DOWNHOLE MEASUREMENTS

Downhole logs are continuous and in situ records of physical and structural properties of the formation penetrated by a borehole. The logs are made using a variety of probes, combined into several tool strings (Fig. F24). These strings are lowered down the hole on heave-compensated electrical wireline and then pulled up at constant speed to provide continuous measurements as a function of depth of several properties simultaneously.

Downhole logging was used during Leg 197 to address issues concerning core reorientation and volcanic stratigraphy and morphology. Whereas core recovery is often biased and incomplete in variable lithology such as alternating pillows and massive flows, logging data are continuous and therefore provide useful information over intervals of low core recovery. During Leg 197, we were particularly interested in determining the number of flow units, which has implications for how well geomagnetic secular variation has been sampled and, hence, how well paleomagnetic paleolatitudes can be constrained. Logging data were used to create synthetic seismograms, which then led to improved correlation between the seismic records and the lithologic units recovered from the boreholes. Moreover, to achieve these objectives, the logging plans also included a three-component fluxgate magnetometer and magnetic susceptibility tool. In comparison to core analysis, magnetic borehole logging yields a dense vertical profile of magnetic variations. A disadvantage of three-component magnetic logging is the lack of horizontal orientation because the tool spins around its z-axis during a log run. Consequently, only the strength of the horizontal magnetic field can be routinely obtained. In a strongly magnetized formation, resolution of the direction of the spinning horizontal components is poor. Therefore, information about the declination at depth remains inaccessible. If the orientation of the horizontal components of the magnetic field during the log run could be referred to a fixed reference system (geographic system or Global Positioning System [GPS]), it would allow us to resolve the horizontal field into its components and to constrain magnetic declination.

Logging Tool Strings

The tool strings planned for Leg 197 were

  1. The triple combination (triple combo) tool string (resistivity, density, and porosity) (Fig. F24), which consists of the accelerator porosity sonde (APS), the hostile-environment natural gamma sonde (HNGS), the high-temperature lithodensity tool (HLDT), and the dual laterolog (DLL). The Lamont-Doherty Earth Observatory (LDEO) temperature/acceleration/pressure (TAP) tool was attached to the bottom of the tool string.
  2. The FMS/sonic tool string (Figs. F24, F25), which consists of the FMS, the GPIT, a dipole shear sonic imager (DSI), and the natural gamma tool (NGT). This last tool was included between the DSI and part of the FMS.
  3. The three-component fluxgate magnetometer: Goettingen Borehole Magnetometer (GBM).
  4. The magnetic susceptibility tool: SUSLOG 403-D.

Each tool string includes a telemetry cartridge for communicating through the wireline with the logging laboratory on the drill ship and a natural gamma ray sonde, which is used to identify lithologic markers, providing a common reference for correlation and depth between multiple logging runs.

In the following sections, the basic principles of the tools and types of measurements are summarized. The principal data provided by the tools, their physical significance, and units of measure are listed in Table T10. The operating principles, applications, and approximate vertical resolution of the tools are summarized in Table T11. More information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and on the LDEO Borehole Research Group (BRG) World Wide Web site (http://www.ldeo.columbia.edu/BRG/).

Natural Radioactivity

Two spectral gamma ray tools were used to measure and classify natural radioactivity in the formation: the HNGS and the NGT. The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million), 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 spectroscopy, which significantly improves the tool precision. Spectral analysis in the HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or potassium chloride in the drilling mud and improving measurement accuracy. Because the NGT 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 to diminish these effects during processing at LDEO.

For both tools, gamma ray values are measured in American Petroleum Institute (API) units. These units are derived from the primary Schlumberger calibration test facility in Houston, Texas, where a calibration standard is used to normalize each tool.

Density

Formation density was determined from the density of electrons in the formation, which was measured with the 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 experience 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 number 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 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 <150 keV after being repeatedly scattered by electrons in the formation. Photoelectric absorption is strongly dependent on the atomic number of the constituents of the formation; it varies according to the chemical composition and is essentially independent of porosity. For example, the PEF of pure calcite = 5.08 b/e-, illite = 3.03 b/e-, quartz = 1.81 b/e-, and kaolinite = 1.49 b/e-. PEF values can be used in combination with NGT curves to identify different types of clay minerals. The PEF values, therefore, can give an indication of the chemical composition of the rock. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values.

Porosity

Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator, which produces fast (14.4 MeV) neutrons, and five neutron detectors (four epithermal and one thermal) positioned at different spacings. The tool is pressed against the borehole wall by an eccentralizing bow spring. Emitted neutrons are slowed by collisions. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus nearly equal to its own mass, such as hydrogen, which is mainly present in the pore water. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times, which act as a measure of formation porosity. However, as hydrogen bound in minerals such as clay or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.

