DOWNHOLE LOGGING

Operations

Initial Logging Program

When the ship reached Site 735, Hole 735B was reentered, a logging BHA was set at 49 mbsf, and two logging tool strings were deployed. The first tool string contained density (HLDS), porosity (APS), resistivity (DLL), spectral gamma-ray (HNGS), and temperature (TLT) probes as well as a one-arm caliper. The second logging run consisted of the NGT, DSI, and FMS.

First Run: HNGS-APS-HLDS-DLL-TLT

The tool string was assembled with an 80-ft bridle, necessary for the deep current return of the DLL, and lowered to 9 mbsf, or approximately 40 m above the bottom of the logging BHA. At this depth, the tool string was kept stationary for 8.5 min to allow the temperature probe to reach thermal equilibrium. After the temperature station, the tool was lowered to 1220.5 mbsf at a logging speed 180 m/hr. Immediately after we started lowering the tool string, an open-line message from the deep laterolog was recorded, meaning that this tool was not properly grounded. Even after exchanging panels in the MAXIS unit and checking the electrical grounds, the problem persisted, so the deployment continued. We began the up-log from the bottom of the hole at a speed of 500 m/hr. At 385 mbsf the density measurements from the HLDS became erratic and the long-spacing spectra ceased to work properly.

The tools checked perfectly in combination aboard the ship before deployment; however, the DLL problem surfaced after it came in contact with seawater. After checking the electronics thoroughly, we found that the specific density tools (HLDS and HLDT) used in ODP operations utilize channel 10 for the caliper electrical ground, and the DLL assumes that this channel is open. This tool string configuration is commonly used in industry wells, but the modifications made to the slimmer version of density tools, which are needed for the smaller-diameter holes drilled by ODP, make this configuration incompatible. Therefore, this tool string configuration cannot be used in future ODP logging operations unless modifications are made.

The quality of the porosity, temperature, shallow-resistivity, and gamma-ray measurements is good. The portions of the density data below 385 mbsf seem to be good, but the data above this depth and the deep resistivity measurements are poor.

Second Run: NGT-Centralizer-DSI-FMS

The 80-ft bridle used in the first run was exchanged for a conventional cable head before deployment. After rig-up and on-deck calibrations, the tool string was lowered to 492 mbsf at a speed of 1600 m/hr. Once on bottom, the first of two planned passes began at a speed of 300 m/hr. As soon as the FMS arms were open, the data being monitored at the MAXIS unit showed very poor images and an automatic "EMEX saturated, increase gain and reconnect EMEX" message was recorded. After adjusting gains to maximum and minimum values and then checking electrical grounds and offset settings, the problem persisted. Nevertheless, we decided to continue acquiring the sonic data and caliper information before retrieving the tool string.

At this point we began to decrease the logging speed because of telemetry transmission problems. The problems persisted until reaching 275 mbsf, where telemetry was lost for the entire tool string. The FMS arms were closed manually but no confirmation of their status was obtained until 30 m before reentering the BHA. After the tool string was retrieved safely, we determined that the telemetry problem was in the configuration of the computer in the MAXIS unit. A few hours before reentering the hole, the air conditioning system in the MAXIS unit had leaked water onto the main computer panels, shorting out parts of the main computer system including the main screen and intercom. At the beginning of logging operations, the companion computer had to be used for monitoring the downhole logs, and the configuration was not properly optimized for handling the large amounts of data generated from the FMS-DSI combination. At this point we also thought that the problems with the FMS logs were due to an exposed joint. In preparing for the next logging opportunity later during the cruise, the computer configuration problems were corrected and all the tools were thoroughly checked individually and in combination.

