Previous   |    Next

doi:10.2204/iodp.proc.330.105.2012

Downhole logging

Downhole logging measurements obtained from Hole U1374A include natural total and spectral gamma ray, density, neutron porosity, electrical resistivity, electrical images, P-wave velocity, acoustic images, and three-component magnetic field. Following some initial issues with rock bridges in the borehole and an extensive period of borehole preparation, an open hole section of 393.9 m was logged with four tool strings.

Operations

Downhole logging of Hole U1374A started once rotary coring to a total depth of 522 mbsf ended on 18 January 2011 at 0545 h (all times are New Zealand Daylight Time, Universal Time Coordinated [UTC] + 13 h). The drill pipe was initially set at 101 mbsf for logging. For details on hole preparation for logging, please see “Operations.”

In a first attempt, the triple combination (triple combo) tool string was lowered into Hole U1374A at 1101 h on 19 January. The wireline heave compensator was optimized while the tool string was in the pipe because of a possible drill pipe sticking issue. At 1622 h, following some effort, the run was aborted because of the occurrence of a rock bridge in the hole immediately below the drill pipe. The tool was returned to the surface and was rigged down by 1855 h. To remove the bridging issue in the hole, mud was circulated in the hole and the pipe was run down to 143.6 mbsf. The pipe was set slightly deeper for a second logging attempt at 110.8 mbsf to avoid the rock bridge. The triple combo was run into the hole at 0037 h (20 January), but again numerous tight spots were encountered and only a wireline log depth below seafloor (WSF) of 139 m was reached, only 28.52 m below the end of pipe. Logging was aborted at 0249 h, and the tools were rigged down at 0400 h. A short string comprising only the Hostile Environment Natural Gamma Ray Sonde (HNGS) and the Hostile Environment Litho-Density Sonde (HLDS) caliper was made up, but again logging was unsuccessful and the tools were rigged down by 0745 h.

Because Hole U1374A was the deepest hole drilled during Expedition 330 at the old end of the Louisville Seamount Trail, it was decided that one final attempt should be made at a successful logging program. All drill pipe was tripped to the surface and then run back down with a standard bit and mechanical bit release (0815–1730 h). The hole was washed and reamed from top to bottom and swept with 35 bbl of sepiolite mud. The hole was displaced with 165 bbl of 1.26 g/cm³ heavy mud (barite), the pipe was set at 128.1 mbsf, and the bit was released at the bottom of the hole. This significantly improved hole conditions, allowing four tool strings to be deployed in Hole U1374A: (1) the triple combo, (2) the Göttingen Borehole Magnetometer (GBM), (3) the Formation MicroScanner (FMS)-sonic, and (4) the Ultrasonic Borehole Imager (UBI). For tool and measurement acronyms, see “Downhole logging” in the “Methods” chapter (Expedition 330 Scientists, 2012a).

The triple combo tool string, which includes the HNGS, Accelerator Porosity Sonde, HLDS, General Purpose Inclinometry Tool (GPIT), and Dual Induction Tool (DIT), was lowered into the hole at 0545 h on 21 January. The tool string reached the bottom of the hole at 520.6 m WSF, and a first uphole pass started at 0804 h (Fig. F80). The tool string was lowered back down to 170 m WSF, and a repeat pass started at 0952 h. This repeat control pass ended when the tool string crossed the seafloor, marked by a peak in natural radioactivity clearly visible in the HNGS gamma ray measurement. The seafloor was detected at 1570 m wireline log depth below rig floor (WRF), which concurs with the drillers seafloor depth estimate. The triple combo reached the rig floor and was rigged down at 1250 h on 21 January.

The second tool string deployed in Hole U1374A was the GBM (Fig. F81). The GBM was rigged up, and the tool was oriented by simultaneously sighting the tool along the ship’s long axis while recording the position of two GPS receivers of known location and also the ship’s heading using the ship’s main gyro. The GBM began measuring at 1325 h on 21 January while still on the rig floor and was lowered into the hole at 1550 h, taking continuous measurements from the rig floor to the bottom of the hole and back up to the rig floor. The GBM reached the bottom of the hole (2096.4 m WRF) at 2015 h, but unfortunately we lost communication with the GBM at 2250 h (1035 m WRF) on the uplog journey to the rig floor. This will not, however, affect further data processing. The GBM reached the rig floor and was rigged down by 0015 h on 22 January.

