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doi:10.2204/iodp.proc.336.105.2012

Downhole logging

Downhole logging measurements obtained from Hole U1383C include total and spectral natural gamma ray, density, compressional velocity, electrical images, and deep UV–induced fluorescence. An open-hole section of 274.47 m was logged with two tool strings (Figs. F40, F41) over a period of ~22.5 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 U1383C started on 3 November 2011 at 0025 h (all times are ship local, UTC – 3 h) after RCB coring ended at a total depth of 331.5 m core depth below seafloor (CSF). A summary of logging operations is presented in Table T11; see “Operations” for full operational details on hole preparation for logging.

Two tool strings were deployed in Hole U1383C: (1) the AMC II and (2) the FMS-DSI (FMS-sonic). The AMC II tool string included the standard logging equipment head-q tension (LEH-QT) (cablehead), the Enhanced Digital Telemetry Cartridge (EDTC; with total gamma ray measurement), the HLDS, the Lamont Multifunction Telemetry Module (MFTM), and the DEBI-t (for tool string details, see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012a]) (Fig. F40). The AMC II was lowered into the borehole at 0255 h on 3 November. The wireline heave compensator (WHC) was optimized with the AMC II in open hole at 4510.1 m wireline log depth below rig floor (WRF). The AMC II completed a downlog to a total depth of 4750.1 m WRF (~7 m above the bottom of the hole [drillers depth]; note that we deliberately did not tag bottom with the DEBI-t), an uplog to 4486.5 m WRF, a second downlog to 4751.4 (tagged bottom), and a final uplog that terminated at 4412.8 m WRF after logging the seafloor. The AMC II tool string was rigged down by 1310 h.

The second tool string deployed was the standard FMS-sonic tool string, which was composed of the HNGS, the DSI, the General Purpose Inclinometry Tool (GPIT), and the FMS (Fig. F41). The FMS-sonic tool string was lowered into the hole at 1410 h on 3 November and reached the bottom of the borehole (4757.1 m WRF) at 1746 h, following logging down from the seafloor. The FMS-sonic tool string performed two successful full passes of the hole and was returned to the surface and rigged down at 2245 h on 3 November, at which time logging operations in Hole U1383C 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 standard final depth-matched log data were done on board (see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012a]).

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 U1383C, the base log was the gamma ray profile from the second uplog of the AMC II tool string. The original logs were first shifted to the seafloor (4421.5 m WRF), which was determined by the step in gamma ray values. This depth did not differ markedly from the seafloor depth given by the drillers (4 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 U1383C. 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 be used only qualitatively. However, the data are adequate to help pick 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 U1383C (iodp.ldeo.columbia.edu/​DATA/).

A wide (>30.5 cm) or irregular borehole can affect most log data, particularly measurements taken with tools like the HLDS (bulk density) that require decentralization and good contact with the borehole wall. The density log correlates well with the velocity and apparent 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) (Fig. F42). Both calipers showed a very reasonable hole with a diameter ranging from 23.3 to 48.2 cm and averaging 31.5 cm (LCAL data). The main breakouts were observed in the upper portion of the hole at ~60–130 and ~154–165 m wireline log matched depth below seafloor (WMSF); however, only certain sections in the uppermost depth range were out of the range of the FMS caliper arms (38.1 cm diameter). Good repeatability was observed between the two downlogs and two uplogs of the AMC II and the two passes of the FMS-sonic, particularly for measurements of compressional velocity and bulk 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 DSI was operated in the following modes: P&S monopole, upper and lower dipole, and Stoneley for both Pass 1 and Pass 2 (all with standard frequency). The slowness data from DTCO and DTSM (Table T6 in the “Methods” chapter [Expedition 336 Scientists, 2012a]) are of good quality for these passes and were thus converted to acoustic velocities (VELP and VELS, respectively) (for a full list of acronyms, see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012a]). Postexpedition reprocessing of the original sonic waveforms is highly recommended to obtain more reliable velocity results.

