f) is also measured by the tool.
Downhole logs are used to determine physical, chemical, and structural properties of the formation penetrated by a borehole. The data are rapidly collected, continuous with depth, and measured in situ; they can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, log data may provide the only way to characterize the borehole section; where core recovery is good, log and core data complement one another and may be interpreted jointly.
Downhole logs are sensitive to formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. They are useful in calibrating the interpretation of geophysical survey data (e.g., through the use of synthetic seismograms) and provide a necessary link for the integrated understanding of physical properties on all scales. Wireline logging was performed at Sites 1207 and 1213.
During wireline logging, the logs are made with a variety of Schlumberger logging tools combined into several "tool strings," which are run down the hole after coring operations are complete. Three wireline tool strings were used during Leg 198: the triple combination (triple combo) (resistivity, density, and porosity); the Formation MicroScanner (FMS)-sonic (resistivity image of the borehole wall and sonic velocities); and the geological high-resolution magnetic tool (GHMT) (magnetic field strength and magnetic susceptibility) (Fig. F8; Table T7).
Each tool string also contains a telemetry cartridge for communicating through the wireline to the Schlumberger minimum configuration MAXIS (MCM) unit on the drillship and a natural gamma radiation tool that provides a common reference for correlation and depth shifting between multiple logging runs. Logging runs are typically conducted at 250-300 m/hr.
In preparation for logging, the boreholes were flushed of debris by circulating a "pill" of viscous drilling fluid (sepiolite mud mixed with seawater; approximate weight = 8.8 lb/gal, or 1.055 g/cm3) through the drill pipe to the bottom of the hole. The bottom-hole assembly (BHA) was pulled up to a depth of 160 mbsf for the Hole 1207B and 90 mbsf for Hole 1213B, and then run down to the bottom of the hole again to ream borehole irregularities. The holes were subsequently filled with more sepiolite mud, and the pipe was raised to 160 mbsf for Hole 1207B and 126 mbsf for Hole 1213B and kept there to prevent hole collapse during logging. The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs. A wireline heave compensator (WHC) was employed to minimize the effect of ship's heave on the tool position in the borehole (Goldberg, 1990). During each logging run, incoming data were recorded and monitored in real time on the MCM logging computer. The tool strings were then pulled up at constant speed to provide continuous measurements as a function of depth of several properties simultaneously.
In addition to the Schlumberger tools, we also used the LDEO multisensor spectral gamma ray tool (MGT) at Hole 1207B; it was positioned at the top of the triple combo tool string. Data from this tool are recorded in real time on the acquisition system in the downhole measurements laboratory (DHML). Data from the MGT and the Schlumberger tools cannot be recorded simultaneously: the MGT requires a second pass in the hole.
The logged properties, and the methods that the tools use to measure them, are briefly described below. The main logs taken by the tools are listed in Table T8. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989).
Three wireline spectral gamma ray tools were used to measure and classify natural radioactivity in the formation: the natural gamma ray tool (NGT), the hostile environment natural gamma ray sonde (HNGS) and the LDEO MGT. The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of K, Th, and U, the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for a significantly improved tool precision. The HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. Although the NGT response is sensitive to borehole diameter and the density of the drilling mud, corrections for these effects are routinely made during processing at LDEO. See "LDEO Multisensor Spectral Gamma Ray Tool" for a description of the MGT.
Formation density was determined with the hostile environment lithodensity sonde (HLDS). The sonde contains 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. Gamma rays emitted by the source undergo Compton scattering, which 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 density of electrons in the formation, which is in turn related to bulk density. Porosity may also be derived from this bulk density if the matrix (grain) density is known.
The HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of the gamma rays occurs when they reach <150 keV after being repeatedly scattered by electrons in the formation. Because PEF depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present. For example, the PEF of calcite = 5.08 b/e-; illite = 3.03 b/e-; quartz = 1.81 b/e-; and kaolinite = 1.49 b/e-. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.
Formation porosity was measured with the accelerator porosity sonde (APS). The sonde incorporates a minitron neutron generator that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one thermal) positioned at different spacings from the minitron. The measurement principle involves counting neutrons that arrive at the detectors after being slowed by atomic nuclei in the formation. The highest energy loss occurs when neutrons collide with hydrogen nuclei, which have practically the same mass as the neutron (the neutrons simply bounce off of heavier elements without losing much energy). If the hydrogen (i.e., water) concentration is low, as in low-porosity formations, neutrons can travel farther before being captured and the count rates increase at the detector. The opposite effect occurs in high porosity formations where the water content is high. However, because hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.
