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doi:10.2204/iodp.proc.318.102.2011

Downhole logging

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 measure 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

During wireline logging operations, the logs are recorded with a variety of Schlumberger logging tools combined into several tool strings, which are lowered into the hole after completion of coring operations. Three tool strings were used during Expedition 318: the triple combination (triple combo; (gamma radiation, porosity, density, and resistivity), the Formation MicroSanner (FMS)-sonic (FMS resistivity image of the borehole wall and sonic velocities), and the Versatile Seismic Imager (VSI; seismic check shots) (Fig. F20; Table T6). Each tool string also contains a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system (MAXIS unit) on the drillship.

In preparation for logging, the boreholes were flushed of debris by circulating viscous drilling fluid and filled with a seawater-based logging gel (sepiolite mud mixed with seawater and weighted with barite; approximate density = 10.5 lb/gal) to help stabilize the borehole walls. The BHA was pulled up to ~100 m WSF to cover the unstable upper part of the hole. The tool strings were then lowered downhole on a seven-conductor wireline cable before being pulled up at constant speed, typically 250–550 m/h, to provide continuous measurements of several properties simultaneously. A wireline heave compensator (WHC) was used when necessary to minimize the effect of ship’s heave on the tool position in the borehole (see below). During each logging run, incoming data were recorded and monitored in real time on the MCM MAXIS logging computer.

Logged sediment properties and tool measurement principles

The logged properties, and the principles used in the tools to measure them, are briefly described below. The main logs are listed in Table T7. 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), and Ellis and Singer (2007). A complete online list of acronyms for the Schlumberger tools and measurement curves is at www.slb.com/​modules/​mnemonics/​index.aspx.

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde was used on the triple combo and FMS-sonic tool strings to measure and classify natural radioactivity in the formation. It 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 Scintillation Gamma Ray Tool (SGT) uses a sodium iodide scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of potassium, thorium, and uranium concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray measurements.

The inclusion of a gamma ray sonde in every tool string allows use of the gamma ray data for depth correlation between logging strings and passes.

Density

Formation density was measured 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, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source undergo Compton scattering, in which gamma rays are scattered by electrons in the formation. 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 the 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 and can be used for the identification of some minerals. For example, the PEF of calcite = 5.08 barns per electron (b/e^–), illite = 3.03 b/e^–, quartz = 1.81 b/e^–, and kaolinite = 1.49 b/e^–.

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde. The sonde includes 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 highest energy loss occurs when neutrons collide with hydrogen nuclei, which have practically the same mass as the neutron (the neutrons 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 (Σ_f) is also measured by the tool.

Electrical resistivity

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 ohm-meters). The spherically focused resistivity is measured by an electrode device that sends a current into the formation while trying to maintain a constant voltage drop. The amount of current necessary to keep the 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. Calcite, silica, and hydrocarbons are electrical insulators, whereas ionic solutions like pore water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity for a given salinity and resistivity of the pore water.

Acoustic velocity

The Dipole Sonic Imager 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 sediments that is relatively free from the effects of formation damage and of an enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it 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 velocity of sound in the borehole fluid. Such slow formations are typically encountered in deep ocean drilling.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity 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. Features such as bedding, stratification, 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.

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 contact with the wall is poor.

Accelerometry and magnetic field measurement

Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometry 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 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. During Expedition 318, it was also used to record the downhole tool motion and evaluate in real time the performance of the new WHC.

Vertical seismic profile

In a vertical seismic profile (VSP) experiment, a borehole seismic tool is anchored against the borehole wall at regularly spaced intervals and records the full waveform of elastic waves generated by a seismic source positioned just below the sea surface. These “check shot” measurements relate depth in the hole to traveltime in reflection seismic lines. The VSI used here contains a three-axis geophone. In the planned VSP/check shots, the VSI was to be anchored against the borehole wall at 25 m station intervals, with 5–10 air gun shots typically taken at each station. The recorded waveforms were stacked and a one-way traveltime was determined from the median of the first breaks for each station. The seismic source used was a Sercel G. Gun Parallel Cluster, composed of two 250 in³ air guns separated by 1 m. It was positioned on the port side of the JOIDES Resolution at a water depth of ~7 mbsl with a borehole offset of ~30 m. The VSI was deployed in Hole U1359D, but the caliper arm failed to open and the geophone could not be clamped against the borehole wall. However, one valid check shot one-way time was obtained with the tool resting on the bottom of the hole.

