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doi:10.2204/iodp.proc.330.102.2012

Downhole logging

Downhole logs are used to determine the physical, chemical, and structural properties of the formation penetrated by drilling. The data are rapidly collected, continuous with depth, and measured in situ; they can then be interpreted in terms of stratigraphy, lithology, mineralogy, magnetic characteristics, 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 site 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.

During Expedition 330, downhole measurements were taken in Holes U1374A and U1376A.

Wireline logging

During wireline logging, logs were made with a variety of Schlumberger logging tools and the third-party Göttingen Borehole Magnetometer (GBM) (Steveling et al., 1991). These tools were combined into several tool strings that were run down the hole after coring operations were completed. Four wireline tool strings were used during Expedition 330 (Fig. F18; Table T11):

  1. The triple combination (triple combo), which recorded density, porosity, electrical resistivity, and gamma ray;

  2. The Formation MicroScanner (FMS)-sonic, which recorded gamma ray, microresistivity images of the borehole wall, and compressional and shear wave velocities;

  3. The Ultrasonic Borehole Imager (UBI), which recorded transit times, amplitudes, and borehole radii; and

  4. The GBM, which logged a three-component magnetic field.

The first three tool strings also contained a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system on the drillship. The GBM communicates with its own dedicated acquisition unit on the drillship through the standard logging wireline.

In preparation for logging, the boreholes were flushed of debris by circulating viscous drilling fluid (sepiolite) mud mixed with seawater (approximate density ≈ 1.08 g/cm3) through the drill pipe to the bottom of the hole. The boreholes were reamed and displaced with heavy (approximate density ≈ 1.26 g/cm3) mud (barite). The bottom-hole assembly was pulled up to ~130 mbsf in Hole U1374A and ~80 mbsf in Hole U1376A. The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs. A wireline heave compensator was employed to minimize the effect of ship heave on tool position in the borehole (see “Wireline heave compensator”). During each Schlumberger logging run, incoming data were recorded and monitored in real time on the MCM MAXIS logging computer. The tool strings were then pulled up at constant speed, typically 250–300 m/h, to provide continuous measurements as a function of depth of several properties simultaneously.

Logged formation properties and tool measurement principles

The logged properties and the methods by which they are measured are briefly described below. The main logs taken by the tools are listed in Table T12. More detailed information on individual tools and their geological applications may be found in Serra (1984, 1986, 1989), Schlumberger (1989, 1994), Rider (1996), Goldberg (1997), Lovell et al. (1998), Ellis and Singer (2007), and Robinson et al. (2008). A complete list of acronyms for Schlumberger tools and measurement curves is available at www.slb.com/modules/mnemonics/index.aspx.

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS), a spectral gamma ray tool, uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of 40K, 232Th, and 238U in the formation. The isotopes of these elements dominate the natural radiation spectrum. The HNGS filters out gamma ray energies <500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. The computation of the elemental abundances uses a least-squares method of extracting thorium, uranium, and potassium elemental concentrations from the spectral measurements.

Density and photoelectric effect

Formation density was determined with the Hostile Environment Litho-Density Sonde (HLDS), which 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 decentralizing arm. Gamma rays emitted by the source undergo Compton scattering, which involves the transfer of energy from the 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 bulk density if the matrix density is known.

The HLDS also measures photoelectric absorption as the photoelectric effect. Photoelectric absorption of gamma rays occurs when their energy is reduced below 150 keV after being repeatedly scattered by electrons in the formation. Because photoelectric effect depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present and therefore can be used to identify some minerals (for example, the photoelectric effect of calcite is 5.08 barns per electron [b/e–] and that of quartz is 1.81 b/e–). Good contact between the tool and borehole wall is essential for good logs. Poor contact typically results in underestimation of density values.

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde (APS), which 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 tool’s detectors count neutrons that arrive at the detectors after being scattered and slowed by collisions with atomic nuclei in the formation. The near detector is centered in the tool and is used to normalize the count rates for differences in the minitron source. The remaining detectors are decentered, and neutron backshielding is used to focus them toward the formation to minimize the effect of the borehole environment.