The third tool string deployed in Hole U1374A was the FMS-sonic tool string, comprising the HNGS, Dipole Shear Sonic Imager (DSI), GPIT, and FMS (Fig. F82). The FMS-sonic tool string was lowered in the hole at 0015 h on 22 January and reached the bottom of the hole (2090.1 m WRF) at 0310 h. The first uphole logging pass started at 0311 h and ended at 0348 h. After the tool string was returned to the bottom of the hole, the second pass started at 0411 h and ended after reaching the seafloor at 1570.1 m WRF. Rig down was completed at 0723 h.

The fourth tool string deployed was the UBI, comprising the HNGS, GPIT, and UBI (Fig. F83). Because of the quality of the borehole and the tool requirement that the borehole be <30 cm in diameter, only specific optimal sections (five in total) of the hole were targeted. The tool string was lowered in the hole at 0823 h. Section 1 was logged at 1001 h (2086–2076 m WRF). However, in order to check the UBI’s rotating sub, we returned to bottom for a repeat of this section and extended the log accordingly (2087–2046 m WRF). Section 2 (2035–1994.4 m WRF) was logged at 1042 h, Section 3 (1885–1866.6 m WRF) was logged at 1112 h, Section 4 (1827–1790 m WRF) was logged at 1126 h, and finally Section 5 (1775–1730 m WRF) was logged at 1147 h. The last imaged section was completed by 1208 h, and the tool string was rigged down by 1420 h on 22 January.

The final tool string deployed was the GBM for its second full pass. The GBM was rigged up, and the tool was again oriented by simultaneously sighting the tool along the ship’s long axis and recording the position of two GPS receivers of known location and the ship’s heading using the ship’s main gyro. The tool began its measurement on the rig floor at 1439 h and was lowered into the hole at 1500 h. The tool reached a total depth of 2090 m WRF at 1759 h and began its journey back up to the surface with no major communication problems. The tool was back on the rig floor at 2100 h and rigged down by 2144 h on 22 January following a final tool sighting and orientation check, at which time logging operations in Hole U1374A were completed.

Data processing and quality assessment

The standard logging data were recorded on board the R/V JOIDES Resolution by Schlumberger and archived in DLIS format. Data were sent via satellite transfer to the Borehole Research Group of the Lamont-Doherty Earth Observatory, processed, and transferred back to the ship for archival in the shipboard database. Processing and data quality notes are given below. The GBM data were acquired from the tool using GBMlog software and processed with GBMdatenverarbeitung software (see “Downhole logging” in the “Methods” chapter [Expedition 330 Scientists, 2012a]).

Depth shifts applied to the logging data were performed by selecting 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 aligning features in equivalent logs from other tool string passes by eye. In the case of Hole U1374A, the base log was the gamma ray profile from Pass 1 (main) of the triple combo (HNGS-HLDS-GPIT-DIT) tool string. The original logs were first shifted to the seafloor (1570 m WRF), which was determined by the step in the gamma ray value, consistent with the seafloor depth given by the drillers.

Proper depth shifting of wireline logging depths relative to core depths was essential to correlate downhole logging data with all other measurements and observations made on core recovered from Hole U1374A. The seafloor was the only target that offered potential wireline logging depth references. However, it should be noted that data acquired through the seafloor resulted from logging through the bottom-hole assembly (BHA), so data from this interval are of poor quality and highly attenuated and should only be used qualitatively. However, they are adequate to pick out the seafloor. The quality of wireline logging data was assessed by evaluating whether logged values were reasonable for the lithologies encountered and by checking consistency between different passes of the same tool. Specific details of the depth adjustments required to match logging runs/data are available in the logging processing notes on the log database for Hole U1374A (iodp.ldeo.columbia.edu/DATA/).

A wide (>30 cm) or irregular borehole affects most recordings, such as the HLDS (bulk density), which requires decentralization and good contact with the borehole wall, and the UBI, which requires a borehole diameter of <30 cm. Density logs typically correlate well with resistivity logs, but this correlation is strongly affected by borehole conditions. Borehole diameter was recorded by the hydraulic caliper on the HLDS tool and by the FMS calipers. The FMS calipers only show one area in the hole where the borehole is >40 cm in diameter (~363–374 m wireline log matched depth below seafloor [WMSF]), and thus there are large areas where the borehole is “in gauge.” These areas were targeted for UBI logging (2087–2046 m WRF; 2035–1994.4 m WRF; 1885–1866.6 m WRF; 1827–1790 m WRF; and 1775–1730 m WRF; Fig. F83). Good repeatability was observed between the main and repeat passes of the triple combo and FMS-sonic tool strings, particularly for measurements of electrical resistivity, gamma ray, density, and compressional wave velocity (V_P) (Fig. F84).

