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Downhole logging

Logging operations

Coring in Hole U1414A ended at 1205 h on 9 December 2012 (all times are local Costa Rica time, UTC – 6 h). Hole U1414A was cored and reentered without problems, and a wiper trip was deemed unnecessary. Ten barrels of heavy mud were pumped to the bottom to stop backflow, the RCB bit was dropped in the hole, and the pipe was raised to logging depth. Rig-up of the wireline system started at 1645 h. Two tool strings were run into Hole U1414A: a triple combo–UBI followed by a FMS-sonic combination. For an illustration of the tool strings deployed in Hole U1414A, explanation of the acronyms, and details on the wireline tools, see the “Methods” chapter (Harris et al., 2013b).

The triple combo–UBI string included (from top to bottom) the Enhanced Digital Telemetry Cartridge (EDTC), Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Litho-Density Sonde (HLDS), High-Resolution Laterolog Array, General Purpose Inclinometry Tool (GPIT), and UBI. Hole U1414A was drilled with an APC/XCB BHA (11 inch bit) from the seafloor to 312 mbsf. The hole was then reentered with a FFF and drilled to 472 mbsf with an RCB BHA (9⅞ inch bit). Because of expected differences in hole diameter, we planned three passes with different UBI settings. Pass 1 covered the entire open hole interval with the UBI set for a 9⅞ inch bit size, a vertical resolution of 0.4 inches, and 180 samples/revolution. Pass 2 was run in the lower part of the hole drilled by RCB with the same settings as in Pass 1 but with a higher vertical resolution of 0.2 inches. Finally, Pass 3 spanned the upper part of the hole drilled by APC/XCB with the UBI set for an 11 inch bit size, a vertical resolution of 0.4 inches, and 140 samples/revolution. The lower number of samples/revolution allows for a longer time window to pick the borehole wall reflection. The UBI frequency was set to 250 kHz in all passes.

The triple combo–UBI tool string started down the hole at 1915 h on 9 December. The seafloor was detected on the gamma ray log at ~2470 mbrf, but the measured natural radioactivity values were very low, and the seafloor could be 1–2 m shallower. In the Hole U1414A cores, there is a sharp transition from sediment to basalt at 375 mbsf (see “Lithostratigraphy and petrology”). A seafloor pick of 2469 mbrf is the same as the driller’s seafloor depth and puts the sediment/basalt boundary at 375 mbsf in the logs. Hence, the mbsf depths in the Hole U1414A logs have been converted from seafloor depth to 2469 mbrf (this corresponds to the wireline log depth below seafloor [WSF] depth scale; see the “Methods” chapter [Harris et al., 2013b]).

After the triple combo–UBI exited the drill pipe at 94 mbsf, its descent stopped when an obstruction was encountered with the base of the tool string at 411 mbsf (61 m above the total depth of the hole). Repeated attempts to pass the obstruction were unsuccessful, and at 2147 h we started to log up. The first pass ended with the base of the tool string at 139 mbsf, which put the top of the triple combo–UBI just below the base of the pipe. The tool string was then lowered back to the obstruction at the bottom. A second pass with a higher vertical resolution was started from 417 mbsf and ended at 311 mbsf, where the borehole became too large to collect useful UBI data. We then moved the tool string down to 331 mbsf, set the UBI to image the larger APC/XCB borehole, and started the third pass at 0038 h on 10 December. During recovery, high tension forced us to stop with the base of the tool string just inside the drill pipe. After pumping to wash the tool string of debris, we were able to resume recovering the tools. The triple combo–UBI reached the rig floor at 0400 h and was rigged down at 0540 h.

The FMS-sonic tool string included (from top to bottom) the EDTC, HNGS, Dipole Sonic Imager, GPIT, and FMS. We planned two passes: a first pass covering the smaller diameter RCB interval at the bottom of the hole followed by a full second pass. The FMS-sonic tool string was rigged up and started down the hole at 0640 h on 10 December. As in the previous run, the obstruction at the bottom was reached with the base of the tool string at 417 mbsf, and we started the first pass at 0905 h. The first pass ended just above the top of the RCB hole at 306 mbsf, and we returned to the bottom for a full second pass. At the end of the second pass, the drill pipe was reentered without problems and the FMS-sonic tool string returned to the rig floor at 1132 h. The wireline system was fully rigged down at 1300 h, ending logging operations for Expedition 344.

Downhole log data quality

The downhole log data collected in Hole U1414A were processed to convert to depth below seafloor and to match depths between different logging runs. The resulting depth scale is wireline matched depth below seafloor (WMSF; see the “Methods” chapter [Harris et al., 2013b]), and from here on “mbsf” denotes the WMSF depth scale.

A key factor that influences downhole log data quality is the size and irregularity of the borehole. Measurements of the diameter of Hole U1414A are summarized in Figure F46. The LCAL track in this figure is measured by a single caliper arm on the HLDS tool (triple combo–UBI tool string), and the C1 and C2 tracks are the hole diameters measured in two orthogonal directions by the two pairs of arms that support the microresistivity pads on the FMS tool (FMS-sonic tool string). As noted earlier, Hole U1414A was drilled with an APC/XCB bit (11 inches) to 312 mbsf and then continued to total depth with an RCB bit (9⅞ inches). The hole is nearly in gauge below 338 mbsf and is as much as ~4 inches larger than the bit diameter in the interval 185–338 mbsf. Above 180 mbsf, the hole is locally enlarged beyond the maximum diameter measurable by the HLDS caliper (~18 inches). In this enlarged hole interval, measurements that rely on good contact with the formation, such as HLDS density, are likely to be unreliable. The notable differences between the hole diameters measured in the two logging runs above ~200 mbsf are discussed below.