Final Logging Program

Although Hole 735B was drilled to 1508 mbsf, about 1400 m of drill pipe had broken off in the hole and only 500 m of pipe was recovered, leaving approximately 600 m of the hole open for logging. Of this, ~500 m had been logged during Leg 118. At the conclusion of fishing operations, the logging BHA was set at 50 mbsf and four logging tool strings were deployed during an approximate total operational time of 42 hr. The first tool string consisted of density (HLDT), caliper, porosity (APS), and spectral gamma-ray (HNGS) probes. The second logging run consisted of an NGT, DSI, GPIT, and FMS probes. The third tool string was composed of NGT, GPIT, and DLL probes. The fourth and final run consisted of the Schlumberger BGKT three-component VSP tool. A modified Kinley sub was placed at 19.5 m above the bottom of the BHA. The inside diameter of this sub had to be modified at the beginning of the leg from 3.810 in to 3.918 in to allow the larger diameter VSP tool (3.85 in) to be deployed. This sub is routinely placed in the BHA during logging operations in case there is a need to cut the logging cable and retrieve the tool.

First Run: HNGS-Bowspring-APS-Caliper-HLDT

During the first run, good density, porosity, and gamma-ray measurements were obtained from the bottom of the logging BHA at 50 to 595 mbsf, or 10 m above the hole obstruction. Measurements were obtained at a logging speed of 500 m/hr starting with a short repeat section from 170 mbsf to 30 m above seafloor at the beginning of the deployment to determine the depth to seafloor from gamma-ray measurements inside the pipe. This repeat was also performed at the beginning of logging to allow enough time for the formation to recover from the neutron activation produced by the minitron in the APS tool before obtaining the main logs in the top section of the hole. The DLL probe was excluded from this first tool string because configuration problems with the density tool (HLDT and HLDS) were encountered at the beginning of the leg.

Second Run: NGT-Centralizer-DSI-FMS

The second run produced good sonic log data, whereas we encountered problems with the FMS. The tool string was set at 270 mbsf for a 7-min station without the wireline heave compensator (WHC) and a 5-min station with the WHC to record acceleration with the GPIT to test the efficiency of the WHC. The DSI recorded cross-dipole and P- and S-wave modes during the first pass and cross-dipole, upper dipole, and Stoneley modes during the second pass. The data seem to be good, and postcruise processing will be performed to produce final results.

After this second string was lowered, the arms of the FMS were opened at 200 mbsf to test the quality of the FMS log. The data were poor and similar to the images obtained at the beginning of the leg. After considerable time was spent testing different gain options, the tool string was brought to the rig floor and replaced with a second FMS probe, in order to rule out any potential problems with the first tool. The second deployment initially went to 270 mbsf to test the performance of the second FMS. The results from the second tool were similar to those recorded with the first. After we logged from 270 to 210 mbsf and tried all possible gain-control and offset-configuration settings, the same automatic EMEX saturation message obtained during the previous runs was recorded. The tool string was lowered to 595 mbsf for the first of two passes in the open section of the hole at a logging speed of 225 m/hr. The poor performance of both FMS tools may have been caused by several factors: (1) the tools could not respond quickly enough to the extreme resistivity contrasts between the oxide gabbros (10 m) and the olivine gabbros (10,000 m); (2) the resistivity contrast between the olivine gabbros (low conductivity) and the borehole fluid (high conductivity) caused the current to travel along the borehole instead of penetrating the formation, thus causing the EMEX current to saturate the measuring electrodes; and (3) the low resolution of the MAXIS displays were degraded to the point where assessments of data quality control during logging operations were not possible. Extensive postcruise data processing was performed, however, and some useful information was obtained.

Third Run: NGT-GPIT-DLL

Cable heads were changed before the deployment of this tool string in order to use the 80-ft bridle necessary for the DLL. The deployment of the third tool string resulted in very good resistivity data. The tool string was set at 270 mbsf for a 48-min station without the WHC and an 8-min station with the WHC to record acceleration with the GPIT in order to test the efficiency of the WHC. The long station without the WHC was because of problems getting the WHC to engage. After several attempts and extensive checks of the WHC system, the WHC began to work properly, and we were able to continue operations. We began logging downward, but there were problems with the quality of the shallow resistivity measurements, and the tool string was brought up to the rig floor. The fiberglass sleeve covering the DLL joint was taped in place to ensure that no current was returning to the tool due to the high conductivity of the Fe-Ti oxides, and the tool was lowered once again. We acquired good shallow and deep profiles from 50 to 595 mbsf at a logging speed of 1500 m/hr.

