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doi:10.2204/iodp.proc.341.103.2014

Downhole logging

Logging operations

Logging operations for Site U1417 began after completion of RCB operations in Hole U1417E at 1315 h (local) on 20 June 2013 to a total depth of 709.5 m DSF. In preparation for logging, the hole was flushed with a 50 bbl sweep of high-viscosity mud and the RCB bit was released. The pipe was pulled to a depth of 81.4 m DSF. Four tool strings were deployed in Hole U1417E during logging operations: the triple combo, the FMS-sonic, the MSS, and the VSI (Fig. F51; see “Downhole logging” in the “Methods” chapter [Jaeger et al., 2014]).

The first deployment was the triple combo tool string, which was made up of gamma ray, porosity, density, resistivity, and magnetic susceptibility tools. The tool string was lowered into the hole at 2120 h on 20 June, completing a downlog to a total depth of 624 m WSF, where it was blocked from downhole progress by a bridge in the hole. The main uplog pass was then conducted at a speed of 900 ft/h and ran up through the pipe and past the seafloor. The tool string was run back down the hole for a short repeat pass from 225 m WSF at a speed of 1800 ft/h.

The second deployment was the FMS-sonic tool string, after an attempt to run the VSI tool string was postponed because of the presence of marine mammals (see “Operations”). The string was run into the hole at 1132 h on 21 June and reached a total depth of 571 m WSF, unable to pass a bridged section of the hole. Two full passes of the hole were made from 571 m WSF: the first at a speed of 1200 ft/h while recording all Dipole Shear Sonic Imager (DSI) modes (monopole compressional, upper dipole, lower dipole, crossed dipoles, and Stoneley) and the second at 1800 ft/h while recording only standard DSI modes (monopole compressional, upper dipole, and lower dipole). The tool string was rigged down by 2230 h.

The MSS tool string was run next in Hole U1417E. This run was the first at-sea attempt to deploy the full MSS-B tool (comprising the deep-reading and high-resolution magnetic susceptibility sensors). The tool string was rigged up at 2240 h and run into the hole, reaching a final depth of 204 m WSF. The high-resolution MSS sensor has a stiff bowspring to keep it eccentralized, which may have prevented the tool string from passing the bridges in shallow sections of the hole. Two full uplog passes were recorded from 204 m WSF, and the tool was rigged down by 0530 h on 22 June.

The last tool run was the VSI tool string. Protected Species Observation began at first light on 22 June, and the air gun ramp-up began 1 h later, as no protected species were observed in the 940 m diameter exclusion zone for this site (see “Operations”). The air guns were positioned ~7 m below the sea surface for the vertical seismic profile (VSP) in Hole U1417E. The rig-up of the VSI tool string started at 0545 h, and the air guns were fired every 5–15 min as the tool string was run into the hole. At 0830 h, the tool string reached a final depth of 218 m WSF; considerable efforts to reach deeper depths were unsuccessful. It was difficult to get a good clamp with the VSI caliper arm because of the irregular borehole diameter, and many of the recorded seismic waveforms were noisy. Two of the station locations, both closer to 211 m WSF, yielded reasonable first arrival times. At 1305 h, the tool string was run back up the hole. The tool string was rigged down by 1551 h, and logging operations were complete by 1605 h on 22 June.

Seas were relatively calm for the duration of logging operations. The average heave was 0.5 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, allowing a unified depth scale to be produced. 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 the seafloor as identified by a stepwise increase in the gamma ray value, leading to wireline log matched depth below seafloor (WMSF). The triple combo main pass identified seafloor at 4200 m water depth.

The quality of the downhole logs was affected by the range in borehole diameter, estimated by the hydraulic caliper on the Hostile Environment Litho-Density Sonde (HLDS) and by the FMS calipers (Figs. F52, F53). The caliper logs show an irregular shape through much of the open borehole (ranging from <5 to ≥18 inches [the maximum extent of the HLDS caliper arm]), with up to 13 inches of horizontal range over just a few meters vertical depth. All four tool strings deployed in Hole U1417E were blocked at various depths in the borehole by narrow sections or bridges. Significant problems occurred at 242, 246, and 624 m WSF for the triple combo and 300 and 366 m WSF for the FMS-sonic.

As a result of varying borehole conditions associated with rugosity and borehole diameter, logging data vary in quality. Most logs exhibit anomalous high-frequency variability in the upper 305 m WMSF section of the hole, where caliper logs indicate that the hole is dominated by thin washouts and bridges, but data quality improves deeper than 305 m WMSF. In general, gamma ray, resistivity, magnetic susceptibility, and P-wave velocity were the least affected by variable hole diameter. Features in gamma ray data associated with very wide or very narrow borehole diameters should be treated with caution; fewer gamma rays reach the tool detector in a wider borehole, and more reach the detector where the borehole is narrower. Density and porosity were highly affected in the upper 305 m WMSF, with average density values close to water density and anomalously high porosity (close to 100%) through most of the interval (Fig. F52). Magnetic susceptibility logs show reasonable responses throughout the borehole (see “Magnetic susceptibility logs”), although these logs exhibit a clear downhole drift presumably related to tool temperature.

