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doi:10.2204/iodp.proc.338.104.2014

Logging while drilling

Hole C0012H was drilled on the Kashinosaki Knoll in the Shikoku Basin with LWD and MWD technology (see the “Methods” chapter [Strasser et al., 2014a]). MWD downhole drilling parameters and LWD gamma ray, azimuthal resistivity, resistivity images, and sonic velocity data were collected from 0 to 710.0 mbsf (3538.0–4248.0 m DRF). A summary of logging tools and data collected are listed in (Table T1). The analysis of LWD and MWD data (Figs. F3, F4) clearly distinguishes basement rock from the Shikoku Basin sediment at 530.3 mbsf. Overall, eight logging units were defined: six within the sediment section and two in the basement.

Data quality

The overall quality of the logging data was determined to be fair. Because of no rotation during wash down and a rate of penetration exceeding 40 m/h, no quality resistivity images were recorded above 58.5 mbsf (3596.5 m DRF). Sharp horizontal lines, artifacts from ship heave and pipe vibration, were observed throughout the processed resistivity images. Missing data due to high stick-slip (>300 cycles/min) were also observed, with a greater concentration in the upper section (0 to ~220 mbsf) and image quality generally increasing toward the sediment/basement interface. Overall, 19.4% of the images were deemed poor quality.

Continuous, good quality compressional sonic velocities were acquired throughout the drilled interval. Through the sediment section (0.0–530.30 mbsf), shear sonic velocity data were not available because it is not possible to extract shear-wave velocities slower than the compressional velocity of the drilling mud velocity when measured by a monopole transmitter. However, shear-wave sonic velocity data were obtained in two faster intervals: 573.9–603.6 mbsf (4111.9–4141.6 m DRF) and 655.4–686.8 mbsf (4193.4–4224.8 m DRF). Quality checks on the data indicate good performance of the sonicVISION tool and good quality of the measurements.

Log characterization and interpretation

Hole C0012H logging units were characterized from visual inspection of the gamma ray, ring resistivity, and sonic velocity logs (Fig. F4). Eight primary logging units were identified from deviations in gamma ray values with respect to a baseline, changes in trend, and changes in overall log character. The logging units were further divided into subunits based on subtle variations in resistivity and sonic velocity (Table T2; Fig. F4). Preliminary lithologic interpretations were made based on log character and relative values, with guidance from coring at Site C0012 during Expeditions 322 and 333 (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c). Prominent changes in overall log character and baseline shifts occur at 530.30 mbsf, where gamma ray values drop to <20 gAPI, resistivity increases from ~3 to >10 Ωm, and sonic velocity increases from ~1800 to >3000 m/s. This boundary is interpreted as the sediment/basement interface, which was cored during Expedition 322 in Hole C0012A and during Expedition 333 in Holes C0012E, C0012F, and C0012G at 537.81, 525.81, 520.46, and 529.69 mbsf, respectively. These cores showed that the lowermost Shikoku Basin sediment overlies basaltic oceanic basement rock, composed of both pillow basalts and sheet flows (Expedition 333 Scientists, 2012c). Given such a contrast in log parameters/character between the sediment and basement (Fig. F5), the sediment section (0–530.30 mbsf) and basement (530.30–710.00 mbsf) are described and interpreted separately.

Shikoku Basin sediment (0–530.30 mbsf)

Six logging units were defined within the sediment section of Hole C0012H (Figs. F4, F6; Table T2). To determine the variation in gamma ray values, a baseline of 95 gAPI was chosen because it reflects the background gamma ray value and is consistent with the baseline defined at Site C0011 in the hemipelagic, silty claystone sediment (Expedition 322 Scientists, 2010b). Lithologic interpretations of logs follow the methodology used at Site C0011 (Expedition 322 Scientists, 2010b). Below this gamma ray baseline value more sand-rich sediment is interpreted, and above this value more clay-rich sediment is interpreted. Volcanic ash and sand horizons were interpreted based on a differentiation between high and low resistivity corresponding to low gamma ray spikes: low gamma ray values with low resistivity were classified as sand and low gamma ray values with high resistivity were classified as volcanic ash. Units and subunits were chosen based on changes in abundance of interpreted sand and ash horizons.

