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doi:10.2204/iodp.proc.336.104.2012

Downhole logging

Downhole logging measurements obtained from Hole U1382A include natural total and spectral gamma ray, temperature, density, electrical resistivity, electrical images, and deep UV (<250 nm)–induced fluorescence. An open-hole section of 105.61 m was logged with two tool strings (Figs. F40, F41) over a period of ~19.5 h. The borehole remained in good condition throughout logging, and no obvious tight spots were encountered in open hole.

Logging operations

Downhole logging of Hole U1382A started on 9 October 2011 at 0215 h (all times are ship local, UTC – 3 h) after RCB coring ended at a total depth of 210 m core depth below seafloor (CSF). A summary of logging operations is presented in Table T14; see “Operations” for full operational details on hole preparation for logging.

Two tool strings were deployed in Hole U1382A: (1) the AMC I and (2) the FMS-HNGS. The AMC I tool string included the logging equipment head-mud temperature (LEH-MT) tool (cablehead with temperature measurement), the Enhanced Digital Telemetry Cartridge (EDTC with total gamma ray measurement), the Hostile Environment Litho-Density Sonde (HLDS), the High-Resolution Laterolog Array (HRLA), the Lamont Multifunction Telemetry Module (MFTM), and the DEBI-t (see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012, for tool string details) (Fig. F40). The AMC I tool string was lowered into the borehole at 0317 h on 9 October. The wireline heave compensator (WHC) was optimized with the AMC I in casing at 4590.1 m wireline log depth below rig floor (WRF). The AMC I completed a downlog to a total depth of 4700 m WRF (~4 m above the bottom of the hole; note that we deliberately did not tag the bottom with the DEBI-t) and an uplog to 4620 m WRF (~20 m below the bottom of casing), where the tool string experienced a power short. Power was lost to the tool string, and the AMC I was brought back up to the surface. The AMC I tool string was rigged down by 1110 h.

The second tool string deployed was the adapted FMS tool string composed of the HNGS and FMS (Fig. F41). The FMS-HNGS tool string was lowered into the hole at 1221 h on 9 October and reached the bottom of the borehole (4703.1 m WRF) at 1517 h, following logging down from the seafloor. The FMS-HNGS tool string performed two successful passes of the hole, and following a short period of difficulty (2 h) in reentering the pipe, the tool string was returned to the surface and rigged down at 2153 h on 9 October, at which time logging operations in Hole U1382A were completed.

Data processing and quality assessment

The logging data were recorded on board the R/V JOIDES Resolution by Schlumberger and archived in Digital Log Interchange Standard (DLIS) format. Data were sent via satellite transfer to shore, processed, and transferred back to the ship for archiving in the shipboard database. Processing and data quality notes are given below. The DEBI-t data recorded to SD memory card (video and full systems and fluorescence data in binary format) were also sent via satellite for archiving; however, data conversion and depth matching of the DEBI-t to the other final depth-matched log data was done on board (see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012]).

Depth shifts were applied to the logging data by selecting a reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed) and aligning features in equivalent logs from other tool string passes by eye. In the case of Hole U1382A, the base log was the gamma ray profile from Pass 2 of the FMS-HNGS tool string. The original logs were first shifted to the seafloor (4497 m WRF), which was determined by the step in gamma ray values. This depth did not differ greatly from the seafloor depth given by the drillers (3 m difference).

Proper depth shifting of wireline logging depths relative to core depths was essential to correlate the downhole logging data with all other measurements and observations made on core recovered from Hole U1382A. The seafloor was the only target that offered a potential wireline logging depth reference. However, note that data acquired through the seafloor resulted from logging through the BHA and casing, so data from this interval are of poor quality and highly attenuated and should only be used qualitatively. However, they are adequate to pick out the seafloor. The quality of wireline logging data was assessed by evaluating whether logged values are reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Specific details of the depth adjustments required to match logging runs/data are available in the logging processing notes in the log database for Hole U1382A (iodp.ldeo.columbia.edu/​DATA/).

