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doi:10.2204/iodp.proc.336.103.2012

Downhole logging

Downhole logging measurements obtained from Hole 395A include natural total and spectral gamma ray, temperature, and deep UV (<250 nm)–induced fluorescence. Following some initial issues with a rock ledge in the borehole, an open-hole section of 405.74 m was logged with the microbiology combination (microbiology combo) tool string (197.76 m [pipe depth]) to 603.5 mbsf. The microbiology combo, which includes the logging equipment head-mud temperature (LEH-MT) (cablehead with temperature measurement), the Hostile Environment Natural Gamma Ray Sonde (HNGS), the General Purpose Inclinometry Tool (GPIT), the Lamont Multifunction Telemetry Module (MFTM), the Lamont Modular Temperature tool (MTT), and the DEBI-t, was the only tool string deployed in Hole 395A (Fig. F14).

Logging operations

Downhole logging of Hole 395A started once the old CORK was removed at 0335 h on 24 September 2011 (all times are ship local, UTC – 3 h) (see Table T8 for more logging operation details). We aimed to log a relatively undisturbed hole with the microbiology combo; therefore, there was no traditional hole preparation. For full logging operational details, please see “Operations.” The drill pipe was fitted with a logging bit that was initially set at 54 mbsf for logging, but note that casing is present in the hole to 112 mbsf.

In a first attempt, the microbiology combo tool string was lowered into Hole 395A at 0500 h on 24 September (see Table T8 for more operational details). The wireline heave compensator (WHC) was optimized while the tool string was in the pipe in order to minimize any disturbance in the hole, but because of the very low levels of heave (<0.5 m peak to peak), the WHC was not used for logging operations. At 0952 h, following some effort, the run was aborted because of an impassable rock ledge in the hole at 174 m wireline log depth below seafloor (WSF). The tool was returned to the surface and was rigged down by 1410 h. To remove the ledge issue in Hole 395A, the logging bit was run down past the obstruction and set lower at 197.76 m drilling depth below seafloor (DSF) in readiness for a second logging attempt.

The microbiology combo tool string was lowered into the hole at 1610 h (24 September). For tool and measurement acronyms, see “Downhole logging” in the “Methods” chapter (Expedition 336 Scientists, 2012a). The tool string performed a full downlog starting from 4448 m wireline log depth below rig floor (WRF) to the total planned logging depth (note that we did not want to tag the bottom of Hole 395A with the DEBI-t), a full uplog into casing, a second downlog to total logging depth, and a second uplog. The final uplog ended when the tool string crossed the seafloor (inside the drill string and casing), marked by a peak in natural radioactivity visible in the HNGS gamma ray measurement. The seafloor was detected at 4496 m WRF, which is within 2 m of the drillers seafloor depth estimate from ODP Leg 174B. The microbiology combo tool string reached the rig floor and was rigged down at 0330 h on 25 September, at which time logging operations in Hole 395A were completed.

Data processing and quality assessment

The logging data were recorded on board the 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) were done on board (see “Downhole logging” in the “Methods” chapter [Expedition 336 Scientists, 2012a]).

Depth shifts applied to the logging data were performed 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 395A, the base log was the gamma ray profile from Pass 1 (main) of the Azimuthal Resistivity Imager (ARI) tool string run during Leg 174B. The original logs were first shifted to the seafloor (4496 m WRF), which was determined by the step in the gamma ray value. This depth did not differ from the seafloor depth given by the drillers.

Proper depth shifting of wireline logging depths measured during Expedition 336 to downhole data collected during Leg 174B was essential for verifying the new data collected and to look for any changes in values in the formation left open to fluid circulation for 14 y. The seafloor was the only target that offered potential wireline logging depth references. However, it should be noted that data acquired at the seafloor/​seawater interface 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 during Leg 174B, by checking the consistency between different passes of the same tool, and by checking the most recently collected data against previous measurements. Specific details of the depth adjustments required to match logging runs/data are available in the logging processing notes on the log database for Hole 395A (iodp.ldeo.columbia.edu/​DATA/).

