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doi:10.2204/iodp.proc.341.104.2014

Downhole logging

Logging operations

Logging operations at Site U1418 began after completion of RCB operations in Hole U1418F at 0730 h (local) on 7 July 2013 to a total depth of 948.7 m DSF. In preparation for logging, a short wiper trip was conducted from 830 to 945 m DSF, the hole was flushed with a 50 bbl sweep of high-viscosity mud, and the RCB bit was released. The pipe was pulled to 98.6 m DSF. Three tool strings were deployed in Hole U1418F during logging operations: the triple combo tool string, the FMS-sonic tool string, and the VSI tool string (Fig. F48; see “Downhole measurements” in the “Methods” chapter [Jaeger et al., 2014a]). The triple combo tool string was modified by omitting the accelerator porosity sonde because the tool is not designed for higher porosity formations and often overestimates porosity in wide boreholes, both of which were expected at Site U1418 based on Site U1417 logging results (see “Downhole logging” in the “Site U1417” chapter [Jaeger et al., 2014b]).

The first deployment was the triple combo tool string, made up of gamma ray, density, resistivity, and magnetic susceptibility tools. The tool string was lowered into the hole at 1949 h on 7 July. With the tool string held stationary at 75 m below the end of the pipe, the wireline heave compensator settings were tested with the aim of minimizing downhole tool motion. A downlog then proceeded at a speed of ~1800 ft/h to a total depth of 600 m WSF, where it was blocked from continued downhole progress by a bridge in the hole. The tool string was stopped in order to address a problem with the logging winch, and, during troubleshooting, the winch brakes released and ~70 m of cable was spooled out. The tool string remained relatively stationary at the blocked section of the borehole. After fixing the winch and pulling up the spooled cable, the hole was logged upward at a speed of 900 ft/h, recording data from ~570 m WSF uphole. No repeat pass was run because of concerns regarding the cable. The tool string was recovered to the rig floor at 0651 h on 8 July, and the cable was found to have several small kinks but not enough damage to warrant replacement for the duration of logging operations at this site.

The second deployment was the VSI tool string. The tool string was rigged up at 0852 h, and Protected Species Observation began at 0900 h. As no protected species were observed in the 940 m diameter exclusion zone (see “Operations”), the air gun ramp-up began at 1030 h, and the air guns were fired every 5–15 min while the tool string was run into the hole. The tool string passed several borehole obstructions or bridges before encountering a completely blocked zone, reaching a final depth of 218 m WSF. Because of the softness of the relatively shallow sediments, the VSI caliper was not able to get a good clamp, even in the in-gauge intervals of the borehole between 192 and 218 m WSF identified by the triple combo caliper log. Consequently, all recorded waveforms were noisy and none of the shots provided clean first arrivals. The tool string was pulled up and rigged down by 1815 h.

The final logging run in Hole U1418F was the FMS-sonic tool string. The tool string was rigged up and run into the hole at 1932 h on 8 July. A downlog was recorded at 1800 ft/h, reaching a total depth of 582 m WSF. The tool string was blocked from additional downhole progress by a blocked zone, ~20 m shallower than the depth reached by the triple combo tool string. One complete full pass of the hole was recorded at a speed of 1800 ft/h before the tool string became stuck in the pipe while trying to log upward to record the seafloor. The FMS-sonic tool string was retrieved to the rig floor and rigged down by 0733 h, and logging operations were completed by 0830 h on 9 July.

Seas were relatively calm for the duration of logging operations. There was an average heave of 0.85 m (peak to peak).

Data processing and quality assessment

All logging curves were depth-matched using the total gamma ray log from the main pass of the triple combo as a reference log, facilitating the generation of a unified depth scale. Features in gamma ray logs from the other tool string passes were aligned to the reference log to produce a complete depth-matched data set. Logging data were then depth-shifted to wireline log matched depth below seafloor (WMSF), based on the step increase in gamma radiation in the main pass of the triple combo that indicated the seafloor, measured at 3677.5 m wireline log depth below rig floor (WRF).

The quality of the downhole logs was affected by the range in borehole diameter (Figs. F49, F50, F51), which often exceeded the 18 inch limit of the Hostile Environment Litho-Density Sonde caliper arm. The caliper log from the triple combo run shows that borehole size varies from ~10 to >18 inches in diameter (Figs. F49, F50). Between the base of the pipe and 218 m WMSF, the borehole presents large variability in shape and numerous intervals through which the caliper arm was fully extended (i.e., washouts). Between 218 and 460 m WMSF, borehole conditions are better, with many intervals close to or at the drill bit diameter. Deeper than 460 m WMSF, hole diameter increases again in size and variability. The tool strings deployed in Hole U1418F encountered blockages at different depths between the first logging run (triple combo) and the last logging run (FMS-sonic). FMS caliper data confirm that hole conditions were deteriorating over the course of logging operations, as indicated by the appearance of bridges in the borehole that were not apparent in the earlier triple combo caliper log (see ~370, 380, and 487–551 m WMSF in Fig. F51).

