) in capture units (cu). This is a useful indicator for the presence of elements of high thermal neutron capture cross section such as boron, chloride, and rare earth elements.
Downhole wireline logs are spatially continuous records of the in situ physical, chemical, and structural properties of the formation penetrated by a borehole. They provide information on a scale that is intermediate between laboratory measurements on core samples and geophysical surveys. The logs are recorded rapidly using a variety of probes or sondes combined into tool strings. These tool strings are lowered downhole on a heave-compensated electrical wireline and raised at a constant speed (typically 250-300 m/hr) to provide continuous simultaneous measurements of the various properties as a function of depth. The vertical sampling interval ranges from 2.5 mm to 15 cm.
Logs can be used to interpret the stratigraphy, magnetic stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, log data may provide the only means to characterize the borehole section. Where core recovery is good, log and core data complement one another and may be interpreted jointly.
During ODP Leg 206, five different tool strings were deployed (Fig. F19; Tables T17, T18):
Explanations of tool name acronyms and their measurement units are summarized in Table T17. The parameters measured by each tool, the sample intervals used, and the vertical resolution are summarized in Tables T18 and T19. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), Bosum (1992), and the LDEO-Borehole Research Group (LDEO-BRG) Wireline Logging Services Guide (1994).
Two gamma ray tools were used to measure and characterize natural radioactivity in the formation: the HNGS and the SGT. The HNGS measures the natural gamma radiation from isotopes of potassium, thorium, and uranium using five-window spectroscopy to determine concentrations of radioactive potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The spectral analysis filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. Corrections to the HNGS log account for variability in borehole size and borehole potassium concentrations. All of these effects are corrected for during processing of HNGS data at LDEO-BRG. The HNGS also provides a measure of the total gamma ray emission (in American Petroleum Institute [gAPI] units) and the uranium-free or computed gamma ray (in gAPI units).
The SGT uses a sodium iodide (NaI) scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of potassium, uranium, and thorium concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging strings. It is included in the FMS-sonic and UBI tool strings to provide a reference log to correlate depth between different logging runs.
Density is measured with the HLDT, which consists of a radioactive cesium (137Cs) gamma ray source (622 keV) and far and near gamma ray detectors mounted on a shielded skid, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm (Fig. F19). Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering involves the transfer of energy from gamma rays to the electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is a function of the bulk density.
The HLDT measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma ray energies drop to <150 keV after being repeatedly scattered by electrons in the formation. As the PEF depends on the atomic number of the elements in the formation, it is essentially independent of porosity. Thus, the PEF varies according to the chemical composition of the sediment. Some examples of PEF values are pure calcite = 5.08 b/e-, pyrite = 16.97 b/e-, quartz = 1.81 b/e-, illite = 3.03 b/e-, and kaolinite = 1.49 b/e-. The PEF values can be used in combination with HNGS curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.
The APS consists of a minitron neutron generator that produces fast neutrons (14.4 MeV) and five neutron detectors (four epithermal and one thermal) positioned at different spacings along the tool. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted high-energy (fast) neutrons are slowed by collisions with atoms, and the amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. Significant energy loss occurs when the neutron strikes a hydrogen nucleus of equal mass, which is mainly present in pore water. Degrading to thermal energies (0.025 eV), the neutrons are captured by the nuclei of silicon, chlorine, boron, and other elements, resulting in a gamma ray emission. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and the neutron arrival times, which act as a measure of formation porosity. Hydrogen bound in minerals such as clays or in hydrocarbons, however, also contributes to the measurement, and so the raw porosity value is often an overestimate. In sediments, hydrogen is mainly present in pore water, so the neutron log is essentially a measure of porosity, assuming pore fluid saturation. In igneous and hydrothermally altered rocks, hydrogen may also be present in alteration minerals such as clays; therefore, neutron logs may not give accurate estimates of porosity in these rocks.