Logging Unit I (0–144.3 mbsf)

Logging Unit I is characterized by a gradually increasing trend in gamma ray values from ~65 to 95 gAPI with minor fluctuations (Fig. F6). Resistivity is low and roughly constant (~0.7 Ωm), with a few minor increases and decreases that correspond to low gamma ray peaks. Overall, sonic P-wave velocity exhibits a gradual increase from ~1530 to 1650 m/s. Low gamma ray values and low resistivity exhibited through logging Unit I are interpreted to reflect sandy mud lithologies, which is supported by the low sonic velocity that is close to the velocity of seawater (1500 m/s), indicative of poorly consolidated, porous sediment (Rider, 2002).

Logging Unit II (144.3–188.1 mbsf)

The logging Unit I/II boundary was placed at 144.3 mbsf, which corresponds to a change in the character of the gamma ray and resistivity logs. Also, the sonic log shows an increase from ~1650 to 1750 m/s across this logging unit boundary. An increase in frequency and thickness of low gamma ray spikes and corresponding resistivity spikes in logging Unit II is interpreted to represent a heterogeneous mixture of sand and ash beds. The most prominent layer occurs between 147.8 and 151.9 mbsf, where gamma ray values drop to <38 gAPI and resistivity increases gradually to 1.16 Ωm before a sharp drop back to a low resistivity of ~0.9 Ωm. This 4 m thick interval is interpreted as volcaniclastic sand based on low gamma ray values and high resistivity. The base of logging Unit II (188.1 mbsf) is placed below the last interpreted ash horizon, where there is a small, stepped increase in resistivity (~1.0 Ωm) and gamma ray values reach a constant baseline.

Logging Unit III (188.1–339.6 mbsf)

At 188.1 mbsf, gamma ray values reach ~95 gAPI and remain fairly constant throughout logging Unit III with only minor fluctuations. These minor fluctuations around the constant background value of 95 gAPI are interpreted to be varying silt versus clay content alternations within the dominant hemipelagic mud. Resistivity and sonic P-wave velocity exhibit overall low variability and constant values (~0.9 Ωm and ~1700 m/s, respectively) throughout this unit. Within logging Unit III, occasional occurrences of thin (1–2 m) low gamma ray spikes (~80 gAPI) correspond to high-resistivity spikes (>1.2 Ωm), interpreted to be thin ash layers in a dominantly hemipelagic lithology.

Logging Unit IV (339.6–403.3 mbsf)

From 339.6 mbsf, there is an increase in abundance of thin gamma ray and resistivity spikes, with the first major downhole occurrence defining the logging Unit III/IV boundary. Unit IV was divided into two subunits based on changes in gamma ray log character. Subunit IVA (339.6–372.1 mbsf) is characterized by an overall increasing gamma ray trend above the 95 gAPI baseline and repeating thin gamma ray and resistivity spikes, suggesting an increase in the number of ash and sand beds. The last downhole occurrence of sand layers in this section marks the base of Subunit IVA (372.1 mbsf). Subunit IVB (372.1–403.3 mbsf) is characterized by overall high gamma ray values (~115 gAPI) with minor fluctuations and one interpreted thin ash layer at ~380 mbsf with low gamma ray values of 79 gAPI and a high resistivity of 2.5 Ωm. With the exception of this interpreted ash layer, resistivity remains fairly uniform in character and constant in value (~1 Ωm). Through Subunit IVB, sonic velocity exhibits an overall increase from ~1780 to 1900 m/s (Fig. F6). The character of the sonic velocity log is more variable than observed in the overlying units (Fig. F5), and the higher values may indicate a more consolidated sediment. The base of logging Unit IV is placed at 403.3 mbsf, where a step down in gamma ray values, resistivity, and sonic velocity occurs (to ~95 gAPI, ~0.8 Ωm, and ~1790 m/s, respectively).

Logging Unit V (403.3–463.5 mbsf)

