Hole 1253A was logged with two tool strings, the triple combo and the FMS-sonic string. Previous logging-while-drilling measurements taken during Leg 170 at Site 1039 did not include the sonic or FMS tool strings and reached to only 407 mbsf (Shipboard Scientific Party, 1997). As expected, logging at Site 1253 was technically challenging. The first logging attempt failed to pass beneath 150 mbsf. After the entire sediment section and part of the upper sill was cased (necessary for eventual CORK-II installation), a second logging attempt was made but was stopped at 564 mbsf when the tool string lost communication as a result of a broken Hostile Environment Litho-Density Sonde (HLDS). The subsequent triple combo run and two FMS-sonic runs were successful.
Downhole logging was performed in Hole 1253A after it had been drilled to a depth of 600 mbsf with a
9
-in RCB drill bit. The hole was opened to a depth of 423 mbsf using a 14
-in bit, and a 10-in casing was installed beneath the sedimentary section into the upper sill to a depth of 413 mbsf. The casing was then cemented in place (see "Operations"). The weather was good, with a maximum heave of 1.1 m, and the wireline heave compensator was used throughout the logging operations.
Downhole logging operations began with the deployment of the triple combo tool string (Table T15; Fig. F85). The tool string initially reached a depth of 530 mbsf. However, communication to the tool's power was lost after several attempts to pass the bridge. The triple combo tool string was pulled out of the hole, and it was found that the HLDS was leaking oil. The HLDS was replaced with another density tool (Hostile Environment Litho-Density Tool [HLDT]), and the tool string was run back into the hole. The tools encountered the bridge at the same depth of 530 mbsf, and upward logging was started from that depth.
For the second logging run in Hole 1253A, the FMS-sonic string was deployed. During the first pass, the tools reached the same bridge depth as in the triple combo run (530 mbsf). During the second pass of the FMS-sonic tool string, it penetrated through the 530-mbsf bridge and reached another bridge at 564 mbsf. Upward logging began from 564 mbsf.
Data from the logging runs were transmitted to the LDEO Borehole Research Group (BRG) for depth shifting, environmental correction, and evaluation of the acoustic data. To aid comparison with core observations, all plots and text in this report will use the drillers depth of 4387 m (water depth). However, all data on the "Log and Core Data" CD-ROM assumed a 4390 m seafloor depth, which was determined by LDEO-BRG based on the step in gamma ray values at the sediment/water interface. The FMS-sonic pass 2 was depth matched to the triple combo run using the gamma ray (environmentally corrected gamma ray and Hostile Environment Natural Gamma Ray Sonde [HNGS]) logs and checked with the caliper logs. The gamma ray log from the FMS-sonic pass 1 was depth matched to that from the FMS-sonic pass 2. The HNGS and Scintillation Gamma Ray Tool (SGT) data were corrected for hole size during the recording. The Accelerator Porosity Sonde and HLDT were corrected for standoff and hole diameter, respectively, during the recording.
Enlarged hole size degrades the quality of density measurements if the density pad does not contact the formation. The measurement of neutron porosity is also degraded because of attenuation by the borehole fluids. Gamma ray counts and resistivity may be underestimated in enlarged intervals. Hole diameter was recorded by the hydraulic caliper on the HLDT and by the caliper on the FMS string. The second caliper on the FMS string did not record the diameter correctly. Because of the mechanical problem with caliper 2 in the FMS-sonic tool string, a second pass of the tool string was made. It had a 90° rotation from the orientation of the first pass, thus providing a more complete picture of the borehole shape (Fig. F86).
The caliper data indicate that the hole diameter in the intervals between 417-431 mbsf and 461-541 mbsf was relatively uniform, ranging mostly between 10 to 12 in except for isolated rougher sections (Figs. F86, F87). An interval of increased diameter is observed above 424 mbsf within the upper igneous unit, corresponding to the 14-in hole that was drilled prior to the casing installation. Intervals of very large hole diameter are observed from 433 to 463 mbsf, and the caliper reached maximum extension between 435 and 461 mbsf. These intervals roughly correspond to the sedimentary section observed in cores. Thin intervals of increased hole diameter are present within the section where igneous rock was recovered.