The pulsing of the neutron source provides the measurement of the thermal neutron cross section () in capture units (cu). This is a useful indicator for the presence of elements of high thermal neutron capture cross section such as boron, chloride, and rare earth elements.
The DLL tool provides two resistivity measurements with different depths of investigation: deep and shallow. In both devices, a 61-cm-thick current beam is forced horizontally into the formation by using focusing (also called bucking) currents. Two monitoring electrodes are part of the loop that adjusts the focusing currents so that there is no current flow in the borehole between the two electrodes. For the deep laterolog (LLD) measurement, both measuring and focusing currents return to a remote electrode on the surface; this configuration greatly improves the depth of investigations and reduces the effect of borehole and adjacent formation conductivity. In the shallow laterolog (LLS) measurement, the return electrodes that measure the focusing currents are located on the sonde, and therefore the current sheet retains focus over a shorter distance than the LLD. Because of high resistivity expected in an igneous environment, the DLL is recommended over the Dual Induction Tool (DIT), as the DLL tool response ranges from 0.2 to 40,000 
m, whereas the DIT response range is 0.2-2,000 m.
Fracture porosity can be estimated from the separation between the LLD and LLS measurements, based on the observation that the former is sensitive to the presence of horizontal conductive fractures only, whereas the latter responds to both horizontal and vertical conductive structures. Because the solid constituents of rocks are essentially infinitely resistive relative to the pore fluids, resistivity is controlled mainly by the nature of the pore fluids, porosity, and permeability. In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and is strongly dependent on porosity.
The TAP tool is deployed in low-resolution memory mode with the data being stored in the tool and then downloaded after the logging run is completed. Temperatures determined using the TAP tool are not necessarily the in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole. From the spatial temperature gradient, however, abrupt temperature changes can be identified that may represent localized fluid flow into the borehole, indicating fluid pathways, fracturing, and/or changes in permeability at lithologic boundaries.
The DSI tool employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of formations. In addition to a robust and high-quality measurement of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to estimate shear wave velocity even in highly unconsolidated formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at the shear wave velocity and is the most reliable way to estimate a shear velocity log. The omnidirectional source generates compressional, shear, and Stoneley waves in hard formations. The configuration of the DSI tool also allows recording of both in-line and cross-line dipole waveforms. In hard rocks, the dipole sources can result in a better or equivalent estimate of shear wave velocity to that from a monopole source. These combined modes can be used to estimate shear wave splitting caused by preferred mineral and/or structural orientation in consolidated formations. A low-frequency (80 Hz) source enables Stoneley waveforms to be acquired as well. The DSI tool measures the transit times between sonic transmitters and an array of eight receiver groups with 15-cm spacing, each consisting of four orthogonal elements that are aligned with the dipole transmitters. During acquisition, the output from these 32 individual elements are differenced or summed appropriately to produce in-line and cross-line dipole signals or monopole-equivalent (compressional and Stoneley) waveforms, depending on the operation modes.
Preliminary processing of DSI data estimates monopole and dipole mode velocities using waveform correlation of the digital signals recorded at each receiver. In most instances, the shear wave data should be reprocessed postcruise to correct for dispersion, which is caused by the variation of sound velocity with frequency. Processing techniques must be applied to account for a dispersive model without assumptions or to compute a bias correction to minimize any frequency effects on the velocity. In addition, information such as mode amplitudes, shear wave polarization, and Poisson's ratio can be extracted postcruise to provide information about lithology, porosity, and anisotropy. Amplitude processing and stacking of Stoneley wave reflections may also be used to identify fractures, fracture permeability, and aperture in the vicinity of the borehole. The DSI tool is particularly important for determining shear wave velocities for the upper parts of the basalt flow units. The VP/VS ratio in basalts is typically 1.8-2.0. Thus, the part of the lava flow with VP < 3.0 km/s will have a VS < 1.5 km/s, which cannot be determined without using the dipole source of the DSI tool.