Upon reaching thermal energies (0.025 eV), the neutrons are captured by the nuclei of Cl, Si, B, and other elements, resulting in a gamma ray emission. This neutron capture cross section (f) is also measured by the tool.
The phasor dual induction/spherically focused resistivity tool (DIT) was used to measure electrical resistivity. The DIT provides three measures of electrical resistivity, each with a different depth of investigation into the formation. The two induction devices (deep and medium depths of penetration) transmit high-frequency alternating currents through transmitter coils, creating magnetic fields that induce secondary currents in the formation. These currents produce a new inductive signal, proportional to the conductivity of the formation, which is measured by the receiving coils. The measured conductivities are then converted to resistivity (in units of ohm-meters). For the shallow penetration resistivity, the current necessary to maintain a constant drop in voltage across a fixed interval is measured; it is a direct measurement of resistivity. Sand grains and hydrocarbons are electrical insulators, whereas ionic solutions and clays are conductors. Electrical resistivity, therefore, can be used to evaluate porosity (via Archie's Law) and fluid salinity.
The dipole shear sonic imager measures the transit times between sonic transmitters and an array of eight receivers. It combines replicate measurements, thus providing a direct measurement of sound velocity through sediments that is relatively free from the effects of formation damage and an enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it also has two crossed dipole transmitters, which allow the measurement of shear wave velocity in addition to the compressional wave velocity, even in the slow formations typically encountered during ODP legs.
The FMS provides high-resolution electrical resistivity-based images of borehole walls. The tool has four orthogonal arms and pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording (Fig. F8). The electrodes are arranged in two diagonally offset rows of eight electrodes each. A focused current is emitted from the button electrodes into the formation, with a return electrode near the top of the tool. The intensity of current passing through the button electrodes is measured. Processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous spatially oriented high-resolution images that mimic the geologic structures of the borehole wall. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation.
The development of the FMS tool has added a new dimension to wireline logging (Luthi, 1990; Lovell et al., 1998; Salimullah and Stow, 1992). Features such as bedding, fracturing, slump folding, and bioturbation can be resolved; the fact that the images are oriented means that fabric analysis can be carried out and bed orientations can be measured.
The maximum extension of the caliper arms is 15 in. In holes with a diameter larger than 15 in, the pad contact will be inconsistent, and the FMS images may appear out of focus and too conductive. Irregular (rough) borehole walls will also adversely affect the images if contact with the wall is poor.
Three-component acceleration and magnetic field measurements were made with the general-purpose inclinometer tool. The primary purpose of this tool, which incorporates a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the FMS-sonic tool string during logging. Thus, the FMS images can be corrected for irregular tool motion, and the dip and direction (azimuth) of features in the FMS image can be determined.
The susceptibility measurement sonde (SUMS) measures magnetic susceptibility by means of low-frequency induction in the surrounding sediment. It responds primarily to magnetic minerals (mainly magnetites, hematite, and iron sulfides), which are typically contained in the detrital sediment fraction.
The nuclear resonance magnetometer sonde (NMRS) measures the total magnetic field using a proton precession magnetometer. The data from the SUMS and the NMRS tools can be used to construct a polarity stratigraphy, using the method outlined below.
The total magnetic field (B) measured in the borehole depends on position (p) and time (t) (Pozzi et al., 1988, 1993):
where Bf (p) = Bfi (p) + Bfr (p).
Br (p) is the Earth's main magnetic field, generated in the Earth's liquid outer core. The field intensity is ~41970 nT for Site 1207. Ba (p) is the magnetic field caused by the BHA (up to ~2000 nT, decaying away from the BHA) and crustal heterogeneities. Bt (p,t) is the time-varying field (e.g., magnetic storms). Two passes of the GHMT are run to check that this is negligible. Bfi (p) is the field produced in the borehole by the induced magnetization (Ji) of the sediment that is parallel to B0 (p,t) and proportional to the magnetic susceptibility (k):
Bfi (p) is then given by
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 (Jr) of the sediment, whose polarity we aim to determine. Jr is either 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).
