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doi:10.2204/iodp.proc.341.107.2014

Downhole logging

Logging operations

Logging operations for Site U1421 began after completion of APC/XCB operations in Hole U1421A, drilled to a total depth of 702.7 m DSF at 1910 h on 26 July (Fig. F30). In preparation for logging, a 75 bbl mud sweep was circulated to clean the hole. A go-devil was then pumped down the hole, and the hole was displaced with 50 bbl of high-viscosity mud, followed by 328 bbl of barite-weighted mud (11.8 ppg). The pipe was raised to 96.6 m DSF for logging.

Two tool strings were deployed in Hole U1421A on the basis of potentially unstable borehole conditions and limited time at the end of the expedition: the Sonic-induction tool string and the VSI tool string. The Sonic-induction tool string, also deployed in Hole U1420A, was composed of the Enhanced Digital Telemetry Cartridge, Hostile Environment Litho-Density Sonde (HLDS) without neutron source, Dipole Shear Sonic Imager (DSI), and Phasor Dual Induction–Spherically Focused Resistivity Tool, respectively measuring total gamma ray, borehole diameter, sonic velocity, and resistivity. The second tool string was the VSI tool string, run without the Hostile Environment Natural Gamma Ray Sonde because of concerns about borehole instability.

The Sonic-induction tool string was rigged up at 0130 h on 27 July and run into the hole. A downlog was recorded at a speed of 2800 ft/h, reaching a depth of 699 m wireline log depth below seafloor (WSF) only a few meters above the total drilled depth (Fig. F30). The first uplog pass (the repeat pass) was recorded over a limited depth interval (699–366 m WSF) as a check for the repeatability of logging data. The tool string was run back to the bottom depth of 699 m WSF, and a full complete uplog pass (the main pass) was recorded, ending just above the seafloor. The tool string reached the rig floor and was rigged down at 0845 h.

As the caliper measurement from the first logging run indicated that much of the borehole was <14 inches in diameter and thus suitable for a VSP experiment, the VSI tool string was the second run in Hole U1421A. Protected Species Observation began at 0715 h, and the air gun ramp-up began 1 h later, as no protected species were observed in the 1410 m diameter exclusion zone for this site (see “Operations”). The air guns were positioned ~3 m below the sea surface for the VSP, as the subseafloor targets were relatively shallow, and the hydrophone was suspended 2 m below the air guns. Rig up of the VSI tool string began at 0845 h, and the tool string was run into the hole, reaching total depth at 1150 h. Shots were fired at nine depth stations in the open borehole, and six of the station locations yielded reasonable first arrivals. The recorded waveforms at the shallowest depth stations were noisy, likely because of unconsolidated sediment that prevented the VSI caliper from making firm contact with the borehole wall. The tool string was pulled back into the pipe at the end of the VSP and reached the rig floor at 1549 h. Rig down was complete at 1625 h, which concluded logging operations for Site U1421 and Expedition 341. During logging operations in Hole U1421A, heave ranged from ~0.8 to 2.0 m (peak to peak).

Data processing and quality assessment

Logging data were depth-matched using the resistivity measurement from the main pass of the Sonic-induction tool string as a reference log, allowing a unified depth scale to be produced. Logging data were then depth-shifted to the seafloor reference frame based on the step increase observed in gamma ray from the main pass of the Sonic-induction tool string. The seafloor was recorded at 729 m wireline log depth below rig floor (WRF) and the resulting depth scale is wireline log matched depth below seafloor (WMSF).

Figures F31, F32, F33, and F34 show the main logging data recorded in Hole U1421A. The log data are good quality, likely because of the relatively smooth borehole wall, as shown by the caliper measurements from the Sonic-induction tool string (Fig. F31). With the exception of some thin washouts, borehole diameter varied smoothly and rarely exceeded 18 inches, the limit of the HLDS caliper arm. The character of the borehole wall in Hole U1421A on the upper continental slope is a distinct change from the rugose character observed at the deeper water sites (U1417 and U1418). Above 500 m WMSF, borehole diameter ranges from 10 to 17 inches with an average diameter of ~15 inches. The only exception is one narrow spot indicated by the caliper log at ~300 m WMSF. The hole was nearly in gauge below ~500 m WMSF, with the exception of a washed-out zone between 583 and 591 m WMSF.

The medium and deep resistivity curves show the same trends downhole and are very close in value (Fig. F31), indicating good-quality resistivity data. There is a distinct separation between the shallow resistivity curve and the medium and deep resistivity curves through much of the logged interval. Given that the borehole diameter is within the depth of investigation of all three curves, this separation could indicate that the shallow borehole wall may be invaded by logging mud or imply the presence of a mud cake, which has relatively low resistivity. Although porosity data from Site U1421 cores are limited, relatively high porosity values were measured in recovered material (24%–56%; see “Physical properties”), which would be consistent with shallow mud invasion of the borehole wall during logging operations.