The overall variation of the log data is more pronounced in logging Unit V compared to the above units (Fig. F5), especially in sonic velocity (Fig. F6). Unit V is interpreted to be a heterogeneous mixture of hemipelagic mudstone, sand, and ash. Throughout this interval, sonic velocity fluctuates between ~1800 and 2000 m/s. The top of logging Unit V is at 403.3 mbsf, where gamma ray values are ~95 gAPI. Gamma ray values exhibit an overall gradual increase to ~99 gAPI at 445.3 mbsf, with occasional low spikes (<75 gAPI) coincident with high-resistivity spikes (>1.2 Ωm). Resistivity is variable and fluctuates around 0.9 Ωm to 421.5 mbsf, where it gradually increases to ~1.3 Ωm at 426.5 mbsf before decreasing back to ~0.7 Ωm at 430.9 mbsf. From 445.3 to 450.0 mbsf, gamma ray values drop (~70 gAPI) whereas resistivity stays fairly constant at ~0.75–0.85 Ωm to the very base of the gamma ray value low at 450.0 mbsf, where there is a prominent resistivity spike (>3.0 Ωm). From 450.0 mbsf to the base of the unit (463.5 mbsf), there are no more distinguishable sand or ash horizons and gamma ray values increase from ~95 to ~105 gAPI. Resistivity exhibits a gradual decrease (from ~1.0 to 0.85 Ωm), and sonic velocity decreases from a unit high (~2100 m/s) at 452.0 mbsf to ~1900 m/s. The base of Unit V is placed at the base of this sonic velocity decrease, where gamma ray values start to drop back below the 95 gAPI baseline (Fig. F6).

Logging Unit VI (463.5–530.3 mbsf)

Logging Unit VI is the lowermost unit in the sediment section of Hole C0012H and is divided into three subunits. In the upper part of Subunit VIA, from 463.5 to 467.94 mbsf, gamma ray values exhibit a gradual decrease (from ~95 to ~60 gAPI), whereas resistivity and sonic velocity are fairly constant (~0.85 Ωm and ~2000 m/s, respectively). The base of the low gamma ray values at 467.94 mbsf corresponds to a thin (<1 m) resistivity spike (0.96 Ωm) indicating a probable ash layer. Below 467.94 mbsf, resistivity is roughly constant, with only minor fluctuations around 0.9 Ωm, and sonic velocity fluctuates between ~1900 and 2000 m/s. Gamma ray values increase to ~85 gAPI at ~483.95 mbsf. Between 483.95 and 520.4 mbsf, gamma ray values fluctuate from ~75 to 85 gAPI and resistivity is roughly constant at ~0.8 Ωm, whereas sonic velocity exhibits an overall decrease from ~2000 to ~1730 m/s. Because gamma ray values between 463.5 and 520.4 mbsf are lower than the baseline of 95 gAPI and sonic velocity is ~2000 m/s, this subunit (VIA) is interpreted as a more consolidated, sandier lithology than the overlying units (Fig. F5). Subunit VIB is defined from 520.4 to 526.7 mbsf because through this section, gamma ray values are constant at ~95 gAPI, resistivity gradually increases to ~1–1.15 Ωm, and velocity gradually decreases to 1740 m/s (Fig. F6). Subunit VIC (526.7–530.3 mbsf) is characterized by decreasing gamma ray values (to <90 gAPI), gradually increasing resistivity (to ~1.2 Ωm), and sharply increasing sonic velocity (>2200 m/s). Because this subunit is directly in contact with the basement rock below, it is possible that some alteration of the sediment has occurred, as observed in the core samples previously obtained at this site. Therefore, this subunit is interpreted to represent the altered red clay samples identified during Expeditions 322 and 333 in Holes C0012A and C0012G (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c).

Basement rock (530.3–710.0 mbsf)

Within the basement section, gamma ray values are low (<60 gAPI), which is typical of basaltic rock (Barr et al., 2002; Bartetzko et al., 2001) recovered during Expeditions 322 and 333 (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c). Therefore, the reference gamma ray baseline for this section was set at 18 gAPI to reflect the average value (Fig. F7). Based on this, the basement was divided into two logging units (Table T2). Subunits in logging Unit VII were defined based on large-scale changes in the resistivity and sonic velocity logs, which are positively correlated. Subunits VIIA and VIIB may correspond to the igneous Units I and II defined in cores (igneous Unit I: pillow lavas; igneous Unit II: sheet flows with pillow lava interlayers [Expedition 333 Scientists, 2012c]); however, without more compositional information from logs (e.g., neutron density logs, photoelectric factor, or spectral gamma ray), a detailed lithologic interpretation of the basement is not possible.