The quality of the logs can also be assessed by comparison with measurements made on cores from the same site (Fig. F52). Total natural gamma ray from the triple combo show good agreement with scaled NGR core logging data from the base of the pipe to ~220 m WMSF. Deeper than 220 m WMSF, the two data sets show similar trends but the core NGR data are offset, likely due to the change in coring technique at ~220 m WMSF (from the APC system to the XCB system) that can result in a lower volume of sediment being counted (see “Physical properties”). Density logs appear to underestimate formation density in the shallowest 305 m WMSF interval where borehole diameter is highly variable, as shown by comparison with the core-based MAD data shown in Figure F52. However, these data show good correspondence deeper than 305 m WMSF, where the borehole condition and log quality are improved. Porosity logs are anomalously high throughout the hole, as compared to MAD porosity data. The Accelerator Porosity Sonde was not designed for high-porosity formations (>50–60 pu) and often overestimates porosity in wide and rugose boreholes; therefore, these data should not be considered in interpretation. Resistivity logs show reasonable responses throughout the hole, despite poor borehole conditions, and postcruise density and porosity estimates may be attempted with caution from resistivity using Archie’s relationship (Archie, 1942).

FMS data quality relies on a number of factors, including an in-gauge hole, regular borehole walls, and good contact between the tool’s pads and the borehole wall. Processing of the FMS image data allows a speed correction to be applied that takes account of variations in the speed of the tool, including stick and slip, measured by the General Purpose Inclinometry Tool incorporated into the tool string. Two processing methods were applied to the speed-corrected images. Static processing normalizes the entire measured resistivity range for the full depth of the borehole to allow for assessment of large-scale resistivity variations. Dynamic processing rescales the color intensity over a sliding 2 m depth window to highlight local features. Despite borehole conditions, FMS images seem to be of good quality, with the pads making contact with the borehole walls through much of the logged interval. Large depth shifts (>0.5 m) were needed through much of the borehole, so the absolute depth reference for these images (wireline log speed-corrected depth below seafloor; WSSF) should be considered with caution.

The DSI recorded P&S monopole, upper dipole, and lower dipole modes in Hole U1417E. To optimize sonic velocity measurements in this sediment, the monopole and upper dipole utilized standard (high) frequency and the lower dipole transmitted/received at a lower frequency. The resulting slowness data were subsequently converted to acoustic velocities (V_P [monopole] and V_S [upper and lower dipole]). In Figure F53, distinctive orange-red areas in the V_P and V_S tracks indicate greater coherence in recorded sonic waveforms, and blue colors indicate little or no coherence. These data show that although the DSI was able to capture both compressional and flexural arrivals in the deeper intervals of the hole (deeper than 305 m WMSF), there was no coherence for flexural arrivals in the shallower section, and thus no V_S data were recorded.

In addition, at some depths between the base of the pipe and ~260 m WMSF, the automatic picking of the wave arrivals (black curve in Fig. F53) failed to recognize the compressional wave because of its proximity to the fluid wave in this very slow formation, and thus no V_P data were recorded. Postcruise processing could refine the V_P profile in the shallowest 305 m WMSF to provide a better estimate of compressional velocity in the poorly picked interval.

Logging stratigraphy

Downhole logging data for Hole U1417E are summarized in Figures F52, F53, F54, and F55. The logged interval is divided into two logging units primarily on the basis of borehole condition. Logging Unit 2 is further divided into two subunits on the basis of trends and distinctive features in the gamma ray, resistivity, magnetic susceptibility, and sonic logs.

Logging Unit 1 (base of drill pipe to 305 m WMSF)

Logging Unit 1 is characterized by highly variable borehole diameter, ranging from <5 inches to the 18 inch limit of the HLDS caliper arm. This irregular borehole shape has an influence on the responses of all logging tools, so the logs in this interval may be compromised by the poor borehole condition. Within this unit, the gamma ray, magnetic susceptibility, and P-wave velocity logs appear to display coherent character despite centimeter-scale noise (Figs. F52, F53, F54). The gamma radiation signal is coherent between the triple combo and FMS-sonic tool strings and ranges from 10 to 66 gAPI. The signal shows a relatively consistent trend from the base of the pipe to 255 m WMSF, with a mean value of 38 gAPI. Between 255 and 280 m WMSF, borehole diameter changes dramatically, and all logs are dominated by the effects of hole size. Deeper than 280 m WMSF, the gamma radiation signal varies around higher values (typically ~50 gAPI) relative to the shallower section of the hole. Gamma radiation is dominated by the radioactivity of potassium and thorium, with uranium contributing a relatively minor component (Fig. F54). The potassium and thorium curves follow similar patterns throughout this unit and may be tracking clay content, as both potassium and thorium are found in clay minerals. Isolated peaks in thorium at 165 and 235 m WMSF may be indicative of volcanic ash beds or interbedded sand/silt layers.