As a result of varying borehole conditions, logging data vary in quality. Most of the logs exhibit high variability in the upper 220 m WMSF, especially where the hole is dominated by washouts of up to several meters in height. Data in intervals where NGR, density, and resistivity show inverse patterns with the caliper log should be interpreted with caution, as they likely reflect washed-out intervals (Figs. F49, F50). The presence of washouts, however, is likely to reflect lithology, as some sediment types are more easily washed out than others. Gamma ray, density, resistivity, magnetic susceptibility, and velocity tools seem to record good quality data outside of washed-out intervals. Magnetic susceptibility logs show reasonable responses throughout the borehole, and measurements were repeatable between the downlog and uplog passes of the triple combo. However, magnetic susceptibility measurements display drift with depth likely related to internal tool temperature, as seen in Hole U1417E Magnetic Susceptibility Sonde (MSS) data. As tool temperature increases linearly with depth, we applied the same method for linear correction as for Site U1417 (Fig. F52; see “Downhole logging” in the “Site U1417” chapter [Jaeger et al., 2014b]). Moreover, the factor applied to correct the temperature drift for the magnetic susceptibility measurement was the same for both sites (Fig. F52). That the same linear relationship was observed between susceptibility and temperature at two different sites allows us to be confident in the reliability of this correction, as well as in the quality of magnetic susceptibility data.

The quality of logs can also be assessed by comparison with measurements made on cores from the same site (Fig. F49). NGR from the triple combo shows good agreement with scaled shipboard core logger data, with the exception of the washed-out intervals where gamma ray log values are lower because fewer gamma rays reach the detector in a wider borehole. Between the base of the pipe and 270 m WMSF, logs appear to underestimate bulk density (giving values close to water density in washouts), except in discrete intervals where hole diameter is smaller (Fig. F49). From 270 to 462 m WMSF, density logs and bulk density from MAD samples show good correspondence. Deeper than 462 m WSF, density logs likely underestimate density formation again but trends between logging and core data are similar. Resistivity logs show reasonable responses throughout the borehole, although higher amplitude responses are likely due to increased borehole size. The FMS resistivity images were also dominated by poor contact with the borehole wall in the wide intervals and are generally of intermediate quality, although some well-imaged intervals exist (Fig. F53).

The Dipole Shear Sonic Imager (DSI) recorded P&S monopole, upper dipole, and lower dipole modes during the one full pass in Hole U1418F. To optimize measurements in this slow sediment, the monopole and upper dipole utilized standard (high) frequency and the lower dipole transmitted and received lower frequencies. Data show that the DSI was able to capture both compressional and flexural wave arrivals through much of the borehole (Fig. F51). Shear wave data are limited in the shallowest 220 m WMSF because of patchy coherence, but flexural arrivals are more coherent below this depth. Gaps in coherence in both compressional and flexural data, along with anomalously low gamma ray signals, correspond in large part to washed-out intervals of the borehole. Postcruise processing of the sonic waveforms could improve the velocity data, particularly in intervals where P-wave arrivals cannot be distinguished from fluid wave arrivals.

Logging stratigraphy

The logged interval in Hole U1418F is assigned to one logging unit (Figs. F49, F50, F51) because the character of the logs changes gradually downhole with no major stepwise changes. At the scale of this unit, the total gamma ray signal ranges from 35 to 55 gAPI, with the exception of anomalously low values corresponding to washed-out intervals. The signal is generally dominated by K and Th content, with a minor U contribution (Fig. F50). For the most part, the three radioactive elements behave in a consistent fashion, suggesting that they are mainly responding to clay mineralogy or content. Density measurements do not show distinctive characteristics in the logged interval. Resistivity data slightly decrease with depth, which is counter to the expected increasing trend with depth due to compaction; however, slightly decreased values below 462 m WMSF may simply be a response to the larger borehole diameter in this interval. Magnetic susceptibility data do not trend noticeably downhole. Velocity measurements show a generally increasing trend with depth.

Logging Unit 1

Logging Unit 1 is divided into five subunits, mainly on the basis of subtle changes in the gamma ray, resistivity, and magnetic susceptibility logs (Figs. F49, F50, F51).

Logging Subunit 1A (base of drill pipe to 217 m WMSF)

The upper logging Subunit 1A is dominated by high variability in most logging data, alternating on the scale of meters between washouts and in-gauge borehole diameter (Fig. F49). Natural gamma ray logs track hole diameter, as does the resistivity log. Magnetic susceptibility is relatively low in this subunit. FMS images show alternations between moderate resistivity and very low resistivity (associated with washouts).

Logging Subunit 1B (217–240 m WMSF)

Logging Subunit 1B is distinguished by a slight increase in the baseline of the gamma ray and resistivity logs (Fig. F49). Resistivity data also decrease in amplitude compared to Subunit 1A. Magnetic susceptibility increases significantly within this subunit. FMS images (see US Implementing Organization [USIO] Log Database at iodp.ldeo.columbia.edu/DATA/) show a general increase from moderate to higher resistivity.