Logging Unit VII (530.3–626.6 mbsf)

Through logging Unit VII, the gamma ray log exhibits significant variation with depth (Fig. F7). Immediately below the sediment/basement contact (530.3 mbsf), gamma ray values decrease from ~90 to ~20 gAPI. At 536.7 mbsf, gamma ray values become constant with only minor (±5 gAPI) fluctuations to ~559.4 mbsf, where gamma ray values spike to ~35 gAPI coincident with drops in resistivity and sonic velocity to 1.3 Ωm and 3500 m/s, respectively. At 562.12 mbsf, gamma ray values increase from ~30 to 45 gAPI before gradually decreasing to ~15 gAPI by 575.57 mbsf. Gamma ray values show a minor increase to ~25–30 gAPI between 575.57 and 589.01 mbsf before another increase to ~50 gAPI at 595 mbsf. Below 595 mbsf, gamma ray values gradually decrease to ~15 gAPI at 607 mbsf before gradually increasing to ~40 gAPI at 624.62 mbsf. A decrease in gamma ray values to ~15 gAPI over a 2 m interval marks the base of logging Unit VII at 626.6 mbsf. These fluctuations may represent volcaniclastic sediment intercalated between lava flows or varying alterations to the basalt, as identified from petrologic investigation of core samples (Expedition 333 Scientists, 2012c).

Three subunits have been defined within logging Unit VII based on fluctuations in the resistivity and sonic velocity logs (Fig. F7; Table T2). Subunit VIIA (530.3–572.9 mbsf) exhibits gradually decreasing resistivity (from 3.0 to 1.6 Ωm), whereas velocity remains fairly constant (~3500 ± 200 m/s) to 564.6 mbsf, where it drops to ~3000 m/s (±300 m/s). At 572.9 mbsf, both resistivity and sonic velocity increase over a 7 m interval to ~20 Ωm and ~4900 m/s, respectively. Throughout Subunit VIIB (572.9–604.1 mbsf), resistivity and sonic velocity remain high (~30 Ωm and 4900 m/s, respectively). At 595.5–596.5 mbsf, low spikes in both resistivity (~6.5 Ωm) and sonic velocity (~3400 m/s) correspond to a high in gamma ray values (~50 gAPI), possibly indicating a highly altered pillow basalt or a volcaniclastic sediment horizon. The base of Subunit VIIB is placed where resistivity and sonic velocity drop back to lower values similar to those in Subunit VIIA. From 604.1 to 613.17 mbsf, resistivity remains constant at ~3.2 Ωm (±0.5 Ωm) and sonic velocity remains constant at ~3200 m/s (±300 m/s), characterizing the upper part of Subunit VIIC. There is an increase in both resistivity and sonic velocity to 7.0 Ωm and 4400 m/s, respectively, between 613.17 and 614.0 mbsf. Between 614.0 and 619.14 mbsf, resistivity gradually decreases to ~5.0 Ωm, and resistivity then remains constant through the rest of Subunit VIIC. Sonic velocity exhibits an overall decrease (to ~4100 m/s) to the base of Subunit VIIC.

Logging Unit VIII (626.6–710.0 mbsf)

Below 626.6 mbsf, gamma ray values become consistently low (~15 gAPI) with only minor fluctuations (±5 gAPI). Resistivity exhibits some variability with depth but remains high relative to all the other units (Fig. F4). From 626.6 to 632.0 mbsf, resistivity and sonic velocity gradually increase to ~8.5 Ωm and ~4700 m/s, respectively. From 632.0 to 656.5 mbsf, resistivity remains consistently around >8.5 Ωm with minor fluctuations (±4.0 Ωm). Sonic velocity decreases over the same depth interval (632.0–656.5 mbsf) from ~4700 to 4000 m/s. A shift to a higher average resistivity (~15 Ωm) is observed from 656.5 to 684.4 mbsf with fluctuations between 7 and 25 Ωm, reaching a high (~25 Ωm) around 682.0–684.0 mbsf. At 684.5 mbsf, resistivity returns to an average value of 8.5 Ωm and maintains low variability to the base of the hole (710 mbsf), with a wide spike to ~20 Ωm at 700 mbsf. Below 656.5 mbsf, sonic velocity exhibits two increasing profiles. From 656.5–665.9 mbsf, sonic velocity increases from ~4000 to 5300 m/s before sharply decreasing to ~4000 m/s at 668.7 mbsf. Another gradual increase to ~4800 m/s is observed to 683.88 mbsf before sonic velocity decreases to ~3750 m/s at 687.0 mbsf. There are no sonic velocity measurements below this depth because of the placement of the sonicVISION tool in the BHA (see the “Methods” chapter [Strasser et al., 2014a]). In the absence of further logging data (e.g., density) and no core data, no subunit classification was applied to this logging unit.