Bulk density (HLDS) data were recorded with a sampling rate of 197 measurements per minute (2.54 cm at 300 m/h), in addition to the standard sampling rate of 32 measurements per minute (15.24 cm at 300 m/h). The enhanced bulk density curve is the result of the Schlumberger enhanced processing technique performed on the MAXIS system on board the JOIDES Resolution. In normal processing, short-spaced data are smoothed to match long-spaced data (depth matched and resolution matched); in the enhanced processing, the raw detail obtained from the short-spaced data is added to the standard compensated density (Flaum et al., 1987). Where there is good contact between the HLDS pad and the borehole wall (low-density correction) the results are improved because the short spacing has better vertical resolution (i.e., it has the capability to resolve thinner beds/units).

The DSI was operated in the following modes: P&S monopole and upper dipole for both the downlog and Pass 1 (all with standard frequency). For Pass 2 the DSI was operated in the two above-mentioned modes in addition to Stoneley. The slowness data from DTCO and DT2 (see Table T12 in the “Methods” chapter [Expedition 330 Scientists, 2012a]) are of good quality for these passes and were thus converted to acoustic velocities (VCO and VS2, respectively). Reprocessing of the original sonic waveforms is highly recommended to obtain more reliable velocity results.

The FMS images are of excellent quality over the entire hole, with the exception of a very short interval between 363 and 374 m WMSF, where the borehole diameter is larger than the 40 cm limit of the FMS calipers. This prevents some FMS pads from being in direct contact with the formation, resulting in poor image resolution or, in this case, dark images. Hence, FMS images (and the high-resolution resistivity logs) should be used with caution in this short depth interval.

Preliminary results

Downhole logging measurements obtained from Hole U1374A include natural total and spectral gamma ray, density, neutron porosity and electrical resistivity, electrical images, P-wave velocity, acoustic images, and three-component magnetic field. The results are summarized below.

Electrical resistivity measurements

Two main electrical resistivity curves were obtained with the DIT: deep induction phasor-processed resistivity (IDPH) and medium induction phasor-processed resistivity (IMPH). The IMPH and IDPH resistivity profiles represent different depths of investigation into the formation (76 and 152 cm, respectively) and different vertical resolutions (152 and 213 cm, respectively). The DIT reached the bottom of the logged interval in Hole U1374A because it was the bottommost tool in the logging tool string (Fig. F80). The IDPH measurement is the most reliable for lithologic interpretation because it has a deeper investigation depth and hence is less influenced by drilling-induced fractures and the presence of mud cake in the borehole.

Both resistivity curves show considerable variability throughout the hole (Fig. F84). Below 128.1 m WMSF Hole U1374A is composed mostly of breccia units interlayered with more massive basaltic lava flows/intrusions as thick as 3.12 m. Values of IMPH range from 2.16 to 965.24 Ωm, and IDPH values range from 9.48 to 1866.36 Ωm. Higher resistivities seem to relate to more solid layers in the breccia and well-cemented breccia units. Some of the highest resistivities correlate to intrusive units observed in the recovered core (e.g., lithologic Units 139 and 140; ~492 m WMSF). Two notable areas of low resistivity are consistent across both resistivity curves. These are located at 370 and 480 m WMSF (log Units VI and VIII; see “Log units”). The former relates to a washed out zone in the borehole, which indicates a more friable, poorly cemented breccia (stratigraphic Unit XVII), and the latter relates to a frothy, glassy lithology (stratigraphic Unit XIX), observed in the recovered core from Hole U1374A.

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1374A with the HNGS (see Table T12 in the “Methods” chapter [Expedition 330 Scientists, 2012a]). The total gamma ray measurements through the BHA show one main anomaly from the seafloor to ~20 m WMSF (log Unit I), and the open hole gamma ray measurements covered a total of 361.9 m downhole (log Units III–IX). The shorter overall coverage of this tool compared with the DIT is the result of the topmost position of the HNGS in the tool string (Fig. F80).

