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doi:10.2204/iodp.proc.330.107.2012

Downhole logging

Downhole logging measurements obtained from Hole U1376A include natural total and spectral gamma ray, density, neutron porosity, electrical resistivity, electrical images, P-wave velocity, and three-component magnetic field. An open hole section of 101.9 m was logged (starting at 80.4 mbsf) with three tool strings over a period of ~21 h. The borehole remained in good condition throughout logging, and no tight spots were encountered.

Operations

Downhole logging of Hole U1376A started on 2 February 2011 at 1720 h (all times are New Zealand Daylight Time, UTC + 13 h) after rotary coring ended at a total depth of 182.8 mbsf. For details on hole preparation for logging, see “Operations.”

Three tool strings were deployed in Hole U1376A: (1) the triple combination (triple combo), (2) the third-party Göttingen Borehole Magnetometer (GBM), and (3) the Formation MicroScanner (FMS)-sonic. The triple combo tool string, which included the Hostile Environment Natural Gamma Ray Sonde (HNGS), Accelerator Porosity Sonde (APS), Hostile Environment Litho-Density Sonde (HLDS), General Purpose Inclinometry Tool (GPIT), and Dual Induction Tool (DIT) (see “Downhole logging” in the “Methods” chapter [Expedition 330 Scientists, 2012a] for tool string details), was lowered into the hole at 2323 h on 2 February. The wireline heave compensator was optimized with the triple combo in open hole at 1675.1 m wireline log depth below rig floor (WRF). The tool string reached its target depth of 1696.6 m WRF at 0139 h on 3 February and began a first uphole pass at 0143 h (Fig. F57). The tool string was lowered back down for a full repeat pass, which started at 0243 h (following a 15 min break between measurements in order to allow the formation to equilibrate following neutron activation). This repeat control pass ended when the tool string crossed the seafloor, which was marked by a peak in natural radioactivity clearly visible in the HNGS gamma ray measurement. The seafloor was detected at 1514 m WRF, which agrees well with the drillers seafloor depth estimate of 1514.3 mbrf. The triple combo was returned to the rig floor and rigged down at 0511 h.

The second tool string deployed in Hole U1376A was the GBM (Fig. F58). The GBM was rigged up and 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 (see “Downhole logging” in the “Methods” chapter [Expedition 330 Scientists, 2012a] for details on sighting). The GBM began measuring at 0531 h and was lowered into the hole at 0554 h on 3 February. Unfortunately, this run had to be aborted because the drill pipe was stuck in the borehole, and the tool was brought back to the rig floor at 0631 h. The GBM was not rigged down but was held in one of the available storage holes. After the drill pipe was freed and the hole further conditioned, the GBM was rigged back up, sighted, and began taking measurements by 0733 h. The tool string was run into the hole at 0759 h and reached its target depth of 1696.7 m WRF at 0956 h. The GBM ran without error and was returned back to the rig floor, resighted, and rigged down by 1218 h on 3 February.

The third and final tool string deployed in Hole U1376A was the FMS-sonic combination, which was composed of the HNGS, Dipole Shear Sonic Imager (DSI), GPIT, and FMS (Fig. F59). The FMS-sonic tool string was lowered into the hole at 1338 h on 3 February and reached the bottom of the hole (1695.8 m WRF) at 1454 h. The first uphole logging pass started at 1455 h and ended at 1519 h. After the tool was returned to the bottom of the hole, the second pass started at 1531 h and ended at 1558 h (1475.9 m WRF) after passing the seafloor at ~1514 m WRF. Rig down was completed at 1755 h on 3 February, at which time logging operations in Hole U1376A 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 archiving 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 GBM datenverarbeitung software (see “Downhole logging” in the “Methods” chapter [Expedition 330 Scientists, 2012a]).

Depth shifts applied to 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 by aligning features in equivalent logs from other tool string passes by eye. In the case of Hole U1376A, 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 (1514 m WRF), determined by the first clear step in the HNGS gamma ray values.

Proper depth shifting of wireline logging depths relative to core depths was essential to correlate the downhole logging data with all other measurements and observations made on core recovered from Hole U1376A. The seafloor was the only target that offered a potential wireline logging depth reference. However, note 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 U1376A (iodp.ldeo.columbia.edu/DATA/).

