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doi:10.2204/iodp.proc.320321.110.2010

Downhole measurements

Logging operations

Downhole logging of Hole U1338B started after APC/XCB coring to a total depth of 416.1 m DSF ended on 9 June 2009 at 1400 h (all times are U.S. Pacific Daylight Savings Time; UTC – 7 h). While drilling the last three cores, the hole was checked for fill at the bottom and no fill was detected. In preparation for logging, the hole was flushed with a ~50 bbl sweep of sepiolite/attapulgite mud (~9 ppg) followed by a wiper trip up to 80 m DSF and down to the bottom. No tight spots were encountered during the reaming, and no fill was noted at the bottom. A go-devil was then pumped through the drill string to open and lock the lockable flapper valve located above the bit. Finally, the hole was displaced with barite/attapulgite mud (~10 ppg) and the bit was raised to the final logging depth of 84.7 m DSF.

We deployed three downhole tool strings in Hole U1338B: a modified triple combo that did not include a neutron porosity measurement, a FMS-sonic combination, and a VSI tool with a SGT-N sonde. For tool and measurement acronyms, see "Downhole measurements" in the "Methods" chapter.

The modified triple combo was lowered into the hole at 2337 h on 9 June. The tool string reached the bottom of the hole at 4621 m wireline log depth below rig floor (WRF) and started logging uphole at 0301 h on 10 June (Fig. F45). This logging pass was stopped at 0310 h when the flying head of the wireline heave compensator (WHC) took a series of sudden large oscillations. These oscillations were caused by flaws of the control algorithm of the WHC and are potentially dangerous for the tool integrity if tension on the cable drops and the tool string hangs on caliper arms. We interrupted this first logging attempt, restarted heave compensation, and went back to the bottom of the hole (4622 m WRF) to start another logging pass at 0339 h.

This main triple combo logging pass proceeded without incident until the base of the tool string was reached at ~4340 m WRF. At this point, the wireline cable tension measured at the surface and at the head of the tool increased by 1500–2000 lb, indicating that the tool string was stuck in a borehole restriction (Fig. F45). We kept pulling up carefully, and after a couple of attempts the tool string was freed with its base at ~4325 m WRF. As the log data acquired below the obstruction were of high quality, we decided that the risk of passing again through the restriction outweighed the benefit of an additional complete logging pass. Therefore, we reentered the drill pipe, logging the seafloor from a peak in the gamma ray at 4207 m WRF. The triple combo tool string reached the drill floor and was rigged down at 0848 h.

During the ascent of the triple combo tool string, options to continue logging Hole U1338B safely were discussed with the operations superintendent and the Co-Chief Scientist on watch. The final decision was to lower the base of the drill pipe below the base of the obstruction. Before rigging up the FMS-sonic tool string, the base of the pipe was deepened to 4351 m DRF (141 m DSF).

The second wireline tool string deployed in Hole U1338B was the FMS-sonic, which was rigged up and started its descent at 1100 h (Fig. F46). The tool string reached the bottom of the hole and started a first logging pass from 4626 m WRF at 1624 h. The first pass ended with the base of the tool string at 4394 m WRF (1716 h), and, after some testing the efficiency of heave compensation with the tool string midway in the open hole interval, the tool string was lowered again to the bottom of the hole to start a second logging pass from 4628 m WRF at 1925 h. The tool string reentered the drill pipe without incident, and the second pass ended when the seafloor was detected at 4209 m WRF. The FMS-sonic tool string was recovered and rig-down completed at 0220 h on 11 June.

The third and final tool string was deployed in Hole U1338B to shoot a VSP and consisted of the VSI tool plus a SGT-N tool for correlation. The Site U1338 VSP was shot during the daylight hours of 11 June so that a watch for the presence of marine mammals could be maintained following IODP procedure. While the VSI/SGT-N combination was being lowered in the hole, the marine mammal watch started at 0645 h and the air guns began ramping up the shooting a half-hour later. The first VSI station was taken at 1008 h immediately above the base of the hole (4623 m WRF). We took as many as five shot records at each station for stacking and moved uphole by 15 m between stations. We acquired 14 successful VSP stations up to 4398 m WRF; a shallower station was attempted, but the VSI tool could not be coupled to the borehole wall because of an enlarged hole and soft formation. At 1308 h we decided to end VSP operations and started logging with the SGT-N, detecting the seafloor at 4207.5 m WRF. Rig-down was completed at 1615 h.