Temperature, Acceleration, and Pressure

Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP tool. When attached to the bottom of the triple combo string, the TAP tool 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 the slotted protective cover. The time constant of the thermistor assembly in the water is ~0.4 s. The tool includes a pressure transducer (0-10,000 psi), which is used to activate the tool at a specified depth and perform pressure measurements. The TAP tool also incorporates a high-sensitivity vertical accelerometer, which provides data for analyzing the effects of heave on a deployed tool string, and an internal temperature sensor for monitoring the temperature inside the electronic cartridge. Temperature and pressure data are recorded once per second, and accelerometer data can be recorded at a 4- or 8-Hz sampling rate.

The borehole temperature record provides information on the thermal regime of the surrounding formation. The vertical heat flow can be estimated from the vertical temperature gradient combined with measurements of the thermal conductivity from core samples. 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 drilling fluid circulation. The data recorded under such circumstances may differ significantly from the thermal equilibrium of that environment. Nevertheless, from the spatial temperature gradient it is possible to identify abrupt temperature changes that may represent localized fluid flow into the borehole, indicative of fluid pathways and fracturing, and/or breaks in the temperature gradient that may correspond to contrasts in permeability at lithologic boundaries.

Acoustic Measurements

Sonic velocities were measured with the DSI tool, which employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1995). The DSI measures the transit times between sonic transmitters and an array of eight receivers. It averages replicate measurements, providing a direct measurement of sound velocity through formation that is relatively free from the effects of formation damage and enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it also has two crossed dipole transmitters. The DSI excites a flexural mode in the borehole, which can be used to determine shear wave velocity in all types of formations. The configuration of the DSI also allows recording of cross-line dipole waveforms, which can be used to estimate shear wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well. These "guided" waves are associated with the solid/fluid boundary at the borehole wall, and their amplitude exponentially decays away from the boundary in both the fluid and the formation.

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 measuring shear wave velocities of the upper parts of the basalt flow units.

Magnetic Field Measurement

Downhole magnetic field measurements were made with the GPIT. The GPIT is used in combination with the FMS. 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/sonic tool string 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 of FMS data to correct the images for irregular tool motion.

Local magnetic anomalies, generated by high remanent magnetization of the basalt in the basement section of a borehole, can interfere with the determination of tool orientation. However, these magnetic anomalies can be useful for constraining the magnetic stratigraphy of the basement section.

Electrical Resistivity Measurements

The DLL provides two resistivity measurements with different depths of investigation: deep (LLD) and shallow (LLS). 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 no current flows in the borehole between the two electrodes. For the LLD measurements, 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 LLS, 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, as the tool response ranges from 0.2 to 40,000 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. Electrical resistivity data can therefore 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 f -m,

where,

FF = formation factor (i.e., the ratio of the formation resistivity to that of the pore fluids),
f = porosity,
m = cementation factor, which is dependent on the tortuosity and connectivity of pore spaces, and
a = a constant that varies with rock type.

Electrical Images: Formation MicroScanner

The FMS provides high-resolution electrical resistivity-based images of borehole walls (Figs. F24, F25). The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or "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. 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, however, the pads must be firmly pressed against the borehole wall. In holes with a diameter >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. 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 Measurement"). 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. Further processing can provide measurements of 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, and positioning of core sections with poor recovery. 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 sedimentary structures and for constraining volcanic stratigraphy during previous ODP legs (Ayadi et al., 1998; Brewer et al., 1999). Detailed interpretation of FMS images in combination with other log data and core imaging will be carried out postcruise.

The Goettingen Borehole Magnetometer

The GBM tool was designed and developed in 1989 by the Geophysical Institute of the University of Goettingen, Germany (Fig. F26). The original application was to continuously monitor magnetic field variations in a borehole for several weeks and to compare these with field variations at depth (Steveling et al., 1991). The maximum operation pressure and temperature is 70 MPa and 100C, respectively. The tool consists of three fluxgate sensors that log the horizontal (x and y) and the vertical (z) components of the magnetic flux density. The tool is equipped with an angular rate sensor to monitor the spin history around the z-axis and variations around the x- and y-axis during a log run. The tool connects directly to the Schlumberger cable head. Centralizers can be optionally applied. The housing is made of low-magnetic monel and is not affected by pressure and temperature up to 70 MPa at 100C, respectively. Specifications are listed in Table T12.

