Downhole logs are used to determine physical, chemical, and structural properties of formations penetrated by drilling. The data are rapidly collected, are continuous with depth, and measure in situ properties; they can be interpreted in terms of the formation's stratigraphy, lithology, and mineralogy. Where there is no core recovery, logs provide the only means to describe the formation; where the XCB recovers biscuited core, logs will often provide physical and chemical measurements superior to those from the core; where core recovery is good, log data complement core data. Logs also provide a link between core and seismic section: sonic velocity logs improve depth to traveltime conversion, and synthetic seismograms may be compared directly to the seismic section.

Logging tools are joined together in "tool strings" and are run sequentially into the hole on wireline cable. During Leg 178, we used the triple combination, Formation MicroScanner (FMS)-sonic, geological high-sensitivity magnetic tool (GHMT), and the well seismic tool (WST) (Table T7). Examples of the use of downhole logs for paleoceanographic objectives are given in the Leg 167 Initial Reports volume (Shipboard Scientific Party, 1997b), the GHMT is described in the Leg 162 Initial Reports volume (Shipboard Scientific Party, 1996), and the integrated porosity-lithology tool (IPLT) and WST are described in the Leg 166 Initial Reports volume (Shipboard Scientific Party, 1997a).

Every Schlumberger tool and tool measurement has a three- or four-letter acronym associated with it. Table T8 lists those used by ODP, tool by tool. Often, there is no convenient translation of the acronym; the acronym itself becomes the name of the tool or measurement. This has the disadvantage of making the meaning of many of the acronyms completely opaque to the uninitiated. In the site chapters, we have tried to combat this by referring to, for example, "density" as well as "RHOM" (the corrected density measurement). The GHMT is always called the GHMT, and the FMS is always called the FMS. The advantage of using the acronyms is that the tool, units, and measurement method are completely specified by the three- or four-letter acronym.

The principle of operation and uses of the tools are described in detail in Serra (1984, 1986), Timur and Toksöz (1985), Ellis (1987), and Rider (1996), and briefly below.

The hostile environment natural gamma-ray sonde (HNGS) and natural gamma-ray tool (NGT) measure the natural gamma radiation from isotopes of potassium, thorium, and uranium in the sediment surrounding the tool.

The accelerator porosity sonde (APS) emits fast neutrons, which are slowed by hydrogen in the formation, and the energy of the rebounded neutrons is measured at detectors spaced along the tool. Most hydrogen is in the pore water, hence porosity may be derived. Hydrogen bound in minerals such as clays also contributes to the measurement, however, so that the raw porosity value is often an overestimate. The neutrons slowed to thermal energies are captured by nuclei, especially those of chlorine and the heavier elements; this effect is measured by the APS as the neutron capture cross section, _f.

The hostile environment lithodensity sonde (HLDS) emits high-energy gamma rays, which are scattered by the electrons in the formation. The electron density, and hence the bulk density, is derived from the energy of the returning gamma rays. Porosity may also be derived from this bulk density, if the matrix density is known. In addition, the HLDS measures the photoelectric effect (PEF, absorption of low-energy gamma rays), which varies according to the chemical composition of the sediment (e.g., PEF of pure calcite = 5.08, illite = 3.03, quartz = 1.81, and kaolinite = 1.49 barns/e^-). The HNGS, APS, and HLDS together comprise the IPLT.

The dual induction tool measures the formation resistivity at three different penetration depths, by electromagnetic induction for deep and medium resistivity, and by current balancing for shallow resistivity. Porosity, fluid salinity, clay content, and gas hydrate content all contribute to the resistivity.

The digital sonic tool measures the traveltime of sound waves through the formation between two transmitters and two receivers, over distances of 2.4, 3.0, and 3.6 m. The sonic velocity increases with compaction and lithification. An impedance log (density × velocity) can be used to generate synthetic seismograms for comparison with the seismic survey sections.

The FMS produces high-resolution images of the microresistivity of the borehole wall. The tool has four orthogonally oriented pads, each having 16 button electrodes that are pressed against the borehole wall (Serra, 1989). Approximately 30% of a 25-cm-diameter borehole is imaged. The vertical resolution is ~5 mm, which allows features such as clasts, thin beds, fractures, and veins to be imaged. The images are oriented; thus, both strike and dip can be obtained for the sediment fabric.

The susceptibility magnetic sonde (SUMS) measures magnetic susceptibility using low-frequency induction in the surrounding sediment. The tool responds to magnetic minerals (mainly magnetites, hematite, and iron sulfides), which are often contained in the detrital sediment fraction and can be a proxy for paleoenvironmental change.

The nuclear magnetic resonance sonde (NMRS) measures the total magnetic field using a proton precession magnetometer. A polarity stratigraphy can usually be determined by comparing the variations in total field and susceptibility downhole, after removal of the background geomagnetic reference field and the effect of the pipe (also see "Magnetic Polarity"). The SUMS and NMRS together comprise the GHMT.

The WST contains a geophone that records the traveltime of sonic waves from an air gun or similar source fired next to the ship. The tool provides the best available conversion from depth to traveltime. Additionally, reflectors below the geophone can be located, if the recorded waveform is of good quality. The WST differs from the other tools in that it remains in a fixed position (clamped against the borehole wall) during measurement at each station. Station separation was ~33 m at Site 1095.

