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doi:10.2204/iodp.proc.301.106.2005

Wireline logging

Hole U1301B

A wiper trip was completed throughout open hole before the start of the wireline logging operations. The hole was circulated with 50 bbl of sepiolite mud, which was not displaced, and the pipe was pulled out of the hole so that the bit could be dropped on the seafloor. Wireline logging operations required 35.25 h beginning at 0415 h on 1 August. The wireline logging operations consisted of four tool string deployments in the following order (Table T22):

1. The triple combo tool string consisting of an LEH-MT cable head with sensors for measuring spontaneous potential (SP), temperature, and tension; the HNGS; the HLDS; the APS; and the QAIT.

2. The UBI tool string consisting of the LEH-MT with sensors for measuring SP, temperature, and tension; the Scintillation Gamma Ray Tool (SGT); the GPIT; and the UBI.

3. The FMS-sonic tool string consisting of the LEH-MT with sensors for measuring SP, temperature, and tension, the SGT; the DSI; the GPIT; and the FMS.

4. The WST string.

First deployment: triple combo string

The drill pipe was set in casing at a depth of 265.2 mbsf (2933 mbrf), which is below the depth in casing where we had problems passing the bit during reentry (257.8 mbsf; 2925.6 mbrf). During the rig-up, a grounding cable was connected from the winch unit to the drill pipe to serve as the contact for the surface electrode that was used for SP measurements. The tool string was then assembled and run in the hole at 2500 m/h.

A downgoing log was recorded, starting before reaching seafloor to a total depth of 578.2 mbsf (3246 mbrf), where ~4 m of fill at the bottom of the hole was encountered, and the APS minitron source was put in standby mode 50 m above total depth and before commencing the first uphole log. Two uphole logs were then recorded, with the first pass recording from 578.2 to 298 mbsf (2965.3 mbrf) and the second pass recording from 454.2 mbsf (3122 mbrf) to past seafloor on the way back to the rig floor. The caliper arm was opened at 574.2 mbsf (3242 mbrf) during first uphole log, and a caliper check was done inside the casing to confirm the calibration but was closed before entering the drill pipe. Once the first uphole log was finished, the APS minitron was shut down after closing the caliper arm inside the casing, and it was put in standby mode 50 m above the total depth of the second uphole log. The second uphole logging pass was planned to repeat a section of the borehole that was measured during the first pass for quality control purposes. Caliper measurements showed that the upper 50 m of the open hole was enlarged and irregular, so a larger interval was chosen for the repeat pass to ensure that data were collected over a depth interval that included good borehole conditions. No problems were encountered during either pass.

Second deployment: UBI tool string

Operations began by picking up the top drive because this would allow us to raise the drill string while logging so that we could image a section of 10¾ inch casing that was thought to either have a hole in it or to have separated from the rest of the casing. This procedure required us to break and reconnect the wireline torpedo connection. We lowered the tool string at 400 m/h because it was so light but increased the speed to ~2500 m/h as cable weight increased. An obstruction was found in the hole at 428.2 mbsf (3096 mbrf). Several attempts to get past the obstruction by running down the tool string at different speeds failed, and we decided to log the open hole and casing above this depth.

The open hole interval (428.2–346.1 mbsf) was logged with two full passes. The second pass stopped at 353.2 mbsf (3021 mbrf) to change the data acquisition configuration of the UBI to casing mode, the pipe was raised to a depth of 246.2 mbsf (2914 mbrf), and data were recorded inside the casing and up to seafloor. The tool was returned to the surface without incident, although some cable speed fluctuations were observed in the winch unit during the tool deployment. Variations in tool acceleration during acquisition were ±1 m/s².

Third deployment: FMS-sonic tool string

Rig-up was completed and the tool string was deployed through the pipe at ~2500 m/h. The deployment was stopped at 232.2 mbsf (2900 mbrf) to check operation of the new wireline heave compensator (WHC). Surface and downhole acceleration measurements were recorded with the surface moonpool instrumentation and the GPIT, respectively. The station consisted of 15 min with the WHC on and 15 min with the WHC off. At the conclusion of the WHC station, the tool string was lowered into the open hole to begin logging operations.

The first pass reached nearly the same depth as the UBI deployment, ~427.2 mbsf (3095 mbrf). Several attempts were made to go past the hole obstruction, but they all failed and logging operations began from this depth. Cable speed fluctuations were also observed throughout most of the deployment as during the UBI deployment. The first pass continued into the 10¾ inch casing to check the caliper calibrations, and the tool string was then lowered to the bottom of the unobstructed interval to begin the second pass.