A wide (>30.5 cm) or irregular borehole affects most recordings, particularly tools like the HLDS (bulk density) that require decentralization and good contact with the borehole wall. The density log correlates well with the resistivity logs but is largely affected by hole conditions. Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the FMS calipers (C1 and C2). Both calipers show a highly variable hole with a diameter ranging from 26.6 to 47.95 cm (LCAL data). The main breakouts were observed at ~142–145, ~153–155, and ~162–163 m wireline log matched depth below seafloor (WMSF); however, only the uppermost breakout was out of the range of the FMS caliper arms (38.1 cm diameter). Good repeatability was observed between the downlog and uplog of the AMC I and the two passes of the FMS-HNGS, particularly for measurements of electrical resistivity, gamma ray, and density.

Bulk density (HLDS) data were recorded with a sampling rate of 197 measurements per minute (2.54 cm at 300 m/h), in addition to the standard sampling rate of 32 measurements per minute (15.24 cm at 300 m/h). The enhanced bulk density curve is the result of the Schlumberger enhanced processing technique performed on the MAXIS system on board the JOIDES Resolution. In normal processing, short-spaced data are smoothed to match long-spaced data (depth and resolution matched). In enhanced processing, the raw detail obtained from the short-spaced data is added to the standard compensated density (Flaum et al., 1987). In a situation where there is good contact between the HLDS pad and the borehole wall (low density correction), the results are improved because the short spacing has better vertical resolution (i.e., it has the capability to resolve thinner beds/units).

The FMS images are of good quality over the majority of the hole; the only images that should be treated with caution are those taken where the main borehole breakouts are located (see above), because the pad may not have maintained good contact with the borehole wall.

Preliminary results

Downhole logging measurements obtained from Hole U1382A include natural total and spectral gamma ray, temperature, density, electrical resistivity, and deep UV–induced fluorescence. The results are summarized below.

Electrical resistivity measurements

Five main electrical resistivity curves were obtained with the HRLA: RLA1–5 (shallower through deeper penetrating measurements). The HRLA reached a total depth of 194.99 m WMSF in the logged interval because it was situated immediately above the DEBI-t (Fig. F40). The RLA3, RLA4, and RLA5 measurements are the most reliable for lithologic interpretation because of their deeper investigation depths and are hence less influenced by drilling-induced features.

Hole U1382A is composed of several lithologic units, so the resistivity values are variable. RLA3 values range between 1.48 and 479.54 Ωm (averaging 38.36 Ωm), RLA4 values range between 1.68 and 731.18 Ωm (averaging 38.77 Ωm), and RLA5 values range between 2.26 and 774.97 Ωm (averaging 43.86 Ωm) (Fig. F42). Some of the highest resistivity values correlate to more massive units (e.g., at ~147 and ~165 m WMSF, which correspond to a massive fine-grained basalt horizon [lithologic Unit 4] and peridotite interval [lithologic Unit 5] in log Units IV and VIII, respectively). All resistivity curves show considerable variability throughout the hole (Fig. F42). However, there is a notable downhole contrast in resistivity, consistent across all resistivity curves, at ~143 and 163 m WMSF (log Units III and VII; see “Log units”) in lithologic Units 3 and 5, which relates to a more visibly conductive (from FMS images) sedimentary breccia that can be considered a rubble zone.

Density

Density ranges from 1.09 to 3.19 g/cm³ (average value = 2.33 g/cm³) in Hole U1382A (Fig. F42). A comparison between discrete physical properties samples and the downhole density log shows relatively good agreement (values measured on core average 2.87 ± 0.05 g/cm³). Low density values correspond to intervals with larger borehole dimensions and sections that exhibit much lower resistivities (Fig. F42). Pronounced high density values correspond to two distinct horizons (log Units IV and VIII; see “Log units”) that relate to massive fine- to medium-grained aphyric basalt in lithologic Unit 3 and a peridotite interval in lithologic Unit 5.