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 U1383C include natural total and spectral gamma ray, density, apparent electrical resistivity, and deep UV–induced fluorescence. The results are summarized below.

Density

Density ranges from 1.10 to 3.12 g/cm³ (average = 2.40 g/cm³) in Hole U1383C (Fig. F42). A comparison between discrete physical properties samples and the downhole density log shows relatively good agreement (average values measured on the cores range between 2.43 and 2.90 g/cm³). Low density values correspond to intervals with larger borehole dimensions and sections that exhibit much lower acoustic velocities (Fig. F42). Considerably variable density values in the upper portion of the open hole (~60–130 m WMSF) occur where the borehole is much more irregular (log Units I and II; see “Log units”) and correspond to nonvesicular to sparsely vesicular aphyric to sparsely phyric basalt pillow lava in lithologic Unit 1. Bulk density values vary throughout Hole U1383C; however, density generally increases with depth.

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1383C with the HNGS and the EDTC (total gamma ray only) (see Table T6 in the “Methods” chapter [Expedition 336 Scientists, 2012a]). The total gamma ray measurements through the BHA (pipe to ~52 mbsf) and casing show two main anomalies from the seafloor to ~59 m WMSF (where casing was set) (Figs. F42, F33).

Basaltic oceanic crust typically has low gamma ray intensities (e.g., Bartetzko et al., 2001; Barr et al., 2002), and the lithologic units penetrated and logged in Hole U1383C follow this trend. Total spectral gamma ray (HSGR) values obtained in Hole U1383C range from 0.01 to 20.21 gAPI and average 6.55 gAPI. Potassium values are relatively low and range from 0 to 0.66 wt%, with an average value of 0.26 wt% (Fig. F33). These values agree well with NGR measured on the whole-round cores (0.1–0.46 wt%; average = 0.19 wt%; see “Physical properties”) and values obtained using geochemical analysis (0.11–0.25 wt%) (see “Hard rock geochemistry”). Uranium concentrations average 0.16 ppm and have a maximum value of 1.02 ppm. Thorium concentrations average 0.16 ppm and reach a maximum value of 1.40 ppm.

The gamma ray data collected in Hole U1383C do not exhibit any particular trends, in contrast to Holes 395A and U1382A, which are eruptive sequences that exhibit distinctive uphole-increasing gamma ray intensity. The overall trend in the gamma ray data from Hole U1383C seems be dominated by the influence of potassium.

Elastic wave velocity

Compressional wave velocity (V_P) ranges from 2.62 to 6.45 km/s, with data collected downhole generally exhibiting much lower velocities than those collected on discrete physical properties cubes (Fig. F42). There is a clear relationship between V_P and density (and apparent resistivity); V_P is higher where density (and apparent resistivity) is higher. The lowest V_P values can be related to the pillow basalts in lithologic Unit 1 (log Unit II; see “Log units”), whereas the highest V_P values correspond to a section of more massive basalt units (visible in FMS data) associated with lithologic Unit 2 (log Unit III). Compressional wave velocity measured on discrete samples taken from the core (values range from 4.74 to 7.63 km/s) does not correlate that well with downhole logging data. However, velocity values obtained by downhole logging give an overall value for the formation measured, including fractures and basalt and matrix (e.g., hyaloclastite) mixtures, and are therefore generally lower.

Fluorescence

The fluorescence spectrum for Hole U1383C is uniform throughout the borehole except below ~290 m WMSF, where it was likely influenced by drilling mud (sepiolite). The structure of the fluorescence pattern is much different from the spectra collected in the previous holes and is not consistent with bacterial signatures, although there does appear to be a spectral feature consistent with a sporelike signal. The 455 nm band showed the highest degree of variability, particularly during the uplogs, when the calipers were deployed. This may reflect changing lithology as the upper fluorescence bands in DEBI-t will respond to aluminosilicates with high amounts of K, Na, and Mg. Additionally, in the lowermost 50 m of the hole, signal intensities for Hole U1383C are lower than those observed in the previous two holes measured (Fig. F43).