The FMS provides high-resolution electrical resistivity-based images of borehole walls (Fig. F20). The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or "buttons," which are pressed against the borehole wall during recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced ~2.5 mm apart. A focused current is emitted from the four pads into the formation, with a return electrode near the top of the tool. Array buttons on each of the pads measure the current intensity variations. The FMS image is sensitive to structure within ~25 cm of the borehole wall and has a vertical resolution of 5 mm with a coverage of 22% of the borehole wall on a given pass. FMS logging commonly includes two passes, the images of which are merged to improve borehole wall coverage. The pads must be firmly pressed against the borehole wall to produce reliable FMS images. In holes with a diameter >38 cm (>15 in), the pad contact will be inconsistent and the FMS images can be blurred. The maximum borehole deviation where good data can be recorded with this tool is 10° from vertical. Irregular borehole walls will also adversely affect the images, as contact with the wall is poor. FMS images are oriented to magnetic north using the GPIT (see "Magnetic Field"). Processing transforms these measurements of the microresistivity variations of the formation into continuous, spatially oriented, and high-resolution images that mimic geologic structures behind the borehole walls. This allows the dip and azimuth of geological features intersecting the hole to be measured from the processed FMS image. FMS images can be used to visually compare logs with core to ascertain the orientations of bedding, fracture patterns, and sedimentary structures. FMS images have proved to be particularly valuable in the interpretation of volcanic stratigraphy during previous ODP legs (Ayadi et al., 1998; Lovell et al., 1998; Brewer et al., 1999; Haggas et al., 2001). Detailed interpretation of FMS images in combination with other log data and core imaging will be carried out postcruise.
The UBI features a high-resolution transducer that provides acoustic images of the borehole wall. The transducer emits ultrasonic pulses at a frequency of 400 kHz, which are reflected at the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined (Fig. F21). A continuous rotation of the transducer and the upward motion of the tool produce a complete map of the borehole wall.
The amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the UBI tool in the borehole, the shape of the borehole, and the roughness of the borehole wall. Changes in the borehole wall roughness (e.g., at fractures intersecting the borehole) are responsible for the modulation of the reflected signal; therefore, fractures or other variations in the character of the drilled rocks can easily be recognized in the amplitude image. The recorded traveltime image gives detailed information about the shape of the borehole, which allows calculation of one caliper value of the borehole from each recorded traveltime. Amplitude and traveltime are recorded together with a reference to magnetic north by means of a magnetometer, permitting the orientation of images. If features (e.g., fractures) recognized in the core are observed in the UBI images, orientation of the core is possible. The UBI orientated images can also be used to measure stress in the borehole through identification of borehole breakouts and slip along fault surfaces penetrated by the borehole (i.e., Paillet and Kim, 1987). In an isotropic, linearly elastic rock subjected to an anisotropic stress field, drilling a subvertical borehole causes breakouts in the direction of the minimum principal horizontal stress (Bell and Gough, 1983).
Downhole magnetic field measurements are made with the GPIT. The GPIT is used in combination with the FMS and the UBI. This sonde incorporates a three-component accelerometer and a three-component magnetometer, and its primary purpose is to determine the acceleration and orientation of the FMS and UBI tool strings. The acceleration data allow more precise determination of log depths than is possible on the basis of cable length alone, as the wireline is subject to both stretching and ship heave.
BGR (Germany) has developed three-component borehole magnetometers with gyro orientation since the 1980s. These tools were originally built for mineral exploration purposes (Bosum et al., 1988). The current tool (Fig. F22) has been designed for super-deep boreholes like the German KTB borehole and can withstand pressures as high as 800 bar and temperatures up to 230°C (Bosum, 1992). Modified versions of the BGR magnetometer have been deployed during previous ODP legs: Leg 102, Hole 418A (Bosum and Scott, 1988), Leg 109, Hole 395A (Bosum and Kopietz, 1990) and Leg 148, Holes 504B and 896A (Worm et al., 1996).