Neutrons leaving the tool travel through the surrounding borehole and formation and interact primarily with hydrogen atoms present in the common formation fluids. Because neutrons and hydrogen atoms have about the same mass, successive collisions rapidly reduce the energy of the neutrons to the thermal energy level of the formation (0.025 eV). Because water contains about the same quantity of hydrogen per unit volume, the detector count-rate ratios can be calibrated in terms of liquid-filled porosity in clean formations. However, because hydrogen bound in minerals such as clay or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.

A mineral’s ability to absorb thermal neutrons is defined as its capture cross section, Sigma (Σf). Formations and formation fluids containing chlorine atoms are the most effective capturer of thermal neutrons. Thus, the rate of thermal neutron decay in the formation can be measured and used to differentiate between hydrocarbons in the pore space and salt water. Sigma is measured in capture units. Higher values of Sigma equate to a greater ability to capture thermal neutrons.

Electrical resistivity

The phasor Dual Induction Tool (DIT) was used to measure electrical resistivity. This tool 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 shallow-penetration resistivity, the current necessary to maintain a constant drop in voltage across a fixed interval is measured, which is a direct measurement of resistivity. Typically, igneous minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals, as well as ionic solutions like pore water, are conductors. Electrical resistivity, therefore, can be used to evaluate porosity (via Archie’s law) and fluid salinity.

Acoustic velocity

The Dipole Shear Sonic Imager (DSI) 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 formations 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 compressional wave velocity. Dipole measurements are necessary to measure shear velocities in slow formations whose shear velocity is less than the velocity of sound in the borehole fluid. Such slow formations are typically encountered in deep ocean drilling (e.g., for Expedition 330 the sediments overlying igneous basement).

Formation MicroScanner

The FMS tool 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. 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. Resistivity of the formation at the button electrodes is derived from the intensity of current passing through the button electrodes. Processing transforms these measurements into oriented high-resolution images that reveal the geologic structures of the borehole wall. Further analysis 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; Salimullah and Stow, 1992; Lovell et al., 1998). Features such as vesicles, veins, fractures, and volcaniclastic breccia can be resolved, and the fact that the images are oriented means that fabric analysis can be carried out and structural feature (e.g., fracture) orientations can be measured. If the same features in these high-resolution electrical images can be identified in the recovered core samples, individual core pieces can be reoriented with respect to true north.

The maximum extension of the caliper arms is 38.1 cm (15 inches). For holes with a diameter larger than 38.1 cm, pad contact at the end of the caliper arms 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. Standard procedure is to make two full passes up the borehole with the FMS to maximize the chance of getting full borehole coverage with the pads.

Ultrasonic Borehole Imager

The UBI features a high-resolution transducer that provides acoustic images of the borehole wall. The transducer emits ultrasonic pulses at a frequency of 250 or 500 kHz (low and high resolution, respectively) that are reflected at the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined. 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 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 (General Purpose Inclinometry Tool [GPIT]), 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 oriented 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 (Paillet and Kim, 1987). In an isotropic, linearly elastic rock formation that is 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). Because of time constraints, the UBI was only run in Hole U1374A.

Borehole inclination and magnetic field measurement

Three-component acceleration and magnetic field measurements were made with the GPIT. 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 and UBI tool strings during logging. This information allows the FMS and UBI images to be corrected for irregular tool motion and the dip and direction (azimuth) of features in the images to be determined. The GPIT was also run on the triple combo tool string in order to provide data to optimize the wireline heave compensator before logging began and to acquire the best possible downhole data. The GPIT is run with other tools on the triple combo and FMS-sonic tool strings that can carry remanent or induced magnetization; therefore, its magnetic measurements can be affected. However, on the FMS-sonic tool string the GPIT has greater nonmagnetic insulation from the other tools, which greatly reduces the effects on its magnetic measurements.