A wide (>30.5 cm) or irregular borehole affects most recordings, particularly by tools like the HLDS (bulk density) that require decentralization and good contact with the borehole wall. The density log correlates well with the resistivity logs but is largely affected by hole conditions. Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the FMS calipers. Both calipers show a mostly “in gauge” borehole with a diameter varying between 24 and 31 cm and very few enlarged portions. Some small breakouts were observed at ~142, ~145, and ~171 m WRF; however, these were never out of the range of the FMS caliper arms (40 cm diameter). Good repeatability was observed between the main and repeat passes of the triple combo and FMS-sonic, particularly for measurements of electrical resistivity, gamma ray, density, and compressional wave velocity (V_P) (Fig. F60). However, the repeat gamma ray data (obtained with the triple combo) are offset to higher values because the advised 15 min wait time between runs was insufficient to allow activation of the borehole formation (from the neutron source used for porosity measurements in the main run) to sufficiently decay.

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). In a situation 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) (see “Downhole logging” in the “Methods” chapter [Expedition 330 Scientists, 2012a], for a full list of acronyms). Postexpedition 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, and all the images collected downhole in Hole U1376A can be used with confidence. Caliper measurements from the FMS show that the pads should have maintained good contact with the borehole wall.

Preliminary results

Downhole logging measurements obtained from Hole U1376A include natural total and spectral gamma ray, density, neutron porosity, electrical resistivity, electrical images, compressional velocity, 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 U1376A because it was the bottommost tool in the logging tool string (Fig. F57). The IDPH measurement is the most reliable for lithologic interpretation because of its deeper investigation depth and hence is less influenced by drilling-induced fractures and the presence of drilling mud cake.

Below 80.4 m wireline log matched depth below seafloor (WMSF) Hole U1376A is composed of two main petrologically distinct units: a 33 m thick massive basaltic lava flow (stratigraphic Unit III), which is found down to ~110 m WMSF, and stratigraphic Unit IV, which is dominated by breccia and interlayered more massive basaltic lava flows and a couple of intrusive sheets or dikes with recovered thicknesses up to 8.96 m. IMPH generally ranges from 3.29 to 579.18 Ωm, and IDPH ranges from 8.0 to 404.8 Ωm (Fig. F60). Some of the highest resistivity values correlate to more solid layers in the breccia (e.g., at ~145 m WMSF in stratigraphic Unit IV) and the massive basalt flow unit (stratigraphic Unit III). Both resistivity curves show considerable variability throughout the hole (Fig. F60). However, there is a notable downhole contrast in resistivity, consistent across both resistivity curves, at ~122.5 m WMSF (log Unit VIII; see “Log units”) in stratigraphic Unit IV, which relates to a more visibly conductive (from FMS images) hyaloclastite breccia observed in the recovered core from Hole U1376A.

Gamma ray measurements

Standard, computed, and individual spectral contributions from ⁴⁰K, ²³⁸U, and ²³²Th were part of the gamma ray measurements obtained in Hole U1376A (Fig. F60) with the HNGS (see Table T12 in the “Methods” chapter [Expedition 330 Scientists, 2012a]). Downhole open hole gamma ray measurements cover a total of 75.49 m of the hole. The shorter overall coverage of the HNGS compared with the DIT results from the topmost position of the HNGS in the tool string (Fig. F57).

The lithologic units penetrated and logged in Hole U1376A follow the same low natural gamma ray trend seen in previously measured basaltic crust (e.g., Bartetzko et al., 2001; Barr et al., 2002) (Fig. F61). Total gamma ray (HSGR) values obtained with the HNGS in Hole U1376A range from 10.68 to 29.26 gAPI, with a mean of 18.93 gAPI. Potassium values are relatively low, with values ranging between 0.27 and 0.85 wt% and a mean of 0.54 wt% (Fig. F61). This is a much smaller range than that obtained using inductively coupled plasma–atomic emission spectroscopy on core samples (0.47–1.56 wt%; see “Geochemistry”) and is largely explained by the fact that the downhole logging HNGS sonde takes measurements every 15 cm uphole compared to the very targeted discrete samples used for geochemical analysis (which typically avoid alteration, veins, vesicles, and voids). Uranium values range between 0.0037 and 0.7941 ppm and have a mean of 0.36 ppm. In contrast, thorium values are relatively high, ranging from 0.076 to 3.27 ppm, with a mean of 1.97 ppm. Potassium abundance drives the general downhole correlation seen in total gamma ray, with some greater influence by uranium at ~105 and ~145 m WMSF. Areas of elevated potassium, such as ~108–138 m WMSF, relate to the upper portion of stratigraphic Unit IV, where there is a downhole disappearance of augite and the onset of an olivine-dominated basaltic section of mostly breccia—in combination with a large quantity of fresh volcanic glass (see “Alteration petrology”). Two coinciding peaks of potassium and uranium at ~105 and ~145 m WMSF correlate well to two lava flow units. A peak in uranium at ~112 m WMSF is closely associated with the occurrence of the intrusive unit or dike (Fig. F61) of lithologic Unit 20. Finally, comparison of natural gamma radiation measured on whole-round cores with downhole gamma ray data shows good agreement (Fig. F61).

