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doi:10.2204/iodp.proc.344.102.2013

Downhole logging

Downhole logs measure physical, chemical, and structural properties of the rock formation penetrated by a borehole. The data are collected continuously with depth and measured in situ; they can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the rock formation. Downhole logs also provide information on the condition, shape, and size of the borehole and on possible deformations induced by drilling or by the stress field. 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 each other and can be interpreted jointly.

The scales of downhole log measurements are intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. Logs 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

During wireline logging, measurements are made with a variety of Schlumberger and Lamont-Doherty Earth Observatory (LDEO) logging tools combined into several tool strings, which are run into the hole on a wireline cable after coring operations are complete. Two wireline tool strings were used during Expedition 344: a modified triple combination or “triple-combo” (gamma ray, bulk density, resistivity, and ultrasonic imaging of the borehole wall) and the Formation MicroScanner (FMS)-sonic (resistivity imaging of the borehole wall and sonic velocities) (Fig. F18; Table T4). Each tool string also contains a telemetry cartridge to communicate through the wireline to the Schlumberger data acquisition system on the drillship.

In preparation for logging, the boreholes were flushed of debris by circulating through the drill pipe to the bottom of the hole a “pill” of viscous drilling fluid (sepiolite mud mixed with seawater; density ~8.8 ppg, or ~1.055 g/cm³; if weighted with barite, density ~10.5 ppg, or ~1.258 g/cm³). The BHA was pulled up to 80–100 mbsf. The tool strings were then lowered downhole by a seven-conductor wireline cable in sequential runs. During a run, each lowering or hauling-up of the tool string while collecting data constitutes a pass. During each pass, the tool strings were pulled up at constant speed, typically 250–550 m/h, to simultaneously measure several properties. During the logging runs, measured data were recorded and monitored in real time on the MCM MAXIS logging computer. A wireline heave compensator (WHC) was employed to minimize the effect of ship’s heave on the tool position in the borehole (see below).

Logged properties and tool measurement principles

Logged properties and the methods that the tools use to measure them are briefly described below. The main measurements taken by the tools are listed in Table T5. More detailed information on individual tools and their geological applications is found in Ellis and Singer (2007), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986). A complete online list of acronyms for the Schlumberger tools and measurement curves is available at www.apps.slb.com/​cmd/.

Natural radioactivity

Two wireline gamma ray tools were used to measure natural radioactivity in the formation. The Hostile Environment Natural Gamma Ray Sonde (HNGS) is a spectral tool that uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of K, Th, and U. The radioactive isotopes of these three elements dominate the natural radiation spectrum. The HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy.

The Enhanced Digital Telemetry Cartridge (EDTC), which is used primarily to communicate data to the surface, includes a sodium iodide scintillation detector to measure the total natural gamma ray emission. It is not a spectral tool, but it provides a high-resolution total gamma ray measurement that allows for precise correlation and depth matching between different logging runs.

Bulk density

Formation bulk density was determined with the Hostile Environment Litho-Density 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. The skid is pressed against the borehole wall by a hydraulically activated eccentralizing caliper arm, which also measures the borehole diameter. Gamma rays emitted by the source are scattered by electrons in the formation (Compton scattering). The number of scattered gamma rays that reach the detectors is proportional 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. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.

The HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of gamma rays occurs when their energy is reduced below 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. The PEF of calcite = 5.08 b/e^–; illite = 3.03 b/e^–; quartz = 1.81 b/e^–; and kaolinite = 1.49 b/e^–. For example, in a clastic sequence, PEF will typically be lower in sand-rich intervals and higher in intervals containing clay, mostly because of Fe in clay minerals (Fe has a high PEF of 31.2 b/e^–) (Ellis and Singer, 2007).

Electrical resistivity

Typical igneous rock minerals, silica, calcite, and hydrocarbons are electrical insulators, whereas ionic solutions like pore water are conductors. Variation in electrical resistivity, therefore, is dominated by water content, and the measured resistivity can be used to evaluate porosity (via Archie’s equation) for a given pore water resistivity. Two wireline tools were used to measure formation resisitivity, the High-Resolution Laterolog Array (HRLA) and the Phasor Dual Induction Tool (DIT).