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1382A with the HNGS and the EDTC (total gamma ray only) (see Table T6 in the “Methods” chapter [Expedition 336 Scientists, 2012]). The total gamma ray measurements through the BHA (pipe down to ~60 mbsf) and casing show two main anomalies from the seafloor to ~99 m WMSF (where casing was set) (Figs. F42, F35). The open-hole gamma ray measurements cover a total of 85.88 m (AMC I) and 92.98 m (FMS-HNGS) downhole. The slightly shorter overall coverage of the gamma ray sondes compared with the DEBI-t and FMS is the result of the EDTC and HNGS being ~20 and ~14 m higher in the tool string, respectively (Figs. F40, F41).

Gamma ray measurements in basaltic oceanic crust are typically low (e.g., Bartetzko et al., 2001; Barr et al., 2002), and the lithologic units penetrated and logged in Hole U1382A follow this trend. Total spectral gamma ray (HSGR) values obtained in Hole U1382A average 5.87 gAPI, with a maximum of 23.83 gAPI. Potassium values are relatively low, with an average value of 0.21 wt% and a maximum of 0.53 wt% (Fig. F42). These values agree very well with NGR measured on the whole-round cores (0–0.35 wt%, average = 0.16 wt%; see “Physical properties”) and values obtained using geochemical analysis (0.07–0.26 wt%) (see “Petrology, hard rock and sediment geochemistry, and structural geology”). Uranium concentrations average 0.16 ppm and have a maximum value of 1.71 ppm. An elevation in uranium abundance together with a low concentration of potassium was observed at ~165 m WMSF, relating to an interval of peridotite (slightly serpentinized). The same trend was observed in the NGR data on Section 336-U1382-8R-4 (see “Physical properties”).

Thorium concentrations average 0.39 ppm and reach a maximum value of 2.47 ppm. The potassium, thorium, and uranium concentrations are all very low and sometimes negative, a phenomenon that is common in formations with low radioactivity (Bartetzko et al., 2001).

Comparison of gamma ray data collected in Holes U1382A (Expedition 336) and 395A (ODP Leg 174B) allows correlation between particular features in the data set (Fig. F43). Because Hole U1382A is 50 m west of Hole 395A, there are some lateral differences in the extent of some units and their depths below seafloor. However, very similar downhole variations in gamma ray intensity were observed.

Temperature

Temperature measurements taken using the LEH-MT cablehead temperature sensor have very little variability downhole (Fig. F42). These data should be used qualitatively because the cablehead was shown to measure temperature 2.75°C higher than the Modular Temperature tool (MTT) (and following tests, the MTT was shown to be the more accurate of the tools). The data presented here were corrected using this factor, but observed values are lower than expected (~2°C). However, it is possible that these data are not entirely reliable given the subsequent failure (power short) that we experienced in the cablehead during logging.

Fluorescence

There was no observable trend in fluorescence in the downlog (Fig. F44); the relative ratio of fluorescence from all bands appeared the same. Video taken during the AMC I logging operation indicated that this hole was much cleaner than the previous hole, corroborating the difference in spectral features observed in Hole U1382A. There were no observable flocs, and the wall appeared to be relatively dark, in contrast to the more oxidized nature of the borehole wall in Hole 395A.

The fluorescence data from Hole U1382A were markedly different from those of Hole 395A. The overall intensity of fluorescence in Hole U1382A was greater than that seen in Hole 395A, with the largest increase being observed in the 360 nm band. Additionally, the signal intensity is more uniform throughout Hole U1382A, with the exception of the 455 nm band, which shows an increase in intensity with depth. A “bump” in intensity of the 455 nm band was observed between 160 and 170 m WMSF (Fig. F45), which may be related to calcareous ooze described near this horizon. This band also appears to respond to borehole size variations based on caliper data because the regions with the noisiest information occur at depths where the borehole diameter varies most. The other significant difference is that the overall structure of the fluorescence spectra differs between the two holes. Furthermore, the 340 and 360 nm bands are more intense as a function of the spectra in Hole U1382A compared to Hole 395A.