Density

Density ranges from 2.08 to 3.45 g/cm³ in Hole U1376A (Fig. F60). A comparison between discrete physical property samples and the downhole density log shows good agreement. Low density values correspond to intervals with larger borehole dimensions and sections that exhibit much higher porosities (Fig. F60). Pronounced high density values relate mostly to intrusive units, lava flows, and a large 33 m thick massive basalt unit (stratigraphic Unit III) (see log Unit IV, below).

Neutron porosity

Neutron porosity ranges from 3.99% to 57.70%, with a mean of 23.09%. 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. F60). Overall, moisture and density porosity measurements are slightly lower, but this is because they are biased in the respect that they do not take into account fractures in the formation, whereas downhole measurements provide an overall in situ porosity measurement for the entire formation. A general trend of slightly increasing neutron porosity was observed from ~122 to 145 m WMSF. There is a general correlation of lower neutron porosity with higher density, resistivity, and compressional wave velocity (V_P) (Fig. F60). One main high-porosity zone is clear in Hole U1376A, correlating to a transition to breccia from massive basalt (at the bottom of stratigraphic Unit III) and a change to low density and velocity values.

Elastic wave velocity

Compressional wave velocity (V_P) ranges from 2.88 to 6.89 km/s, with trends in the data correlating well with discrete physical property measurements (Fig. F60). There is a clear relationship between V_P and density, porosity, and resistivity, with V_P being higher where resistivity and density are higher and porosity is lower. Peaks in V_P can be related to the large section of massive basaltic lava in stratigraphic Unit III and lava/intrusive units found in the breccia sequences (stratigraphic Unit IV). The lowest V_P values can be related to large fracture zones in the massive basalt and within the brecciated units. Compressional wave velocity measured on discrete samples taken from the core correlates well with downhole logging data. It should be noted that velocity values obtained by downhole logging give an overall value for the formation measured, including fractures and clast and matrix mixtures, and are therefore generally lower.

Magnetic field measurements

Two different tools were used to obtain magnetic field data in Hole U1376A. The GPIT was run as part of the FMS-sonic tool string, 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 F62. The deviation differs slightly between the tool strings because of their different lengths and geometries. The mean deviations from vertical measured by the GBM and GPIT are 3.83° ± 0.44° and 4.38° ± 0.26°, respectively, and are within error.

The approximate value for the borehole azimuth in Hole U1376A is 92° ± 2°. This value is determined by the GPIT using magnetic field measurements. However, these values are influenced by the magnetic/magnetized neighboring components on the sonde’s associated tool string, and thus this value for azimuth and its uncertainty are only preliminary.

Magnetic measurements

The magnetic logs of the GPIT and GBM show the same trend in the formation. Raw magnetic field data of the different tools are shown in Figures F63 and F64. These data were corrected for all sensor errors (sensor offsets, scale factors, and errors in orthogonality) but not for the deviation from vertical of the tool string of ~4° (see above). This means that the shown horizontal and vertical fields are not yet entirely aligned to the Earth’s reference frame. The horizontal magnetic components measured by the two instruments show only small differences, but the vertical components have noticeably different offsets. This is mainly caused by the influence of magnetized components above the GPIT magnetometers in the FMS-sonic tool string (Fig. F59).

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. F58). 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 done in Houston, Texas (USA), in August 2010. The aluminum sinker bar lowers this influence to <50 nT by almost doubling the distance between the magnetometer and the magnetized/magnetizable parts of the tool string. 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 sonde (Fig. F65). The actual magnetic field inclination in the borehole should be steeper than the average magnetic field inclination given by the GPIT. 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 F66 shows the accumulated rotation angle for all gyros during the GBM run. 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. The tool turned ~25 times counterclockwise during the downlog and 8 times clockwise during the uplog. In the open hole, the tool hardly rotated at all (a total of ~1 turn). The low rates of rotation in the open hole are likely caused by the stabilizing effect of the drilling mud and the rougher surface of the open hole.

