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, F6). 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.
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 system supplied 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 confirm 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 Integrated Drilling Evaluation and Logging (IDEAL) sonic-while-drilling (ISONIC) velocity 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 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. Some of these variations might be 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 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 due to 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 (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 of resistivity and decrease of 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 the 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 agree with core data from Leg 190 Hole 1173A. Fractures are high angle (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 increased bedding dip. The upper limit of this zone corresponds to the stratigraphic equivalent of the décollement zone. 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 to propagating compressional deformation from the 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 the differential caliper 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) to III (lower Shikoku Basin). 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 nearly constant with depth while density increases. All LWD resistivity logs show a similar overall trend, in good agreement with available wireline logs. Shallow-focused button resistivities that are consistently higher than medium and deep resistivities are unusual and an 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 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 log 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 (log Unit 3) and may be due to a sampling bias toward more cohesive, higher velocity samples in the core velocity measurements.
A four-packer, five-screen, 728-m-long ACORK string (Fig. F6) was deployed through the sediment section in Hole 1173B, configured to emphasize long-term observations of pressures in three principal zones, as follows:
After ACORK installation, the rotary core barrel coring bottom-hole assembly was successfully deployed through the ACORK casing (Fig. F34 in the "Site 1173" chapter) 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 a video inspection confirmed that there is no broken pipe outside the ACORK head to inhibit future data recovery operations (Fig. F35 in the "Site 1173" chapter).
We drilled Hole 808I (Table T1) to obtain LWD data through the frontal thrust and décollement zones at the deformation front of the Nankai Trough and to install an ACORK long-term subseafloor hydrological monitoring experiment (Figs. F5, F7). This hole complements cores recovered at Site 808 during Leg 131. Coring, logging, and monitoring here are intended to document the physical and chemical state of the Nankai accretionary prism and underthrust sediments through the frontal thrust zone, the décollement zone, and into oceanic basement.
The overall quality of the LWD logs recorded in Hole 808I is variable. We recorded at least one sample per 15 cm over 99% of the total section. Sections of enlarged borehole estimated by the differential caliper yield unreliable density data and associated derived porosity that is confirmed by comparison to core data. This problem is primarily associated with the depth intervals 725-776 and 967-1057 mbsf, in which there was a long duration between drilling and recording of the logs due to wiper trips or poor hole conditions. Density and density-derived porosity should be used cautiously until more complete corrections and editing are completed postcruise. Although the LWD ISONIC tool worked well, the processing of the waveforms was not straightforward and postcruise processing is required to yield reliable sonic data.
A combination of visual interpretation and multivariate statistical analysis defined four log units and six log subunits.
Log Unit 1 (156-268 mbsf) is characterized by the overall lowest mean values of gamma ray, density, and photoelectric effect and overall highest mean values of resistivity and neutron porosity. These values coincide with very fine grained sandstone, siltstone, and clayey siltstone/silty claystone observed in the cores. Log Unit 2 (268-530 mbsf) shows a constant value range of gamma ray and neutron porosity and a decreasing resistivity log. A high variability in the differential caliper log and a large number of values >1 in reflect poor borehole conditions. Log Unit 3 (530-620 mbsf) is marked by a significant increase in mean values of gamma ray, density, and photoelectric effect logs. Log Unit 4 (620-1035 mbsf) is characterized by the overall highest mean values of gamma ray, photoelectric effect, and density and the lowest mean values of resistivity and neutron porosity. Generally, a positive correlation between gamma ray and photoelectric effect is observed. A continuous increase in gamma ray and photoelectric effect, which reflects an increase in clay and carbonate content, is observed from log Unit 1 to log Unit 4. A positive correlation between resistivity with density defines log Units 3 and 4.
RAB tools imaged fracture populations and borehole breakouts throughout much of the borehole. We identified both resistive and conductive fractures, respectively interpreted as compactively deformed fractures (leading to porosity collapse) and open fractures. Fractures are concentrated in discrete deformation zones that correlate with core analysis performed during Leg 131: the frontal thrust zone (389-414 mbsf), a fractured interval (559-574 mbsf), and the décollement zone (~940-960 mbsf). Only relatively sparse deformation occurs between these zones. The major deformation zones are characterized by conductive fractures and overall high resistivity, with resistive fractures between these zones. Fractures are steeply dipping (majority >30°) and strike predominantly east-northeast-west-southwest, close to perpendicular to the convergence vector (~310°-315°; Seno et al., 1993). Bedding dips are predominantly low angle (<50°) but are difficult to identify in the highly deformed zones, therefore biasing this result. Bedding strike is more random than fracture orientation, but, where a preferred orientation is recorded, beds strike subparallel to fractures and approximately perpendicular to the convergence vector.