Fourth Run: VSP

The deployment of the Schlumberger BGKT three-component VSP tool and data acquisition system was successful even though operational problems were encountered during the experiment. Other components of the VSP operation consisted of a 1000-in3 air gun, a 400-in3 water gun, and an over-the-side (OTS) source monitoring hydrophone. These three elements were suspended from the Number 3 crane, aft of the rig floor on the port side.

The objectives of the VSP were (1) to cover the 100-m section between 600 and 500 mbsf, (2) to identify seismic reflectors below the 1500-m depth of the hole, and (3) to provide high-quality data for seismic attenuation studies.

Three test phases were conducted in advance of the downhole VSP operation. Before the cruise, a 2-day orientation was held at the Schlumberger Houston District Offshore office on 29 and 30 September. The other two test phases were on board JOIDES Resolution. On 9 November, an 80-in3 water gun was lowered from the fantail to test the Schlumberger firing box, the blast phone configuration, and the MAXIS acquisition system. On 24 November, we tested the 1000-in3 air gun, the 400-in3 water gun, and the OTS hydrophone as they were suspended from the Number 3 crane. These tests went well, and in both sets of tests, data sets were acquired and used in an attempt to decipher the Schlumberger SEG-Y format. Before the experiment, we arranged to have logs of engine room and dynamic positioning activity during the experiment.

The VSP tool was rigged up with the 80-ft bridle used during the DLL run, and approximately an hour was spent testing the system on the rig floor. Because there was considerable ship vibration, all channels were overloaded, and it was difficult to test the tool on deck. However, a test of the clamping arm was performed, and the VSP acquisition software was initialized before the deployment. The tool was deployed to 595 mbsf at a speed of 480 m/hr. As the light weight of the tool (253 lb) could not be monitored in the tension measurements at the logging winch, the noise that the geophones recorded during deployment was monitored with a speaker at the MAXIS unit to ensure that the tool did not get hung up in ledges while the wireline was being lowered. In addition, 30 gpm of drilling fluid was pumped while the tool was inside the pipe to help the tool past the Kinley sub.

The 1000-in3 air gun, a 400-in3 water gun, and the OTS monitoring hydrophone were lowered into the water after the tool passed the Kinley sub and while the tool was still being lowered to the bottom. When the tool reached the bottom, the guns, the clamping arm, the three orthogonal components, and the monitoring hydrophone were tested for signal quality and gain controls. At this point, the OTS monitoring hydrophone was not working properly. It was brought back to the ship, an electrical short was fixed, and the hydrophone was lowered once again to a depth of 300 mbsl. During the tests and the duration of the experiment, the y-component of the tool did not work properly; postcruise data processing will determine if any of the data from this channel is useful. However, the y-component apparently had cross-talk with the z-component (same character as the z-component with 60-Hz noise superimposed on it), and it is more than likely that the signals from this channel are not useful.

The advantage of having horizontal channels on a normal-incidence VSP is that the instrument can detect vertically traveling shear waves that may be generated by scattering near the seafloor. This was documented during previous VSP experiments at Hole 504B. Because the x-component was functioning and this one channel could provide some information on the shear wave arrivals, we decided it was not worth retrieving the tool to the rig floor and losing more time attempting to fix the y-channel. Aside from the y-channel problem, good signals from the air gun on both the x and z components were recorded. However, signals from the water gun, which is a source with lower amplitude and higher frequency, were weak and only sporadically detected. This was consistent with the observations from Leg 118, during which similar effects were attributed to the high seismic attenuation of these lower crustal rocks. However, we expected to see the water gun return signals get stronger shallower in the hole. Postcruise processing may improve the water gun return signals beyond those which we observed during the experiment.