A brief indication of low overpull (~340 lb) was recorded during the second pass at ~404.2 mbsf (3072 mbrf), and shortly thereafter, one FMS caliper (C2) began to behave erratically. After the second pass, operations in the open hole were finished, the calipers were closed, and data acquisition was stopped shortly before entering the 10¾ inch casing shoe. The DSI was reconfigured to the cement bond mode to look at the bottom of the casing shoe and the gap in the casing, and the pipe was raised to 242.2 mbsf (2910 mbrf). The caliper arms were opened inside the 10¾ inch casing at 268.2 mbsf (2936 mbrf) to check caliper C2, and this caliper read 4 inches, whereas C1 read ~10 inches. The tool string was pulled up past the casing gap.

Once above the casing gap, the caliper arms were closed and the tool string was brought to the end of the pipe, where it became stuck. The tool string was lowered back inside the casing, and the caliper arms were opened and closed once again. The tool string was brought back inside the end of pipe, and once again it became stuck. A maximum of 7800 lb was pulled to try to get the tool string free, and after failing, the pipe was lowered several meters in case there was an obstruction in the casing that may have impeded the tool's motion. At this time, the tool string was wedged and partially sticking out of the end of the drill pipe with no ability to move in or out. We then began pumping for several minutes followed by a series of hard pulls in the range of 6500–7800 lb. After several iterations of this technique, the tool string began to move inside the pipe and was finally retrieved to the surface. We spent ~2 h trying to get the tool string past the end of the drill pipe. When the tool string was recovered on the rig floor, the FMS C2 caliper had lost part of the imaging pad, the outer cover of the caliper that serves as a housing for the pad wiring, and the arm expansion springs.

Preliminary observations suggest that during the second pass and while still in the open hole, the tool string probably hit a ledge or otherwise damaged the upper joint of the C2 and the closing mechanism. This caused the arm to show erratic responses and prevented complete closure. In addition, the lower 50 m of cable was kinked. We cut and reterminated the cable and prepared a new rope socket for the next deployment.

Fourth deployment: WST tool string (vertical seismic profile)

The original logging plan included use of the WST-3 for obtaining a zero-offset VSP. However, when we powered up the WST-3 on deck, it did not respond, even though a deck check 2 days earlier did not show any problems. The clamping arms were opened and closed several times using the manual control box, but the tool did not respond once power was supplied. After the unsuccessful attempts, the WST-3 was replaced with the WST. The WST powered up on deck and all diagnostic checks were positive. The Schlumberger winch unit was used for the WST data acquisition.

Rig-up procedures began after sunrise with several observers in place for compliance with the IODP marine mammal policy. The procedure included a 1 h observation period prior to the use of the seismic source, where the mate on watch and the marine mammal observers on the aft end of the ship began observations. Observations continued throughout the duration of the seismic experiment, and no marine mammals were sighted within the 700 m safety zone. After the initial observation period, the "soft start" procedure began with the seismic source being fired at 30 s intervals starting at a pressure of 500 psi and gradually increasing the pressure to the "operational" pressure of 2000 psi over a 30 min period.

A GI seismic source consisting of a 45 inch³ generator chamber volume and a 105 inch³ injector chamber volume was used for the experiment. The generator produced the primary pulse, while the injector controlled the oscillation of the bubble produced by the generator. The GI seismic source was operated at 2000 psi air pressure with a time delay between the generator and injector shots of 40 ms. The GI seismic source was deployed using the JOIDES Resolution portside crane number 3 and suspended with a floating buoy at a depth of 2 m below the sea surface (Fig. F73). The seismic source was located 14.6 m from the ship, and the length between the GI seismic source and the center of the moonpool was 44 m. The over-the-side monitoring hydrophone was placed 2.3 m below the GI seismic source, and we confirmed that the source wavelet had minimum phase. After each shot, 5 s was recorded with a starting point of 600 ms and a sampling rate of 1 ms.

The WST took 2 h to reach the seafloor because of the tool's light weight. On several occasions, we stopped to close the clamping arm, as we suspected that it opened periodically during descent. The initial downgoing speed was ~305 m/h and increased to ~2350 m/h with depth. Based on caliper observations from previous logging runs, three potential intervals were identified for WST stations. These were at 407.2 mbsf (3075 mbrf), 382.2 mbsf (3050 mbrf), and 357.2 mbsf (3025 mbrf). Shots were taken at all three approximate depths, and details are presented in Table T22.