The frontal thrust zone (389-414 mbsf) represents the most highly deformed zone in Hole 808I and contains predominantly south-dipping fractures (antithetic to the seismically imaged main thrust fault) and a few north-dipping east-northeast-west-southwest striking fractures. The highly fractured interval at 559-574 mbsf contains similar fracture patterns to the frontal thrust zone. Both deformation zones are characterized by high-conductivity (open?) fractures within a zone of overall high resistivity.
Deformation at the décollement zone is more subdued and is represented by a series of discrete fracture zones. The décollement zone in the RAB images (937-965 mbsf) is defined by a general increase in fracture density and variability in physical properties.
Borehole breakouts are
recorded throughout Hole 808I and are particularly strongly developed within log
Unit 2 (270-530 mbsf), suggesting lithologic control on sediment strength and
breakout formation. Breakouts indicate a northeast-southwest orientation for the
minimum horizontal compressive stress (
2),
consistent with a northwest-southeast convergence vector (~310°-315°, parallel
to 1;
Seno et al., 1993). Breakout orientation deviates slightly from the dominant
strike of fractures (east-northeast-west-southwest), but this deviation may be
within error of measurements.
The Hole 808I LWD density log shows a good fit to the core bulk density, slightly underestimating core values in the upper 550 mbsf and slightly overestimating core values between 550 and 970 mbsf. Below 156 mbsf the LWD density log shows a steady increase from ~1.7 to ~1.95 g/cm3 at 389 mbsf. Between 389 and 415 mbsf density varies greatly, corresponding to the frontal thrust zone. The low density values here are probably spurious, produced by washout of the borehole. Below the frontal thrust zone density decreases sharply to ~1.85 g/cm3. Below 530 mbsf it increases to ~2.1 g/cm3. Between 725 and 776 mbsf density drops sharply to ~1.75 g/cm3, corresponding to a period of borehole wiper trips. Density increases more rapidly from ~1.95 g/cm3 at 776 mbsf to 2.25 g/cm3 at 930 mbsf, decreasing steadily to ~2.15 g/cm3 before stepping down to 1.4 g/cm3 at 965 mbsf. This corresponds to the base of the décollement zone; below, density increases steadily from ~1.7 g/cm3 at 975 mbsf to ~2.0 g/cm3 at 1034.79 mbsf.
The downhole variation of
LWD resistivity measurements shows identical trends in all five resistivity logs
of Hole 808I. All log unit boundaries are clearly identified. Log Unit 1 has an
average resistivity of 0.8-0.9 
m.
After a sharp decrease in resistivity from 1.3 to 0.5 m
between 156 and 168 mbsf, the signal shows an increasing trend downward to the
boundary between log Units 1 and 2. Log Unit 2 is characterized by an overall
decreasing trend in resistivity from ~0.9 to ~0.6 m.
However, this trend is sharply offset from 389 to 415 mbsf (log Subunit 2b),
where resistivity values are ~0.3 m
higher. This zone seems to correspond to the frontal thrust; the higher
resistivity here may reflect compactive deformation in the frontal thrust zone.
Log Unit 3 shows a higher degree in variability of the resistivity signal.
Resistivity exhibits less variation again in log Unit 4, where it averages ~0.6 m.
At ~925 mbsf resistivity changes trend from a gradual increase to a gradual
decrease. This decreasing trend persists to the base of the décollement zone at
~960 mbsf. As observed in Hole 1173B, shallow-focused resistivity is
systematically higher than both medium and deep resistivity.
Beneath the casing (~150 mbsf), correlations between the synthetic seismogram and seismic reflection data are only broadly consistent. The details of amplitude and waveform throughout most of the section do not match the seismic data. Amplitudes of the intervals between ~200 and ~400 mbsf, between ~750 and ~850 mbsf, and below ~925 mbsf are significantly higher in the synthetic seismogram than in the seismic data and are probably generated by numerous anomalously low velocity and density values. The seismic data cannot be matched with the synthetic seismograms produced from the logs because of the large variations in velocity and density. These results and the poor hole conditions raise doubts about the validity of portions of these logs.
In Hole 808I we assembled a 964-m-long ACORK casing string incorporating two packers and six screens for long-term observations of pressures in three principal zones, as follows:
Drilling conditions during installation of the ACORK steadily worsened, starting ~200 m above the intended total depth. Despite all efforts, progress stopped 37 m short of the intended installation depth. This left the screen sections offset above the intended zones (Fig. F7), not an ideal installation but still viable in terms of scientific objectives. In addition, this left the ACORK head 42 m above the seafloor, unable to support its own weight once we pulled the drilling pipe out (Fig. F31 in the "Site 808" chapter). Fortunately, when the ACORK head fell over, it landed on the seafloor such that all critical components remained in good condition (Fig. F32 in the "Site 808" chapter). This includes the critical hydraulic umbilical, data logger, and the underwater-mateable connector, which remains easily accessible by a remotely operated vehicle or by submersible for data download.