The DSI recorded P&S monopole and lower and upper dipole modes in Hole U1421A, with standard (high) frequency for the monopole and upper dipole and low frequency for the lower dipole. High coherence in sonic waveforms is indicated by orange to red areas in the compressional velocity (V_P) and flexural velocity (V_S) tracks in Figure F31. These data indicate that the DSI was successful in capturing clear compressional arrivals through the entire logged interval. Coherence in the flexural arrivals is spotty in the upper half of the logged interval but higher deeper than ~440 m WMSF.

A comparison between the main pass and repeat pass of the Sonic-induction tool string shows excellent agreement for all measurements (Fig. F32). This agreement provides further evidence that log data quality is good for this site.

Core recovery was limited at Site U1421, preventing a direct comparison with logs for most of the logged interval. However, gamma ray data were recorded through the drill pipe at Hole U1421A in an interval where core recovery was high. The gamma ray measurement is highly attenuated when the logging tool is inside the BHA (above ~96 m WMSF in this hole). Despite the log signal attenuation and a slight (<1 m) depth offset between log and core data, there is still reasonable agreement in NGR between logs and cores (Fig. F33). Variations observed in total gamma radiation could be linked to lithology in this interval, with lower gamma radiation corresponding to diatom-rich sediment in the core and higher gamma radiation corresponding to layers with low microfossil content (see “Lithostratigraphy” for description of diatom-bearing mud).

Logging stratigraphy

The logged interval in Hole U1421A is divided into four units based on distinct changes in character and trends in gamma radiation, resistivity, and P-wave velocity data (Fig. F31). Gamma radiation shows downhole variability but no consistent trend with depth. Resistivity and P-wave velocity show very similar variations through the entire borehole. P-wave velocity generally increases downhole, consistent with a typical compaction trend with depth.

Logging Unit 1 (base of pipe to 202 m WMSF)

Logging Unit 1 does not display any trends with depth for all log curves (Fig. F31). Gamma ray values are relatively high, with an average value of ~43 gAPI. Values decrease slightly from ~170 m WMSF to the base of the unit. Resistivity and P-wave velocity data both vary around mean values, ~3.6 Ωm for medium and deep resistivity and ~1900 m/s for P-wave velocity.

Logging Unit 2 (202–433 m WMSF)

Logging Unit 2 is distinguished by a general decrease in gamma ray values and increased variability in all log data (Fig. F31). Within this unit, gamma ray values range from 26 to 60 gAPI, with an abrupt increase at ~298 m WMSF. Both resistivity and P-wave velocity show two distinct trends within the unit: intervals of high, relatively constant values (e.g., 220–290 m WMSF) separated by intervals of very low values (e.g., 290–302 m WMSF). Deep resistivity values range from 1.7 to 5.2 Ωm. P-wave velocities range from ~1600 m/s in the low intervals to as high as ~2400 m/s.

Logging Unit 3 (433–625 m WMSF)

The boundary between logging Units 2 and 3 is marked by a dramatic decrease in resistivity and velocity (Fig. F31). Logging Unit 3 is characterized by the greatest fluctuations in borehole diameter, gamma radiation, resistivity, and P-wave velocity compared to all other logging units. A series of thin (<5 m) washouts are observed in the shallowest 70 m of this unit, and one thick (~10 m) washed-out interval is recorded in the caliper log between 583 and 591 m WMSF. Gamma ray values range from 17 to 55 gAPI, with the lowest values associated with washed-out intervals. Deep resistivity values range from 1.2 to 6.4 Ωm, and velocities range from ~1600 to 2600 m/s.

Logging Unit 4 (625–699 m WMSF [base of logged interval])

Logging Unit 4 is characterized by distinctly less emergent resistivity and P-wave velocity data (Fig. F31). The average gamma ray value in this unit is 39 gAPI. Deep resistivity values are similar to values in logging Unit 1 and the high-resistivity plateaus in logging Unit 2, with typical values of ~4 Ωm. P-wave velocity is relatively high, with an average value of 2300 m/s.

Vertical seismic profile

The VSP in Hole U1421A establishes a link between core and log data (recorded in depth) and seismic surveys (recorded in two-way traveltime) at the location of the borehole. Data acquired during the VSP are summarized in Table T12 and Figures F30 and F34.

Six out of nine stations yielded reasonable traveltimes, ranging from 1.278 s two-way traveltime below sea level at 284.7 m WMSF to 1.641 s at the deepest station at 687 m WMSF. Many of the seismic waveforms at shallower stations were noisy, but the waveform stacks from these six stations appear to be good. Table T12 lists measured and corrected arrival times. 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. Corrected traveltimes are the times from the sea surface to the borehole receiver and account for the depth of the air guns (3 m below sea level for Hole U1421A) and for the depth of the hydrophone below the air guns (2 m). A linear trend is observed in the traveltime data, consistent with the increase in P-wave velocity with depth (Fig. F31).

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