Göttingen Borehole Magnetometer

The GBM was designed and developed in 1989 by the Geophysical Institute of the University of Göttingen, Germany (Fig. F19A). The tool consists of three fluxgate sensors that log the horizontal (x and y) and the vertical (z) components of magnetic flux density (Steveling et al., 1991). In the first version of the tool, orientation was determined by two inclinometers, which still can be used to measure deviation of the tool from vertical. In 2001 the first fiber-optic gyro was installed in the GBM to measure rotations around the vertical axis of the tool. Since then, two additional angular rate sensors have been added to the tool to monitor variations around the tool’s x- and y-axes during a logging run.

The tool connects directly to the Schlumberger cable head and is deployed with a centralizer and two sinker bars (a Schlumberger sinker bar and a new virtually nonmagnetic aluminum sinker bar, the latter being especially developed for Expedition 330 by the Scripps Institution of Oceanography). The nonmagnetic sinker bar was found to be essential following extensive tests in August 2010 in Houston, Texas. The tests revealed that the Schlumberger sinker bar and the centralizer both carried nonnegligible remanent magnetization (with the centralizer being much more magnetic than the sinker bar) and, together with induced magnetization, caused an error of up to 350 nT in the magnetic measurements of the GBM when used in a common tool string configuration. The truly nonmagnetic aluminum sinker bar reduced this influence to <50 nT by almost doubling the distance of the magnetometers from the other parts of the tool string. The Schlumberger sinker bar had to be deployed in conjunction with the nonmagnetic aluminum sinker bar to increase the weight of the tool string, and the centralizer was necessary to both center the tool in the borehole and reduce tool rotation. The GBM housing is made of low-magnetic monel and is not affected by pressures or temperatures up to 70 MPa and 100°C, respectively. Specifications are listed in Table T13.

The GBM was deployed during Expedition 330 because the tool’s fiber-optic gyros allow independent determination of both inclination and declination of the magnetic field in the borehole (and hence estimation of declination and inclination of the magnetization of penetrated lava flows). In addition, the better quality of the GBM magnetic data, especially the low magnetic influence of the other parts of the tool string (thanks to the aluminum sinker bar), can possibly help to better orient the FMS and UBI data (postexpedition). The fluxgate magnetometers incorporated in the GPIT are sufficient for orienting the FMS tool string; however, these sensors have relatively poor sensitivity (50 nT) and, more importantly, substantial (~1000 nT) offsets that limit interpretation of the data (e.g., Ito et al., 1995). The fluxgate magnetometers in the GBM have better resolution (12 nT) and are well calibrated. For example, when the ambient field was measured with the GBM above the HSDP-2 drill hole on Hawaii (Steveling et al., 2003), the inclination of 36.5° compares well with the International Geomagnetic Reference Field inclination of 36.6°, and the measured total field was compatible with that determined by aeromagnetic surveys.

GBM angular rate sensors

LITEF miniature fiber-optic rate sensors were used to provide angular rate output during the entire run (downlog and uplog) of the GBM. The tool contains three of these gyros, each of which has a small volume and low weight and requires very little power (2 VA) (Fig. F19B). Free from gravity-induced errors and with no moving parts, the sensors are insensitive to shock and vibration. The rate sensors are unconventional gyros because they do not have a spinning wheel. The sensor detects and measures angular rates by measuring the frequency difference between two contra-rotating light beams. The light source is a superluminescent diode, and its broad spectrum provides light with a 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 a 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. 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, sampled at 5 Hz, is a function of time and corresponds to the accumulated angle.

The angular rate measured by the sensor is influenced by the Earth’s rotation, which depends on latitude (ø) and varies from 15.04°/h at the poles to 0°/h at the Equator (Fig. F19C). From Equator to pole, Earth’s measured rotation increases by sin(ø). To obtain the rotation rate about an inertial system, the effect of Earth’s rotation must be eliminated. To do so, the orientation of the tool relative to the Earth’s reference frame at the beginning of a measurement has to be known exactly. The x-gyro of the GBM was aligned with the axis of the ship using a scope mounted to the tool and a sighting plate positioned in the center of the helideck at the aft of the ship. Knowing that the tool was identically oriented with the ship, information from the ship’s gyro and two GPS antennas was then used to determine the heading of the ship (at the time of sighting) and thus the orientation of the tool at the start of logging. This procedure was repeated at the end of each logging run to compare the true heading of the gyro with the heading calculated by the data-processing algorithm. This was done to check for errors in the reorientation procedure. If the corrected rotation rate around each axis is known, the orientation of the tool can be derived as a function of depth from the rotation history, and thus the three components of the magnetic field can be calculated for every data point collected by the GBM.