Downhole log data quality

Figures F47 and F48 show a summary of the downhole log data acquired in Hole U1338B. These data were processed to convert to depth below seafloor and to match depths between different logging runs. The resulting depth scale is WMSF (see "Downhole measurements" in the "Methods" chapter).

The overall quality of the logging data can be assessed from the repeatability of measurements acquired in different passes, by the consistency of different measurements (e.g., density and resistivity), and by comparing downhole log data to core measurements. A key factor that influences downhole log data quality is the size and irregularity of the borehole, especially for measurements that require good contact with the borehole wall (e.g., the density measured by the Hostile Environment Litho-Density Sonde (HLDS) and the resistivity images obtained by the FMS tool). The "hole diameter" track in Figure F47 is measured by a caliper arm on the Hostile Environment Natural Gamma Ray Sonde (HNGS) and shows a hole at least 14 inches in diameter (bit diameter = 11.4375 inches), with values locally near the maximum caliper range (~18 inches). Despite the occasionally large hole, the HLDS density data are high quality, as shown by their close correlation with MAD core measurements on discrete core samples and with the electrical resistivity logs, which are relatively insensitive to hole size. As noted earlier, because of a hole obstruction we could only run one complete pass of the triple combo tool string in Hole U1338B, so we cannot compare passes to assess the repeatability of density and resistivity measurements.

Resistivities obtained by the spherically focused resistivity (SFLU) electrode measurement were lower than those obtained in induction measurements (e.g., medium induction phasor-processed resistivity in Fig. F47), probably because of current loss at the electrodes. The higher induction resistivities are closer to values typically measured in deep-sea sediments, and we applied a simple empirical correction to the SFLU data by multiplying them by a constant factor of 1.16. This correction brings the overall measured resistivity values in close agreement.

We measured P-wave velocities with the Dipole Sonic Imager (DSI) using two different transmitting frequencies in the two passes of the FMS-sonic tool string. Occasionally, the waveform processing algorithm that determines the P-wave velocity picks the borehole fluid velocity. The P-wave velocity log in Figure F47 is a combination of the two passes obtained by choosing the higher value of P-wave velocity determined at each depth, which corresponds to the velocity of the formation. Above 210 m WMSF, the sonic log measured low formation velocities of 1550–1600 m/s, near those of the fluid circulating in the hole (~1500 m/s). Core measurements (see "Physical properties") and results of the VSP experiment (see below) also indicate velocities near or just above 1500 m/s in the top 210 m of the hole.

Figure F48 compares spectral gamma ray logs acquired by the HNGS in the triple combo and FMS-sonic tool strings. The gamma ray measurement is highly attenuated when the tool is inside the bottom-hole assembly and the drill pipe (above 83 m WMSF for the triple combo and above 139 m WMSF for the FMS-sonic tool string), and data in this interval should only be used qualitatively. The total gamma ray log is very similar in the two passes illustrated in Figure F48; in the spectral data, some features are repeatable (e.g., the U peak at the seafloor), while others are less so (e.g., the Th peak at ~150 m WMSF). Spectral gamma ray features that are not repeatable in different passes may not be reliable.

In Hole U1338B, we also acquired FMS electrical resistivity images. The quality of these measurements depends on close contact between the measuring pads on the tool and the borehole wall. FMS borehole images are good quality between ~190 m WMSF and the base of the logged interval, where they reproduce sediment layers in detail.

Logging units

Downhole log measurements of bulk density, electrical resistivity, and P-wave velocity in Hole U1338B correlate very well (Fig. F47). The likely reason is that variations in sediment composition result in variations of porosity, and changes in porosity affect similarly bulk density, resistivity, and P-wave velocity. High porosities obviously result in low bulk densities. Also, resistivity variations in water-saturated sediments are controlled by variations in pore water content, which is the component of bulk sediment that conducts electricity. Finally, laboratory measurements and rock-physics models show that P-wave velocities in sediments decrease with increasing porosity. Details on these fundamental relationships between porosity and logged properties are given by Hearst and Nelson (1985) and Ellis and Singer (2007).

Downhole logs of gamma ray radioactivity (Fig. F48) are controlled by the sediment content of naturally occurring radioactive elements (K, U, and Th) and do not closely depend on porosity. The most significant feature in the gamma ray logs is the peak in U content at the seafloor. The seafloor peak is notably large, as it is attenuated because the tool is measuring through the drill pipe (see above) and it agrees with natural gamma ray counts made on the cores immediately below the seafloor (see "Physical properties"). Gamma ray data at Site U1337 also showed a clear U peak at 240 m WMSF over a ~40 cm thick chert layer; this chert layer was also seen at Site U1338 in the same stratigraphic position, but no corresponding U peak is apparent in the downhole gamma ray data (Fig. F48). The reason could be that the chert layer at Site U1338 is only ~16 cm thick, as shown by the FMS resistivity images (see below). The vertical resolution of the HNGS measurements is ~50 cm (see "Downhole measurements" in the "Methods" chapter).