Both resistivity (Fig. F52) and P-wave velocity (Fig. F53) logs show slightly increasing trends with depth in logging Unit 1. Local peaks in P-wave velocity between ~200 and 305 m WMSF may be correlated with sand/silt layers.

Magnetic susceptibility data from Hole U1417E generally exhibit lower frequency variations from the base of the drill pipe to 280 m WMSF, with the exception of local peaks (Fig. F55). High-resolution data show many centimeter-scale features with elevated susceptibility in logging Unit 1; most have sharp boundaries, but several features have gradational boundaries. Deeper than 280 m WMSF, the amplitude of the background variability in susceptibility increases slightly.

FMS images (example intervals in Fig. F56) show that logging Unit 1 is generally more conductive than Unit 2. Within Unit 1 there are sharply bounded submeter to multiple meter-scale alternations between highly resistive and highly conductive layers. A distinct interval of higher resistivity observed in the FMS images between ~260 and 305 m WMSF coincides generally with the increase in gamma ray signal in this unit (Fig. F52).

Logging Unit 2 (305–624 m WMSF)

Logging Unit 2 is distinguished from Unit 1 primarily by improved condition of the borehole wall. Consequently, the logging data are of better quality throughout Unit 2. Although there are still thin washed-out intervals and bridged sections, there are also intervals between 305 and 624 m WMSF in which the borehole is nearly in gauge (~10 inches). Logging Unit 2 is divided into two subunits.

Logging Subunit 2A (305–476 m WMSF)

Logging Subunit 2A is characterized by total gamma ray values ranging from 25 to 66 gAPI (mean = 45 gAPI) (Fig. F54). The gamma ray signal is dominated by Th and K content, similar to the signal in logging Unit 1. A slightly higher contribution from U in Unit 2 may indicate a greater content of organic matter. The density log gradually increases with depth in this subunit (Fig. F52).

Resistivity (Fig. F52) and P-wave velocity logs (Fig. F53) show similar trends, generally increasing in value with depth in Subunit 2A. P-wave velocity increases from ~1500 m/s at the top of the subunit to >2000 m/s at the base. Local resistivity peaks correspond in most cases to P-wave velocity peaks, suggesting that both are responses to lithologic variations.

Magnetic susceptibility logs (Figs. F52, F55) display strong variability, with higher amplitude changes than in the overlying unit. Local peaks may indicate the presence of increased detrital sediments and/or centimeter-scale silt beds within a lower susceptibility background. FMS images show that Subunit 2B is a transition zone between a more conductive shallower formation and more resistive deeper formation (Fig. F56). These images highlight an alternation between sharp and gradational contacts at the meter scale.

Within Subunit 2A, there is a distinctive interval between ~409 and 425 m WMSF in which many of the logs show unusual behavior. Gamma radiation, density, and resistivity values decrease sharply within this interval (Fig. F52) and then increase at the base, whereas P-wave increases throughout the interval (Fig. F53). The magnetic susceptibility signal drops significantly until the base, which coincides with a susceptibility peak (Fig. F52). The FMS images show a moderately resistive interval, with a highly resistive layer at the base. Together, these data suggest that there is a discrete layer of unusual character at this depth, with corresponding characteristics in core physical properties data (Fig. F52). However, there is no clear evidence of such a distinct feature observed in cores.

Logging Subunit 2B (476–624 m WMSF)

Logging Subunit 2B is mainly characterized by higher values in gamma radiation and density (Fig. F52). The upper boundary of this subunit is distinguished by an abrupt change in the character of magnetic susceptibility (Fig. F52) to lower amplitude variability and a step decrease in P-wave velocity (Fig. F53). The mean gamma ray value is 52 gAPI, and the downhole patterns in U, Th, and K measurements correspond within this subunit (Fig. F54). A change in the ratio of Th and K to the total gamma ray counts relative to Subunit 2A may indicate a change in the source of radioisotopic inputs this interval.

Density values are slightly higher than in Subunit 2A, increasing to >2.0 g/cm³ at the base of the logged interval (Fig. F52). The P-wave log shows a sharp decrease to ~1600 m/s at 480 m WMSF, followed by a general increase to the base of the logged interval (Fig. F53). Within Subunit 2B, magnetic susceptibility (Fig. F52) displays low to moderate amplitude variation but generally higher values from 540 to 580 m WMSF, which coincides with a general increase in gamma ray and density logs. FMS images display very resistive material with few sharp contacts and few conductive zones.