The maximum operation temperature for the fiber-optic gyros is 70°C. The gyros have a temperature-dependent drift that is lowest between 35° and 50°C, so the gyros were heated. The temperature of the gyros was measured during the logging run, and the temperature drift was corrected for during data processing. However, depending on the resistance of the wireline, the voltage received by the tool is not sufficient to heat against the cold seawater. As a result, during Expedition 330 the temperature of the gyros typically decreased to values below 35°C but never fell below 28°C, which is still an acceptable temperature range (Fig. F20). Data were acquired from the tool using GBMlog software (written by E. Steveling, University of Göttingen) and processed with GBMdatenverarbeitung software (developed by S. Ehmann, University of Braunschweig).

Logging data quality

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 or ledges caused by layers of harder material) the logs from some tools (e.g., FMS, density, and porosity tools) may be degraded. Deep (0.23–1.5 m) 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 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, performing a full wiper trip, and logging as soon as possible after drilling and conditioning are completed.

The quality of the depth determination depends on a series of factors. 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 ray log by the abrupt decrease in gamma ray count at the sediment/water interface (“mudline”). Discrepancies between drillers and core depth and wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, and drill pipe stretch, in the case of drillers depth. In the case of log depth, discrepancies occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. Tidal changes in sea level also have an effect. To minimize the wireline tool motion caused by ship heave, a hydraulic wireline heave compensator (see below) was used to adjust the wireline length for rig motion during wireline logging operations.

Wireline heave compensator

The new wireline heave compensator system installed during the recent refit (first used during Expedition 320T in February 2009) is designed to compensate for the vertical motion of the ship and maintain a steady motion of the logging tools. It uses vertical acceleration measurements made by a motion reference unit located under the rig floor near the ship’s center of gravity to calculate the vertical motion of the ship. It then adjusts the length of the wireline by varying the distance between two sets of pulleys through which the cable passes. Real-time measurements of uphole (surface) and downhole acceleration are made simultaneously by the motion reference unit and the GPIT tool, respectively. A software package developed by Lamont-Doherty Earth Observatory allows these data to be analyzed and compared in real time, displaying the actual motion of the logging tool string and enabling monitoring of the efficiency of the compensator. In addition to an improved design and smaller footprint compared to the previous system, the location of the wireline heave compensator with the winch unit on the starboard side of the derrick contributes to a significant reduction in the time necessary to prepare for logging operations.

Logging data flow and log depth scales

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS 500 system. The initial logging data were referenced to the rig floor (wireline log depth below rig floor). After logging was completed, the data were shifted to a seafloor reference (wireline log depth below seafloor), which was based on the step in gamma radiation at the sediment/water interface. These data were made available to the science party as a provisional data set.

The data were transferred on shore to Lamont-Doherty Earth Observatory, where standardized data processing took place. The main part of the processing is depth matching to remove depth offsets between data from different logging runs, which results in a new depth scale (wireline log matched depth below seafloor). In addition, corrections are made to certain tools and logs, documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII for the conventional logs and GIF for the FMS images. Schlumberger Geo-Quest’s GeoFrame software package is used for most of the processing. The data were transferred back to the ship within a few days of logging, and this processed data set was made available to the science party (in ASCII and DLIS formats) through the shipboard IODP logging database.

Core-log-seismic integration

GeoFrame’s IESX seismic interpretation software package was available during Expedition 330 to display site-survey seismic sections acquired before the expedition. Velocity and density logs, together with equivalent measurements made on core in the physical properties laboratory, can be used to create synthetic seismograms. The depth-traveltime relation must be adjusted until the features in the synthetic seismogram match the features in the seismic section. In this way, lithostratigraphic units in the core may be correlated with reflectors and sequences in the seismic section. Should the quality of the shipboard sonic and density logs be sufficient, synthetic seismograms can be produced postexpedition.

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