The experiment concluded at 1150 h on 2 August, when fog covered the area and poor visibility did not allow us to monitor the 700 m radius stipulated in the marine mammal policy. The tool's retrieval speed was slowed down to ~760 m/h to allow the rig floor crew to work on the AHC system, after which we increased the speed to ~2740 m/h. Once the tool was back on the rig floor, we noticed that the clamping arms were fully extended, although they had been closed before entering the pipe.

Depth shifting of logs

Depth shifts begin with the selection of 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 features in equivalent logs from other runs are aligned by eye. The depth adjustments that are required to bring the match log in line with the base log are applied to all the other data sets from the same tool string.

Proper depth shifting of wireline logging depths relative to pipe depths was essential both to achieve scientific goals and to assess the state of the 10¾ inch casing string, as described below. Local tides during Expedition 301 created ambiguity as to relative depths for all measurements, with typical uncertainties of 1–3 m (see "Operations" and Fig. F6). In addition, because of uncertainty as to the depth of the 10¾ inch casing shoe, we could not use this as a depth reference from which we could "hang" wireline logging data.

There are two other targets that offered potential wireline logging depth references: the seafloor and the sediment/basement interface. However, data acquired across the seafloor resulted from logging through the drill string and a reentry cone, so data from this interval are of poor quality. Depth matching between logging passes across this interval was difficult because of the low gamma radiation levels and general lack of significant features in the gamma ray logs that could be matched. The natural gamma ray record across the sediment/basement contact proved to be more useful, with caliper logs used for additional interlog comparison. Drilling data (penetration rate and torque) suggested that the sediment/basement interface was located at 265 mbsf (2933 mbrf), and this coincided with an abrupt (local) decrease in natural gamma radiation as detected with the FMS tool string. Caliper logs from the triple combo and FMS strings suggested that these two strings were matching within 1 m of each other, with the FMS string being ~1 m deeper. Based on these observations and the relatively short logged interval, differential depth matching, which includes stretching and squeezing the depth scale to line up common features, was not done and only block shifts were applied.

Some common features could be identified in the caliper logs—for example, the base of a zone where the hole widened (404 mbsf) and the base of the 10¾ inch casing (352 mbsf). As shown in Table T22, depth shifts were applied to align these log features with each other, to match the step in gamma ray in the casing logs to the sediment/basement interface at 2933 mbrf (268 mbsf) and to fit the driller's seafloor depth of 2667.8 mbrf. All logging depths discussed in the rest of this report have been depth shifted accordingly.

Logging data used to assess 10¾ inch casing geometry

We had two casing depth "targets" we hoped to identify using the various logging tools. First, we intended to identify the depth of the 10¾ inch casing shoe. This was originally cemented into place at 346 mbsf (3014 mbrf), but it was possible that the lower section of casing had slipped downward after cementing if the casing had parted somewhere in the middle of the string. Second, we hoped to identify the dimensions and geometry of a suspected gap in the casing that was responsible for obstructing the drill pipe during previous operations (see "Operations") at 250–258 mbsf (2918–2926 mbrf). Even taking into account uncertainties associated with local tides (1–3 m), the variations in the depth of the obstruction in casing suggested that the gap had changed shape over time.

Results from the triple combo and FMS strings were relatively consistent with regard to the depth of the 10¾ inch casing shoe, but these results were inconsistent with results from the UBI tool. The caliper logs from the triple combo tool string and the FMS tool string demonstrate convincingly that the casing shoe is located at 352 mbsf (3020 mbrf). UBI data suggest that the casing shoe is located at 357 mbsf (3025 mbrf), 11 m deeper than originally placed and 7 m deeper than the 14¾ inch rat hole. The casing shoe depth indicated by the triple combo and FMS calipers makes more sense operationally than that indicated by the UBI tool, since the former places the casing shoe just 2 m deeper than the 14¾ inch rat hole. For these reasons, we place the depth of the casing shoe based on logging data at 352 mbsf (3020 mbrf).