The density correction (DRHO) curve (Fig. F88) is calculated from the difference between the short- and long-spaced density measurements and provides a further indication of the quality of the bulk density data. DRHO values in excess of 0.1 g/cm3 indicate unreliable density data. Intervals of unreliable density data occur between 440 and 458 mbsf, corresponding to high caliper readings and probable poor contact with the sedimentary formation (Fig. F88). In addition, poor data quality is indicated below 513 mbsf by DRHO values of 0.30-0.35 g/cm3. Thinner (1 to 2 m) spikes of unacceptably high DRHO values occur throughout the logging run. Despite high caliper values in the interval between 433 and 443 mbsf, low DRHO values suggest that density values may be reliable in this interval.
The caliper from the HLDT indicates a thin enlarged hole section between 433 and 434 mbsf. In contrast, caliper data from the FMS-sonic runs do not indicate an enlarged borehole at this depth (Figs. F86, F87). The caliper data from the two runs indicate that the caliper from run 1 is slightly greater than that from run 2 through sections of the lower igneous unit. Azimuth data from the runs suggest that the FMS tool rotated throughout the runs, indicating that there is not likely to be a consistent ellipticity to the borehole.
The logging data show a good correlation with the physical property measurements from cores, particularly for bulk density. The contacts between the sediment and gabbro zones are distinct with sharp increases in density, velocity, and resistivity and a sharp decrease in porosity. The two passes of the FMS images are generally in agreement except at depths where the hole conditions are bad.
Hole inclination can also affect log quality, especially during the FMS-sonic run, as the FMS pad can lose contact because of tool weight if the inclination is >10°. Measured inclinations range from 0.5° to 1.6°.
Plots of logging results are presented in Figures F87, F88, F89, and F90, and the complete logging data are available (see "Related Leg Data"). As explained above, the depths calculated in the complete, depth-shifted logging data will be 3 m below the drillers depth that we use in this report.
Recovered cores from the logged section of Hole 1253A indicated two intervals of igneous rocks separated by nannofossil chalk. The Hole 1253A logs can be clearly separated into three large intervals on the basis of obvious changes in hole diameter, velocity, resistivity, bulk density, and porosity. These intervals generally correspond to the major lithology changes identified in the recovered cores. Between the bottom of the casing (413 mbsf) and 431 mbsf, porosities are low and densities, resistivities, and P-wave velocities are high. This interval corresponds to the upper igneous unit. Between 431 and 461 mbsf, high porosities and low bulk densities, resistivities, and P-wave velocities identify sediments. This interval also has an enlarged diameter. A return to high bulk densities, resistivities, and P-wave velocities at 461 mbsf indicates the top of the lower igneous unit. The boundaries identified in the logs vary from those shown by the core descriptions, probably because of uncertainty in depths of recovered core pieces in Cores 205-1253A-10R and 13R, which had low recovery. The NGR is not distinctly different between the lower igneous and the sedimentary rocks in Hole 1253A. In contrast, there does appear to be a difference in natural gamma values between the upper igneous unit and the sediments and lower igneous units.
In addition to identification of the major intervals, the logging data identify a change in the character of the resistivity and P-wave velocity logs that occurs at ~491 to 493 mbsf. High-resistivity intervals are present between 493-497 and 503-507 mbsf. A significant interval of decreased resistivity and sonic velocity is present between 513 and 517 mbsf. Unfortunately, porosity information is not available for these depths and the bulk density data may be unreliable, as discussed above. FMS images indicate a change in character at a depth of ~508 mbsf. Above that depth, conductive features are generally discontinuous and are generally randomly oriented. Below that depth, more closely spaced, thin, near-horizontal to slightly dipping conductive features are present in several intervals that are separated by intervals of poor images and irregular borehole size.
