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doi:10.2204/iodp.proc.320321.102.2010

Downhole measurements

Downhole logs are used to determine 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 each other 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, logs are made with a variety of Schlumberger and Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) logging tools combined into "tool strings," which are run down the hole after coring operations are complete. Four wireline tool strings were used during Expedition 320/321: a triple combination (triple combo: gamma ray, density, and resistivity); a modified triple combo, termed the "paleocombo" (gamma ray, density, and magnetic susceptibility); the Formation MicroScanner (FMS)-sonic (resistivity image of the borehole wall and elastic wave velocities); and the Versatile Seismic Imager (VSI) (Fig. F16; Table T12). Each tool string also contains a telemetry cartridge for communicating through a seven-conductor wireline cable to the Schlumberger data acquisition system (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, or 1.055 g/cm³) through the drill pipe to the bottom of the hole. The BHA was pulled up to a depth of 60–80 m drilling depth below seafloor (DSF). The tool strings were lowered downhole on a wireline cable during sequential runs and pulled up at constant speed, typically 250–300 m/h, to provide continuous measurements of several properties simultaneously. A new wireline heave compensator (WHC) was employed to minimize the effect of ship's heave on the tool position in the borehole (see below).

Logged sediment properties and tool measurement principles

The logged properties and the methods that the tools use to measure them are briefly described below. The main logs taken by the tools are listed in Table T13. 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), Robinson et al. (2008), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989). A complete online list of acronyms for the Schlumberger tools and measurement curves is at www.apps.slb.com/cmd/.

Natural radioactivity

Two wireline gamma ray tools were used to measure and classify natural radioactivity in the formation. The Hostile Environment Natural Gamma Ray Sonde (HNGS) is a spectral gamma ray tool, and the Scintillation Gamma Ray Tool (SGT-N) is a total gamma ray tool.

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-N uses a sodium iodide scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of K, U, and Th concentrations in the formation. The SGT-N is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging strings. It was included in the VSI tool string for this purpose.

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, 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. For example, the PEF of calcite = 5.08 barns/e^–, illite = 3.03 b/e^–, quartz = 1.81 barns/e^–, and kaolinite = 1.49 b/e^–.

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 (10–40 kHz) 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). 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. 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 equation) for a given resistivity of the pore water, which depends on salinity and temperature.

Magnetic susceptibility

The Magnetic Susceptibility Sonde (MSS), a new wireline tool designed by the LDEO-BRG, was run for the first time in IODP during Expedition 320. It measures the magnetic susceptibility of the formation, which depends on the concentration and composition (size, shape, and mineralogy) of magnetic minerals, principally magnetites. These measurements provide one of the best methods for investigating stratigraphic changes in mineralogy and lithology because the measurement is quick, repeatable, and nondestructive and because different lithologies often have strongly contrasting susceptibilities. High-resolution susceptibility measurements aid significantly in paleoclimatic and paleoceanographic studies, where an accurate and complete stratigraphic framework is critical to reconstruct past climatic changes.

The MSS measures at two vertical resolutions (Fig. F17). A single-coil sensor provides high-resolution measurements (~10 cm) but reads shallow; therefore, bowsprings are used to push the tool against the borehole wall (additionally, the HLDS caliper arm aids in eccentralizing the tool). A dual-coil sensor provides lower resolution (~40 cm) and deeper reading measurements and is minimally affected by standoff; therefore, it acts as a quality control for the high-resolution readings. It also measures the electrical conductivity of the formation with an induction measurement similar to that provided by the DITE-SFL described above. The MSS can be run as a component of a Schlumberger tool string using a specially developed data translation cartridge (ELIC), saving hours of operation time. For quality control and environmental corrections, the MSS also measures internal temperature and z-axis acceleration.

Elastic wave 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 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, in which shear velocity is less than the compressional velocity of 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 images with a resolution of ~0.5 cm that reveal the geologic structures of the borehole wall.

The development of the FMS tool has added a new dimension to wireline logging (Luthi, 1990; Lovell et al., 1998; Salimullah and Stow, 1992). Features such as bedding, fracturing, slump folding, and bioturbation can be resolved; 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, the pad contact 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.

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.

