Downhole logs are continuous records of the in situ physical properties of the formation penetrated by a borehole. The logs are made using a variety of sensors, called sondes, which are stacked together in combinations known as tool strings. These are lowered down the hole on a heave-compensated electrical wireline and then pulled up at constant speed while acquiring data from which many properties can be determined as a function of depth.
Log data are used to interpret formation structure, stratigraphy, lithology and mineralogy. 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 Formation MicroScanner (FMS) images may be used for core orientation. Synthetic seismograms constructed from borehole sonic data provide the best-known method for traveltime-to-depth conversion of surface seismic data. Finally, downhole logs are sensitive to formation properties on scales that are intermediate between those of laboratory measurements on core samples and those of surface geophysical surveys. They therefore provide a necessary link in the integrated understanding of physical properties on all scales.
Two combinations of downhole sensors were used during Leg 192: the geophysical tool string and the FMS/sonic tool string (Fig. F7). With the exception of the Lamont-Doherty high-resolution temperature/acceleration/pressure (TAP) tool, the tool strings used during Leg 192 were made up exclusively of Schlumberger sondes.
The geophysical tool string (Fig. F7) measures formation electrical resistivity, density, porosity, and radioactivity, and is therefore also known as the "quad combo." The tools that make up this string are the accelerator porosity sonde (APS), the hostile-environment natural gamma ray sonde (HNGS), the hostile-environment lithodensity sonde (HLDS), and the phasor dual induction-spherically focused resistivity tool (DITE-SFR). The TAP tool was attached to the bottom of the Schlumberger tool string.
The FMS/sonic tool string (Fig. F7) provides electrical images of the borehole walls and determines acoustic velocity in the formation and the downhole magnetic field vector. The tools that make up this string are the FMS logging tool (Fig. F8), the general-purpose inclinometer (three-axis magnetometer-inclinometer) tool (GPIT), and the dipole shear sonic imager (DSI). The natural gamma spectrometry tool (NGT), which measures radioactivity, was included in this tool string.
Each tool string includes a telemetry cartridge for communicating through the wireline with the logging laboratory on the drillship and a natural gamma ray (NGR) sonde that identifies lithologic markers, providing a common reference for depth-matching between multiple logging runs.
The logging tools, their operating principles, log applications, and factors affecting data quality are briefly described below. Principal data channels of the tools, their physical significance, and units of measurement are listed in Table T8. The logging speeds used during Leg 192, properties of the formation determined by each tool, measurement sampling intervals, approximate vertical resolutions, and depths of investigation are summarized in Table T9. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Hearst and Nelson (1984), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and on the Lamont-Doherty Borehole Research Group Web site (see the "Related Leg Data" contents list).
We used two spectral gamma ray tools to measure natural radioactivity in the formation: the NGT and the HNGS. The NGT uses a sodium iodide scintillation detector and five-window spectrometry to determine concentrations of potassium, thorium, and uranium, 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 and 256-window spectrometry for significantly improved tool precision. The hostile-environment natural gamma ray sonde derives its name from the fact that it is rated to a much higher temperature (260°C) than the natural gamma spectrometry tool (149°C).
The gamma ray log is one of the few that provides useful data in cased wells. It is therefore used as a correlation curve for depth matching between individual logging runs, with lithologic markers or the seafloor acting as reference points. Gamma ray logs may also be used for clay typing, mineralogy, and ash-layer detection. Clean sedimentary formations usually have a very low level of radioactivity. Potassium and thorium tend to concentrate in clays and shales. An increase of potassium in carbonates can be related to either the presence of algal material or glauconite, whereas the presence of uranium is often more closely associated with organic matter. Thorium is commonly found in ash layers.
Because the natural gamma ray response is sensitive to borehole diameter and the weight and concentration of bentonite or potassium chloride present in the drilling mud, corrections are routinely made for these effects during data processing by the Borehole Research Group at Lamont-Doherty Earth Observatory.