Ideally, fractured igneous rock can be distinguished from sediment layers by comparing the sonic velocity log to the bulk density log. Because acoustic waves transmit through the solid portion of fractured rock, intervals of decreased bulk density but no corresponding velocity decrease may indicate a fractured interval. In contrast, sediment interlayers should cause a velocity decrease. However, the vertical resolution of the sonic tool is 107 cm, so small sediment interlayers may not be clearly distinguishable in this log. Based on the bulk density and sonic logs, potential fractured intervals are observed at 466-468, 484-486, 490-493, and 506-508 mbsf.
Among the two gamma ray tools run in Hole 1253A, the HNGS on the triple combo string can be considered to more accurately measure true formation natural gamma ray values than the SGT on the FMS-sonic string, which is not corrected for borehole diameter effects. The SGT log is used for depth comparisons with the triple combo run. In addition, the SGT data from the second FMS-sonic run provide information at greater depths than reached by the triple combo tool string.
Gamma ray data (Fig. F87) show elevated values from 413 to 432 mbsf, corresponding to the upper igneous section. In this section, thorium values range between 0.8 and 2.3 ppm compared to 0.2-1.8 ppm in the lower part of the borehole, corresponding to the sediments and the lower igneous unit. Within the lower igneous unit, narrow thorium spikes (~2 ppm) are observed at 469-473 and 477-481 mbsf. Uranium and potassium also show elevated values in the upper igneous section. A thin spike in the gamma ray values occurs at 434 mbsf, reaching the highest value (25 gAPI) of the run. Maximum values in uranium (1.3 ppm) and potassium (0.56 wt%) are also associated with this high gamma ray spike. This spike is also observed in the gamma ray data from both of the FMS-sonic runs.
Below 435 mbsf to the deepest measurement, the gamma ray curve shows little variation, despite the lithologic change between sediments and igneous rocks apparent in the recovered cores. The NGR values from the MST run on recovered cores indicates a shift to slightly higher values at ~513 mbsf. This depth is below that logged by the HNGS on the triple combo string but can be compared to gamma ray data from the second FMS-sonic run that were recorded to 523 mbsf. Although some values above 10 gAPI were recorded in the interval between 508 and 518 mbsf, no consistent shift is distinguishable.
Bulk density data were recorded in high-resolution mode, which increased the sampling rate to 2.54 cm from 15 cm in normal mode, and Schlumberger-enhanced processing was performed on the Schlumberger minimum configuration maxis system onboard (Fig. F88) the ship. Bulk density values are variable between the bottom of the casing and 425 mbsf and may be affected by the enlarged borehole. Between 425 and 432 mbsf, most bulk density values are between 2.8 and 2.9 g/cm3, with some thin spikes of low values. Bulk density in the interval between 432 and 460 mbsf is between 1.3 and 2.0 g/cm3, corresponding to the recovered sediments.
In the interval from 460 to 513 mbsf (the lower igneous unit), bulk density values generally vary between 2.7 and 2.9 g/cm3, with spikes to lower values. Most but not all spikes are associated with zones of enlarged borehole diameter. Physical property measurements taken on recovered cores are consistent with the density logs in both the igneous and sediment intervals.
Neutron porosity data (Fig. F88) also were recorded in high-resolution mode, which increased the sampling rate to 5.08 cm from 15 cm in normal mode, and an enhanced processing technique was used. Porosities for Hole 1253A were estimated from bulk density log data and compared to the neutron porosity data. The density log (
b) was converted to porosity (
) using the relationship
= (s - b)/(s - w),
where water density, w , and grain density, s, were assumed to be 1.03 and 2.95 g/cm3, respectively. This estimate for grain density was based on core moisture and density property measurements of dry mass and volume on the igneous cores (see "Physical Properties").