The HRLA sonde sends a focused current beam into the formation and measures the current intensity necessary to maintain a constant drop in voltage across a fixed interval, providing a direct resistivity measurement. The HRLA electrode array has one central source electrode and six electrodes above and below it, which serve alternately as focusing and returning current electrodes. By rapidly changing the role of these electrodes, five resistivity profiles at different depths of investigation are simultaneously measured. The HRLA uses these resistivity profiles to estimate the true formation resistivity and also measures the resistivity of the drilling fluid in the borehole.

The DIT 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 field, proportional to the conductivity of the formation, which is measured by the receiving coils. The measured conductivities are then converted to resistivity (in ohm-meters). The spherically focused resistivity is measured by an electrode device that sends a current into the formation. The amount of current needed to maintain a constant drop in voltage gives a direct measure of resistivity. This device uses several electrodes to focus the current flow into the formation so that equipotential surfaces are spherical, and has a higher vertical resolution than the induction measurements.

Elastic wave velocities

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 measurement of compressional velocity through sediment that is relatively free from the effects of formation damage and of borehole enlargements (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, the DSI also has two cross-dipole transmitters, which allow an additional measurement of shear wave velocity. Dipole measurements are necessary to measure shear velocities in slow formations, whose shear velocity is less than the compressional velocity of the borehole fluid. Such slow formations are often encountered in deep ocean drilling.

Electrical resistivity images

The FMS provides high-resolution electrical resistivity images of the borehole wall (Chen et al., 1987). 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 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. Fine-scale features such as bedding, fracturing, slump folding, and bioturbation can be resolved; the images are oriented to magnetic north so that fabric analysis can be carried out and the dip and direction (azimuth) of planar features in the formation can be measured (Serra, 1989; Luthi, 1990; Lovell et al., 1998).

The maximum extension of the FMS caliper arms is 15 inches. In holes with a diameter larger than 15 inches, the pad contact will be inconsistent, and the FMS images may appear out of focus and too conductive. Irregular borehole walls will also adversely affect the images if the pad contact with the wall is poor. The aperture and orientation of the FMS caliper arms can also be used to identify borehole breakouts and measure stress orientation in the borehole (see below).

Ultrasonic images

The Ultrasonic Borehole Imager (UBI) is the latest generation of acoustic borehole televiewer (Zemanek et al., 1970). The UBI features a rotating transducer that emits ultrasonic pulses at a frequency of 250 or 500 kHz. The pulses are reflected by the borehole wall and then received by the same transducer. The continuous rotation of the transducer and the upward motion of the tool produce a complete image of the borehole wall.

The UBI measures both the amplitude and traveltime of the reflected signal. The measured amplitude is mostly affected by the roughness of the borehole wall, with an additional minor contribution due to the contrast in acoustic impedance between the formation and the borehole fluid. Fractures intersecting the borehole wall are typically surrounded by rough areas and can be recognized in the amplitude image. The traveltime is sampled every 2°–2.6° during the transducer rotation and gives detailed information on the borehole radius. Amplitude and traveltime are recorded together with a reference to magnetic north, permitting the orientation of images. If features (e.g., fractures) recognized in the core can be matched to those observed in the UBI images, orientation of the core is possible. The UBI measurements can also be used to measure stress orientation in the borehole through identification of borehole breakouts (see below).

Accelerometry and magnetic field measurement

Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometer Tool (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 during logging of imaging tools such as the FMS and the UBI. The measured images can then be corrected for irregular tool motion and the dip and direction (azimuth) of imaged features can be determined.

Log data quality

The principal influence on log data quality is the condition of the borehole wall. If the borehole diameter varies over short intervals because of washouts, ledges made of harder layers, or narrow (“bridged”) sections, the logs from those tools that require good contact with the borehole wall (e.g., bulk density and FMS) 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 irregularity of the borehole. The quality of the borehole is improved by minimizing the circulation of fluid while drilling, flushing the borehole to remove debris, and logging as soon as possible after drilling and hole conditioning are completed.