After the hole is cored, it is usually filled with viscous drilling fluid. The base of the drill string is raised to ~90 mbsf, run back to the full depth, and returned to ~90 mbsf to clean the hole and stabilize the walls. Two to four tool strings are run sequentially down each hole on wireline cable, and the hole is logged on the upward pass, typically at speeds of 300 m/hr for the triple combination and FMS-sonic, and at 400 m/hr for the GHMT. The wireline heave compensator is used to minimize the effect of the ship's motion on the tool position.

Incoming data for each logging run were recorded and monitored in real time on the Schlumberger Maxis 500 logging computer. Schlumberger's GeoFrame software was used to process the FMS images. All logging data except FMS and WST were transferred by satellite high-speed data link (Inmarsat B) to LDEO, where the logging runs were shifted to a common depth scale and the NGT logs were recomputed. These processed logs were returned to the ship ~1 week after logging and are displayed in the logging summary figures (Figs. F40, F41, both in the "Site 1095" chapter; Figs. F43, F45,  both in the "Site 1096" chapter; Figs. F27, F28, both in the "Shelf Transect" chapter). The processed logs can be obtained from this World Wide Web address: http://www.ldeo.columbia.edu/BRG/ODP/.

The principal influence on data quality is the state of the borehole wall. If the borehole width varies much over short intervals, or is >15 in wide, results from those tools (density, porosity, and FMS) that require good contact with the wall may be degraded. Very narrow ("bridged") sections will also cause irregular log results. The quality of the borehole is helped by minimizing the circulation of fluid during drilling and by logging a young hole. The RCB bit (10-in diameter) is narrower than the APC/XCB bit (11.75-in diameter) and thus will ordinarily create a narrower hole.

Measurements that penetrate deeply into the formation, such as resistivity and susceptibility, are least sensitive to borehole conditions. Sonic velocity is more reliable in more compacted sediment. The maximum extent of the FMS pads is 15 in; boreholes wider than this cannot be well imaged. Of the two natural gamma tools, the HNGS has the more sensitive detector, and the data are corrected for borehole width in the tool itself; the NGT data require shore-based reprocessing.

The depth of the logged measurements is calculated from the length of cable, minus the cable length to the seafloor (seafloor is identified by the step reduction in gamma-ray activity at the sediment/water boundary). Differences between the drill-pipe depth and the log depth occur for a number of reasons. Drill-pipe depth may be inaccurate owing to core expansion, incomplete core recovery, and nonrecovery of the mudline. The log depth may be inaccurate because of incomplete heave compensation, cable stretch (~1 m/1 km), and cable slip. Other differences may reflect tidal effects, which may differ between when the drill-pipe measurement and logging measurement are made. Hence, there can be small but significant offsets between the drill-pipe and log depths, which should be considered when using the logs.

APS porosities often overestimate the actual porosities, because of water in, and bound to, the clays. An alternative porosity () was derived from the bulk density log (RHOM), assuming a constant grain density:

= (_gr - _b)/(_gr - _w),

where _gr (mean grain density) is given by the index physical properties measurements (e.g., 2.76 g/cm³ for Hole 1095B), _w (pore-water density) is taken to be 1.03 g/cm³, and _b (bulk density) is provided by the density log. This typically gives an improved estimate of porosity, except where tool contact with the borehole wall is poor.

Where the sonic velocity tool was not run, for example at Hole 1095B, an alternative method estimates the velocity of the formation (v_pseudo), using the porosity ():

1/v_pseudo = F · (1/v_fluid) + (1 - F) · (1/v_grain).

The total magnetic field (B) measured in the borehole depends on position p and time t (Pozzi et al., 1988):

B(p,t) = Br(p) + Ba(p) + Bt(p,t) + Bf(p),

where Bf(p) = Bfi(p) ± Bfr(p). Br(p) is the Earth's magnetic field, generated in the Earth's core (39,000 to 45,000 nT for Leg 178 sites); Ba(p) is the magnetic field resulting from the bottom-hole assembly (BHA) (up to ~2000 nT, decaying away from the BHA) and crustal heterogeneities; and Bt(p,t) is the time-varying field. We run two passes of the GHMT to check that Bt(p,t) is negligible. Bfi(p) is the field produced in the borehole by the induced magnetization (J_i) of the sediment, which is parallel to B(p,t) and proportional to the magnetic susceptibility ():

J_i = B(p,t) · ,

and Bfi(p) is given by

Bfi(p) = (J_i /2) · (1 - 3sin² I),

where I is the inclination of the Earth's field at the site. Bfr(p) is the field produced in the borehole by the remanent magnetization (J_r) of the sediment, whose polarity we aim to determine. J_r is either approximately parallel (normal polarity) or antiparallel (reversed polarity) to B(p,t), if the site has not moved significantly (relative to the magnetic poles) since sediment deposition.

We find Bfr(p) by subtracting Br(p), Ba(p), and Bfi(p) from the total field measurement, B(p,t). Then, for preliminary interpretation of the Leg 178 data, if Bfr(p) is negative, the polarity is normal, and if Bfr(p) is positive, the polarity is reversed. Often, the assumptions involved in modeling Ba(p) and calculating Bfi(p) are not accurate enough to allow reliable determination of polarity directly from the sign of Bfr(p). An extra step can then be performed to calculate a linear regression between Bfr(p) and Bfi(p) within a sliding depth window: correlation indicates normal polarity; anticorrelation indicates reversed polarity.

The LDEO Borehole Research Group, Leicester University Borehole Research, United Kingdom, and the Laboratoire de Mesures en Forage, Aix-en-Provence, France, in conjunction with Schlumberger Well Logging Services, provided the wireline logging services aboard the JOIDES Resolution.