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doi:10.2204/iodp.proc.317.102.2011

Downhole logging

Downhole logs are used to determine the physical, chemical, and structural properties of the formation penetrated by a borehole. 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 those from 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

Logs are recorded during wireline logging operations with a variety of Schlumberger logging tools, which are combined into several tool strings and run down the hole after coring operations are completed. During Expedition 317, the following three tool strings were planned for deployment (Fig. F12; Table T17):

1. The triple combination (triple combo) tool string, which consists of the Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Litho-Density Sonde (HLDS), Phasor Dual Induction–Spherically Focused Resistivity Tool (DIT), and Accelerator Porosity Sonde (APS). During Expedition 317, the General Purpose Inclinometry Tool (GPIT) was also included in the tool string;

2. The Formation MicroScanner (FMS)-sonic tool string, which consists of the FMS, GPIT, HNGS, and Dipole Sonic Imager (DSI); and

3. The Versatile Seismic Imager (VSI) with the Scintillation Gamma Ray Tool (SGT).

Each tool string also contains a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system (the MAXIS unit) on the drillship. Because of difficult hole conditions, some of these tool strings had to be reduced or reconfigured. See individual site chapters for details.

In preparation for logging, the boreholes were flushed of debris by circulating viscous drilling fluid. In most cases, the boreholes were then filled with a seawater-based logging gel (sepiolite mud mixed with seawater; approximate density = 8.8 lb/gal, or 1.055 g/cm³) to help stabilize the borehole walls. The limited supply of logging gel available during Expedition 317 required that the appropriateness of its use be evaluated at some sites. The BHA was pulled up to 60–100 m wireline log depth below seafloor (WSF). The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs before being pulled up at constant speed, typically 250–400 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 heave on the tool position in the borehole (see "Wireline heave compensator"). During each logging run, incoming data were recorded and monitored in real time on the Schlumberger MAXIS 500 system.

Logged sediment properties and tool measurement principles

The logged properties, and the principles used by the tools to measure them, are briefly described below. Tool name acronyms, the parameters measured by each tool, the sampling interval, and the vertical resolution are summarized in Table T18. More detailed information on individual tools and their geological applications may be found in Ellis and Singer (2007), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989). A complete online list of acronyms for Schlumberger tools and measurement curves is available at www.apps.slb.com/​cmd/​index.aspx.

Natural radioactivity

The HNGS was used in the triple combo and the FMS-sonic tool strings to measure and classify natural radioactivity in the formation. The HNGS 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 SGT uses a sodium iodide scintillation detector to measure total natural gamma ray emissions, combining the spectral contributions of K, U, and Th concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray.

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

Density

Formation density 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 both Compton scattering and photoelectric absorption. Compton scattering involves the ricochet of gamma rays off electrons in the formation via elastic collision, transferring energy to the electron in the process. 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.

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 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^–, hornblende = 10.49 b/e^–, and plagioclase (albite) = 1.68 b/e^–.

Good coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in the underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.

Porosity

Formation porosity was measured with the APS, which includes a minitron neutron generator that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one thermal) positioned at different spacing 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 simply bounce off 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 count rates increase at the detector. The opposite effect occurs in high-porosity formations where 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 DIT provides three measures of electrical resistivity, each with a different depth of investigation into the formation. 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 ohmmeters). A third device, a spherically focused resistivity instrument with higher vertical resolution than the induction devices, sends a current into the formation while trying to maintain a constant voltage drop. The amount of current necessary to maintain 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. 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 pore water.

Acoustic velocity

The DSI measures 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, the DSI also has two cross-dipole transmitters, which allows for 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 images of borehole wall microresistivity. The tool has four orthogonal arms and pads, each containing 16 button electrodes that are pressed against the borehole wall during 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. The 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 structure 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 >15 inches, 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 are 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 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 images can be determined. During Expedition 317, the GPIT was also used to record downhole tool motion and evaluate the performance of the new WHC in real time.

Vertical seismic profile

In a vertical seismic profile (VSP) experiment, a borehole seismic tool is anchored against the borehole wall at regularly spaced intervals to record 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 is usually composed of multiple shuttles separated by acoustically isolating spacers, each containing a three-axis geophone, but only one shuttle was to be used during Expedition 317. The plan for Expedition 317 was to anchor the VSI against the borehole wall at 20 m intervals, and to take 5–10 recordings at each station. The recorded waveforms could then be stacked and a one-way traveltime determined from the median of the first breaks for each station. The seismic source to be used was a Sercel G. Gun Parallel Cluster, composed of two 250 in³ air guns separated by 1 m. The cluster is generally positioned on the port side of the JOIDES Resolution at a water depth of ~7 m with a borehole offset of ~30 m. However, no VSP experiments were carried out during Expedition 317 because of poor hole conditions.

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 or ledges 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 that do not require contact with the borehole wall (i.e., gamma ray, resistivity, and sonic velocity) are generally less sensitive to borehole conditions. Very narrow (bridged) sections also cause irregular log results. The quality of the borehole can be 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 log by the abrupt reduction in gamma ray count at the water/sediment boundary. Discrepancies between drilling depth and wireline log depth occur because of core expansion, incomplete core recovery, or incomplete heave compensation for drillers depth. In the case of log depth, discrepancies between successive runs occur because of incomplete heave compensation, insufficient 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 also have an effect. To minimize wireline tool motion caused by ship heave, a WHC was used to adjust the wireline length for rig motion during wireline logging operations (Goldberg, 1990).

Wireline heave compensator

Evaluation of a new hydraulic WHC system continued during Expedition 317. This 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), which is located under the rig floor near the center of gravity of the ship, to calculate the ship's vertical motion. 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-Doherty Earth Observatory (LDEO)-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 compensator's efficiency. In addition to the WHC's improved design and smaller footprint compared to the previous system, its location with the winch unit on the starboard side of the derrick contributes to a significant reduction in the time needed 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 (WRF). After logging was completed, the data were shifted to a seafloor reference (WSF) based on the step in gamma radiation at the sediment/water interface. These data were made available to the science party.

The downhole log data were also transferred onshore to LDEO for standardized data processing. The main processing task was depth matching to remove depth offsets between different logging runs, which resulted in a new depth scale: wireline log matched depth below seafloor (WMSF). Also, corrections were made to certain tools and logs, documentation for the logs was prepared (with an assessment of log quality), and the data were converted to ASCII for the conventional logs and GIF for the FMS images. Schlumberger GeoQuest's GeoFrame software package was used for most of the data 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, which are a function of traveltime. For example, a direct measurement of the depth–traveltime relationship is given by the first arrival times in the VSP (see above).

The depth–traveltime relationship can also be estimated by constructing synthetic seismograms, which are computed from reflection coefficients obtained from 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 interactive adjustments of 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 allows for correlation of borehole stratigraphy with seismic reflection features (e.g., assignment of ages to prominent seismic reflectors that can then be correlated away from the drill site).

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