Logging Subunit 1C (240–282 m WMSF)

Logging Subunit 1C is distinguished from the subunit above by an increased contribution of U to the total natural gamma ray signal (Fig. F50). The resistivity log shows alternating trends on the scale of meters in this subunit, from low to high amplitude (Fig. F49). The magnetic susceptibility log appears similar to that of Subunit 1A, with lower values than the overlying Subunit 1B. FMS images (see USIO Log Database at iodp.ldeo.columbia.edu/DATA/) show relatively high resistivity values throughout the subunit.

Logging Subunit 1D (282–500 m WMSF)

Gamma ray logs are generally constant with depth, varying around a mean value of 45 gAPI, outside of washouts (Fig. F49). Spectral gamma ray logs show a higher contribution of U relative to the shallower and deeper subunits (Fig. F50), which may indicate a change in lithology or a decrease in the oxidation state of the upper sediment column near the time of deposition. The resistivity log displays no overall trend with depth. The mean magnetic susceptibility value is higher in this subunit, with distinct intervals of higher susceptibility between 379 and 385 m WMSF and at 405, 421, and 480 m WMSF. The interval between 379 and 385 m WMSF corresponds to a diamict interval in lithostratigraphic Unit IIB (see “Lithostratigraphy”), whereas the narrower high-susceptibility intervals have no direct association with distinct features described in cores. The P-wave velocity log shows an interval of low velocity at the logging Subunit 1C/1D boundary but returns to a generally increasing trend with depth within Subunit 1D. FMS images are highly variable, ranging from moderate to high resistivity (Fig. F53).

Logging Subunit 1E (500–570 m WMSF [base of logged interval])

Logging Subunit 1E is distinguished from the shallower subunits in most of the logging measurements by the addition of a lower frequency trend with a decameter wavelength (Fig. F49). NGR counts decrease at ~520 m WMSF and then an increase from 535 m WMSF to the base of the logged interval. The density log shows a similar trend that is repeated in the MAD bulk density data from cores despite the washed-out intervals (Fig. F49). The resistivity log trends with gamma ray and density logs. Resistivity decreases deeper than ~550 m WMSF. The magnetic susceptibility log trend is generally inverted relative to the trends in gamma ray and resistivity. The P-wave velocity log indicates that velocities in this subunit are lower than those in Subunit 1D, but gaps in the data prevent the distinction of additional trends (Fig. F51). FMS images (see USIO Log Database at iodp.ldeo.columbia.edu/DATA/) show variability between high and low resistivity, which may reflect the development of bridges (high resistivity and small borehole diameter) between the washed-out zones (low resistivity and large borehole diameter) within this depth interval over the course of logging operations.

Formation MicroScanner images

Despite the variability in borehole size and the presence of washed-out intervals, the FMS resistivity images reveal some local transitions between alternating resistive and conductive intervals at the meter to submeter scale. Some of these alternations correspond to the resistivity log from the triple combo (Fig. F53) and may reflect changes in sediment electrical properties related to changes in lithology from mud to diamict (see “Lithostratigraphy”). FMS resistivity images also show local features, such as individual clasts at depths corresponding to intervals with large clasts described in Site U1418 cores (Fig. F54; see “Lithostratigraphy”). In general, however, the FMS resistivity images remain dominated by poor contact with the borehole wall in the wide, washed-out intervals of the logged section (seen as dark bands that correspond to large-diameter intervals) and what may be inadequate compensation for downhole tool motion (evidenced by blurry images, even in regions of relatively small borehole size).

Vertical seismic profile

The VSI tool string was deployed in Hole U1418F in hopes of conducting a vertical seismic profile (VSP) experiment focused on in-gauge intervals of the borehole identified by the triple combo caliper log. However, the tool string was obstructed at ~218 m WSF either by a blocked zone or by the base of the tool string lodging in a washed-out portion of the borehole wall. The VSI was not able to progress deeper into the hole where the formation was more indurated and/or lithified. The VSI tool string is relatively small and light compared to other standard IODP tool strings (i.e., VSI: ~12 m length, ~525 lb in fluid; FMS-sonic: ~33 m length, ~1900 lb in fluid), which is important for maximizing the anchoring force directed into the borehole wall by the VSI caliper during a VSP. However, this may also prevent the lighter tool string from passing narrow spots in the borehole and/or intervals of abrupt change in borehole diameter that can be worked past by heavier tool strings, as was the case in Hole U1418F. In the shallow interval of the hole that was accessible during the VSI run in Hole U1418F, it was difficult to achieve sufficient anchoring force with the VSI caliper. The caliper was most likely pressing into the soft formation to its maximum extent without establishing a solid clamp, and therefore the sensor package in the sonde could not be fully isolated from the tool string. Although 35 shots were fired during the VSP at approximately six depth stations, all the sonic waveforms were noisy and no clear first arrivals could be distinguished.

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