We determined formation density from gamma ray attenuation with the hostile-environment lithodensity sonde. The sonde contains a radioactive cesium (137Cs) gamma ray source (622 keV) and far and near gamma ray detectors mounted on a shielded skid that is pressed against the borehole wall by a hydraulically activated arm. Gamma rays emitted by the source experience Compton scattering, which involves the transfer of energy from gamma rays to electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is related to the density of electrons in the formation, which is, in turn, related to bulk density. Porosity may be derived from this bulk density if the matrix density is known.
The HLDS also measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach energies of <150 keV after being repeatedly scattered by electrons in the formation. Because photoelectric absorption depends strongly on the mean atomic number of the elements in the formation, it varies according to chemical composition and is essentially independent of porosity. For example, the PEF of pure calcite = 5.08 barn/electron, illite = 3.03 barn/electron, quartz =1.81 barn/electron, and kaolinite = 1.49 barn/electron. PEF values can therefore give an indication of the chemical composition of the formation and can be used in combination with natural gamma ray data to identify different clay minerals.
Formation porosity was determined using the accelerator porosity sonde. The sonde incorporates a minitron neutron generator which produces fast (14.4 MeV) neutrons. Five neutron detectors (four epithermal and one thermal) measure the number and arrival times of neutrons at different distances from the source. Neutrons emitted from the source are slowed by collisions with nuclei in the formation, experiencing an energy loss that depends on the relative mass of the nuclei with which the neutrons collide. Maximum energy loss occurs when a neutron strikes a hydrogen nucleus. As hydrogen is mainly present in pore water, the neutron log essentially measures porosity, assuming pore-fluid saturation. However, as clays and hydrocarbons also contain hydrogen, the log often overestimates raw porosity and the results should be treated with caution.
The electrical resistivity of the formation was measured with the phasor dual induction-spherically focused resistivity tool. This sonde provides three measures of electrical resistivity based on different depths of investigation: shallow, medium, and deep (Table T9). Shallow-penetration measurements with a high vertical resolution are made with a spherically focused laterolog. This is a constant-current device that uses bucking electrodes to focus the path taken by the measurement current in the formation. Medium- and deep-penetration measurements are made inductively using transmitter coils energized with high-frequency alternating currents, creating time-varying magnetic fields that induce secondary Foucault currents in the formation. The strength of the induced ground currents is inversely proportional to the resistivity of the formation through which they circulate, as are the secondary inductive fields that they create. Formation resistivity is deduced from the amplitude and phase of the secondary magnetic fields, which are measured with receiving coils.
Induction sondes are most
accurate in low- to medium-resistivity formations (<100 
m),
whereas focused continuous-current devices such as the Schlumberger Dual
Laterolog (DLL) are preferable for measurements in resistive basalts. As the
greater part of the logged interval was in sediments during Leg 192, the dual
induction tool was selected over the DLL to measure electrical resistivity.
The solid constituents of
crustal rocks are highly resistive relative to the pore fluids. Electricity is
conducted primarily by ion transport through pore fluids, and electrical
conductivity strongly depends on porosity and pore connectivity. Electrical
resistivity data can accordingly be used to estimate formation porosity using
Archie's Law (Archie, 1942) if the formation does not contain clay. Archie's Law
is expressed as FF = a 
-m,
where FF is the formation factor (the ratio of the formation
resistivity to that of the pore fluids);
is the porosity; m is known as the cementation factor and depends on
the tortuosity and connectivity of pore spaces; and a is a constant that varies
with rock type.
Downhole temperature, acceleration, and pressure were measured with the Lamont-Doherty high-resolution temperature/acceleration/pressure tool. Attached to the bottom of the geophysical tool string, this sonde operates in an autonomous mode, with data stored in built-in memory. A two-component thermistor (for different temperature ranges) is mounted near the bottom of the tool in a slotted protective cover. The time constant of the thermistor assembly in water is ~0.4 s. The tool also includes a pressure transducer (0-10,000 psi), which is used to activate the tool at a specified depth and to measure pressure, and a high-sensitivity vertical accelerometer, which provides data for analyzing the effects of heave on a deployed tool string. Temperature and pressure data are recorded once per second, and accelerometer data can be recorded at a 4- or 8-Hz sampling rate.
