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doi:10.2204/iodp.proc.317.106.2011

Downhole logging

Operations

Preparations for downhole logging of Hole U1354C began after APC/XCB coring to a total depth of 384.2 m DSF (~509 m DRF) was completed at 1800 h on 1 January 2010 (all times are ship local time, UTC + 13 h). In preparation for logging operations, the hole was swept and circulated with 50 bbl of high-viscosity mud and displaced with 320 bbl of heavy mud (~10.5 ppg). The bit was raised to the logging depth of 231 m DRF (106 m DSF). Because of the potential for unstable hole conditions, our previous experience logging shelf Sites U1351 and U1353, and time constraints at the end of the expedition, logging was limited to a single tool run without radioactive sources. A modified tool string ("sonic combo"), which consisted of the HNGS, the DSI, the General Purpose Inclinometry Tool (GPIT), and the DIT, was rigged up by 0135 h on 2 January and run into the hole (RIH) at a speed of 2000 ft/h. While the tool string was being lowered, data were recorded from the seafloor to the total depth of 505 m WRF. The first logging pass was started at total depth at 0245 h at a speed of 900 ft/h and stopped at 440 m WRF, at which point the tool string was again RIH to total depth for a full pass. The main pass began at 0308 h from total depth at a speed of 900 ft/h and was completed at 0500 h when the seafloor was identified in the gamma ray log at 122 m WRF. The tool string was brought back to the surface and rigged down completely by 0607 h. By 0625 h, the rig floor was clear of all logging equipment and ready for the cementing protocol in Hole U1354C that concluded drilling operations for Expedition 317.

Data quality

Figures F38, F39, and F40 summarize the main logging data recorded in Hole U1354C. These data were converted from original field records to depth below seafloor and processed to match depths between different logging runs. The resulting depth scale is wireline log matched depth below seafloor (WMSF; see "Downhole logging" in the "Methods" chapter).

Because the DSI requires borehole centralization to properly measure sonic velocities in the formation, an eccentralizing caliper was not included in the tool string so that the quality of the acoustic logs and the smooth motion of the tool string would not be adversely affected. Without a caliper, the first indicator of the quality of the data is the correlation between the different measurements. If any lows in the gamma ray log were due to hole enlargement, the resistivity and velocity logs should also display lower values or remain unchanged because they are least affected by borehole irregularities. Instead, the significant increases in resistivity and velocity associated with the gamma ray excursions (Figs. F38, F39) show that these intervals are likely consolidated sand-rich layers and that the change in gamma ray is representative of true changes in lithology. A comparison with the logs recorded at the other shelf sites (U1351 and U1353) also shows that the trends and changes in the gamma ray logs can be visually correlated across the shelf (see Fig. F25 in the "Expedition 317 summary" chapter), indicating the good quality of the logs recorded in Hole U1354C. Such correlations between sites will be fully characterized postcruise.

The gamma ray logs agree reasonably well with the NGR track data measured on cores from Hole U1354C (Fig. F38), validating the overall gamma ray log trend. However, poor core recovery in intervals with significant gamma ray variability, particularly in the sand-rich intervals, makes it difficult to precisely match log and core measurements.

The quality of some of the logging data sets can also be evaluated by internal consistency. The agreement between the deep- and medium-penetration resistivity curves is an indication that the borehole was not anomalously large or irregular (Fig. F38). The clear arrivals in acoustic logging waveforms and the high coherence indicated by distinct red areas in the V_P and V_S tracks in Figure F39 show that the DSI was able to measure reliable V_P and V_S values. Additional postcruise processing will refine these profiles and characterize some of the high-coherence events that were not labeled automatically at the time of acquisition. Finally, the comparison in Figure F41 between the data recorded during the downlog and the uplog shows that all measurements repeat very well between the two passes. The notable discrepancies in the V_P and V_S logs are related to the failure of the automatic processing to identify the correct arrivals and will likely be reduced by postcruise processing. A depth offset in the resistivity curves between ~150 and 130 m WMSF is the result of variations in wireline tension and cable stretch when the top of the tool string entered the drill pipe.