The overall quality of the logging data can also be assessed from the match between measurements acquired in different runs or passes. In general, the downhole log data acquired in Hole U1414A show excellent repeatability. Total gamma ray logs measured in different passes of the same tool string are very similar (Fig. F46), and spectral gamma ray data are also consistent (Fig. F47). The only differences are observed where the borehole diameter changed between different logging runs (e.g., around 160 mbsf), and these are discussed next.

Changes in borehole size

Above ~200 mbsf, there are several narrow borehole intervals with obvious differences between the hole diameters measured by the HLDS tool in the triple combo–UBI tool string and the FMS in the FMS-sonic tool string (Fig. F46). These hole size measurements were taken ~11 h apart. The differences in hole size and the observation that the hole diameter is smaller than the bit in these narrow hole intervals suggest that the formation was actively swelling and spalling off. For example, the hole restriction at ~160 mbsf became narrower between the first and second run, and the sharp hole restriction at ~200 mbsf that was measured in the FMS-sonic run was not present in the earlier triple combo–UBI run. Another restriction observed in the triple combo–UBI run at ~145 mbsf disappeared in the subsequent FMS-sonic run, implying that pieces of the swelling formation fell into the hole. These marked changes in hole diameter affected the natural radioactivity measurements: for example, the small total gamma ray peak observed at ~160 mbsf in the triple combo–UBI runs became much more prominent in the FMS-sonic measurements, likely because of the decreased distance between the borehole wall and the detector. Similar changes attributable to changes in borehole diameter can be observed at ~145 and ~200 mbsf both in the total gamma ray (Fig. F46) and spectral gamma ray data (Fig. F47).

Log characterization and logging units

The logging measurements acquired in Hole U1414A are summarized in Figure F48. The total gamma ray data measured by the HNGS (track HSGR in the figure) generally correlate with NGR measurements made on cores. The total gamma ray values measured by the HNGS are expressed in an American Petroleum Institute (gAPI) scale based on a standard artificial formation built to simulate about twice the radioactivity of a typical shale and conventionally set to 200 gAPI (Ellis and Singer, 2007). The units of the NGR measurements made on whole-core sections are in counts per second (cps; for a detailed description of the NGR apparatus, see Vasiliev et al., 2010). The comparison of log and core natural gamma ray measurements in this figure shows that their curves overlap if 1 cps equals ~1 gAPI. The figure also shows tracks of measured bulk density, electrical resistivity, compressional and shear wave velocity, and FMS images. The density, resistivity, and elastic wave velocities show very similar patterns because the variation of these physical properties is similarly controlled by changes in formation porosity and lithology.

We distinguish four logging units in the interval logged in Hole U1414A. Logging Unit 1 (94–259 mbsf) is characterized by total gamma ray values that decrease from between ~40 gAPI at 120 mbsf to ~10 gAPI at the base of the unit. Bulk density, resistivity, and elastic wave velocities are generally low in this unit and show a decreasing trend toward the base of the unit, where bulk density is 1.5 g/cm3, resistivity is 0.5 Ωm, and compressional and shear velocities are 1.6 km/s and 0.4 km/s, respectively. As noted earlier, the low and variable densities observed above 185 mbsf are likely unreliable because of the enlarged borehole. Logging Unit 2 (259–335 mbsf) displays an increase with depth of bulk density (1.6–1.8 g/cm3), resistivity (0.5–1 Ωm), compressional velocity (1.7–2.1 km/s), and shear velocity (0.5–0.7 km/s). Natural gamma ray values are generally 10–30 gAPI. Logging Unit 3 (335–375 mbsf) contains larger variations in physical properties, with gamma ray values ranging between 20 and 60 gAPI, bulk densities between 1.8 and 2.2 g/cm3, resistivities between 1 and 10 Ωm, compressional velocities between 2 and 4 km/s, and shear velocities between 0.5 and 2.7 km/s. Finally, Logging Unit 4 (375–410 mbsf) corresponds to the volcanic basement (see “Lithostratigraphy and petrology”). This basalt interval features very low natural radioactivity (10 gAPI or less) and high bulk density (2.3–2.8 g/cm3), resistivity (2–100 Ωm), compressional velocity (3.2–6.7 km/s), and shear velocity (1.7–3.8 km/s).

Borehole images

Borehole images displaying FMS microresistivity data collected in two runs and UBI reflection amplitudes (related to the small-scale roughness of the borehole wall) are shown in Figure F49. These images span the lower interval logged in Hole U1414A, containing the consolidated sediments in Logging Unit 3 and the basalt of Logging Unit 4. In the sediment interval (340–375 mbsf), the FMS and UBI images show a sedimentary formation with generally horizontal layers, except for an interval between 360 and 370 mbsf where there is evidence of a westward dip. The UBI image also shows vertical fractures oriented ESE–WNW, which could be drilling-induced tensile fractures (Zoback et al., 2003). The top of a steep fracture intersecting the borehole is also visible at 354–355 mbsf. The base of the sediment column at ~375 mbsf is marked by a thin (<0.5 m) borehole enlargement, which has also been measured by the caliper logs (Fig. F46). In the basalt (375–410 mbsf), the image logs show a complex set of fractures.