Preliminary results

Downhole logging measurements obtained from Hole 395A include natural total and spectral gamma ray, temperature, and fluorescence. The results are summarized below.

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole 395A with the HNGS (see Table T6 in the “Methods” chapter [Expedition 336 Scientists, 2012a]). The total gamma ray measurements through the BHA and casing show three main anomalies from the seafloor to ~197 m wireline log matched depth below seafloor (WMSF) (Fig. F15), and the open-hole gamma ray measurements cover a total of 388.22 m downhole (from 112 mbsf, where casing was set). The slightly shorter overall coverage of this tool compared with the DEBI-t is the result of the HNGS being ~17 m higher in the tool string (Fig. F14).

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 395A follow this trend. Total spectral gamma ray (HSGR) values obtained with the HNGS in Hole 395A range from 0.012 to 16.451 gAPI, with a mean of 6.73 gAPI. Potassium values are relatively low, with values ranging between 0.0002 and 0.73 wt% and a mean of 0.32 wt% (Fig. F16), which is a much larger range than values obtained from core measurements during Leg 174B (0.10–0.29 wt%; Shipboard Scientific Party, 1979). Uranium values range between 0.001 and 1.54 ppm and have a mean of 0.52 ppm. Thorium values range from 0.001 to 2.02 ppm, with a mean of 2.63 ppm. Comparison of gamma ray data collected during Expedition 336 to that collected during Leg 174B shows good agreement (Fig. F16). The downhole variation in gamma ray intensity collected most recently displays the same saw-tooth pattern recorded during Leg 174B (Bartetzko et al., 2001). However, in open hole, total gamma ray values are marginally higher (~5 gAPI) in the most recently collected data set than those taken 14 y ago. This increase could be driven by alteration of the formation nearest the borehole (over the last 14 y), or it could be simply instrument related. The data collected during this expedition and Leg 174B both used the HNGS; however, calibrations can vary.

Temperature

Temperature ranges from 1.94° to 18.42°C in Hole 395A. Downhole logging measurements were taken as soon as the old CORK was removed so that an undisturbed temperature profile could be obtained. Over the drilling and downhole measurement history of Hole 395A, temperature has been measured four times, including this expedition (DSDP Leg 78B and ODP Legs 109 and 174B) (Fig. F17). Considering all of these temperature measurements, Hole 395A has behaved consistently over time. Each of the temperature logs shows strongly depressed borehole temperatures that are near isothermal to ~300 m WMSF. From 404 m WMSF all of the temperature profiles taken in Hole 395A exhibit temperatures above 3°C (Fig. F17). The temperature profiles recorded during Expedition 336 are nearly identical to those determined during Leg 174B. Most notable is the sudden change in the temperature gradient below a brecciated zone at ~425 mbsf. That zone has been proposed to correspond to the deepest aquifer below which seawater can no longer recharge into the formation and heat transport is conductive (Becker et al., 1998). The similarity of the temperature records indicates that the hydrological state has not changed in the past 14 y.

Fluorescence

Fluorescence data from the DEBI-t indicate the presence of particulate organic matter throughout Hole 395A (Fig. F18). There is a general trend in the signal intensity as the instrument descends through the borehole, and this trend appears to be a reflection of changes in the intensities of the 360, 380, and 455 nm bands. It is possible that the initial intensity variation that occurs in these channels between 197 and 250 m WMSF is a result of the decreased influence of material in the fluids diffusing the logging pipe. However, the change in intensity that occurs between 340 and 455 nm is quite sudden at the horizon where the instrument clears the pipe; thus, any subsequent influence from any material discharging from the logging pipe may be minimal.

A second change in the intensity of specific bands is discernible at ~450 m WMSF. This change coincides with the temperature profile, reflecting a shift in the influence of ocean bottom water compared to water from the surrounding aquifer. At this horizon there is a decrease in intensity in the 300 and 455 nm bands and a slight increase in intensity in the 340 and 360 nm bands. These changes in intensity may reflect a change in the organic concentration, as well as the types of organics found in the lower portions of Hole 395A, where the water has reached a state of equilibrium.