The quality of the logging depth determination depends on several factors. The depth of wireline log measurements 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). As depth in the cores is determined from the length of the drill pipe (drillers depth), there may be discrepancies between the core depth and the wireline depth scale. Inaccuracies in the core depth scale can be due to core expansion, incomplete core recovery, incomplete heave compensation, and drill pipe stretch. The wireline log depth can be inaccurate because of incomplete heave compensation, incomplete correction for cable stretch, 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.

Wireline Heave Compensator

The WHC system 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 (MRU), located under the rig floor near the center of gravity of the ship, to calculate the vertical position of the ship. The WHC adjusts the length of the wireline by varying the distance between two pulleys that the cable passes through. Real time measurements of uphole (surface) and downhole acceleration are made simultaneously by the MRU and by the GPIT tool, respectively. An LDEO-developed software package allows these data to be analyzed and compared in real time, displaying the actual motion of the tool string and enabling the efficacy of the compensator to be evaluated.

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 logging data are initially referenced to the rig floor (WRF). After logging was completed, the data were shifted to a seafloor reference (WSF) based on the step in natural gamma radiation at the sediment-water interface. These data were made available to the science party within a few days of their acquisition.

The downhole log data were also transferred onshore to LDEO for standardized data processing. The main part of the processing is depth matching to remove depth offsets between different logging runs, which results in a new depth scale (WMSF). Also, corrections are made to certain tools and logs (e.g., FMS images are corrected for tool acceleration), documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII format. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing. The data are then transferred back to the ship and made available through the shipboard IODP logging database (in ASCII and digital log information standard [DLIS] formats and GIF for borehole images).

Core-log-seismic integration

A depth-traveltime relationship must be determined at each site to correlate core and log data acquired in depth with seismic reflection measurements that are a function of traveltime. This relationship can be estimated by constructing synthetic seismograms, which are computed from reflection coefficients obtained from contrasts in P-wave velocity and density. These velocities and densities may be measured in situ with downhole logs or on cores in the physical properties laboratory. Synthetic seismograms can be calculated using the IESX seismic interpretation package (part of the Schlumberger GeoFrame software suite), which allows for interactively adjusting the depth-traveltime relationship until a good match is achieved between features in the synthetic seismogram and the measured seismic data. A calibrated depth-traveltime relationship provides an accurate correlation of core and log data with features in the seismic reflection data (e.g., to assign ages to seismic horizons that can be correlated away from the drill site).

Borehole breakout analysis

Drilling a vertical borehole through a rock mass that is under different principal compressive horizontal stresses induces a compressive stress along the borehole wall that is strongest at the azimuth of the minimum horizontal stress. If the stress reaches the rock strength, the borehole wall can fail and develop characteristic breakouts that are located 180° apart and mark the minimum horizontal stress direction. Therefore, borehole breakouts are key indicators of the state of stress in the subsurface (Zoback et al., 2003). Downhole logging measurements can indicate the presence and measure the orientation of borehole breakouts from the orientation of four-arm caliper tools and from borehole imaging. Due to cable torque, four-arm caliper tools such as the FMS rotate while they are being pulled uphole. If breakouts are present, a pair of caliper arms will tend to remain within the breakout, stopping tool rotation. The breakout direction can be determined from the orientation of the pair of caliper arms that measures the larger borehole diameter (Bell and Gough, 1979; Plumb and Hickman, 1985; Lin et al., 2010). Data acquired by an ultrasonic borehole televiewer such as the UBI can also image breakouts. Breakout surfaces are rough and appear as persistent vertical stripes of low reflectivity 180° apart in reflection amplitude images. Traveltime data can provide detailed cross sections of the borehole radius that show the width and depth of breakouts (Plumb and Hickman, 1985; Zoback et al., 2003).

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