Angular Rate Sensor

The LITEF miniature fiber-optic rate sensor (FORS) provides angular rate output. It has a small volume and low weight and requires little power (2 VA) (Fig. F27). Free from gravity-induced errors and with no moving parts, the sensor is insensitive to shock and vibration. The rate sensor is an unconventional gyro, since it does not have a spinning wheel. It detects and measures angular rates by measuring the frequency difference between two contra-rotating light beams. The light source is a superluminescent diode. Its broad spectrum provides light with short coherence length to keep the undesirable backscattering effects in the optical path to sufficiently low levels. The beam is polarized, split, and phase modulated. The output light travels through a 110-m-long fiber coil. The light travels to the detector, which converts the light into an electronic output signal. When the gyro is at rest, the two beams have identical frequencies. When the gyro is subjected to an angular turning rate around an axis perpendicular to the plane of the two beams, one beam then has a greater optical path length and the other beam has a shorter optical path length (Fig. F27). Therefore, the two resonant frequencies change and the frequency differential is measured by optical means, resulting in a digital output. Readings are output at 1 Hz. The angular rate is a function of time sampled with 5 Hz and the accumulated angle. The angular rate measured by the sensor is influenced by the Earth's rotation, which depends on the latitude () and varies from 15.04/hr at the poles to 0/hr at the equator (Fig. F28). From equator to pole, Earth's measured rotation increases by sin. To obtain the rotation rate about an inertial system, the effect of Earth's rotation must be eliminated. If the rotation rate around each axis is known, the orientation of the tool can be derived as a function of depth from the rotation history.

Magnetic Susceptibility: SUSLOG 403-D

The magnetic susceptibility tool (Krammer and Pohl, 1991) was previously deployed during Leg 109 (See Krammer, 1990, for Hole 395A, Mid-Atlantic Ridge). It consists of three vertically oriented coils at the lowermost part of the tool locked inside a nonconductive and nonmagnetic pressure tube (Fig. F26). This housing is made of a pressure-resistant ceramic surrounded by an elastomeric layer of silicon rubber for shock absorption and a fiberglass-reinforced epoxy resin to prevent leakage. The lowermost coil produces and transmits an alternating magnetic field with a frequency of 1000 Hz. In a second coil wound on the transmitter coil, a voltage is induced. This voltage is used to compensate that part of the receiver coil voltage that is induced in free air corresponding to the magnetic permeability (0) (susceptibility = 0). The remaining voltage is preamplified by a factor of 1000 and band-pass filtered to decrease induced noise. The spacing between transmitter and receiver coil is 40 cm. Two phase-sensitive detectors, with 90 phase shift between them, convert the amplified receiver signal to two DC voltages. The in-phase part of the signal is proportional to the electrical conductivity, and the quadrature part is proportional to the magnetic susceptibility of the surrounding formation. Both parts are separated by phase detectors and are displayed separately. Table T13 summarizes the specifications of the tool, which is designed to operate under temperatures and pressures up to 60C and 40 MPa, respectively.

Log Data Quality

The quality of log data may be seriously degraded by excessively wide sections of the borehole or by rapid changes in hole diameter. Resistivity and velocity measurements are the least sensitive to borehole effects, whereas the 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 these conditions and, generally, any departure from the conditions 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 the NGT, run on every log tool string, provide calibration points and relative depth offsets among the logging runs and can be correlated with distinctive lithologic contacts observed in the core recovery or drilling penetration (e.g., basement contacts).

Data Recording and Processing

Data for each logging run were recorded, stored digitally, and monitored in real time using the Schlumberger Minimum Configuration Maxis system. On completion of logging at each hole, data were transferred to the downhole measurements laboratory for preliminary interpretation. Basic processing was then carried out to provide scientists with a comprehensive, quality-controlled downhole log data set that can be used for comparison and integration with other data collected during each ODP leg. This processing is usually conducted onshore at LDEO after the data are transmitted by satellite from the ship. It 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 and GIF for the FMS images). Schlumberger GeoQuest's "GeoFrame" software package is used for most of the processing. Further postcruise processing of FMS log data is performed, and data are available 1 month after the cruise.

The magnetic susceptibility data recorded with the SUSLOG 403-D required several corrections. The signal depends on the borehole diameter. Variations of the borehole caliber or breakouts must be corrected. For correction, the recorded apparent susceptibility is multiplied by a factor that takes into account the borehole diameter and the deviation from the borehole axis. This correction is achieved by means of a correction chart (Krammer and Pohl, 1991). The application of the correction transforms the apparent susceptibility into a "true" susceptibility; however, it does not take into account the thickness of the layer. The vertical resolution increases with decreasing distances between transmitter and receiver coil but decreases the depth of investigation simultaneously.

Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in are available in ASCII format directly from the LDEO-BRG World Wide Web site at http://www.ldeo.columbia.edu/BRG/ODP/DATABASE. Access to logging data is restricted to Leg 197 participants for 12 months following the completion of the leg, and a password is required to access data during this period. Thereafter, access to these log data is openly available. A summary of logging highlights is also posted on the LDEO-BRG web site at the end of each leg.

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