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doi:10.2204/iodp.proc.340.109.2013

Downhole logging

After Hole U1399 had to be abandoned, a hole dedicated solely to downhole logging was drilled (see “Operations”). By 0045 h on 2 April 2012, Hole U1399C had been drilled to a final depth of 240 mbsf and the hole was prepared for logging operations. Three tool strings were deployed in the hole over a 25 h period: the triple combo-Magnetic Susceptibility Sonde (MSS), the VSI, and the FMS-sonic (Fig. F13; see Fig. F11 in the “Methods” chapter [Expedition 340 Scientists, 2013a]). All of these tool strings were slightly modified versions of previous tool strings used during Expedition 340. No radioactive sources were used in this hole as a consequence of unstable hole conditions encountered at Site U1399.

Operations

Hole U1399C was prepared for logging activities by sweeping with high viscosity mud, pumping a go-devil, and displacing with heavy barite-weighted mud. The pipe was pulled to a bit depth of 80.8 mbsf, leaving a ~159 m section of open hole for logging.

The modified triple combo was the same as the triple combo-MSS but without the Hostile Environment Natural Gamma Ray Sonde (HNGS; spectral gamma ray). The HNGS, with the largest diameter of the tools, presents the greatest challenge in passing successfully through the BHA. Because the bottom ~0.5 m of the last core barrel twisted off into the hole at the end of drilling (see “Operations”), potentially damaging the lockable float valve, it was decided to keep tool string diameter to a minimum by not including the HNGS. The modified triple combo was rigged up at 0442 h and run in the hole to a total depth of 237 mbsf (3 m of infill). Two uplogs were then completed, acquiring magnetic susceptibility, electrical resistivity, gamma ray, and caliper measurements.

The second tool string to be deployed in Hole U1399C was the VSI tool string. Because of the trouble passing the VSI through the BHA in Hole U1394B, it was decided to add a weight bar to the tool string in Hole U1399C. Preparations for the vertical seismic profile (VSP) experiment commenced with deployment of the air guns to a water depth of ~7 m off the port side of the R/V JOIDES Resolution. Rig up commenced at 1115 h, shortly followed by the start of the protected species watch at 1130 h. The tools were run in hole at ~1200 h, and a series of uplogs were taken to ensure an accurate depth reference frame, using gamma ray measurements from the triple combo run for comparison. After some delays because of the sighting of a whale within the exclusion zone, the experiment started at 1500 h. Eight stations were taken, with spacing between stations ranging from ~9 to 27 m.

The final tool string deployment was a modified FMS-sonic tool string. Because good hole conditions were experienced throughout the first two runs, it was decided to run the HNGS with the FMS-sonic in order to add spectral gamma ray to the data set for Hole U1399C. The ~35 m tool string was lowered into the hole at 2100 h, reaching a total depth of 211 mbsf. Because of problems opening the FMS caliper arms during the first pass and rapidly deteriorating hole conditions, three passes of the open hole section were conducted to get a complete FMS data set. Problems were encountered with reentering pipe and passing the tool string through the BHA, meaning that tools did not reach the surface until 0407 h, with rig down complete by 0546 h on 3 April 2012.

Data processing and quality assessment

Logging data from Hole U1399C are summarized in Figures F14, F15, F16, and F17. A full data set was achieved from the tool strings deployed with the exception of density data, which is absent because the radioactive source was omitted from the triple combo. As indicated by the HLDS caliper data (Fig. F14), Hole U1399C is relatively in gauge, with a profile similar to Holes U1394B and U1395B. Hole diameter generally increases from ~10 inches at the base of the hole to ~14 inches at the top of the open hole section (~90 mbsf). The exceptions are three washed out zones with hole diameters >18 inches that were encountered at 114–115, 129–133, and 137–140 mbsf. FMS caliper data from three subsequent passes suggest that hole conditions deteriorated significantly between the VSI tool string run and the FMS-sonic run. This deterioration is also clear from difficulties encountered in getting the tool string through the open hole section and from periods of increased head tension that suggest that material was falling into the hole or that the hole was closing in during logging operations.

Despite the hole conditions, the repeatability of all of the logging measurements is very good between the multiple passes for both tool strings (Fig. F18). In addition, gamma ray measurements from both tool strings agree well (Fig. F15).

The acquisition of good VSP data relies on coupling of the VSI geophone to the borehole wall, which is best achieved in an in-gauge hole. Hole diameter was sufficiently small in Hole U1399C (Fig. F14) to acquire some good quality data. However, it was clear that maintaining a strong anchoring force over successive shots at any station was challenging, particularly in the shallowest interval between 80 and 140 mbsf. This poor anchoring potential is likely a reflection of the soft formations present and was most evident in the upper four stations. Figure F17 shows the stacked VSP waveforms from the eight stations in Hole U1399C. Corrected one-way traveltimes (green crosses) take into account the geometry of the VSP experiment.

Acoustic velocity data in Hole U1399C are of variable quality, most likely due to the deteriorating hole conditions encountered during the FMS-sonic tool string run. The automatic picking function was not consistently able to identify valid compressional wave (V_P) values between 80 and 145 mbsf. The picking function did a somewhat better job for shear wave velocity (V_S) in this hole, with the exception of an interval between 102 and 112 mbsf. Despite these breaks, there are intervals of good quality V_P and V_S data within the logged portion of the hole, as evidenced by the high coherence regions (red) in Figure F15. Postcruise reprocessing of this data set will improve the data quality, particularly in the problematic zones.