Physical properties

Estimation of porosity and bulk density from resistivity

Porosity was estimated using Archie’s law (Archie, 1947; see the “Methods” chapter [Strasser et al., 2014a]). Seawater electrical resistivity was calculated using the temperature profile that was estimated for Site C0012 during Expedition 333. The temperature at the seafloor was estimated to be 2.85°C, with an average thermal gradient of 135°C/km (Expedition 333 Scientists, 2012c). Archie’s law parameters were found by fitting the Archie equation to reliable resistivity measurements of discrete samples (conducted at 2 kHz) and moisture and density (MAD)-derived porosity measurements taken during Expedition 333 on sediment in the upper 180 m of Site C0012 (Expedition 333 Scientists, 2012b). Resistivity and MAD-derived porosity measurements taken at Site C0012 during Expedition 322 were not used in the Archie’s law fit because the resistivity measurements were taken at a current frequency of 25 kHz (Expedition 322 Scientists, 2010a). The Schlumberger geoVISION tool measures resistivity at the bit at a current frequency of 1.5 kHz (Bonner et al., 1996), making use of the Expedition 333 data appropriate. The parameters that best fit the data are a = 1 and m = 2.95 with a coefficient of determination (R2) of 0.78 (Fig. F8). Archie parameters employed at Site C0002 (a = 1 and m = 2.4) (Expedition 314 Scientists, 2009; see the “Site C0002” chapter [Strasser et al., 2014b]) are similar to those estimated here for Site C0012. These parameters are applied to the entire drilled section of Hole C0012H; thus, the a and m values do not account for lithologic variations with depth or the presence of igneous basement below 530.3 mbsf. Bulk density was estimated from the resistivity-derived porosity using a grain density (ρg) value of 2.67 g/cm3. This grain density value is an average (with a standard deviation of 0.12 g/cm3) of all the MAD-derived grain densities for sediment above the igneous basement measured during Expeditions 322 and 333 (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c).

Resistivity-derived porosity and bulk density depth trends are shown in Figure F4. Porosity is somewhat scattered in the upper 70 m but averages 71% ± 2%. Porosity decreases linearly from 72% to 58% from 70 to 107 mbsf and then increases to 66% at 123 mbsf. Between 123 mbsf and the base of Unit II (188.1 mbsf), porosity decreases to 54% and then remains generally constant to 372.1 mbsf. Within Subunit IVB (372.1–403.3 mbsf), there is a slight negative excursion in porosity, decreasing to 49% at ~388 mbsf before increasing to 54% at the base of Unit IV (403.3 mbsf). Within Units V and VI (403.3–530.3 mbsf), porosity values decrease gradually from 54% to 45% at the base of Unit VI (530.3 mbsf) before decreasing precipitously to <30% at the top of Unit VII (530.3 mbsf). This rapid decrease corresponds to the upper surface of the basement. Within Unit VII (530.3–626.6 mbsf), porosity increases gradually to 38% at 570 mbsf before decreasing suddenly to 17% at 578 mbsf, increasing rapidly to 29% at 604 mbsf, and finally decreasing to 23% at the base of Unit VII (626.6 mbsf). Porosity is generally scattered at ~20% within Unit VIII (626.6–710 mbsf), reaching as low as 15% between 665 and 683 mbsf and 23% at the base of Hole C0012H at 710 mbsf. Note that estimated porosity in the basement units (VII and VIII) are subject to errors because of the assumed constant values for a and m throughout the hole.

Trends in resistivity-derived bulk density mirror those described above for the resistivity-derived porosity because reported bulk density values are a mathematical manipulation of the derived porosity values (Fig. F4). Generally, bulk density increases from ~1.4 g/cm3 at the seafloor to 1.7 g/cm3 at 107.2 mbsf (within Unit I). Bulk density decreases to 1.6 g/cm3 at 123 mbsf before generally increasing to 1.9 g/cm3 at the base of Unit VI (530.3 mbsf). Within the basement (Units VII and VIII; 530.3–710 mbsf), bulk density averages 2.3 ± 0.1 g/cm3 but is subject to the same errors as resistivity-derived porosity in the basement units.