Data collected with the FMS and UBI tool strings also can be used to assess the geometry of the 10¾ inch casing gap, but unfortunately results from both tools are inconsistent with operational data. Both the FMS and UBI tools suggest that the base of the casing gap is located at 263 mbsf (2931 mbrf), whereas repeated trips across the gap with the drill string indicate that the primary obstruction, interpreted to be the top of the lower section of casing, is located at 258 mbsf (2926 mbrf), a difference of 5 m. Interestingly, this difference is the same as that found for the casing shoe, comparing the triple combo and FMS calipers to the UBI tool. Because the most likely explanation for the casing gap is separation at a casing joint, we can use the known lengths of casing joints to determine the most likely geometry for the gap. With the 10¾ inch casing shoe placed at 352 mbsf (3020 mbrf), based on caliper logs described above, the bottom of the 10¾ inch casing gap should be at 257–258 mbsf (2925–2926 mbrf), consistent with the location of the obstruction determined repeatedly with the drill pipe. Both the FMS and UBI tools place the top of the casing gap at ~252 mbsf (2920 mbrf), and this depth is inconsistent with consideration of casing joint lengths and the estimated depth of the 10¾ inch casing hanger. The most reliable indicators suggest that the casing gap is ~5–6 m thick. It remains unclear why the FMS and UBI tools indicate a greater gap thickness (~10–11 m) and a downward shift of the base of the gap.

Results and data quality

The quality of wireline logging data was assessed by evaluating whether logged values are reasonable for the lithologies encountered, by checking consistency between different passes of the same tool, and by correspondence between logs affected by the same formation property (e.g., the resistivity log should show similar features to the sonic velocity log). Gamma ray logs recorded through BHA and drill pipe should be used only qualitatively because of signal attenuation and noise. A wide (>12 inch) and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS and APS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the calipers on the FMS tool (C1 and C2). The triple combo was the only tool string that could be lowered to the bottom of Hole U1301B. Good repeatability was observed between the main and repeat passes from 450 to 350 mbsf, particularly for measurements of neutron porosity, density, and photoelectric factor. The caliper arm from the HLDS shows that the borehole is almost in gauge below ~464 mbsf but very irregular and oversized between 352 and 464 mbsf, reaching >18 inches between 395 and 405 mbsf (Fig. F72). The sonic velocity logs contain minor intervals of poor-quality data but are mostly reliable in sections where the borehole is not enlarged.

Electrical resistivity measurements

A total of 15 electrical resistivity curves were obtained with the QAIT. These resistivity profiles represent five different depths of investigation into the formation (10, 20, 30, 60, and 90 inches) and three different vertical resolutions (1, 2, and 4 ft). Some of the resistivity curves show small spikes of high electrical resistivity values reaching up to 1950 Ωm; however, most of the resistivity values within the basement range between 0.3 and 150 Ωm. Among the set of resistivity curves, the 10 inch depth of investigation curves show lower electrical resistivity values than the other resistivity curves of the same vertical resolution. Despite some scattering within the data, most curves follow similar trends. For example, there is remarkably little variation in formation electrical resistivity with depth (Fig. F74). Interestingly, the shorter-spaced log (AF-10) suggests a systematic increase in resistivity with depth through the logged interval.

Neutron porosity measurements

Neutron porosity values show a large range from 4% to 100%. Neutron porosity values are particularly high above ~462 mbsf, where the borehole is enlarged, and are lower in the bottommost part of the hole, where values are mostly between 5% and 20%. The latter values are closer to porosities measured on core samples, which range from 2% to 9% and have a mean value of 5% (see "Physical properties"). High neutron porosity values measured above 462 mbsf can be attributed either to the tool losing contact with the borehole wall, the high degree of fracturing, or the presence of clays in veins, hydrous vesicle fillings, or altered groundmass (see "Igneous and metamorphic petrology").

Density and photoelectric factor

Density values range from 1.23 to slightly more than 3.00 g/cm³ over the entire section of the open hole. Below 462 mbsf, density values are between 2.5 and 3.0 g/cm³ and close to the range of values measured on core samples, which average 2.78 ± 0.08 g/cm³ (see "Physical properties"), except for some local intervals that may be associated with fractures. Above 460 mbsf, density values are lower because of the irregular and enlarged shape of the borehole. Photoelectric factor values range between 0.89 and 6.69 b/e^–. Values tend to be higher below 462 mbsf than in the shallower section.

Gamma ray measurements

Gamma ray measurements in basaltic oceanic crust are typically low (e.g., Bartetzko et al., 2001; Barr et al., 2002). Total gamma ray values (HSGR) obtained with the triple combo tool string range from 5.3 to 13.2 gAPI. Potassium values are low, with values between 0 and 0.48 wt%. Thorium and uranium values are mostly between 0 and 1 ppm.