Porosity and density estimation from the resistivity log

In order to provide a measure of porosity and density from the logs without nuclear sources, we used Archie's (1942) relationship to calculate porosity from the phasor deep induction log (IDPH), which is the log least affected by borehole conditions (Schlumberger, 1989) and combined it with MAD grain density data to derive a density profile. Archie (1942) established an empirical relationship between porosity (ϕ), formation resistivity (R), and pore water resistivity (R_w) in sandy formations:

where m and a are two empirical parameters often called cementation and tortuosity (or Archie) coefficients, respectively. The resistivity of seawater (R_w) was calculated as a function of temperature and salinity, as described by Fofonoff (1985). Pore water salinity was assumed to be 35.5 ppt (or 3.55%; see "Geochemistry and microbiology"), and temperature was assumed to follow a local linear gradient of 40°C/km, as suggested by in situ measurements at Site U1352 (see "Heat flow" in the "Site U1352" chapter) and in the Clipper-1 well (Shell BP Todd, 1984). The most realistic value for the cementation coefficient is a = 1 because this gives a resistivity equal to formation water resistivity when porosity is 100%. A value of m = 1.9 was chosen iteratively to provide the best baseline match with MAD porosity data. Although Archie's relationship was originally defined for sand-rich formations, Jarrard et al. (1989) showed that the effect of clay minerals is moderate, and the relationship is commonly used to estimate porosity in clay-rich formations with poor borehole conditions (Collett, 1998; Jarrard et al., 1989). The resulting porosity log is shown in Figure F38, where it compares well with MAD porosity data. Using MAD grain density, we used this resistivity-derived porosity to calculate a new density curve, which is in good agreement with core measurements (Fig. F38).

Logging stratigraphy

The combined analysis of gamma ray, resistivity, and velocity logs can be used to identify logging units defined by characteristic trends. In addition, the proximity of Site U1354 to Site U1353 and the similarities between the logs recorded at these two sites reflect stratigraphic continuity across the shelf, allowing for the definition of a common logging stratigraphy between the two sites. The two following logging units were identified in Hole U1354C.

Logging Unit 1 (110–285 m WMSF), similar to logging Unit 1 in Hole U1353C, is characterized by an increasing trend in gamma ray from the top of the unit to ~185 m WMSF, followed by a generally decreasing trend to the base of the unit. These trends are interrupted by intervals of low gamma ray and high resistivity and velocity that are interpreted as sandy intervals, most of which coincide with intervals of low core recovery and the occurrence of sand in the recovered material. In Figure F39, the two most prominent intervals with very high velocity values (185–195 and 220–230 m WMSF) have the same character as two analogous intervals in Hole U1353C (178–185 and 202–208 m WMSF; see "Downhole logging" in the "Site U1353" chapter) and coincide with two seismic unconformities across these two sites (see "Log-seismic correlation").

Logging Unit 2 (285–384 m WMSF) is characterized by slightly decreasing trends with depth in gamma ray and resistivity, with only limited variability, and increasing velocity along a likely compacting trend (Figs. F38, F39). As at Site U1353, this unit is also characterized by very low core recovery associated with the predominance of muddy sands and sandy mud in the recovered intervals (see "Lithostratigraphy").

Log-seismic correlation

A depth–traveltime relationship can be determined from sonic logs and used to correlate features in the logs, recorded in the depth domain, with features in seismic stratigraphy, recorded in the time domain. A synthetic seismogram was constructed for Hole U1354C from the sonic log and the density curve calculated from the resistivity log using Archie's relationship. Figure F42 shows relatively good agreement between the synthetic waveform and reflections in the seismic line closest to Site U1354. High variability in the logs in logging Unit 1 corresponds to an interval of strong seismic reflections between ~0.27 and ~0.43 s two-way traveltime. In particular, seismic sequence boundaries U10–U13, interpreted from seismic data (see "Seismic stratigraphy" in the "Expedition 317 summary" chapter), are well resolved in the synthetic seismogram and correspond to distinct features in sonic, calculated density, and gamma ray logs. U9 is somewhat deeper in the seismic section in logging Unit 2, where low variability in the logs results in fewer distinctive waveforms in the synthetic seismogram. U12, U11, and U10 all have similar log characteristics, as at Site U1353, and are located at abrupt transitions from high gamma ray, lower density, and low velocity below to low gamma ray, higher density, and very high velocity above. Although core recovery through these high-amplitude features is poor in Hole U1354C, at Site U1353 these boundaries correspond to lithologic changes from dominantly muddy sediments below to sand-, shell-, and gravel-dominated sediments above (see "Lithostratigraphy" in the "Site U1353" chapter). As at Site U1353, U13 is less prominent in both the synthetic seismogram and the seismic stratigraphy than the deeper boundaries, but at Site U1354 it has the same polarity as U10–U12. The difference in polarity between sites could indicate a change in the character of the boundary itself but may also simply be the result of trying to resolve a surface that is thinner than the resolution of the seismic data. Additional postcruise research will refine these correlations by reprocessing the sonic logs and providing more detailed synthesis of core-log correlations at this site.

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