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doi:10.2204/iodp.proc.323.102.2011

Downhole measurements

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. When core recovery is incomplete or disturbed, logging data may be the only means to characterize the borehole section. When core recovery is good, log and core data complement one another and may be interpreted jointly.

Downhole logs measure formation properties on a scale intermediate between the scales obtained from laboratory measurements of 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, downhole logs were made with a variety of Schlumberger logging tools that were combined into tool strings and run down the hole after coring operations were complete. Two tool strings were used during Expedition 323: a triple combination or "triple combo" tool string, which measured gamma radiation, porosity, density, and resistivity, and the Formation MicroScanner (FMS)-sonic tool string, which gathered resistivity images of the borehole wall and sonic velocities (Fig. F11; Table T6). Each tool string also contained a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system (the MAXIS unit) on the drillship.

In preparation for logging, the boreholes were flushed of debris by circulating a "pill" of viscous drilling fluid (sepiolite mud mixed with seawater; approximate density = 8.8 lb/gal [1.055 g/cm³]) through the drill pipe to the bottom of the hole. The BHA was pulled up to 60–80 mbsf. The tool strings were then lowered downhole by a seven-conductor wireline cable in sequential runs. A new wireline heave compensator (WHC) was employed to minimize the effect of the 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 Schlumberger Minimum Configuration Maxis (MCM) logging computer. The tool strings were then pulled up at constant speed, typically 250–300 m/h, to provide continuous measurements of several properties simultaneously.

Logged sediment properties and tool measurement principles

The logged properties and the methods by which they were measured 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 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 provided at www.slb.com/modules/mnemonics/index.aspx.

Natural radioactivity

The Hostile Environment Gamma Ray Sonde (HNGS) was included in both 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 potassium, thorium, and uranium (K, Th, and U). The radioactive isotopes of these 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. Its inclusion in both tool strings allows the use of gamma ray data for depth correlation between logging strings and passes.

Density

Formation 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, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source undergo Compton scattering, where 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 also may be derived from bulk density if the matrix (grain) density is known. Good contact between the tool and the 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). Gamma rays are photoelectrically absorbed when their energy is reduced below 150 keV by 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 to identify 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 (APS). The APS 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 highest energy loss occurs when neutrons collide with hydrogen nuclei, which have practically the same mass as the neutron (the neutrons simply bounce off of heavier elements without losing much energy). If 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 water content is high. However, because hydrogen that is 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), 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 APS.

Electrical resistivity

The phasor dual induction–spherically focused resistivity tool (DITE-SFL) was used to measure electrical resistivity. The DITE-SFL 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, that is measured by the receiving coils. The measured conductivities are then converted to resistivity (in units of ohm-meters). 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 current flow into the formation so that equipotential surfaces are spherical, and it has a higher vertical resolution than 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 (via Archie's law) for a given salinity and resistivity of the pore water.

Acoustic velocity

The Dipole 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 sediments that are relatively free from the effects of formation damage and an enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, the DSI also contains two cross-dipole transmitters that allow for the additional measurement of shear wave velocity. Dipole measurements are necessary to measure shear velocity in slow formations whose shear velocities are 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 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. Formation resistivity at the button electrodes is derived from the intensity of the 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, 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 having >15 inch diameters, pad contact is inconsistent and the FMS images may appear out of focus and too conductive. Irregular (rough) borehole walls 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 323, the GPIT was also used to record downhole tool motion and evaluate in real time the performance of the new heave compensator.

Logging data quality

The principal influence on logging data quality is the condition of the borehole wall. If borehole diameter varies over short intervals because of washouts during drilling or ledges made of layers of harder material, the logs from tools that require good contact with the borehole wall (i.e., FMS and density tools) 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 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, 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 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 NGR log by the abrupt reduction in gamma radiation at the water/sediment boundary (mudline). Discrepancies between drillers depth and wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, and, for drillers depth only, drill pipe stretch. Discrepancies in log depth between successive runs occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. Tidal changes in sea level also affect depth determination. Wireline tool motion caused by ship heave is minimized by a new hydraulic WHC that adjusts for rig motion during wireline logging operations.

Wireline heave compensator

Expedition 323 continued evaluation of the new WHC system aboard the JOIDES Resolution. The WHC system is designed to compensate for the vertical motion of the ship and maintain the 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 ship's center of gravity) to calculate the vertical motion of the ship, and it 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 tool, respectively. A software package developed by Lamont-Doherty Earth Observatory (LDEO) 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 its improved design and smaller footprint compared to the previous system, the WHC's placement near 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. Initial logging data were referenced to the rig floor (wireline log depth below rig floor [WRF]). After logging was completed, the data were shifted to a seafloor reference (wireline log depth below seafloor [WSF]) based on the step in gamma radiation at the sediment/water interface.

Downhole logging data were also transferred on shore to LDEO for standardized data processing. This processing consisted primarily of depth matching to remove depth offsets between different logging runs, which resulted in a new depth scale, wireline log matched depth below seafloor (WMSF). In addition, during data processing, corrections were made to certain tools and logs, and documentation for the logs (with an assessment of log quality) was prepared. Schlumberger GeoQuest's GeoFrame software package was used for most of the processing. The data were transferred back to the ship within a few days of logging and were made available through the shipboard IODP logging database.

Measurements of properties such as NGR and density were taken both downhole and on cores. These measurements can be correlated with Correlator, which allows the shifting of core depths onto the wireline depth scale (see "Stratigraphic correlation").

Core-log-seismic integration

Depth–traveltime relationships must be determined at each site to correlate core and logging data acquired at depth with seismic reflection measurements that are a function of sonic traveltime. Although the most direct way to define such a relationship is to acquire a check shot survey or vertical seismic profile, it can also be estimated by constructing synthetic seismograms that are computed from reflection coefficients obtained from contrasts in P-wave velocity and density. Synthetic seismograms were calculated from density and V_P logs using the IESX seismic interpretation package (part of the Schlumberger GeoFrame software suite), which allows interactive adjustment 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 correlation of hole stratigraphy with seismic reflection features (e.g., the assignment of ages to prominent seismic reflectors that can be correlated away from the drill site).

In situ temperature measurements

In situ temperature measurements were made at each site using the APCT-3 temperature tool. The APCT-3 tool fits directly into the coring shoe of the APC and consists of a battery pack, data logger, and platinum resistance-temperature device calibrated over a temperature range of 0°–30°C. Before it enters the borehole, the tool is stopped at the seafloor for 5 min to thermally equilibrate with bottom water. However, the lowest temperature recorded during the run down is preferred to the average temperature at the seafloor as an estimate of bottom water temperature because this measurement is more repeatable and the bottom water is expected to have the lowest temperature in the profile. After the APC penetrated the sediment, it was held in place for 5–10 min while the APCT-3 instrument recorded the temperature of the cutting shoe every second. When the APC was plunged into the formation there was an instantaneous temperature rise from frictional heating. This heat gradually dissipated into the surrounding sediments as the temperature at the APCT-3 equilibrated 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 in the TP-Fit software by Heesemann. However, when the APCT-3 does not achieve a full stroke or ship heave pulls up the APC from full penetration, the temperature equilibration curve is disturbed and temperature determination is more difficult. The nominal accuracy of the APCT-3 temperature measurement is ±0.1°C.

APCT-3 temperature data were combined with measurements of thermal conductivity (see "Physical properties") obtained from core samples to obtain heat flow values. Heat flow was calculated according to the Bullard method to be consistent with ODP Leg 199 analyses and the synthesis of ODP heat flow data by Pribnow et al. (2000).

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