Figure F67 shows the raw magnetic data from the GBM run together with the lithology from the core recovered from Hole U1376A. The GBM does not record data against depth, but both magnetic field and depth were recorded against time, allowing the data to be combined. The log was depth shifted to the approximate core depth but has not yet been matched to the other downhole logs.

The drill pipe extends to 80.4 mbsf. Its magnetic influence decays within ~10 m below the pipe and mostly affects the vertical component. Because of the way the hole is prepared for logging, a drill bit is present at the bottom of the hole; hence, both the horizontal and vertical components are disturbed at the lower end of the logs for approximately the bottommost 5 m. Remarkably, the thick augite-phyric lava flow in stratigraphic Unit III is not as homogeneous in the GBM data as in the other logging data (Fig. F60) or the physical property and paleomagnetic data collected on the cores (see “Physical properties” and “Paleomagnetism”). Nevertheless, the magnetic data (particularly the vertical component) have an anomaly at ~105 mbsf that appears to coincide with the breccia recovered at the base of lithologic Unit 15. These findings will be further examined postexpedition. In addition, Cores 330-U1376A-17R and 18R had low recovery (42% and 9%, respectively), and the magnetic data over this interval show large variations on the order of 2000 nT, which will also be investigated in more detail postexpedition. The section between 112 and 130 mbsf is very homogeneous in the vertical field, with only two sharp peaks.

Further detailed investigations 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 estimating the virtual geomagnetic pole position and the paleolatitude of the Louisville hotspot.

Log units

Preliminary interpretation of the downhole log data divided Hole U1376A into a number of log units (Fig. F60). Log units in the section covered or partially influenced by the BHA were interpreted on the basis of downhole gamma ray, whereas log units in the open hole section were characterized using the resistivity, density, porosity, and compressional wave velocity logs.

Three log units were qualitatively identified in the section covered by the BHA (Fig. F60):

• Log Unit I (seafloor to ~26 m WMSF) shows a significant increase in total gamma ray measurements.

• Log Unit II (~26–40 m WMSF) shows a steep decline to extremely low gamma ray values. This log unit correlates well to stratigraphic Subunit IIA, an algal limestone formation.

• Log Unit III (~40–80.4 m WMSF) shows slightly higher gamma ray values compared to log Unit II. This depth range correlates to a section of conglomerates, volcanic breccia, and the top of the main massive basaltic lava unit. This log unit extends to the end of the drill pipe, where one can see how attenuated the signal is (as it moves through a petrologically similar unit) compared to the open hole section.

The sequence in open hole below the BHA was divided into 10 additional log units (Fig. F60):

• Log Unit IV (80.4–104 m WMSF) has consistently high values for density, resistivity, porosity, and velocity, related to a massive basalt lava unit in stratigraphic Unit III. Lower values of density, resistivity, and velocity with higher porosity values correlate to significant conductive fractures in the basalt unit (which can be seen in FMS images).

• Log Unit V (104–108 m WMSF) exhibits significantly lower values of density and velocity, moderate resistivity, and higher porosity and gamma ray values compared with the previous log unit. Log Unit V relates to a relatively narrow band of breccia found toward the base of stratigraphic Unit III. The GBM data sharply increase in both the vertical and horizontal field in the transition to the brecciated zone.

• Log Unit VI (108–109.5 m WMSF) shows increases in density and velocity values and decreases in natural gamma ray and porosity. Additionally, the GBM data show a prominent decrease in the magnetic field in the vertical component. This correlates to the lowermost basalt lava unit at the very base of stratigraphic Unit III.

• Log Unit VII (109.5–123 m WMSF) exhibits some small stepped increases in resistivity. Density increases very slightly downhole in this unit and porosity decreases, whereas velocity remains fairly constant. A small peak in density, resistivity, and velocity at the top of this log unit relates to lithologic Unit 18 (an intrusion). The overall trend in the data relates to the breccia unit at the top of stratigraphic Unit IV.