Data were recorded at 5-m intervals for the bottommost 50 m, at 10-m intervals for the following 50 m, and at 100-m intervals for the upper 500 m of the open hole. While lowering the OTS hydrophone to its operating depth of 300 mbsl, the cable ran off the sheave and lodged around the axle. Further lowering or raising the hydrophone could not be done without swinging the crane in and taking the air guns out of the water. Because time was running short (estimates were that we would require at least 11 hr of shooting for a complete VSP over the 600-m section of open hole), we began the VSP experiment with the tool clamped at 587 mbsf. Stations at 594, 591, and 588 mbsf had been acquired earlier during testing. The shooting procedure consisted of clamping at a depth, slacking the cable 2-3 m to isolate the tool from cable vibration, and then recording 10 air gun and 10 water gun shots at each level. VSP data were then acquired at depths of 587, 584, 579, and 574 mbsf.

At the beginning of the experiment, the weather conditions were clear and calm with a gentle, long-wavelength swell. However, wind speed and swells increased through the evening. The captain was concerned that weather conditions were deteriorating. Because the currents had been strong and variable throughout the cruise at this site and the hydrophone cable was off the sheave, we would not have been able to retrieve the hydrophone quickly should the cable come near the thrusters and screws. There was also concern that should the cable break at the sheave, remotely operated vehicle operations at this site, planned for late 1998, would be jeopardized. VSP operations were immediately suspended, and, with the assistance of ODP technicians and Sedco personnel, the hydrophone and cable were brought back on board. Under stronger winds and light rain, the OTS hydrophone was secured on deck, and the guns were redeployed. The entire operation was completed in 1 hr. Shooting was interrupted on two more occasions: once to adjust the pressure to recock the water gun, and then again to reattach an air gun hose.

VSP data acquired at 5-m intervals between 594 and 544 mbsf and at 10-m intervals between 544 and 494 mbsf give a good coverage over the new section of hole. As the BGKT tool did not show appreciably better results than the Leg 118 VSP, we decided not to redo the VSP in the upper 500 m. To compare the response between the Leg 118 and Leg 176 VSPs over the upper 500 m of the hole, stations at 100-m intervals between 494 and 94 mbsf were recorded. At the end of the VSP operations, a total of 525 air gun and water gun shots had been fired.

A problem with the VSP data acquisition during this experiment was the presence of noise generated by the pipe banging in the hole. If shots went off during quiet periods, good records were obtained, whereas if shots went off during noisy periods, the noise obscured the shot. Given the larger-than-normal outside diameter of the BGKT tool and the style of reentry cone at Hole 735B, we had no options for reducing pipe noise.

Transcription of data from Schlumberger SEG-Y format (TIF files) and an SEG-Y format that we could read on board the ship was a recurring problem throughout VSP planning, operations, and shipboard analysis. With the help of software specialists at the Woods Hole Oceanographic Institution, data files were translated from the Schlumberger files to a format that could be recognized by the shipboard software package. However, problems still exist in getting the identical waveforms that are produced by the MAXIS. Proper transcription from Schlumberger format to a UNIX-based SEG-Y format at the Lamont-Doherty Earth Observatory will be necessary before any significant processing can occur.

Borehole Condition and Log Data Quality

Shipboard analysis of logging data and cores recovered during Leg 118 and Leg 176 shows that Hole 735B consists of definable units and horizons, reflecting differing influences of magmatic, structural, and metamorphic processes within the lower crustal gabbroic rocks drilled at this site. A selection of most of the logs acquired during Leg 176 is presented in Figures F127, F128, F129, F130, F131. The interval displayed in these figures corresponds to approximately the 545 m of open hole (50-595 mbsf) above the 900 m of pipe still in the borehole and the depth below the logging BHA. Most logs were also run through the BHA to the seafloor, and repeat passes were made for quality control, but these are not shown. Two caliper logs from the FMS (Fig. F127) illustrate two orthogonal dimensions of the borehole with depth. The diameter of Hole 735B generally varies between 10.4 and 15 in, with the largest diameters occurring at approximately 100 and 560 mbsf. Otherwise, the condition of Hole 735B is generally adequate for the acquisition of good logging data. In several sections, the borehole seems to be slightly elliptical in cross section. Only a few intervals have large systematic differences between the two calipers. These intervals are approximately at 100, 485, and 560 mbsf and have a maximum difference of 1.2 in. The orientation of the calipers with respect to magnetic north (P1AZ) illustrates that the tool followed almost identical paths during the two full passes with the FMS. This small rate of rotation may be indicative of hole ellipticity, deviation, or directional borehole damage caused by the extensive fishing operations conducted near the end of the leg.

