C closely match core physical properties data from Hole 1173A, except for the uppermost 60 m, where differential caliper values exceeded 1 in. LWD densities are nearly constant in log Subunit 1b (55-122 mbsf) and Unit 2 (122-340 mbsf), with the notable exceptions of two high-amplitude variations near the transition from lithologic Unit II (upper Shikoku Basin facies) to III (lower Shikoku Basin facies). Log Unit 3 (340-698 mbsf) is characterized by a steady increase in density consistent with normal compaction. The LWD resistivity logs clearly respond to the lithologic boundaries identified in Hole 1173A. Within log Unit 2 resistivity decreases with depth while density is constant, whereas in log Unit 3, resistivity is about constant with depth and density increases. All LWD resistivity logs show a similar overall trend, in good agreement with wireline logs, where available. Shallow button resistivities that are consistently higher than medium and deep resistivities is an unusual and unexplained feature of Site 1173 LWD 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 the seismic reflection data at ~80-100 (trench-basin transition facies), ~175, ~265-270, and ~300-350 mbsf (associated with the upper to lower Shikoku Basin unit boundary and the log Unit 2/3 boundary). A change in physical properties 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 (log Unit 3) and may be caused by a sampling bias in the core velocity measurements.
A four-packer, five-screen, 728-m-long ACORK string (Fig. F1) was deployed through the sediment section in Hole 1173B, configured to emphasize long-term observations of pressures in three principal zones, as follows:
After installation of the ACORK, 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 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.
Following 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. This was intended to be set 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 a video inspection confirmed that there is no broken pipe outside the ACORK head to inhibit future data recovery operations.
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Ms 196IR-103