At ~40 m above the base of the borehole (~290 m WMSF), the signal intensity of the 455 nm band increases significantly (Fig. F44), most likely because of the presence of sepiolite at the bottom of Hole U1383C. It may be that the high signal count is due to both the fluorescence intensity and the reflectivity of the sepiolite. This possibility is corroborated by video data from the DEBI-t, which shows the instrument going from a cleaner environment into a very cloudy, highly reflective environment. As the tool string ascended, the high intensity of the 455 nm band decreased after the 280 m WMSF mark (Fig. F45). The video log also indicates that the hole was not as clean as Hole U1382A because particulates consistent with sepiolite and cement were present throughout the hole. Additionally, uplog video data show an increase in the amount of large particulates in the water compared to the downlog video data. This finding can most easily be explained by the fact that the caliper arm was extended during the uplog, causing it to scrape and break up the cement.

Apparent electrical resistivity measurements

No resistivity sonde was run on the AMC II tool string; however, in order to obtain a qualitative idea of electrical resistivity or apparent electrical resistivity, an average value of very shallow penetrating conductivity from the four FMS pads was obtained during log processing, and the reciprocal of this value was plotted (Fig. F42). Normally, to make a resistivity measurement, a logging tool (e.g., the Dual Induction Tool) creates a voltage between a pair of electrodes and measures the current flowing between them. Provided that the current path is controlled (usually by focusing electrodes), the conductance measured (current/​applied voltage) can be converted to a measure of the average conductivity or resistivity of the formation through which the current flows. The microresistivity buttons are not designed to measure resistivity because the button current is not focused or calibrated. However, the response of a microresistivity button, albeit uncalibrated, normally moves in the same direction as formation conductivity. The data calculated from the FMS pad averages, referred to here as apparent resistivity, agree well with both density and compressional velocity. In regions of high velocity and density, high apparent resistivity (log Unit III; see “Log units”) is also present, and there is an overall subtle trend of increasing apparent resistivity with depth.

Log units

Preliminary interpretation of the downhole log data divided Hole U1383C into a number of log units (Fig. F42). Log units were defined only below the casing (~60 m WMSF) in the open-hole section and were characterized using gamma ray, density, compressional velocity, and apparent resistivity.

Five main log units were qualitatively identified in the open-hole section of Hole U1383C (Fig. F42):

Log Unit I (~60–94 m WMSF) exhibits some of the lowest gamma ray intensities, with a subtle decrease downhole. Some small zones of higher gamma ray may correspond to more altered formations. Gamma ray values range between 4 and 12 gAPI. Bulk density values are highly variable throughout this unit, ranging between 1.25 and 2.75 g/cm³. Borehole diameter is also highly variable, with some areas of washout present where there is a transition from the end of the rathole into a fractured formation. Apparent resistivity values follow a very similar trend as density but show three main areas of increased resistivity at ~80, 88, and 93 m WMSF. Compressional velocity shows an arching trend, with upper unit values of ~4 km/s (~60 m WMSF), moving to lower values of ~3.5 km/s (~75 m WMSF), and then finally increasing to ~4.5 km/s at the base of log Unit I. This log unit correlates to lithologic Unit 1 and relates to nonvesicular to sparsely vesicular aphyric to sparsely phyric pillow lava with intercalated limestone.

Log Unit II (~94–132 m WMSF) exhibits slightly higher, though relatively constant, gamma ray values. Values cluster around 7 gAPI, with the exception of a peak at ~121 m WMSF (which relates to a very conductive section in the FMS electrical images). The borehole caliper is highly variable over the entirety of this log unit, and, in keeping, so are the density values. Just as in the preceding log unit, density values have a wide range between 1.1 and 2.8 g/cm³; however, there is a shift to lower density values at the top of this unit. There is an overall trend of increasing density with depth downhole in this log unit. Apparent resistivity shows a similar trend as density in that there is a decrease in values at the top of the log unit compared to the previous unit and a very slight increase with depth throughout. Acoustic velocity values also show a shift to lower values at the top of the unit. Values range between 3 and 5.5 km/s and, like density and apparent resistivity, show a slight downhole-increasing trend. Log Unit II corresponds to the lower portion of lithologic Unit 1, which is nonvesicular to sparsely vesicular aphyric to sparsely phyric pillow lava. However, FMS imagery shows more frequent conductive zones in the upper portion of this log unit compared to log Unit I (relating to fractured and altered materials) and hence slightly higher gamma ray values and lower density, apparent resistivity, and velocity values. The conductive zones in this unit become slightly less pervasive with depth downhole, which is reflected in the subtle increasing trend in density, apparent resistivity, and velocity.