The HLDS, HLDT, and FMS logs from initial and final logging programs show variations of ~2 in greater than the caliper measurements from Leg 118. The tools deployed during Leg 176 were recalibrated after logging operations, and they appear to be correct. A closer inspection of the Leg 118 caliper log suggests that these measurements are somewhat questionable because in many instances they are less than the bit diameter.

Figure F127 also shows the hole deviation and azimuth logs obtained with the GPIT. Hole 735B shows a deviation approximately varying from 7.1º at 95 mbsf to 5.1º at 545 mbsf. Below 545 mbsf, the hole deviates more, which may be a result of the fishing operations. This slight hole deviation did not affect the operation of the logging tools or the data quality. The direction of this deviation rotates from N3°E at 95 mbsf to ~N35°E at 595 mbsf. The abundance of magnetic minerals in Hole 735B seems to have an influence in the azimuth measurements obtained with the GPIT magnetometer. This is most evident in the inflections measured in Unit 4 from 233 to 278 mbsf. However, the azimuth values obtained in Unit 5 from 325 to 400 mbsf should be more reliable, because this unit has a relatively low abundance of magnetic minerals, as already shown in the susceptibility log obtained during Leg 118 (Shipboard Scientific Party, 1989).

Temperature Measurements

Temperature measurements obtained with the Lamont-Doherty memory temperature tool (TLT) at the beginning of the leg show that the hole is nearly isothermal for 500 m. A slight but steady decrease in temperature of approximately 0.8ºC from 8.9ºC at 49 mbsf to 8.1ºC at 240 mbsf is observed and followed by a steady increase of up to 0.8ºC at a depth of approximately 445 mbsf. Below 445 mbsf, temperature and pressure fluctuations were recorded. These fluctuations may be attributed to difficulties trying to reach the bottom of the hole. The average temperature for the entire upper 500 m of Hole 735B is 8.5ºC.

Nuclear Measurements

The bulk density (RHOB) of the formation and the photoelectric factor (PEF) were measured using the HLDT and are shown in Figure F128 (Track 1). Density values range from 1.47 to 3.27 g/cm3 with a mean value for the entire logged section of 2.88 g/cm3. Low values are related to fractures filled with seawater or to enlarged sections of the hole. The olivine gabbros of Unit 5 exhibit a range of values from 2.25 to 2.98 g/cm3 with a mean of 2.88 g/cm3, whereas the oxide gabbros of Unit 4 show a range of values from 2.95 to 3.27 g/cm3 with a mean of 3.09 g/cm3. The PEF varies from 1.10 to 9.84 barns/e-, which is indicative of the lithologic variations observed in this lower oceanic crustal section. Variations in the density profile correspond to variations in oxide mineralogy (Shipboard Scientific Party, 1989) and increases in porosity (Fig. F128, Track 2). Density values from discrete laboratory measurements show a fairly good correlation with log measurements, exhibiting slightly higher values in the upper 500 m of the hole and more scatter at the bottom of the logged section.