We divided the logged section into three units on the basis of the overall variation of density, resistivity, and P-wave velocity. Logging Unit 1 (from the top of the logged interval to 244 m WMSF) has low densities and resistivities that are variable but show no trend with depth. The transition between logging Units 1 and 2 is marked by a distinct increase in logged densities and resistivities and corresponds to the transition between lithologic Units II and III (see "Lithostratigraphy"). While density and resistivity do not show a clear trend with depth in logging Units 1 and 2, Unit 3 (from 380 m WMSF to the base of the logged interval) displays an increase with depth of density and resistivity. This increase is probably caused by lithification of the pelagic carbonate oozes in lithologic Unit III (see "Lithostratigraphy"). Logging Unit 3 is only ~30 m thick in Hole U1338B, but it can be clearly identified by correlating the density and resistivity curves at Sites U1337 and U1338, which are very similar.

Core-log correlation

Figure F49 shows a notable interval in the FMS images with more conductive layers (darker color in the images) alternating with more resistive layers (lighter color). When compared to other downhole logging data from this site, these conductive bands correlate with lower values of resistivity and bulk density measured by the modified triple combo tool string (Fig. F49C, F49D). Here we make a first attempt to correlate the spliced core images to the continuous downhole log data. The Site U1338 core image splice (see "Stratigraphic correlation and composite section") has been uniformly reduced in length by 11% to account for core expansion on recovery, and we concentrated on the interval spanned by Cores 25H, 26H, 27H, and 28H. On closer inspection, the more conductive bands in the statically normalized FMS images match well with darker, more silica rich units in the core, and the more resistive bands match with the more carbonate rich intervals. Five main conductive bands are observed in this depth interval (230–252 m WSF), highlighted by gray boxes in Figure F49. In these carbonate-rich sediments the logged density and resistivity are primarily influenced by changes in porosity (carbonate-rich sediments being less porous and silica-rich sediments more porous). The presence of these alternating units likely relates to changes in climatic and paleoceanographic conditions, as shown by the cyclicity in carbonate deposition.

At Site U1337 the process of coring the four holes was made more challenging by the presence of a ~40 cm thick chert layer. At Site U1338 evidence of a chert layer was seen in Hole U1338A and was confirmed by the logging data in Hole U1338B. The downhole logging data, particularly the FMS images, clearly identify the chert layer as a very resistive unit ~16 cm thick (Fig. F50, chert highlighted by a gray box); less than half the thickness of the chert unit seen at Site U1337. The high-resolution density measurements also highlight the location of the chert unit as a relatively dense layer. However, because of the very thin nature of the unit, it is probably not fully resolved and its density may be underestimated. The SFLU high, which correlated well with the chert unit imaged in Hole U1337A, is offset below the chert in Hole U1338B. The reason behind this will require further processing of the logging data and will be followed up postexpedition. Additionally, the gamma ray peak (a U peak) that was observed at the chert in the downhole logging data at Site U1337 is not seen here. As noted earlier, the chert layer is too thin to be resolved by the HNGS. Similarly, the thin chert layer at Site U1338 is associated with a silica-rich sediment layer immediately above it, seen in the FMS images as a dark colored, conductive band, and a lower resistivity and density value. This silica-rich layer is clearly visible in the core image splice (see "Stratigraphic correlation and composite section"), which has been uniformly reduced in length by 11%, as a distinctly darker sediment interval.

Vertical seismic profile

Figure F51A shows the stacked waveforms measured at 14 stations (190.6–415.7 m WSF) by the vertical direction geophone in the VSI. The waveforms are noticeably noisier than in the other VSP taken in Hole U1337A. The waveforms show clear first arrivals, but they do not display the basement reflection observed in the Hole U1337A VSP.

Table T26 lists the values of the measured and corrected one-way arrival times in the Hole U1338B VSP. Measured traveltimes are the differences between the arrival of the acoustic pulse at a hydrophone located immediately below the air gun source and the arrival at the borehole receiver. Corrected traveltimes are traveltimes from the sea surface to the borehole receiver and account for the depth of the air guns (7 m below sea level) and the depth of the hydrophone below the air guns (2 m). More details on the VSP measurement procedure are in "Downhole measurements" in the "Methods" chapter.