Vertical seismic profiling

The VSI is a borehole seismic tool optimized for vertical seismic profiles (VSPs) in open or cased holes that are vertical or deviated. The VSI consists of multiple shuttles (each containing a three-axis geophone) separated by acoustically isolating spacers. During Expedition 321, we used the VSI tool with a single shuttle. In a VSP experiment, the VSI records the full waveform of elastic waves generated by a seismic source positioned just below the sea surface. As a seismic source during Expedition 321, we used a Sercel G. Gun Parallel Cluster, which is composed of two 250 inch³ air guns separated by 1 m. The source was positioned on the port side of the JOIDES Resolution at a water depth of ~7 m with a borehole offset of ~30 m. The VSI was clamped against the borehole wall at 15 m intervals, and 1–5 recordings were 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. These "check shot" measurements relate depth in the hole to traveltime in reflection seismic lines.

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 during drilling or ledges made of layers of harder material, the logs from those 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 will also cause irregular log results. Borehole quality 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 paid 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). Discrepancies between the drillers depth and the 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 will also have an effect. To minimize the wireline tool motion caused by ship heave, a new hydraulic wireline heave compensator adjusts for rig motion during wireline logging operations.

Wireline heave compensator

For the first time during a full expedition, the new WHC was used aboard the JOIDES Resolution during Expedition 320/321. The WHC system is designed to compensate for the vertical motion of the ship and to 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 and adjusts the length of the wireline by varying the distance between two pulleys that the wireline cable passes through. Real-time measurements of uphole (surface) and downhole acceleration are made simultaneously by the MRU and GPIT, respectively. A 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 the efficiency of the compensator to be evaluated. In addition to an improved design and smaller footprint compared to the previous system, its location with the winch unit on the starboard side of the derrick contributed to a significant reduction in the time necessary to prepare for logging operations. The WHC was nonoperational during Expedition 320.

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 are referenced to the WRF depth scale. After logging was completed, data were shifted to a seafloor reference (WSF) based on the step in NGR at the sediment/water interface. These initial data were made available to the science party.

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, wireline matched depth below seafloor (WMSF). Also, corrections are made to some tools and logs, documentation for the logs (with an assessment of log quality) is prepared, and 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. Data were transferred back to the ship within a few days of logging and were made available (in ASCII and DLIS formats) through the shipboard IODP logging database.

Measurements such as magnetic susceptibility, natural gamma radiation, and density are taken both downhole and on cores. They can be correlated using the Correlator software, which allows shifting of the core depths onto the wireline depth scale (see "Stratigraphic correlation and composite section").

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 a VSP experiment (see above). This relationship can also 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 property laboratory. The synthetic seismograms were 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 in the measured seismic data. A calibrated depth-traveltime relationship allows for correlating hole stratigraphy with seismic reflection features (e.g., to assign ages to prominent seismic horizons that can be correlated away from the drill site).

In situ temperature measurements

During Expedition 320/321, in situ temperature measurements were made with the APCT-3 and the sediment temperature (SET) tool. At least four in situ temperature measurements were made at each site using the APCT-3. 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 is first stopped at the mudline for 5 min to thermally equilibrate with bottom water. However, the lowest temperature recorded during the run down was occasionally preferred to the average temperature at the mudline as an estimate of the bottom water temperature because it was more repeatable, and bottom water is expected to have the lowest temperature in the profile. 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 SET tool was deployed for the first time on the JOIDES Resolution during Expedition 321, and except for an updated electronics section it is very similar to the Davis-Villinger Temperature Probe used extensively in previous IODP expeditions. The SET tool is run in semiconsolidated sediments that cannot be penetrated by the APCT-3 and measures temperature with a 1.4 m long probe that is pushed into the sediment below the drill bit and held in place for 10 min. The SET tool is run through the drill string on the coring wireline with the colleted delivery system, which allows the probe to be decoupled from the BHA and prevents the ship's heave from moving the probe and disturbing the measurement.

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 and SET 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 and SET tool temperature measurements is ±0.05°C.

The APCT-3 and SET tool temperature 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 Leg 199 analyses and the synthesis of ODP heat flow data by Pribnow et al. (2000).

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