The temperature record must be interpreted with caution, as the amount of time elapsed between the end of drilling and the logging operation is generally not sufficient to allow the borehole to recover thermally from the influence of cold drilling fluid circulation. The temperature profile recorded under such circumstances may differ significantly from that of the surrounding formation. Nevertheless, abrupt changes in the temperature gradient can indicate regions of fluid entrainment in, or inflow from, permeable layers. In cases where the borehole has thermally reequilibrated with the surrounding formation, we can determine vertical heat flow from the temperature gradient combined with measurements of thermal conductivity on core samples.
The dipole shear sonic imager employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of lithologies. The DSI measures the transit times between sonic transducers and an array of eight receivers. Along with the monopole transducers found on most sonic tools, the DSI has two crossed dipole transducers. These allow the determination of shear and Stoneley wave velocities in addition to compressional wave velocity, even in the seismically slow formations commonly encountered in ODP boreholes.
Data from the DSI are used to generate synthetic seismograms for traveltime-to-depth conversions of surface seismic data.
Downhole magnetic field measurements were made with the general purpose inclinometer tool. The primary purpose of this sonde, which incorporates a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the FMS during logging. 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. Acceleration data are also used in processing FMS data to correct the images for irregular tool motion.
Local magnetic anomalies, generated by high remanent magnetization of basalts in the basement section of a borehole, can interfere with the determination of tool orientation. However, these magnetic anomalies are frequently used to infer the magnetic stratigraphy of the basement section.
The FMS provides high-resolution, electrical-resistivity-based images of borehole walls. The tool has four orthogonal arms (pads), each containing 16 microelectrodes known as "buttons," which are pressed against the borehole wall during the 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. Processing transforms these measurements of the microresistivity variations of the formation into continuous, spatially oriented high-resolution images that mimic geologic structures behind the borehole walls. Further processing can provide the dip and direction (azimuth) of planar features in the formation. FMS images are particularly useful for mapping structural features, dip determination, detailed core-log correlation, positioning of core sections with poor recovery, and analysis of depositional environments.
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.
To produce reliable FMS images, the pads must be firmly pressed against the borehole wall. The maximum extension of the caliper arms is 38 cm (15 in). In holes with a diameter larger than this, the pad contact is inconsistent and the images are blurred. Irregular borehole walls also adversely affect the images as contact with the wall is poor.
The quality of log data may be seriously degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. Resistivity and velocity data are the least sensitive to borehole effects, whereas nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays) are most sensitive because of the large attenuation by borehole fluid. Corrections can be applied to the original data to reduce the effects of most borehole conditions that depart from those under which the tool was calibrated.
Logs from different tool strings may have depth mismatches caused by either cable stretch or ship heave during recording. Small errors in depth matching can distort the logging results in zones of rapidly changing lithology. To minimize the effects of ship heave, a hydraulic wireline heave compensator adjusts for rig motion during logging operations. Distinctive features recorded by natural gamma ray tools, included on every tool string, provide correlation and relative depth offsets between logging runs and can be calibrated to distinctive lithologic contacts observed in the recovered core or during drilling operations (e.g., basement contacts).
Data for each logging run are recorded, stored digitally, and monitored in real time using the Schlumberger multitasking acquisition and imaging system (MAXIS 500). On completion of logging at each hole, data are transferred to the shipboard downhole measurements laboratory for preliminary interpretation. FMS image data are processed onboard using Schlumberger GeoQuest's "Geoframe" software package.
Additional processing of the logs is carried out onshore by the Borehole Research Group at Lamont-Doherty Earth Observatory, after the data are transmitted by satellite from the ship. It includes adjustments to remove depth offsets between data from different logging runs; corrections for borehole conditions 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 and GIF for 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 at the Lamont-Doherty Borehole Research Group Web site (see the "Related Leg Data" contents list). A summary of logging highlights is also posted on the BRG Web site.