Excitation with a 224 nm source induces a peak fluorescence emission at 300 nm for spores and 320 nm for bacteria (Bhartia et al., 2010). Although there is evidence of a bacterial signature, the signal intensities suggest cell densities that are at limit of detection for DEBI-t, based on laboratory calibrations. Thus, we are unable to quantify the bacterial biomass within Hole 395A. However, the signal corresponding to spores (300 nm) was much greater, and suggested a spore or sporelike density of up to 10⁶ spores/mL. Although the fluorescence from these channels shows some variation, the general trend indicates that biomass is distributed throughout the borehole.

DEBI-t also recorded video information, which indicated that there was a large amount of particulate material within the hole. The majority of this material appeared to be oxidized iron particles, as well as some floccular material. The high particle load within Hole 395A makes it difficult to ascertain how much of the signal collected by DEBI-t came from the wall and how much from the water column. Nevertheless, the uniform distribution of particulate matter throughout the hole would seem to correlate with the relatively homogenous biosignal seen in the logging runs.

Log units

Electrofacies (EFA) for Hole 395A defined by Bartetzko et al. (2001) were primarily defined using resistivity, P-wave velocity, density, and total gamma ray logs, in addition to a sequence of eruptive units (Fig. F15).

Four EFAs were identified from the logs by Bartetzko et al. (2001): (1) massive basalt, (2) altered lava flows, (3) pillow basalts, and (4) rubble zones. (Note that all of the following log values were taken from Bartetzko et al., 2001.) The massive basalts (1) are characterized by high average density values of 2.8 g/cm³, high formation resistivities ranging from 50 to 500 Ωm, and high P-wave velocities of 6100 m/s, on average. Total gamma ray values are low, with an average value of 1.8 gAPI. Altered lava flows (2) were not identified in the cores, but their log responses show a stronger influence of alteration and fracturing than the massive basalts. Resistivity and density values are lower in the altered lava flows than in massive basalts, with resistivity varying between 25 and 100 Ωm. The average density value is 2.7 g/cm³, and total gamma ray values are higher compared to massive basalts, with a mean value of 3.8 gAPI. Pillow basalts (3) of Hole 395A are described as fine- to very fine grained glassy rocks with varying degrees of alteration. The significantly higher fracturing and associated alteration in pillow basalts, as well as the high porosity of interpillow areas, produce characteristic log responses. Fractures and voids fill with seawater, and conductive clay minerals may also preferentially concentrate in these voids, causing the measured resistivity log to decrease. Typical values range between 15 and 200 Ωm. Additionally, there are associated low density values of ~2.5 g/cm³, low P-wave velocities of ~4.9 km/s, and high total gamma ray values (mean value = 6.2 gAPI). The final rubble zone (4) electrofacies was given to intervals where significant borehole enlargements strongly influenced the log responses.

The EFA log devised by Bartetzko et al. (2001) is divided into three intervals (100–169, 182–417, and 430–590 mbsf) separated by thick rubble zones. The section between 182 and 417 mbsf is composed of alternating pillow basalts and lava flows (both massive and altered). The upper- and lowermost intervals consist exclusively of pillow basalts. In the uppermost interval, the lithology is homogeneous and uniform log responses were noted (Fig. F15). The lowermost interval below 430 mbsf is characterized by increasing log properties with depth, especially electrical resistivity. Bartetzko et al. (2001) attribute this trend to the dominant effect of a decrease in fracture porosity caused by closure from the increasing overburden. Although, Bartetzko et al. (2001) do not provide a classification of different electrofacies for the lowermost interval, they report that the core results indicate that only pillow basalts and breccias are present and the existence of massive basalts is unlikely. Hence, they present a realistic uniform pillow basalt classification for this lower region.

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