Precautions were taken to protect marine mammals. If there were no mammals in or approaching the safety radius (940 m for water depths >1000 mbsl, 1410 m for water depths between 100 and 1000 mbsl), air gun operations commenced using a ramp-up, or “soft start” procedure (gradually increasing the operational pressure and air gun firing interval) to provide time for undetected animals to respond to the sounds and vacate the area. Once the air guns were at full power, the check shot survey proceeded. Marine mammal observations continued during the check shot survey, and if a mammal entered the safety radius, the survey was suspended.

Log data quality

The principal influence on log data quality is the condition of the borehole wall. Where the borehole diameter varies over short intervals because of washouts or ledges made of layers of harder material, the logs from tools that require good contact with the borehole wall (i.e., the FMS and density tools) may be degraded. Deep investigation measurements such as gamma ray, 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 quality of the logging depth determination depends on several factors. The depth of the logging measurements is determined from the length of the cable played out from the winch on the ship. The seafloor (mudline) is identified on the natural gamma ray log by the abrupt reduction in gamma ray count at the water/sediment boundary. Discrepancies between the drillers depth and the wireline log depth occur because of core expansion, incomplete core recovery, or incomplete heave compensation for the drillers depth. In the case of log depth, discrepancies between successive runs occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. In the case of very fine sediments in suspension, the mudline can be an elusive datum. Tidal changes in sea level will also have an effect. To minimize the wireline tool motion caused by ship heave, a new hydraulic WHC was used to adjust the wireline length for rig motion during wireline logging operations.

Wireline heave compensator

Expedition 318 continued evaluation of the new WHC system. Such WHC systems are designed to compensate for the vertical motion of the ship and maintain a steady motion of the logging tools (Goldberg, 1990). The new WHC 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 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 MRU and by the GPIT, respectively. A Lamont-developed software package allows these data to be analyzed and compared in real time, displaying the actual motion of the logging tool string, and enabling evaluation of the efficiency of the compensator. In addition to an improved design and smaller footprint compared to the previous system, location of the WHC 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 depths of the initial logging data are referenced to the rig floor [WRF]). After logging was completed, the data were shifted to a seafloor reference (WSF), based on the step in gamma radiation at the water/sediment interface. These data were made available to the science party.

The downhole log data were also transferred onshore to Lamont-Doherty Earth Observatory 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, wireline matched depth below seafloor. Also, 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 format for the conventional logs and GIF for the FMS images. Schlumberger GeoQuest’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 made available (in ASCII and DLIS formats) through the shipboard IODP logging database.

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. A direct measurement of the depth-traveltime relationship is given by the first arrival times in the zero-offset VSP (see above).

It can also be estimated by constructing synthetic seismograms, which are computed from reflection coefficients resulting from downhole contrasts in P-wave velocity and density, to match the seismic traces closest to the borehole. When the quality of the shipboard sonic logs was sufficient, synthetic seismograms were calculated from the density and V_P logs 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 in the measured seismic data. A calibrated depth-traveltime relationship allows for correlation of the borehole stratigraphy with seismic reflection features (e.g., to assign ages to prominent seismic reflectors that can then be correlated away from the drill site).

In situ temperature measurements

In situ temperature measurements were made with the advanced piston corer temperature tool (APCT-3) on cores from Site U1359. The APCT-3 fits directly into the coring shoe of the APC and consists of a battery pack, data logger, and a platinum resistance-temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool was first stopped at the mudline for 5 min to thermally equilibrate with bottom water and give a bottom water temperature. After the APC penetrated the sediment, it was held in place for 10 min as the APCT-3 recorded the temperature of the cutting shoe every second. When the APC is plunged into the formation, there is an instantaneous temperature rise from frictional heating. This heat gradually dissipates into the surrounding sediments as the temperature at the APCT-3 equilibrates toward the temperature of the sediments.

The equilibrium temperature of the sediments was estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). The synthetic thermal decay curve for the APCT-3 tool is a function of the geometry and thermal properties of the probe and the sediments (Bullard, 1954; Horai and Von Herzen, 1985). The equilibrium temperature must be estimated by applying a fitting procedure (Pribnow et al., 2000). However, where the APC has not achieved a full stroke, or where ship heave pulls the APC up from full penetration, the temperature equilibration curve will be disturbed and temperature determination is more difficult. The nominal accuracy of the APCT-3 measurements is ±0.05°C.

The APCT-3 data were combined with measurements of thermal conductivity (see “Physical properties”) obtained from whole-core samples to obtain heat flow values. Heat flow was calculated according to the Bullard method, to be consistent with the synthesis of ODP heat flow data by Pribnow et al. (2000).

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