Gamma ray measurements in basaltic oceanic crust are typically low (e.g., Bartetzko et al., 2001; Barr et al., 2002), and the lithologic units penetrated and logged in Hole U1374A follow this trend. Total spectral gamma ray values obtained with the HNGS in Hole U1374A range from 11.93 to 37.98 gAPI, with a mean of 25.29 gAPI. Potassium values are relatively high, with values ranging between 0.27 and 1.55 wt% and a mean of 0.73 wt% (Fig. F85), which correlates well with values obtained on the core (0.53–1.20 wt%; see “Geochemistry”). Uranium values range between 0.002 and 2.46 ppm and have a mean of 0.52 ppm. In contrast, thorium values are relatively high, ranging from 0.7 to 4.49 ppm, with a mean of 2.63 ppm. Areas of elevated potassium, such as at ~270–300 m WMSF, relate to a zone of higher neutron porosity and illustrate a higher degree of fracturing and moderate alteration (see “Alteration petrology”). An area of elevated thorium seems related to a frothy and glassy altered unit (in stratigraphic Unit XIX) at ~470–490 m WMSF, where there are also high porosity values. The peak in uranium at ~410 m WMSF is closely associated with the occurrence of a number of intrusive units (Fig. F85) and also coincides with an increase in potassium. Comparison of measurements of natural gamma radiation in counts per second taken on whole-round cores with downhole gamma ray data shows good agreement (Fig. F85).

Density

Density ranges from 1.35 to 2.94 g/cm³ in Hole U1374A (Fig. F84). Comparison between discrete physical property samples and the downhole density log shows good agreement (Fig. F84). Low density values correspond to both intervals with enlarged borehole dimensions and sections that exhibit high porosities (Fig. F84). Pronounced high density values relate mostly to intrusive units, lava lobes(?), and well-cemented breccia zones (e.g., lithologic Units 139 and 140 at 492 m WMSF in stratigraphic Unit XIX).

Neutron porosity

Neutron porosity ranges from 4.92% to 78.34%, with a mean of 29.09%. Neutron porosity correlates very well (inversely) with density, resistivity, and velocity measurements. Additionally, there is good agreement between moisture and density porosity measurements (see “Physical properties”) and downhole logging porosity data (Fig. F84). Overall, moisture and density porosity values are slightly lower, but this is because the measurement is biased in the respect that it does not take into account fractures in the formation. In contrast, downhole measurements give an overall in situ porosity measurement for the entire formation. There is a trend of lower neutron porosity with higher density, resistivity, and V_P (Fig. F84). Two main high-porosity zones are clear in Hole U1374A, and these correlate to the oversized borehole at 360–380 m WMSF and the frothy hyaloclastite material in stratigraphic Unit XIX at 470–490 m WMSF. Low resistivity, density, and velocity were also observed in these two depth intervals.

Elastic wave velocity

V_P ranges from 2.67 to 7.11 km/s, with trends in the data correlating very well with discrete physical property measurements (Fig. F84). There is a clear relationship between V_P and density, porosity, and resistivity, whereby high V_P correlates with high resistivity and density and low porosity. Some peaks in V_P can be related to lava lobes or intrusive units found in the breccia sequences. The lowest V_P values can be related to two main zones at 360–380 m WMSF (oversized borehole region) and 470–490 m WMSF (stratigraphic Unit XIX; frothy unit). Compressional wave velocity measured on discrete samples taken on the core correlates well with downhole logging data (Fig. F84). However, velocity values obtained with logging give an overall value for the formation measured (including fractures and clast and matrix mixtures), unlike measurements on core, which are taken on specific discrete cube samples.

Magnetic field measurements

Two different tools were used to obtain magnetic data in Hole U1374A. The GPIT was run as part of both the FMS-sonic and UBI tool strings, whereas the GBM was run on its own dedicated tool string. Magnetic field data in the drill pipe were only collected by the GBM and were heavily affected by the magnetized pipe, which saturated the magnetometers.

Borehole deviations

Both the GPIT and the GBM measure the deviation of the borehole from vertical, and the results of these measurements are shown in Figure F86 (note that the logs shown are depth shifted to match the depth of GBM Run 1). The deviation differs slightly between the tool strings, primarily because of their different lengths and geometries. Because the GBM tool string is shorter, it shows more pronounced kinks in the deviation caused by washouts in the formation (e.g., at 370 mbsf), and its mean deviation is shallower than that of the longer tool strings. The mean deviation measured by the GBM is 2.64° ± 0.32°, whereas the mean deviation measured by the GPIT is 3.16° ± 0.32°. The borehole azimuth for Hole U1374A ranges from 235°–250°. This range is determined by the GPIT using the magnetic field measurements. However, these values are influenced by the magnetic/magnetized neighboring components on the sonde’s associated tool string, and thus this range is approximate.