Similar to Hole 395A, the fluorescence from the 300 and 320 nm channels shows no large-scale variation (Fig. F46). Additionally, the spectral structure does not correlate with the presence of large quantity of biomass. Because the hole was pumped/​circulated for extended periods with surface seawater and periodic sepiolite mud sweeps, it is unclear at this time what portion of the detected signal is from the formation or from the surface water. Calibration work done on board the ship with drilling mud and surface seawater indicates that some of the signal may be related to the seawater, although there does not appear to be a large influence from the drilling mud.

Log units

Preliminary interpretation of the downhole log data divided Hole U1382A into a number of log units (Fig. F42). Log units were defined only below the casing (~99 m WMSF) in the open-hole section and were characterized using gamma ray, resistivity, and density.

Eleven log units were qualitatively identified in the open-hole section of Hole U1382A (Fig. F42):

• Log Unit I (~99–118 m WMSF) was defined using only gamma ray and resistivity. This unit shows relatively stable gamma ray values between 3 and 5 gAPI. There is a mild decreasing trend in gamma ray intensity from ~99 to 110 m WMSF, below which values become rather uniform. Resistivity values are variable at the top of this log unit; however, below ~110 m WMSF, a steady increasing trend was observed that is likely related to the borehole shape (and the transition from the end of the rathole into cored hole). This log unit correlates to lithologic Unit 1 and relates to massive, nonvesicular, aphyric fine- to medium-grained basalt with fractures.
• Log Unit II (~118–143 m WMSF) exhibits relatively constant gamma ray values, with values clustering around 6 gAPI. With the exception of a steep decrease in resistivity values at the very top of this log unit, resistivity very slightly increases with depth through this log unit. Density values are highly variable and relate to the highly fractured nature of this lithology. Log Unit II corresponds to lithologic Unit 2, a pile of aphyric cryptocrystalline pillow basalt with numerous glassy and fractured flow contacts.
• Log Unit III (~143–146 m WMSF) has considerably lower gamma ray intensities compared to log Unit II. This unit also features the lowest density values measured in this hole and some of the lowest resistivity values. This interval likely represents a section of sediments and brecciated material (related to lithologic Unit 3 but not recovered). Additionally, the borehole is extremely enlarged in this region, and therefore this log unit could be viewed as a rubble zone. This interpretation is corroborated by the very high rate of penetration (3 m in 10 min) during drilling of lithologic Unit 3.
• Log Unit IV (~146–148 m WMSF) has consistently high density and resistivity values. Additionally, gamma ray intensities are marginally higher than in log Unit III. This log unit is correlated to a fine- to medium-grained aphyric basalt unit (lithologic Unit 3) recovered in Core 336-U1382A-6R.
• Log Unit V (~148–153 m WMSF) exhibits significantly lower resistivity values than those from log Unit IV; however, they are by no means low values. Gamma ray values increase downhole in this unit, which may relate to an increase in alteration. Density and resistivity values fluctuate greatly, but they follow the same trend (i.e., high resistivities correlate with high densities). The interval with lower densities and resistivities likely relates to fractures in the aphyric cryptocrystalline basalt pillow lava to which this log unit is correlated (lithologic Unit 4).
• Log Unit VI (153–163 m WMSF) shows a steady increase in resistivity downhole after very low values at the top. Density values vary highly, but a general increasing trend with depth was observed. Gamma ray intensities exhibit a decrease down to the middle section of this log unit. Values then start to increase slightly toward the base. The variable density values reflect both the fractured nature of the formation (pillow lava from lithologic Unit 4) and sections of enlarged borehole, where it is possible that good contact with the borehole was not always maintained.
• Log Unit VII (~163–165 m WMSF) exhibits a small increase in gamma ray intensity and a sharp decrease in density and resistivity. This zone relates to a very dark, conductive section (in the FMS images) and may relate to some interpillow sediments that were not recovered. Consistent with this interpretation is the fact that the interval was drilled in several minutes, without torquing of the drill string.
• Log Unit VIII (~165–167 m WMSF) displays the highest density values measured in Hole U1382A, along with some of the highest resistivity values. Gamma ray values are slightly higher than in log Unit VII, and higher uranium values and lower potassium values in this interval relate to a piece of mildly serpentinized peridotite in lithologic Unit 5. The high uranium-to-potassium ratios and densities of this rock type were also detected in the course of shipboard studies (see “Physical properties”).
• Log Unit IX (~167–169 m WMSF) has very similar gamma ray, resistivity, and density values as log Unit VII and likely relates to another section of sediments or matrix-supported breccia.
• Log Unit X (~169–189 m WMSF) exhibits consistently high density and resistivity values in the top two-thirds of the unit. Toward the base, resistivity values decrease and density values fluctuate more widely. Gamma ray generally increases with depth. From FMS images it is possible to see a gradual change from the highly resistive material containing fractures at the top of the unit to a more conductive, fractured, blocky formation toward the base. This log unit primarily relates to a massive flow of highly plagioclase-olivine-phyric fine-grained basalt (lithologic Unit 6).
• Log Unit XI (~189–195 m WMSF) is defined primarily by a considerable drop in resistivity at the top. Gamma ray values increase downhole from the top of this unit, with total gamma ray values reaching ~9 gAPI. This log unit, and the sudden drop in resistivity, marks the change from a more massive flow to pillow lavas (the transition from lithologic Unit 6 to Unit 7).