Log Unit III (~132–153 m WMSF) has relatively stable values for gamma ray intensities (centered around 6.5 gAPI) compared to log Units I and II. It also features some of the highest density (3.2 g/cm³), resistivity, and acoustic velocity (6.25 km/s) values measured in this hole. This interval represents a section of sparely vesicular plagioclase-olivine-phyric basalt sheet to massive flow (lithologic Unit II). Additionally, the borehole is in gauge in this region and therefore has excellent FMS imagery that shows a zone of high resistivity and a fracture pattern indicative of a massive flow unit.

Log Unit IV (~153–166 m WMSF) is characterized by a significant drop in density (values around 2.4 g/cm³), apparent resistivity, and velocity (values around 3.5 km/s) and an increase in gamma ray intensity. Additionally, gamma ray intensities are marginally higher than in log Unit III and show a slight increase in intensity with depth. It is possible that this log unit reflects the interpillow/​flow sediments and tectonic breccias that were observed in core from this interval (see “Petrology and hard rock geochemistry”). FMS electrical imagery shows an area of very conductive material containing a low proportion of high-resistivity layers and clasts.

Log Unit V (~166–327 m WMSF) exhibits relatively uniform values for density, apparent resistivity, and velocity. Gamma ray intensities have much more structure downhole in this unit, particularly in log Unit V Zones A and B, where higher intensities are present. Log Unit V Zone C shows an area of decreased density, apparent resistivity, and acoustic velocity. There is an overall trend, albeit very subtle, of increasing density, apparent resistivity, and acoustic velocity with depth. Lithologic Unit 3, comprising nonvesicular aphyric pillow lava, relates to this log unit. FMS imagery shows a slight increase in resistive formation with depth. Additionally, electrical images show Zone A to be associated with a very conductive band that may be a large empty fracture or a unit of material with elevated alteration and higher porosity. Zone B also shows a number of thin bands of high conductivity that may relate to empty or filled fractures within more resistive material. Zone C relates to a highly conductive region with relatively small clasts of resistive material (likely basalt). Zones A and B correspond to intervals of high abundance of hyaloclastite in the core. The hyaloclastites are more altered than the basalt flows, and this variability may account for the increased range in gamma ray intensities in this interval.

Electrical images

In Hole U1383C, we also acquired FMS electrical resistivity images (Figs. F46). 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 U1383C are of very good quality; however, sections of the borehole were not in gauge, and in these sections (~60–77, ~71–76, ~96–102, ~108–115, ~118–130, and ~153–166 m WMSF) the quality of the images is diminished.

FMS imagery provides essential information—especially because core recovery in Hole U1383C was only 19%. Using FMS imagery and other log data, we can fill the data gap in the core record, providing a continuous record downhole. Figure F42 shows divisions of units 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 core recovered from Hole U1383C (Fig. F46), including more massive flow units, pillow basalts, fractured sections, and potential sedimentary horizons. However, perhaps more importantly, the FMS images help refine lithologic boundaries when contacts were not recovered in the core. Another strength of FMS imagery is the fact that the images are oriented to geographic north (using the 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 mosaicked together so that an attempt could be made to find the piece in the FMS images. Additional time will be spent postexpedition trying to line up some of these core images from formations recovered from Hole U1383C; however, compared to those from Hole U1382A, the recovered pieces are of much shorter length and hence likely more difficult and time consuming to place.

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