Magnetic susceptibility logs

Site U1417 marks the first at-sea deployment of the full MSS (officially MSS-B), built by Lamont-Doherty Earth Observatory between 2010 and 2012 to replace an earlier version of the tool. Comprising both a deep-reading sensor (MSS-DR), which was also incorporated into the triple combo, and a high-resolution sensor (MSS-HR), the full MSS tool has the capability to make magnetic susceptibility measurements at 10 and 40 cm vertical resolution.

In Hole U1417E, deep-reading magnetic susceptibility measurements were repeatable between multiple passes of the triple combo tool string (the first logging run), as well as with the MSS tool string (the third logging run) (Figs. F51, F55). High-resolution magnetic susceptibility is also repeatable between multiple passes of the MSS tool string.

The trend superimposed upon all magnetic susceptibility logs is most likely related to internal tool temperature. When the temperature of the MSS increases linearly with depth, for example as observed in the MSS data from all passes with the triple combo tool string, a simple linear temperature correction was applied. This correction was also applied to MSS data from the downlog with the MSS tool string but not to MSS data from the uplog passes because of evidence of nonlinear tool temperature effects. Figure F55 shows the uncorrected and corrected MSS-DR data for the main pass of the triple combo, as well as the similarly corrected high-resolution downlog measurement of the MSS run.

The high-resolution magnetic susceptibility signal appears to track natural gamma ray variation (Fig. F55). The small (<1 m) vertical offset observed could come from the discrepancy between the vertical resolution of the Hostile Environment Natural Gamma Ray Sonde (~20–30 cm) and the MSS-HR sensor (~10 cm). As expected, the high-resolution magnetic susceptibility data seem to record more fine-scale features than either gamma radiation or deep-reading susceptibility measurements. For example, at 116 m WMSF, the high-resolution data show multiple higher amplitude peaks, whereas the deep-reading data show only one broad peak in accordance with different vertical resolutions of the two sensors (see “Downhole logging” in the “Methods” chapter [Jaeger et al., 2014]).

In general, the trends and high values of magnetic susceptibility from the deep-reading and high-resolution measurements in Hole U1417E are the same. Postcruise investigations using core-based magnetic susceptibility data from Site U1417, for which a composite splice exists to ~220 m CCSF-D, should help to understand temperature response and improve the calibration of the MSS, as well as better resolve high-resolution susceptibility variations at this site.

Formation MicroScanner images

Despite the rugosity of the borehole wall associated with high-frequency changes in borehole diameter, FMS resistivity images reveal differences in textures and lithologies throughout the logged interval of Hole U1417E (Fig. F56). In the statically processed images from FMS-sonic Pass 1, the submeter-scale alternating intervals of conductive and resistive lithologies characterizing logging Unit 1 are shown in contrast to the moderately resistive to highly resistive units in logging Subunit 2A and the highly resistive lithologies in logging Subunit 2B (example sections in Fig. F56). Based on the images, most if not all of the bedding in the logged interval has horizontal to subhorizontal orientation. The boundaries between different layers are generally distinct, though there are more sharp boundaries observed in Unit 1 than in Unit 2.

Vertical seismic profile and sonic velocity

One objective of the expedition was to establish the age and lithologic origin of the seismic reflections identified in the seismic survey data in the Gulf of Alaska. The VSP provides a good intermediate step in integrating the core and wireline logging data (recorded in depth) with seismic data (recorded in two-way traveltime).

Data acquired during the VSP are summarized in Table T16 and Figures F51 and F57. Although >45 shots were fired, most of the sonic waveforms recorded downhole during the VSP were noisy because of the limited extent of accessible open hole and the rugose borehole wall through this interval. The VSI caliper arm had a difficult time achieving good clamping force because of the soft formation and poor contact with the rough borehole wall. Despite these limitations, two stations close to 211 m WMSF provided check shot arrival times. Figure F57 shows the waveforms measured by the vertical direction geophone of the VSI and the direct arrivals. Table T16 lists the values of the measured and corrected arrival times. The measured traveltimes are the differences between the arrival of the acoustic pulse at a hydrophone located directly below the air gun array and the arrival at the borehole receiver. The corrected traveltimes are the traveltimes from the sea surface to the borehole receiver, which account for the depth of the air guns (7 m below sea level for Hole U1417E) and for the depth of the hydrophone below the air guns (2 m).

Approximating a linear trend, sonic velocities (between 150 and 557 m WMSF, the interval with the best P-wave data quality) increase downhole at ~0.15 km/s per 100 m. The similarity of the resistivity log to the sonic velocity log (Fig. F53) indicates that a pseudosonic log, constructed from the resistivity data to extend to ~615 m WMSF and calibrated with discrete P-wave measurements from cores to extend to the base of the hole, could potentially be used as input for a synthetic seismogram.

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