Spontaneous potential measurements

Values of spontaneous potential from the main triple combo logging run vary between –170 and 24 mV. Values increase within the enlarged borehole sections and decrease within more massive intervals. Particularly low values are observed above 378 mbsf, which is above the strongly oversized interval from 380 to 390 mbsf.

Sonic velocity measurements

Shipboard slowness time coherence analyses were used for estimating P- and S-wave velocities by correlating digital waveform signals recorded at each receiver. P-wave velocities obtained with the DSI range from 4000 to 6000 m/s and correlate well with the average laboratory velocity measurements of ~5300 m/s obtained from core samples (Fig. F75). S-wave velocities range from 2000 to 3000 m/s (Fig. F75). However, several sections have anomalously high or low velocities, especially below 385 mbsf, where both P- and S-wave velocities are low because the borehole is enlarged and irregular. Estimation of accurate velocities within those sections will require postcruise processing. At this point, it is difficult to identify significant trends of P- and S-wave velocity with depth and lithology.

Seismic velocity measurements

Recorded waveforms from the VSP experiment with the WST at 402.2 mbsf (shots 91–94) are too noisy for analysis. The first five waveforms at 413.6 mbsf (shots 72–76) are also noisy and could not be used for analysis. At these stations, the WST may have slipped along the borehole.

The waveforms at each of the good stations were stacked (Fig. F76A), and a traveltime was determined from the first breaks (Orcutt, Schultz, Davies, et al., 2003). In some instances, it is difficult to determine the first break, so we also used the median of the first break for each stacked trace to determine interval velocities (Fig. F76B). The gradient of the traveltime first break used to estimate an interval velocity produced a result of ~5220 m/s, whereas the median yielded an interval velocity of 4990 m/s. Core sample and sonic logging measurements show a slightly higher range of velocities, and the difference may reflect the different scales of core measurements, sonic logs, and seismic experiments.

Lithostratigraphic correlations

Except for the enlarged borehole sections, most measurements do not show statistically significant variations. This may result from the lithology primarily consisting of pillow basalts throughout the logged interval (see "Igneous and metamorphic petrology"). Overall, 21 logging units were identified using mainly density, porosity, and photoelectric effect profiles obtained with the triple combo over the entire open hole section (Fig. F74). Most of the units are characterized by massive sections bounded by fractured or brecciated intervals (Fig. F74). In the narrower borehole intervals, massive flow of Units 2, 4, and 6 can also be identified in the downhole logs as slight increases in electrical resistivity, low neutron porosity, and high density values (Fig. F74). Pillow basalt Subunit 1C and Units 3 and 5 coincide with the enlarged borehole intervals above 462 mbsf. Pillow basalt Unit 7 is located at a depth interval with narrower borehole dimensions. This unit is characterized by low neutron porosity and high density values with small spikes of high neutron porosity and low density that may be caused by fracturing. The slight increase in gamma ray values below 515 mbsf may be caused by slightly higher alteration within Subunit 7C, which is the only depth interval with highly altered rocks identified in the cores. Typical secondary minerals in this interval are saponite, iron hydroxides, and celadonite (see "Igneous and metamorphic petrology"). Celadonite may contain potassium and thus increase the gamma ray values. The two intervals of basalt-hyaloclastite breccia of Subunits 1A and 8A cannot be identified in the downhole measurements, consistent with the subunits being very thin.

Borehole images

Acoustic and microresistivity borehole images were acquired with the UBI and FMS, respectively. The initial quality of both sets of images is poor for two main reasons. The section of the borehole that was imaged is characterized by washouts and irregularities that hinder the acquisition of high-resolution images. In addition, the new heave compensating system (hardware and software) used during Expedition 301 may have not worked properly.

Preliminary examination of the data suggests that there could be depth discrepancies associated with the acceleration data that were collected with the GPIT. The compensated depth data are calculated from a time-indexed data file that uses cable speed for its conversion. Cable speeds fluctuated during the deployments, although it was difficult to assess if these were normal fluctuations because the cable drum is supposed to rotate frontward and backward to compensate for the ship's motion. A 30 min heave compensating station was performed at the beginning of the FMS/DSI tool string deployment and evaluated postcruise in an attempt to understand the compensator's behavior. These and subsequent postcruise tests revealed that the hardware was working properly, but the software was not. It does not appear that postexpedition processing will help to improve the quality of borehole imaging data.

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