Comparison of values indicates an inconsistent depth offset between peaks in neutron and density porosity, which is surprising because these sensors are on the same tool string. At 500.5 mbsf, peaks in neutron and density porosity coincide, whereas the peak in density porosity at 504 mbsf is below a neutron porosity peak at 503.5 mbsf, and a large density porosity peak at 485 mbsf is ~1 m above the corresponding peak in neutron porosity.
In general, values determined from neutron porosity exceed those calculated from the bulk density log. The largest discrepancy (40%) occurs in the upper igneous unit between 416 and 424 mbsf. The difference is also high in the sediments (20%-30% difference). In the lower igneous unit, values are much closer. Both neutron and calculated density porosity values are slightly higher than those determined from core moisture and density properties. The higher log values may reflect fracture porosity that is not reflected in the moisture and density property measurements, which are made on intact samples. The difference may also reflect chemically bound water that is not removed during the drying of the core samples.
Deep and medium induction resistivity data from Hole 1253A (Fig. F89) show lower values than the spherically focused resistivity log (SFLU) in the high-resistivity sections of the hole. This is atypical of most logging applications, because the SFLU has the shallowest penetration and is, therefore, most affected by the conductive borehole fluids. However, the deep and medium induction tools are not accurate in highly resistive (>100 
m) formations, as would be expected for igneous rock. Therefore, in the high-resistivity sections, the SFLU data should be considered to best reflect true formation resistivity.
The SFLU data show an interval of higher values (20-310 m) between the top of the logged section and 432 mbsf. Low resistivities (0.6-5 m) were recorded between 432 and 462 mbsf. The medium and deep induction data are slightly greater than the SFLU values. Between 462 and 493 mbsf, resistivity values are consistently high with minor variations. Below 493 mbsf, the curve shows greater variation, with intervals of elevated resistivity between 493-497 and 503-507 mbsf. No corresponding change is seen in the other logs within these intervals. An interval of low SFLU resistivity, with values of 5-15 m, is present between 513 and 517 mbsf. This interval is also seen clearly in the deep and medium resistivity logs and is associated with decreased P-wave velocity.
The Dipole Sonic Imager was operated with the modes of P and S monopole (standard frequency), lower dipole (low frequency), upper dipole (standard frequency), and Stoneley (standard frequency) in both passes of the FMS-sonic tool string, except no upper dipole logs were taken for pass 1. Velocity data from the deeper second run are presented in Figure F90. Among these measurements, the compressional velocities (P and S mode) generally correlate well between the two passes. The lower dipole shear velocities are generally good too; however, there are a few sections of low-quality values at some depths. In contrast, the upper dipole shear velocities are of poor quality, and Stoneley wave velocities are well determined only in the igneous rocks, not in the sediments. Postcruise analyses of full waveform data will allow a better assessment of data quality.
P-wave velocities from the top of the logged section to 423 mbsf are mostly between 5000 and 6000 m/s. Between 423 and 433 mbsf, P-wave values become more variable and generally decrease. Between 433 and 460 mbsf, velocities range between 2050 and 3400 m/s. Between 460 and 513 mbsf, most values are between 4500 and 6000 m/s. Between 513 and 518 mbsf, the P-wave velocities become more variable, with a number of values as low as 3800-4000 m/s.
Shear wave velocities follow similar trends to those of the P-wave velocity log. However, variation is large, ranging from 220 to 5400 m/s where igneous sections, especially the lower igneous unit, are larger than sediment sections. Unlike that for the P-wave velocities, the boundary between the upper igneous unit and sediment section is not apparent in the shear wave data.
Physical property measurements taken from cores are consistent with P-wave velocities between the top of the logged section and 423 mbsf. The few values of sediment velocity measured in cores between 433 and 453 mbsf (~1600-1900 m/s) are significantly lower than the logging velocities (~3200 m/s between 433 and 443 mbsf), despite good correlation between bulk density values from logging and core moisture and density properties.