The resistivity-derived porosity log is plotted for comparison along with MAD-derived porosity measurements taken during Expeditions 322 and 333 (Expedition 322 Scientists, 2010c; Expedition 333 Scientists, 2012c). The resistivity-derived estimate maintains good visual agreement with MAD-derived measurements to ~186 mbsf, where they become increasingly offset (Fig. F9). Below 186 mbsf, the resistivity-derived values are consistently higher than the MAD-derived values, with the offset Δϕ = ϕMAD – ϕresistivity between the two reaching a maximum of about Δϕ = –0.2 (relative to the resistivity-derived values) near the base of the sedimentary section at 530.3 mbsf. This difference could be due to lithologic changes that are not captured by the Archie’s law model, for which we assumed constant values of a and m. Another possible explanation is the observed salinity increase with depth at Site C0012 (Expedition 322 Scientists, 2010c); although this decreases the pore fluid resistivity, it would not account for the full offset. For example, a salinity of ~4.0% was measured from interstitial water at the base of the sedimentary section at Site C0012 during Expedition 322 (Expedition 322 Scientists, 2010c). Using the thermal gradient reported above, the temperature at 530 mbsf is ~62°C. Under these conditions, seawater resistivity is ~0.23 Ωm, compared to ~0.27 Ωm for seawater with a salinity of 35 (Schlumberger, 2009). From Archie’s law (see the “Methods” chapter [Strasser et al., 2014a]), increasing the salinity from 3.5% to 4.0% corresponds to an ~0.02 decrease in porosity, which is about an order of magnitude less than the observed difference between the resistivity- and MAD-derived porosity values at 530 mbsf.

Resistivity image analysis

The statically normalized shallow, medium, and deep button resistivity images were the primary images used for structural and geomechanical analyses. Three different resistivity ranges were selected to normalize the data: 0.5–1.5 Ωm for the Shikoku Basin sediment and 0–10 or 2–50 Ωm for basement rock. In the absence of a caliper measurement, the bit diameter was used as the borehole diameter and assumed to be constant. Therefore, all reported dip angles should be considered as a maximum bound. Observations are summarized separately for the Shikoku Basin sediment and basement rock.

Shikoku Basin sediment (logging Units I–VI)

Generally, bedding dips are gentle-to-moderate (10°–30°). The dominant dip direction is bipolar, with strong clustering in both west–northwest and east–northeast directions (Fig. F4). Overall, there is low variability in sediment resistivity throughout the entire section, although the number of visible bedding planes increases at depths >330 mbsf as a direct result of the increase in resistivity contrast between adjacent beds (Fig. F6). The average fracture density is <1 per 10 m, although logging Unit VI has a higher than average concentration of ~3 per 10 m (463.5–530.3 mbsf).

Only six bedding planes were visible in logging Unit I, with variable dip direction from north through to west and moderate dip angle (20°–28°; Fig. F6). It is possible that there were more bedding planes with dip angles close to horizontal, but these were not picked, as they were impossible to distinguish from data overlap artifacts caused by tool bounce, especially in regions of high stick-slip (“Data quality”).

Logging Units III and IV have strong evidence for opposite bedding dip directions (Fig. F4). Between 188.1 and 250.0 mbsf, dip direction is to the west, and between 333.0 and 372.1 mbsf, dip direction is to the east. There is a lack of clear bedding planes between 250.0 and 333.0 mbsf (Fig. F6). In this lower ~80 m section of Unit III, only two bedding planes are observed, both dipping gently (~18°) to the southeast. Fractures in Unit III mostly dip toward the east, with a large range of dip angles (18°–80°). Several high-angle (>70°) north- or south-dipping fractures are also observed.

Bedding planes in logging Unit IV dip to the east, with dip angles ranging from 15° to 25°. Logging Unit V has the most variable bedding plane dip directions (Fig. F4), although dip angles were predominantly gentle to moderate (10°–30°). No fractures were observed in the upper section of logging Unit V (403.3–437.0 mbsf), and only five low-angle fractures (<22°) were picked in the lower section (437.0–463.5 mbsf; Fig. F6). Logging Unit VI demonstrates two strong trends in dip direction: northwest to west from 463.5 to 497.0 mbsf and east to southeast from 497.0 to 530.3 mbsf (Figs. F4, F6). Well-defined bedding planes were observed in logging Subunit VIC (526.7–530.3 mbsf), with dip angles of 20°–38° to the southeast. A high number of fractures were also identified in logging Unit VI, with characteristically medium to high dip angles (40°–75°) and variable dip directions that do not appear to correlate with the bedding planes (Fig. F6).