Figure F51B shows the relationship between depth below seafloor in Hole U1338B and the TWT, which is the traveltime to reflectors in surface seismic sections. To construct this relationship, we start from the TWT from sea level to the seafloor (5.5862 s), computed from the uncorrected seafloor depth measured by the ship's echo sounder (4189.6 m). The difference between this time and the arrival time to the shallowest receiver in the VSP gives an average P-wave velocity of 1500 m/s between the seafloor and 214.1 m WSF. At first approximation, the traveltime-depth relationship in Figure F51B assumes this constant velocity in the interval 0–190.6 m WSF. The dependence of the arrival times on depth in the VSP receiver array can be fit very closely by a second-degree polynomial

t(z) = 5.5796 + (1.5037 × 10–3) z – (6.8064 × 10–7)z2,

where t is TWT (s) and z is depth below seafloor (190.6 ≤ z ≤ 415.7 m WSF). The maximum residual on this fit is 1.7 ms and the root-mean-square residual is 0.94 ms.

The variation of P-wave velocity in the depth interval spanned by the VSP receiver array can be obtained from the derivative of the time-depth relationship above, which gives

V(z) = [(0.75186 × 10–3) – (6.8064 × 10–7)z]–1,

where V is P-wave velocity (m/s) and z is depth (190.6 ≤ z ≤ 415.7 m WSF). This relationship is compared to the velocities measured by acoustic logging with the DSI in Figure F47. The acoustic logging measurements contain small-scale details in the velocity structure that cannot be resolved by the VSP arrival time data. Whereas Site U1337 velocities from the VSP and from acoustic logging showed a similar trend in depth, at Site U1338 overall VSP velocities are somewhat higher than those measured by acoustic logging. The reason for this difference is not clear, and resolving it will require further processing of the VSP and acoustic logging data.

The traveltime-depth relationship allows for correlating stratigraphic events at Site U1338 to reflections in the surface seismic data. In Figure F52 we correlate the downhole logging results in Hole U1338B to seismic reflection Line 1 of the AMAT-03 site survey in the proposed Site PEAT-8 area, which crosses the location of Site U1338 at Shotpoint 35658. To make an accurate correlation, the TWTs in the seismic reflection line were shifted so that the seafloor reflection time (originally estimated at 5.63 s) matched the more accurate TWT to the seafloor measured by the JOIDES Resolution echo sounder (5.5862 s; see above).

Reflections in the seismic line correspond to fluctuations in bulk density and velocity; for example, an increase in density at 130 m WMSF matches a reflection event at ~5.76 s TWT. The seismic reflection record at Site U1338 does not show a sequence of reflectors as rich and well resolved as that observed at Site U1337. Nonetheless, the preliminary well to seismic correlation of Figure F52 can be the starting point in assembling an up to date, age-calibrated interpretation of the seismic stratigraphy and of the Neogene sedimentation history in the equatorial Pacific (e.g., Mayer et al., 1985; Mitchell et al., 2003).

Heat flow

Downhole temperature measurements at Site U1338 consisted of seven APCT-3 measurements in Holes U1338A and U1338C (Table T27). Measured temperatures ranged from 3.33°C at 40.7 m DSF to 12.69°C at 316.4 m DSF and closely fit a linear geothermal gradient of 34.4°C/km (Fig. F53). The temperature at the seafloor was 1.65°C, based on the average of the measurements at the mudline in the seven temperature profiles. Thermal conductivity under in situ conditions was estimated from the laboratory-determined thermal conductivity using the method of Hyndman et al. (1974). The estimated in situ thermal conductivities in Figure F53 are as much as 2.3% below the measured laboratory values.

A simple estimate of the heat flow can be obtained from the product of the geothermal gradient times the average in situ thermal conductivity (0.97 W/m°C), which gives a value of 33.4 mW/m2. However, thermal conductivity increases with depth, hence a more accurate heat flow value can be obtained from the slope of the temperature measurements plotted versus thermal resistance, as in the Bullard method (see Pribnow et al., 2000). The variation of thermal conductivity with depth was estimated by fitting a linear trend, and thermal resistance was calculated as the integral in depth of the inverse thermal conductivity (Pribnow et al., 2000). The fit between temperature and thermal resistance gives a slightly higher heat flow of 33.6 mW/m2, which is similar to values of nearby sites in the global heat flow database (Pollack et al., 1993).