Acquisition of good-quality FMS images requires an in-gauge hole, regular borehole walls, and good tool pad/borehole contact. Despite some problems opening the calipers at the beginning of the FMS-sonic tool string run, the third and final uplog allowed the continuous acquisition of FMS images from Hole U1399C between 88 and 204 mbsf (Fig. F15). Preliminary assessment of the images suggests they are of generally good quality. Data processing includes correction for tool motion, which is effective for vertical shifts of as much as ~1 m. Tool dynamics data from Hole U1399C indicate shifts of <1 m, with the exception of a few slightly larger shifts between 88 and 105 mbsf.

Logging stratigraphy

Downhole logging data are summarized in Figures F14, F15, F16, F17, F18, F19, and F20. The logged interval at Hole U1399C is divided into four logging units primarily on the basis of responses from the tools in the triple combo (gamma ray, resistivity, and magnetic susceptibility). A description of these logging units follows.

Logging Unit 1

Logging Unit 1 comprises the uppermost 26 m of the logged interval (80–106 mbsf). This unit is characterized by an overall increase in gamma ray values with depth from ~25 to ~50 gAPI. Average gamma ray values in this logging unit are ~33 gAPI. Magnetic susceptibility and electrical resistivity profiles are similar in character with a small net decrease with depth. True electrical resistivity (RT) in this unit has a mean value of 1.38 Ωm.

Logging Unit 2

The transition from logging Unit 1 to logging Unit 2, at 106 mbsf, is marked by a sharp decrease in gamma ray values that coincides with sharp increases in resistivity and magnetic susceptibility. Similar to logging Unit 1, gamma ray values in logging Unit 2 show a net increase with depth with an average value of ~31 gAPI. Three intervals have lower gamma ray values (114–115, 129–133, and 137–140 mbsf) and correspond to washed out zones >18 inches in diameter (Fig. F14). Between 140 and 150 mbsf there is an interval of increased gamma ray values; this interval is reflected in both the total gamma ray and spectral gamma ray measurements (Figs. F14, F15). Interestingly this increase is not shown in the corresponding natural gamma radiation data from cores recovered in Hole U1399A. Resistivity generally decreases with depth (mean = 1.47 Ωm) through this logging unit with localized high-resistivity features (for example, ~106–110, ~114–116, ~126–129, and ~138–140 mbsf). This trend is repeated in the magnetic susceptibility curve.

Logging Unit 3

Logging Unit 3 (150–181 mbsf) is characterized by higher amplitude variability in gamma ray values, resistivity, and magnetic susceptibility. Similar to the transition between logging Units 1 and 2, the transition from logging Unit 2 to logging Unit 3 is marked by a stepwise decrease in gamma ray values and increases in resistivity and magnetic susceptibility. Average gamma ray values are lower than in the overlying unit, with a value of ~28 gAPI, and average resistivity is higher (1.60 Ωm). Generally lower values of gamma ray coincide with higher values of both resistivity and magnetic susceptibility (Fig. F11). There is no net change in any of the measurements through this logging unit.

The boundary between logging Units 2 and 3 at 150 mbsf coincides with a change in lithostratigraphy from a chaotic unit above to a transition zone below (see “Lithostratigraphy”).

Logging Unit 4

Higher amplitude and more regular variability characterizes logging Unit 4, which extends from 181 mbsf to the bottom of the logged section. More regular gamma ray variations, of ~15 gAPI, continue through the uppermost 25 m of this logging unit, after which variations decrease in amplitude. Overall, the mean gamma ray value is ~29 gAPI. Logging Unit 4 exhibits the most variable magnetic susceptibility profile in Hole U1399C, including some significant peaks, the largest of which (at ~205–208 mbsf) corresponds to similarly elevated values of resistivity. This feature with high resistivity and magnetic susceptibility coincides with the depth at which the coring system in Hole U1399A was changed from the APC to the XCB (Fig. F14), suggesting a change in lithology or induration at this depth.

Vertical seismic profile experiment

One of the expedition objectives is to constrain the seismic stratigraphy of mass transport deposits. The VSP experiment provides a good intermediate step in integrating core and wireline logging data, recorded in depth, with seismic data, recorded in time.

Data acquired during the VSP experiment are summarized in Figures F17 and F19 and Table T6. First arrivals from 31 out of the 123 shots were used in the stack, with the vertical component being the most reliable. Despite several of the shots recorded by the VSI being noisy, eight stations yielded reliable check shot traveltimes ranging from 0.128 s two-way traveltime at 99.1 mbsf to 0.279 s at the bottom of the hole (237 mbsf). A comparison of the data from both the VSP and the sonic tool, with predictions of velocity from seismic data at this site (1800–2200 m/s), indicates that measured velocities are slower than predicted (Fig. F19).

Formation MicroScanner images

Figure F20 shows examples of the FMS images acquired from Hole U1399C. The images indicate a range of textures and features across the logged interval, including a wide range of resistivities. Some surfaces can be identified, ranging from subhorizontal to moderately dipping. Broadly, the images can be divided into four sections alternating between conductive (88–104 and 128–149 mbsf) and resistive zones (104–128 and 149–204 mbsf). Two of these boundaries roughly coincide with the logging unit boundaries (Fig. F15) as determined by the triple combo tool measurements. These are the boundaries between logging Units 1 and 2 and logging Units 2 and 3 where there are downhole shifts from more conductive to more resistive tool responses.

Direct comparison between the FMS and core images is not possible because no cores were collected in Hole U1399C. It is clear from all of the sites cored during Expedition 340 that there are often significant differences between two holes at a site, particularly in terms of the thickness of the different formations encountered. Site U1399 is no exception, with offsets in unit boundaries >10 m (see “Lithostratigraphy”). For the purposes of core-log integration it is advised to use core data from Hole U1399A, as it lies in closer proximity to logging-dedicated Hole U1399C than Hole U1399B.

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