The porosity measurements in the logged section of the hole show variations between 0.03% and 57.37% with a mean value for the entire section of 3.34%. High values correspond to borehole washouts or fractures and generally correlate with low peaks in the density log. Several isolated zones corresponding to high porosity and low density occur in the upper 450 m of the hole and were previously documented as high permeability zones from results of Packer experiments (Shipboard Scientific Party, 1989). Below 450 mbsf, several zones showing decreases in resistivity and density and increases in porosity and borehole size were observed (Fig. F129). As shown in Figure F130, the higher porosity and lower resistivity may correspond to the higher deformation intensity observed in this section of the hole (see "Structural Geology" and "Metamorphic Petrology"). Discrete laboratory measurements do not correlate well with the porosity logs. This may be a direct cause of sampling bias, because fractured intervals are usually poorly recovered and the rocks that are recovered have veins and fractures that are not often sampled for measurements of physical properties.

The spectral gamma-ray logs were measured with both the NGT and HNGS tools (see "Principles and Uses of the Tools"  in the "Explanatory Notes" chapter). Profiles from both tools show excellent correlations; therefore, for simplicity, only the profiles obtained with the HGNS are shown in Figure F128 (Tracks 3 and 4). The total spectral gamma ray (HSGR) varies from 1.8 to 16.9 API in the open section of Hole 735B. The contributions to the natural radioactivity observed in the HSGR and the computed gamma-ray (HCGR) vary throughout the hole. In Unit 2 from 58 to 186 mbsf, most of the contributions seem to be related to an increase in thorium content toward the base of this unit. In Unit 3 from 186 to 239 mbsf, the variations are mostly caused by small increases in both potassium and thorium. Unit 4 from 250 to 278 mbsf is characterized by sharp increases in potassium and smaller variations in thorium, whereas the natural radioactivity of Unit 5 from 281 to 403 mbsf is mainly caused by increases in thorium and uranium. The base of Unit 6 from 488 to 536 mbsf is characterized by a decrease in thorium with increases in both potassium and uranium decay series. Potassium is highly mobile in oceanic crustal environments during low-temperature alteration processes; therefore, the HSGR and potassium logs are good indicators of alteration.

Sonic Measurements

The sonic logs recorded with the DSI tool represent the first use of this tool in the lower oceanic crust. The data obtained during the second logging run at both the beginning and the end of the leg were recorded during three separate passes of the tool through the section of open hole. In total, five different modes of the DSI using different acoustic sources allowed the acquisition of both compressional and shear waveforms (see "Principles and Uses of the Tools" in the "Explanatory Notes" chapter). Both high-frequency compressional and shear modes as well as the low-frequency dipole mode produced good sonic waveforms. Preliminary processing of compressional (DTC) and shear (DTS) traveltimes was completed on board JOIDES Resolution using Slowness-Time-Coherence (STC) software on the Schlumberger MAXIS acquisition system (Kimball and Marzetta, 1984). Postcruise processing must also be applied to the dipole data to account for dispersion effects, which may reduce the traveltimes by 2%-6% (Brie and Saiki, 1996). The low-frequency Stoneley mode also produced high-quality waveforms.

The delta-transit-time compressional (DTC) and delta-transit-time shear (DTS) logs from the monopole source and the delta-transit-time shear measurements (DTSM) from the dipole source are shown in Figure F131. The DTC trends were computed using a high-frequency source over a range from 35 to 150 µs/ft, whereas the DTS trends were computed using a low-frequency source over a range from 40 to 200 µs/ft. Coherence of the transmitter and receiver combinations for the compressional (CHTP and CHRP) and shear-wave (CHTS and CHRS) logs from the monopole source is degraded in washouts and with excursions in borehole size at several intervals. The dipole-shear waveforms (CHR2) have systematically higher coherence than the high-frequency compressional and high-frequency shear waveforms, which in part is the result of less scattering from small fractures.

Compressional and shear traveltime logs show generally uniform values throughout the 550 m of section logged. Variations correlate with changes in both resistivity and density measurements. The largest variation in both compressional and shear traveltimes is observed at approximately 565 mbsf. This apparent low-velocity, low-density zone, which also correlates with high-porosity and caliper readings, may be responsible for the reflector identified during Leg 118 (Swift et al., 1991) at this depth. The average compressional and shear-wave velocities obtained from the monopole source are 6516 m/s and 3697 m/s. The average shear-wave velocity obtained from the dipole source is 3504 m/s. However, these values have not been corrected for dispersion effects, and postcruise processing may have a significant effect on the final results.