Under favorable conditions, a magnetostratigraphy is given simply by the sign of Bfr (p). Further processing, completed on shore, involves regression analysis of Bfr (p) vs. Bfi (p) downhole on intervals of various thickness. Correlation indicates normal polarity, and anticorrelation indicates reversed polarity. In the case of the Shatsky Rise sediments, the polarity interpretation is complicated and the sites have moved 30° northward since the Early Cretaceous, so the induced and remanent magnetizations will not be parallel to each other. The GHMT was run at Site 1207; however, both the magnetic field and susceptibility logs were compromised by tool malfunction (see "Downhole Measurements" in the "Site 1207" chapter).
The MGT was developed by the LDEO Borehole Research Group to improve the vertical resolution of natural gamma ray logs by using an array of four short detector modules with ~2-ft spacing. Each module comprises a small 2 in x 4 in NaI detector, a programmable 256-channel amplitude analyzer, and an 241Am calibration source. The spectral data are later recalculated to determine the concentration of K, Th, and U radioisotopes. The spectral data from individual modules are sampled four times per second and stacked in real time based on the logging speed. This approach increases vertical resolution by a factor of 2-3 over conventional tools while preserving comparable counting efficiency and spectral resolution. The radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity) and the energy of the gamma rays (a higher-energy gamma ray can reach the detector from deeper in the formation).
The MGT also includes an accelerometer to improve data stacking by the precise measurement of tool motion. The MGT is typically deployed on top of the Schlumberger triple combo. It has a specialized telemetry system that requires that the Schlumberger tools be powered off while the MGT is collecting data. Postcruise processing may correct for borehole size and tool sticking, which is assessed by accelerator data recorded in the MGT.
The principal influence on log data quality is the condition of the borehole wall. If the borehole diameter is variable over short intervals resulting from washouts during drilling, clay swelling, or ledges caused by layers of harder material, the logs from those tools that require good contact with the borehole wall (i.e., FMS, density, and porosity tools) may be degraded. Deep investigation measurements such as resistivity and sonic velocity, which do not require contact with the borehole wall, are generally less sensitive to borehole conditions. Very narrow ("bridged") sections will also cause irregular log results. The quality of the borehole is improved by minimizing the circulation of drilling fluid while drilling, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning are completed.
The depth of the wireline-logged measurement is determined from the length of the logging cable played out at the winch on the ship. The seafloor is identified on the natural gamma log by the abrupt reduction in gamma ray count at the water/sediment boundary (mudline). The coring depth (driller's depth) is determined from the known length of the BHA and pipe stands; the mudline is usually recovered in the first core from the hole.
Discrepancies between the driller's depth and the wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, and drill pipe stretch in the case of driller's depth. In the case of log depth, these discrepancies occur because of incomplete heave compensation, cable stretch (~1 m/km), and cable slip. Tidal changes in sea level will also have an effect. To minimize the wireline tool motion caused by ship heave, a hydraulic wireline heave compensator adjusts for rig motion during wireline logging operations. The small but significant differences between drill pipe depth and logging depth should be taken into account when using the logs for correlation with core and log measurements. Core measurements such as susceptibility and density can be correlated with the equivalent downhole logs using the Sagan program, which allows shifting of the core depths onto the log depth scale. Precise core-log depth matching is difficult in zones where core recovery is low, because of the inherent ambiguity of placing the recovered section within the cored interval.
Logs from different wireline tool strings will have slight depth mismatches. Distinctive features recorded by the natural gamma tool, run on every tool string, provide correlation and relative depth offsets among the logging runs.
Data for each logging run were recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging at each hole, data were transferred to the DHML for preliminary interpretation. Basic processing was carried out during the cruise to provide scientists with a comprehensive, quality-controlled downhole logging data set that can be used for comparison, integration, and correlation with other data collected during Leg 198 and other ODP legs. The processing includes depth adjustments to remove depth offsets between data from different logging runs, corrections specific to certain tools and logs, documentation for the logs (with an assessment of log quality), and conversion of the data to a widely accessible format (ASCII for the conventional logs; GIF for the FMS images). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing.
Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available (see the "Related Leg Data" contents list). A summary of logging highlights is also posted on the LDEO-BRG Web site shortly after the end of each leg.
GeoFrame's IESX seismic interpretation software package was used during Leg 198 to display site-survey seismic sections acquired precruise on the Thompson. Velocity and density logs were used to create synthetic seismograms, which were overlaid on the seismic section and used to refine the depth-traveltime relation. In this way, lithostratigraphic units in the core are correlated with reflectors and sequences in the seismic section.