Electrical images

In Hole U1382A, we also acquired FMS electrical resistivity images (Figs. F47, F48). The quality of electrical resistivity image measurements depends on close contact between the measuring pads on the tool and the borehole wall. The FMS borehole images collected in Hole U1382A are of good quality; however, sections of the borehole were not in gauge, and image quality is diminished in these sections (~142–145, ~153–155, and ~162–163 m WMSF).

FMS imagery is an essential data set and becomes invaluable when core recovery is low. In Hole U1382A, overall core recovery was 32%. Using FMS imagery and other standard log data, we can make a competent attempt at filling the data gap in order to have a continuous record downhole. Figure F43 shows unit divisions based on FMS images over the entire section. It is not possible to determine the petrology of the lithology, but the structural and textural make-up is clear.

The FMS images highlight some of the key units observed in the core recovered from Hole U1382A (Fig. F47), including more massive flow units, brecciated sections, and potential sedimentary horizons. However, perhaps more importantly, the FMS images help refine lithologic boundaries when contacts are not recovered in the core. Another strength of FMS imagery is the fact that the images are oriented to geographic north (using the General Purpose Inclinometry Tool [GPIT]), and by picking sinusoidal traces on the images one can obtain important oriented structural information for key boundaries, fractures, and other features of interest. It is possible to differentiate between picked fractures (be they conductive or resistive) and boundaries or pronounced fractures in the lava flows. Structural studies are a key part of postexpedition research. Such structural picks can aid overall interpretation of the lithologic sequence observed in the core and be used to orient observed structures and produce a stress regime model for the drilled formation.

One of our key scientific aims is to integrate core and log measurements, observations, and interpretations. The FMS data permit integration of core observations and images by constraining the location of recovered core pieces when recovery is low; depth uncertainty for core pieces in low recovery can be as high as 9.6 m. Before the cores were split, images of the external surfaces of the whole-round cores were taken on the line-scan imager. These images were then mosaicked together so that we could attempt to find the piece in the FMS images. Figure F48 shows the successful result of integrating external surface whole-round core images with FMS data. This is the very first time such work has been done during an IODP expedition, and it may prove to be extraordinarily useful in repositioning core pieces. Additional time will be spent postexpedition trying to line up some of the other core images from rocks recovered from Hole U1382A.

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