The tool consists of three fluxgate sensors in the lower part of the instrument that log the horizontal (x- and y-) and the vertical (z-) components of the magnetic flux density. Attached to the same mount are two tiltmeters, which record the tilt angle in the x- and y-axis, respectively. The upper part of the tool is equipped with an angular rate sensor (gyro) monitoring the tool's rotation around its z-axis. The tool connects to a Gearhart-Owen GO 7 cable head. The use of centralizers is highly recommended since they reduce rotation and motion in regular halt phases, which are necessary for offset and drift correction of the gyro data. The housing is made of nonmagnetic titanium. Specifications are listed in Table T18. Since rotation angles around x-, y-, and z-axes are measured, the recorded magnetic flux density vector can be unrotated for all axes using principal axes rotation matrices. The processed magnetic data is referenced to the ship's heading (northing) at the start and end of the logging run.
The WST is used to produce a zero-offset vertical seismic profile and/or checkshots in the borehole. The WST consists of a single geophone used to record the full waveform of acoustic waves generated by a seismic source positioned just below the sea surface. During Leg 206, an 80-in3 generator-injector air gun, positioned at a water depth of ~7 m with a borehole offset of 50 m on the port side of the JOIDES Resolution, was used as the seismic source. The WST was clamped against the borehole wall at 30- to 50-m intervals, and the air gun was typically fired between 5 and 15 times at each station. The recorded waveforms were stacked, and a one-way traveltime was determined from the median of the first breaks for each station, thus providing checkshots for calibration of the integrated transit time calculated from sonic logs. Checkshot calibration is required for the core-seismic correlation because P-wave velocities derived from the sonic log may differ significantly from true formation velocity because of (1) frequency dispersion (the sonic tool operates at 10-20 kHz, but seismic data are in the 50- to 200-Hz range), (2) difference in travel paths between well seismic and surface seismic surveys, and (3) borehole effects caused by formation alterations (Schlumberger, 1989). In addition, sonic logs cannot be measured through pipe, so the traveltime down to the uppermost logging point has to be estimated by other means.
Temperature measurements were taken during coring in Hole 1256B to determine the in situ temperatures within the sediment column for the purpose of measuring heat flow. The discrete in situ measurements were made with the Advanced Piston Coring Temperature (APCT) tool, formerly Adara), which is located in an annulus in the coring shoe of the APC during piston coring operations. The components of the tool include a platinum temperature sensor and a battery-powered data logger. The platinum resistance-temperature device is calibrated for temperatures ranging from -20° to 100°C, with a resolution of 0.01°C. During operation, the adapted coring shoe is mounted on a regular APC core barrel and lowered down the pipe by wireline. The tool is typically held for 5-10 min at the mudline to equilibrate with bottom-water temperatures and then is lowered to the bottom of the drill string. Standard APC coring techniques are used, with the core barrel being fired out through the drill bit using hydraulic pressure. The APCT tool (and the APC corer) remains in the sediment for 10-15 min to obtain a temperature record. This provides a sufficiently long transient record for reliable extrapolation back to the steady-state temperature. The nominal accuracy of the temperature measurement is ±0.1°C.
Data reduction for the APCT tool estimates the steady-state bottom-hole temperature by forward-modeling the recorded transient temperature curve as a function of time. The shape of the transient temperature curve is determined by the response function of the tool and the thermal properties of the bottom-hole sediment (Bullard, 1954; Horai and von Herzen, 1985). A synthetic curve is constructed based on the tool geometry, sampling interval, and the properties of the tool and surrounding sediments. It is difficult to obtain a perfect match between the synthetic curves and the data because (1) the probe never reaches thermal equilibrium during the penetration period; (2) contrary to theory, the frictional pulse upon insertion is never instantaneous; and (3) temperature data are sampled at discrete intervals, meaning that the exact time of penetration is always uncertain. As a result, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data.