• Log Unit VIII (123–145 m WMSF) exhibits some of the lowest density, resistivity, gamma ray, and velocity measured in the open hole in Hole U1376A. This log unit primarily correlates to a portion of the hole with poor core recovery (~130–144 mbsf). The aphyric basalt unit at ~128 mbsf is visible in the logging data by a small peak in density, resistivity, and velocity and a small trough in porosity. The log unit shows the section to be variable and correlate to interlayered massive and brecciated units. Overall, porosity steadily increases throughout this log unit to the maximum observed in Hole U1376A.

• Log Unit IX (145–147 m WMSF) has high gamma ray, density, resistivity, and velocity and low porosity. In addition, the GBM data show a decrease in the vertical component of the magnetic field and an increase in the horizontal component of the magnetic field. This log unit correlates directly to a massive olivine-phyric basalt unit in stratigraphic Unit IV.

• Log Unit X (147–158 m WMSF) has overall higher values of density and resistivity than those in log Unit VIII. There is considerable variability in the velocity data and an overall decreasing trend in porosity.

• Log Unit XI (158–160 m WMSF) exhibits high density, resistivity, and velocity and low porosity, which correlates to a massive olivine-phyric basalt unit in stratigraphic Unit IV.

• Log Unit XII (~160–163 m WMSF) is differentiated using only density, resistivity, porosity, and velocity. This log unit exhibits a significant decrease in density, resistivity, and velocity and an associated increase in porosity, which correlates well with a band of heterolithic breccia in stratigraphic Unit IV.

• Log Unit XIII (163–168 m WMSF) was defined using only density and resistivity. This log unit exhibits higher density and resistivity values and correlates very well to two massive aphyric basalt units in stratigraphic Unit IV.

Beneath log Unit XIII only resistivity data are available. Values for resistivity are high immediately beneath log Unit XIII and correspond to the top of lithologic Unit 41, a dike intrusion of aphyric basalt.

Electrical and acoustic images

In Hole U1376A, we also acquired FMS electrical resistivity images (Figs. F68, F69, F70). 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 collected in Hole U1376A are of very high quality owing to the borehole being in gauge throughout the logged section.

The greatest utility of FMS imagery comes when recovery is low. In Hole U1376A core recovery between ~130 and 144 mbsf was very poor. Using FMS imagery and other standard log data, we can make a competent attempt at filling the data gap in order to have a continuous record downhole. Figure F68 shows FMS images over the entire unrecovered section. It is not possible to determine the petrology of the lithology, but the structural makeup is clear. Toward the top of the unrecovered section (130–131 m WMSF) there appears to be a solid banded unit with a concentrated band of vesicles toward its base (Fig. F68B). Beneath this potential flow, the formation appears very blocky (131–133 m WMSF) (Fig. F68C) with some angular blocks ~50 cm long. A portion of very conductive and brecciated material beneath occurs down to ~136 m WMSF, where a highly resistive band of a possible lava lobe (Fig. F68D) is present. Toward the base of the unrecovered section there is a 2 m interval of more resistive brecciated material, followed by a more conductive, resistive-clast-sparse breccia (Fig. F68E). Overall, the formation in this unrecovered section is variable and appears to contain a large amount of conductive sections, which most likely relate to more porous, brecciated materials interlayered with more resistive, massive basalt layers or lava lobes.

The FMS images also highlight some of the key features observed in the core recovered from Hole U1376A (Fig. F69), including massive flow units, intrusive contacts, and smaller features such as clasts and vesicles. Additionally, the FMS images help refine lithologic boundaries when contacts are not recovered in the core. Another strength of FMS imagery is the fact 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. F70). It is possible to differentiate between picked fractures (be they conductive or resistive) and bed boundaries. Extensive structural picking is a key part of postexpedition research. Figure F70 shows some provisional sinusoid picks that highlight some of the true dips and azimuths observed in a number of the stratigraphic and lithologic units. From these preliminary picks one can obtain a true dip for the top surface of lithologic Unit 41 (an intrusion) of 34.5° and a true azimuth of 140.3° (Fig. F70D; note that at this surface the hole deviation is 4° toward 94.1° and has been corrected for hole deviation). Additionally, we observe variable conductive fracture dips and orientations in the massive basalt unit (Fig. F70A; from top to bottom: true dip 46.3° toward true azimuth 289.4°, 43.8° toward 334.6°, 46.3° toward 55.8°, 63.4° toward 279.9°, 46.2° toward 246.2°, and 13.1° toward 320.2°). Such structural picks can aid overall interpretation of the lithologic sequence observed in the core and be used to produce a stress regime model for the drilled formation.

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