Basement (logging Units VII–VIII)

An irregular contact is observed at 530.3 mbsf between the base of the Shikoku Basin sediment (logging Unit VI) with a low resistivity (<1.5 Ωm) and the top of the basement (logging Unit VII) with a higher resistivity (>3 Ωm) (Fig. F10A). Within the ~180 m section of basement that was logged, a wide variety of textures and fracture patterns can broadly be summarized as follows (Fig. F10):

  • Mottled texture, distinct from other regions of clear fracturing;
  • “Turtleshell” texture, approximately circular regions of high-resistivity “clasts” within a lower resistivity network; and
  • Zones of homogeneous background resistivity, often with subvertical fractures (either individual or anastomosing).

The mottled texture could be interpreted as zones containing a multitude of small fractures or, alternatively, as material that is chaotic or irregular, perhaps representing pillow breccia or hyaloclastite. A third interpretation could be different alteration of the basalt. This mottled texture is often observed in conjunction with, or directly overlying, zones of turtleshell texture. The turtleshell texture may also result from a complex fracture network, but this texture differs from other fractured zones by the presence of approximately circular resistive clasts within a lower resistivity network (Fig. F10B, F10E). It is possible that this texture represents various sized pillow basalts. The homogeneous high-resistivity zones containing subvertical fractures may represent sheet flows (Fig. F10C, F10D). Table T3 summarizes the observed textures and their preliminary interpretation according to their depth range and includes the thickness and range of resistivity for each occurrence. To compare intervals of different basement textures with variations in other LWD data, refer to Figure F7.

Both planar and subvertical fractures were picked (Fig. F7). The lower resistivity networks around the pillow basalts were also picked as fractures to check for any correlation between changes in ring/bit resistivity and sonic velocity and the number of fractures or regions of low resistivity in the images.

Logging Subunit VIIA generally has textural variations every 5 m or less, with the exception of two ~10 m thick regions of pillow basalts at 537.5–545.7 and 557.0–566.6 mbsf (Fig. F10B; Table T3). The fracture density within logging Subunit VIIA is ~10 per 10 m, and the subvertical fractures are relatively wide (~0.05 m) and sinuous. Logging Subunit VIIB is dominated by two large (19.0 and 7.2 m thick) zones of relatively high, homogeneous background resistivity with thinner subvertical fractures (~0.02 m) compared to logging Subunit VIIA. These zones still have a moderate number of fractures (~5 per 10 m), including a steeply dipping (~75°), low-resistivity (<7 Ωm) fracture that is ~0.1 m wide (Fig. F10C). Logging Subunit VIIC has a mixture of all three textures (Fig. F10D), although resistivity is generally higher than that of logging Subunit VIIA, reflecting its lower fracture content (Fig. F7).

Logging Unit VIII has an average resistivity of ~10 Ωm, although it contains the same textural features as logging Subunit VIIC. At ~684 mbsf, a sharp decrease in resistivity from ~50 to <10 Ωm correlates with a sharp transition from a sheet flow to pillow basalts (Fig. F10E). Similar basal surfaces can be observed elsewhere within logging Unit VIII (and logging Unit VII to a lesser extent), although these are less distinctive. The repetition of these textures, with pillows and mottled regions overlying sheet flows with sharp basal surfaces, suggests that there have been multiple cycles of pillow basalt and sheet flows, with each set having a variable combined thickness, ranging from ~3 to >25 m (Table T3).

Borehole breakouts

Four breakouts were observed in the Shikoku Basin sediment between 424.5 and 518.6 mbsf. Three of the breakouts occurred in logging Unit V, with heights ranging between 0.45 and 0.85 m and widths of 30° to 60° that were often uneven between the pairs (Fig. F11A). The breakout in logging Unit VI was more diffuse and taller (~1.9 m) than those seen in logging Unit V. Orientations of the breakouts were used to estimate the direction of principal horizontal stress (SHMAX). The uppermost and lowermost breakouts (at 424.8 and 517.7 mbsf) agree with a north–south SHMAX orientation, whereas the central two breakouts (at 442.6 and 446.6 mbsf) both demonstrate a northeast–southwest SHMAX orientation. Drilling-induced tensile fractures (DITFs) were observed in basement rock (logging Units VII and VIII). A clear example of a pair (i.e., 180° apart) of DITFs can be observed between 658.0 and 678.0 mbsf (Fig. F11B).

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