In preparation for logging, a borehole is usually flushed of debris by circulating a "pill" of viscous drilling fluid (sepiolite mud mixed with seawater; approximate weight = 8.8 lb/gal or 1.11 g/cm3) through the drill pipe to the bottom of the hole. The BHA is pulled up to a depth of between 50 and 100 mbsf then run down to the bottom of the hole again to ream borehole irregularities. The hole is subsequently filled with more sepiolite mud, and the pipe is raised to ~50-80 mbsf and kept there to prevent hole collapse during logging. The tool strings are then lowered downhole during sequential runs. The tool strings are pulled uphole at constant speed to provide continuous measurements as a function of depth of several properties simultaneously. After the logs are acquired, the data are transferred to the downhole measurements laboratory (DHML) and also to LDEO-BRG for processing using a high-speed satellite data link.
Each tool string also contains a telemetry cartridge, facilitating communication from the tools along a double-armored seven-conductor wireline cable to the Schlumberger Minimum Configuration Maxis (MCM) computer van on the drill ship. The 9000-m-long logging cable connects the MCM to the tool string through the logging winch and LDEO-BRG wireline heave compensator (WHC). The WHC is employed to minimize the effect of ship's heave on the tool position in the borehole. The logging winch is located aft of the pipe racker. The 160-m-long logging cable fairlead runs from the winch forward to the drill floor, through a sheave back to the heave compensator located alongside the logging winch, then forward to another sheave on the rig floor, up to the crown block on the top of the derrick, and then down into the drill string. As the ship heaves in the swell, an accelerometer located near the ship's center of gravity measures the movement and feeds the data, in real time, to the WHC. The WHC responds to the ship's heave by hydraulically moving the compensator sheave to decouple the movement of the ship from the desired movement of the tool string in the borehole.
Logging data quality may be seriously degraded by changes in the hole diameter and in sections where the borehole diameter greatly decreases or is washed out. Deep-investigation measurements such as resistivity and sonic velocity are less sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data in order to reduce these effects. Very large washouts, however, cannot be corrected for. HNGS and SGT data provide a depth correlation between logging runs, but logs from different tool strings may still have minor depth mismatches caused by either cable stretch or ship heave during recording. Ship heave is minimized by the WHC, designed to adjust for rig motion during logging operations.
Data for each wireline logging run are recorded and stored digitally and monitored in real time using the Schlumberger MCM system. After logging is completed in each hole, data are transferred to the shipboard DHML for preliminary processing and interpretation. FMS image data are interpreted using Schlumberger's Geoframe version 4.0.2 software package. Well seismic, sonic, and density data are used for calculation of synthetic seismograms with GeoQuest's IESX software package in order to relate specific seismic reflectors to depths in the borehole.
Log data are transmitted to LDEO-BRG using a FFASTEST satellite high-speed data link for processing soon after each hole is logged. Data processing at LDEO-BRG consists of (1) depth-shifting all logs relative to a common datum (i.e., mbsf), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO-BRG, log data are transmitted back to the ship, providing near real-time data processing. Log curves of LDEO-BRG-processed data are then replotted on board (see "Downhole Measurements" in "The Sedimentary Overburden (Holes 1256A, 1256B, and 1256C)" in the "Site 1256" chapter). Further postcruise processing of the log data from the FMS is performed at LDEO-BRG. Postcruise-processed acoustic, caliper, density, gamma ray, neutron porosity, resistivity, and temperature data in ASCII are available directly from the LDEO-BRG Internet Web site at www.ldeo.columbia.edu/BRG/ODP/DATABASE/. A summary of "logging highlights" is posted on the LDEO-BRG Web site at the end of each leg.
Downhole logging aboard the JOIDES Resolution is provided by LDEO-BRG in conjunction with Leicester University Borehole Research, the Laboratoire de Geophysique et d'Hydrodynamique en Forage Montpellier, University of Aachen, University of Tokyo, and Schlumberger Well Logging Services.