A single core sample of igneous rock from Core 205-1254A-13R yields a velocity significantly higher than that observed for the same depth in the sonic log. This discrepancy suggests that the recovered interval may be from the bottom of the cored interval (460 mbsf) rather than from the top, as curated. Below 460 mbsf, P-wave velocities from igneous core samples are consistently and slightly lower than the velocities from the sonic log.
The FMS tool produces high-resolution images of the electrical resistivity characteristics of the borehole wall that can be used for detailed structural interpretations (Figs. F91, F92, F93). Because of the malfunction of caliper 2, detailed structural interpretation will require merging of the two FMS runs where they overlap. Shipboard processing provided preliminary FMS images. In Figure F91, static normalized FMS images are shown together with caliper, velocity, and density curves. Resistivity contrasts between the igneous rock and soft sediments are clearly defined and are consistent with density and velocity data. Because hole conditions were good in most of the igneous intervals, textural and structural variations between the upper and lower sections and within the lower igneous unit itself are resolvable.
The upper igneous unit between 419 and 426 mbsf exhibits a blocky texture with a spacing of ~0.5 m between conductive features. Between 426 and 432 mbsf, the formation appears more massive with thin, conductive features at a 0.5- to 2-m spacing (Fig. F93A). In this interval, it is difficult to trace the conductive features across the four FMS pads.
At the top of the lower igneous unit (463 to 467 mbsf), curved conductive features (fractures or irregularities in the borehole wall) are common. Between 467 and 493 mbsf, the formation appears more massive to blocky, with 0.5- to 1-m spacing between thin conductive features. At ~472 to 478 mbsf, these conductive features can be clearly traced across the four pads, whereas in other intervals, the conductive features appear discontinuous. Between 487 and 493 mbsf, irregular to curved vertical conductive features are present, representing possible fractures or irregularities in the borehole wall. From 493 to 498 mbsf, conductive features are rare, becoming more common again between 498 and 508 mbsf.
At 508 mbsf, the character of the FMS image changes to more closely spaced conductive features (<0.5 m spacing). In rare cases, such as from 513 to 514 mbsf, these conductive features can be traced across the four pads and suggest a low dip angle (Fig. F93B). Image quality between 514.5 and 518 mbsf is poor because of a borehole enlargement. Relatively low (3800-4000 m/s) P-wave velocities and low (5-15 m) SFLU resistivities occur at similar depths
Below 518 mbsf, the layered character returns but the absolute value of resistivity increases, as indicated by the static image (Fig. F93B). This high resistivity below 518 mbsf is confirmed by the SFLU measurements in this interval. Image quality is poor from 525 to 527, 534 to 539, and 542 to 555 mbsf. From 539 to 541 and 555 to 563 mbsf, the image is characterized by more closely spaced (<0.5 m) thin, nearly horizontal conductive features. These conductive features appear to dip to the southwest. Although no SFLU data exist for these intervals, the static FMS images indicate that both intervals have high resistivity. Therefore, it appears unlikely that these are sediment layers.
Images from the intervals into which the upper and lower OsmoSamplers were stationed are shown in Figure F93. Dipping conductive features are visible in the section screened for the upper OsmoSampler. The lower OsmoSampler is located in an area with a layered character beneath a zone of poor images because of an irregular borehole shape.
Downhole magnetic field measurements were made with a three-axis fluxgate magnetometer of the General Purpose Inclinometer Tool (GPIT) that is used to orient the FMS images. The tool utilizes a three-axis inclinometer and a three-axis magnetometer to determine magnetic field strength and inclination of the x-, y-, and z-directions and make calculations of the tool deviation, tool azimuth, and relative bearing.
Magnetic field strength recorded by the GPIT is shown with paleomagnetic intensity results (Fig. F94). Even though core measurements (see "Paleomagnetism") are scattered, the general trend of the curve is in good correlation with logging measurements. Sharp intensity changes at the depths of 412, 419, 435, 463, and 515 mbsf may be linked to lithologic changes. Additional detailed analyses are required to verify and quantify these initial, tentative observations.