Electrical Resistivity Measurements

Electrical resistivity measurements and images were obtained with the DLL and FMS probes in Hole 735B during Leg 176, recording two different electrical logs and one type of formation image. As shown in Figure F127 (Track 4), the laterolog deep (LLd) and shallow (LLs) measurements give similar results, with the lowest values being obtained in the Fe-Ti oxide gabbros of Unit 4. Several other low-resistivity measurements were recorded throughout the upper 600 m of the hole, and they correlate with density and porosity variations discussed below.

The quality of the FMS images from both passes at the end of the leg was poor and required extensive postcruise processing. In formations with resistivities greater than 10,000 m, such as these, the FMS current will tend to flow into the mud and along the borehole fluid rather than into the formation (Schlumberger, pers. comm., 1997). A future alternative to this problem in high resistivity environments may be to increase the mud resistivity by pumping fresh-water mud into the hole with the goal of increasing the mud resistivity from 0.1 to 2 m.

The following postcruise corrections and processing were made to the FMS data:

  1. Speed corrections were applied to the data to correct the fact that measurements attributed to a cable depth are actually acquired at a somewhat different depth. This is essentially a depth correction but is also referred to as a speed correction because the depth error would not exist if the tool traveled at the same speed as the cable at the surface winch. The integration of the cable speed and the z-axis accelerometer data were used to estimate the speed and depth of the tool.
  2. Because the tool was sticking for short intervals (i.e., the tool remained stationary while the cable moved for a short distance), a sticking detection threshold and recovery speed factors were applied to correct the fact that in these zones the information from the cable depth and the integration of the accelerometer data are in conflict. The former indicated that the tool is moving at a cable speed, whereas the latter showed that the tool velocity was not changing.
  3. The average response of all the buttons in each pad were equalized to account for the difference in gain and offset of the pre-amplification circuits associated with each button, differences in standoff according to button location due to mismatch of borehole and pad curvature, and the difference in application pressure between pads.
  4. A faulty button detection and correction was made specially for pad 4 because approximately 95% of the buttons failed. This correction interpolated the faulty button values using the values of adjacent good buttons.
  5. The button response is controlled by the EMEX voltage which is applied between the button electrode and the return electrode. Because of the EMEX saturation messages were recorded during this particular logging run, a voltage correction was applied where the button response was divided by the EMEX voltage channel so that the response corresponds more closely to the conductivity of the formation.
  6. Imaging enhancing techniques were used for data display by using the method of histogram equalization. This technique enhances the depiction of details in an image by optimizing the color usage (i.e., the use of colors available with equal frequency). The technique was used in two ways for highlighting different features: static normalization, which is a global optimization with a window covering the entire logged interval, and dynamic normalization, which is a local optimization with separate normalization computations repeated at regularly spaced positions using a 5-m sliding window.

A comparison of the FMS raw and processed data shown in Figure F132 illustrates the difficulties in monitoring data quality during the logging runs and the subsequent improvement. A preliminary interpretation of the processed FMS images revealed strike orientations as well as dip azimuth and magnitude for several hundred structural features. However, two main factors may influence the overall orientation of these features after more detailed postcruise interpretation and processing is performed. First, these orientations were obtained from sinusoid fits based on the assumption that the features were planar. Second, these picks may be significantly influenced by the high concentration and magnetization of the Fe-Ti oxide minerals present throughout the logged interval (Fig. F127). The degree to which the GPIT magnetometer is influenced by the high magnetization of the oxide gabbros will be investigated at a later date. The preliminary strike orientation of the majority of the features range from 280º to 310º. The dip azimuth of these features ranges from 340º to 20º, with several features also dipping from 180º to 220º and the magnitudes mostly ranging from 10º to 50º.