Magnetic measurements

The magnetic logs of the GPIT and GBM show the same trend within the formation and correlate well with the lithology observed in the core recovered from Hole U1374A. The raw magnetic field data of the different tools are shown in Figures F87 and F88. These data were corrected for all sensor errors (sensor offsets, scale factors, and errors in orthogonality) but not for borehole deviation. This means that the horizontal and vertical fields presented here are not yet aligned to the Earth’s reference frame. The horizontal magnetic components in the GPIT and GBM agree well, but the vertical components have differing offsets. This is mainly caused by the influence of magnetized components above the GPIT magnetometers in both the FMS and UBI tool string (Figs. F82, F83).

To reduce the magnetic influence of the tool string on the magnetometers of the GBM, a nonmagnetic aluminum sinker bar was custom-made for this expedition and run directly above the tool (Fig. F81). Thus, the magnetic field data measured with the GBM are more reliable and cleaner than those obtained with the GPIT. The influence of the Schlumberger sinker bar and the centralizer was found to be on the order of 350 nT (total field) during extensive testing in Houston, Texas (USA), in August 2010. The aluminum sinker bar reduces this influence to <50 nT by almost doubling the distance between the magnetometers and the magnetized/magnetizable parts of the tool string. This positive effect can be clearly seen in the difference in the vertical component of the magnetic field measured by the GPIT and the GBM shown in Figure F88. In the case of the GPIT, the surrounding magnetized parts of the associated tool string have an influence on the inclination of the magnetic field measured by this tool (Fig. F89). The actual magnetic field inclination in the borehole is steeper than the average magnetic field inclination given by the GPIT. The UBI tool string, which measures a lower vertical component than the FMS tool string (Fig. F88), also shows a shallower inclination. We expect that a more accurate inclination will be derived from the GBM data (postexpedition).

Determination of rotation history: fiber-optic gyros

In addition to the fluxgate sensors, three angular rate sensors measured the rotations of the GBM. These data will be used to reorient the recorded magnetic field to the Earth’s reference frame. Figure F90 shows the accumulated rotation angle for all gyros during both GBM runs. The data are not yet corrected for the rotation of the Earth. Nevertheless, the Rz gyro already provides an approximate rotation history about the vertical axis of the tool. During the first run, the GBM turned ~27 times counterclockwise in the downlog and started turning clockwise during the uplog. During the second run the GBM turned ~19 times counterclockwise during the downlog and ~11 times counterclockwise (negative) during the uplog. In the pipe, rotations were generally more frequent during the downlog and less pronounced during the uplog. In open hole, the tool hardly rotated at all (a total of ~1 turn). The speed during the first run was ~900 m/h in the drill pipe, compared to >1150 m/h during the second run; increasing the speed seemed to slow the rotation of the tool. The lower rates of rotation in the open hole are likely caused by the influence of the heavy drilling mud and the rougher surface of the open hole.

Figure F91 shows the raw magnetic data for the first GBM run together with the lithology from the core recovered from Hole U1374A. The GBM does not record data against depth, but both magnetic field and depth were recorded against time, and later these data sets were combined. The log was depth shifted to the approximate core depth but has not been matched to the other downhole logs (this will be done postexpedition).

Because of the influence of the drill pipe, the uppermost ~130 m of the hole could not be measured with the GBM. The magnetic influence of the pipe at the start of the open hole section is strongest on the vertical axis, but this influence significantly decays by ~10 m below the end of the pipe. The horizontal component is less disturbed. Because of the way the hole was prepared for logging, a drill bit was present at the bottom of the hole, and as a result both the horizontal and vertical components are disturbed at the lower end of the logs to ~5 m above the bottom of the hole.

Several igneous units observed in core recovered from Hole U1374A were identified in the magnetic data. Examples of these are the basalt intrusive sheets or dikes at 175 and 231 mbsf. Stratigraphic Unit XV can also be clearly separated from the surrounding intervals in the magnetic data (see “Igneous petrology and volcanology” for more details of this interval). At ~460 mbsf the vertical field strength drops considerably downhole, coinciding with a change from a hyaloclastite matrix to an interval of large jigsaw-fit clasts of aphyric basalt. Further detailed investigation of the data collected with the GBM will focus on separating and identifying the magnetic signals of the different flow units and determining the inclination and declination of the natural remanent magnetization, with the intention of determining the virtual geomagnetic pole position and the paleolatitude of the Louisville hotspot.

Log units

Preliminary interpretation of the downhole log data divided Hole U1374A into a number of logging units (Fig. F84). Logging units in the section covered or partially influenced by the BHA were interpreted on the basis of downhole gamma ray. Logging units in the open hole section were characterized using the resistivity, density, porosity, and velocity logs.

