We drilled Hole 808I to obtain logging-while-drilling (LWD) data through the frontal thrust and décollement zones at the deformation front of the Nankai Trough and to install an Advanced CORK (ACORK) long-term subseafloor hydrogeologic monitoring experiment (Fig. F1). This hole complements Site 808 cores, which were recovered during Ocean Drilling Program (ODP) 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 indicated by differential caliper measurements yield unreliable density and associated porosity data, which is confirmed by a comparison to core data. Unreliable data are primarily associated with the depth intervals at 725-776 and 967-1057 meters below seafloor (mbsf), where there was a long gap between drilling and recording of the logs due to wiper trips or poor hole conditions (see "Operations"). Density and density-derived porosity should be used cautiously until more complete corrections and editing is completed postcruise. Although the Anadrill Integrated Drilling Evaluation and Logging (IDEAL) sonic-while-drilling (ISONIC) velocity tool worked well, the processing of the waveforms was not straightforward and postcruise processing is required to yield reliable sonic data.
Four log units and six log subunits were defined through a combination of visual interpretation and multivariate statistical analysis. 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 sandstones, siltstones, and clayey siltstone/silty claystones observed in the cores. Log Unit 2 (268-530 mbsf) has constant values 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 bad borehole conditions. Log Unit 3 (530-620 mbsf) is marked by a significant increase in mean values of gamma ray, density, and photoelectric effect. 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 is observed from log Unit 1 to Unit 4, which reflects an increase of clay and carbonate content. Log Units 3 and 4 are characterized by a positive correlation between resistivity and density.
Resistivity-at-the-bit (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 those seen in Site 808 cores 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 dominated by conductive fractures and overall high resistivity with resistive fractures between the 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, 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 at Hole 808I and contains predominantly south-dipping (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 imaged in the RAB images (937-965 mbsf) is defined by a general increase in fracture density and a marked variability in physical properties.
Borehole breakouts are
recorded throughout Hole 808I. They 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 the measurement error.
The Hole 808I LWD density log shows a good fit to the core bulk density, slightly underestimating core values in the upper 550 m 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 shows large variations 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, and 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 variations of
LWD resistivity measurements show 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
Unit 1/2 boundary. Unit 2 is characterized by an overall decreasing trend in
resistivity from ~0.9 to ~0.6 m.
However, this trend is sharply offset at 389-415 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.
Unit 3 is characterized by a higher degree of variability in the resistivity
signal. Resistivity again exhibits less variation in Unit 4 where it averages
~0.6 m.
At ~925 mbsf resistivity values change from gradually increasing to gradually
decreasing. This change in trend occurs near the top of the décollement zone
and may represent a tendency toward increasing porosity downward within the
décollement zone. As was observed in Hole 1173B, shallow-focused resistivity
values are systematically higher than both medium- and deep-focused resistivity
values.
Correlations between the synthetic seismogram and seismic reflection data are only broadly consistent beneath the cased section (~150 mbsf). The defined lithologic boundaries or units correlate only at a few depth intervals. 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. The velocity and density logs infer reflections that are not observed in the seismic data, so many of the velocity and density values may not reflect true in situ properties and may be a product of poor hole conditions.
At 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. F1), not an ideal installation but still viable in terms of scientific objectives. In addition, this left the ACORK head 42 m above seafloor, unable to support its own weight once we pulled the drilling pipe out. Fortunately, when the ACORK head fell over it landed on the seafloor such that all critical components remained in good condition. This includes the critical hydraulic umbilical, data logger, and the underwater-mateable connector, which remains easily accessible by a remotely operated vehicle (ROV) or by submersible for data download.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 196IR-104