Examples of some of the features identified in the FMS images are displayed in Figures F133, F134, and F135. A 25-m interval from 273 to 297 mbsf shows the variations in character observed in the transition from oxide gabbros of Unit IV, through the breccia zone at the base of Unit IV, and into the olivine gabbros of Unit V. Alternating intervals of olivine and oxide gabbros are observed from 273 to 280 mbsf. The transition from the oxide gabbros into the magmatic breccia zone shows some deformation at the base of Unit IV with most of the features approximately dipping between 17º to 22º (Fig. F133A). Besides the large resistivity contrast between the interlayered subunits of Unit IV, one of the most noticeable features is the abrupt contact between the magmatic breccia zone and the olivine gabbros of Unit V (Fig. F133) at ~292 mbsf.

Several structural features and lithologic boundaries are clearly observed in the bottommost 100 m of the FMS logs (Figs. F134, F135). Highly conductive zones at 557 and 566 mbsf (Fig. F134) may correspond to zones of intense deformation. These 1 m and 4 m zones seem to correlate with high core fault intensity measurements (see "Structural Geology"). Lithologic boundaries are also observed in Figure F135. Oxide gabbros in Sections 176-735B-99R-6 to 101R-2 (see the "Core Descriptions" contents list) seem to correspond to a 3-m low-resistivity interval ranging from 579 to 582 mbsf (Fig. F135A). A smaller 1-m interval at 590 mbsf (Fig. F135B) also seems to correlate with oxide olivine gabbros recovered from Sections 176-735B-101R-3 through 102R-1 (see the "Core Descriptions" contents list). Both of these intervals also seem to be characterized by strong deformation (Fig. F135A, F135B).

Core Imaging

During Leg 176, all whole-core pieces that could be successfully rotated through 360º were imaged on the DMT Digital Color CoreScan system. Contiguous pieces were imaged together wherever possible. In a number of cases, pieces with lengths in excess of 1 m were broken to fit the core scanner. One such piece (Samples 176-735B-150R-1, 0-77 cm, and 176-735B-150R-1, 77-147 cm) was originally 147 cm long. Pieces too small or uneven to be scanned effectively were also measured, to allow for them in the total core barrel lengths.

In total, more than 800 m of whole core was scanned in the unrolled mode. This accounts for approximately 93% of the material recovered during Leg 176. The scanned images were then integrated into core barrel lengths using the DMT CoreLog Software. The individual core images are imported into the CoreLog Software, then rotated so that the red china marker line, marked on the core by the structural geology team, is in the same orientation for each piece. The images can then be inserted together to reconstruct each core barrel length. Initial structural analysis of the images included the picking of pertinent structures, such as veins, fractures, and foliations. These are plotted by the software as sinusoids, from which the dip of the feature is calculated. Examples of the unrolled scanned images are shown in Figure F136. Figure F136A shows four smectite veins with dips ranging from 51º to 61º, and Figure F136B illustrates a highly foliated zone with thick mafic boundary layers. Preliminary reorientention of core pieces shows a good correlation between Leg 118 borehole televiewer (BHTV) data, unrolled core images, and Leg 176 FMS logs (Fig. F137). A westerly steeply dipping fracture is clearly identified in the core and the oriented logs. A second fracture is also identified in the FMS logs but not in the core or the BHTV image. These crosscutting fractures are dipping at 90° from each other but at this time, the lack of evidence for a second fracture in the recovered core and BHTV data prevents a classification as a conjugate pair of fractures. Final structural analysis and correlation with downhole logging data will be completed postcruise.

Approximately 48 m of split half-core was imaged in the slabbed scan mode. This represents only 5% of the core recovered. Slabbed Cores 176-735B-89R, 90R, 93R, 94R, 95R, 97R, 107R, 108R, 109R, 110R, and 111R were imaged in entirety. Selected pieces, with structural and igneous features of particular interest, were also imaged from Cores 176-735B-99R, 104R, 105R, 112R, 113R, 119R, 120R, and 121R.