Two logging units were identified in the section covered by the BHA (Fig. F84):

• Log Unit I (seafloor to 20 m WMSF) shows a significant increase in total gamma ray measurements.

• Log Unit II (20–128.1 m WMSF) shows generally low gamma ray values, with small increases where a sedimentary sequence is present (~65–85 mbsf; stratigraphic Unit IX).

The sequence in open hole was divided into seven additional logging units (Fig. F84):

• Log Unit III (128.1–240 m WMSF) has fluctuating values for density, resistivity, porosity, and velocity, which are related to the interbedded massive lava/intrusions and breccia units in the sequence recovered at this depth interval in stratigraphic Unit XIII. Additionally, the GBM data show strong fluctuations in both horizontal and vertical magnetic field throughout this logging unit.

• Log Unit IV (240–278 m WMSF) has more consistent values for density, velocity, porosity, and resistivity. Resistivity values are slightly higher (~100 Ωm) in this unit than in other logging units and are related to the lower ~16 m of stratigraphic Unit XIII and stratigraphic Unit XIV.

• Log Unit V (278–358 m WMSF) is characterized at its top by a dramatic decrease in resistivity and a general change from the previous log unit, with greater variability observed in all characterizing log data. A significant increase in the vertical and horizontal magnetic field components of the GBM can also be seen at the top of this log unit. This elevation in magnetic field values relates to stratigraphic Unit XV, and the overall log unit relates to stratigraphic Units XV and XVI.

• Log Unit VI (358–380 m WMSF) shows a marked decrease in density, resistivity, and velocity values and an increase in porosity, which correlates to a band of breccia with green alteration and an enlarged section of the borehole (stratigraphic Unit XVII).

• Log Unit VII (380–469 m WMSF) exhibits characteristics similar to log Unit V, with relatively consistent density of ~2.0 g/cm³ in conjunction with higher stable resistivity, lower porosity, and higher velocity values. Peaks in density and velocity correlate to the presence of intrusive sheets or dikes in the surrounding brecciated formation (stratigraphic Unit XVIII).

• Log Unit VIII (469–490 m WMSF) is characterized by a significant decrease in resistivity, density, and velocity and an increase in porosity. The top of this log unit is also marked by a significant positive kick in the GBM data, which is seen in both the horizontal and vertical magnetic field components. This correlates with frothy altered hyaloclastite material in stratigraphic Unit XIX.

• Log Unit IX (490–507 m WMSF) represents the lower limit of the logging data acquired. However, the top of this unit shows a marked increase in density, velocity, and resistivity and a decrease in porosity. Additionally, the top of this log unit is marked by both a large negative kick in the horizontal magnetic field component and a large positive kick in the vertical magnetic field component measured by the GBM. These significant changes correlate well to intrusive units recovered in the core at this depth interval (stratigraphic Unit XIX). Immediately beneath log Unit IX there is a marked increase in resistivity (manifested as a double peak) that correlates well with lithologic Unit 146 (aphyric basalt).

Electrical and acoustic images

In Hole U1374A we also acquired FMS electrical resistivity images (Figs. F92, F93) and UBI acoustic images (Fig. F94). The quality of electrical resistivity image measurements depends on close contact between the measuring pads on the tool and the borehole wall. The FMS borehole images are of high quality throughout the borehole (except between 363 and 374 m WMSF, where the borehole was enlarged), and they accurately reproduce breccia and massive igneous unit layering. UBI images require a borehole with a maximum diameter of 30 cm, and therefore only portions of the borehole were imaged with this tool. However, where images were taken, a good virtual hardness of the entire borehole wall was obtained to complement and enhance the information collected with the FMS (Fig. F94).

Figure F92 gives examples of the quality of the FMS images obtained at different depths in Hole U1374A. The images highlight clast-rich breccia areas compared with more matrix-dominated zones. Moreover, they show texture in more massive layers and help refine lithologic boundaries where contacts are not recovered in the core. Another strength of FMS imagery is that the images are oriented to geographic north (using the GPIT), and by picking sinusoidal traces on the images one can obtain important oriented structural information on key boundaries, fractures, and other features of interest (Fig. F93). Extensive structural picking is a key part of postexpedition research. Figure F93 shows some provisional sinusoid picks that highlight some of the high-angle relationships observed in adjacent units and lithologies. Such structural picks can aid overall interpretation of the lithologic sequence observed in the core.

Top of page   |    Previous   |    Next