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doi:10.2204/iodp.proc.324.102.2010

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 are sensitive to 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, the logs were made with a variety of Schlumberger logging tools combined into several tool strings, which were run down the hole after coring operations were completed. Three wireline tool strings were used during Expedition 324 (Fig. F19; Table T11): the triple combination (gamma ray, density, porosity, and electrical resistivity), the FMS-sonic (gamma ray, microresistivity image of the borehole wall, and compressional- and shear-wave velocities), and the Ultrasonic Borehole Imager (UBI; transit times, amplitudes, and borehole radii). Each tool string also contained a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system on the drillship.

In preparation for logging, the boreholes were flushed of debris by circulating viscous drilling fluid through the drill pipe to the bottom of the hole. Depending on hole conditions, the viscous drilling fluids used during Expedition 324 were either attapulgite mixed with seawater (approximate weight = 8.9 ppg or 1.07 g/cm³) or drilling mud containing barite mixed with seawater (approximate weight = 10.5 ppg or 1.26 g/cm³). The drill bit was released (using the mechanical bit release) and the bottom-hole assembly was pulled up. The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs. A new wireline heave compensator was employed 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. The tool strings were then pulled up at constant speed, typically 250–550 m/h, to provide continuous measurements as a function of depth of several properties simultaneously.

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 T12. 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).

Natural radioactivity

The Hostile Environment Gamma Ray Sonde (HNGS) uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of ⁴⁰K, ²³²Th, and ²³⁸U. The 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.

Density and photoelectric effect

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, which involves the transfer of energy from gamma rays to the electrons in the formation through elastic collision. The number of scattered gamma rays that reach the detectors is directly related 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 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. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values. The use of drilling mud containing barite can significantly affect the PEF measurements.

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde. The sonde incorporates a minitron neutron generator that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one thermal) positioned at different distances 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. Each of the five detectors provides a special feature to enhance the overall measurement. The near detector is centered in the tool and is used to normalize the count rates for differences in the minitron source. The remaining detectors are eccentered, and neutron backshielding is used to focus them toward the formation to minimize the effect of the borehole environment.

Neutrons leaving the tool travel through the surrounding borehole and formation and interact primarily with hydrogen atoms present in the common formation fluids. Since neutrons and hydrogen atoms have about the same mass, successive collisions rapidly reduce the energy of the neutrons to the thermal energy level of the formation (0.025 eV). Since water contains about the same quantity of hydrogen per unit volume, the detector count-rate ratios can be calibrated in terms of liquid-filled porosity in clean formations. 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.

A mineral's ability to absorb thermal neutrons is defined as its capture cross section (Σf). Formations and formation fluids containing chlorine atoms are the most effective capturers of thermal neutrons. Thus, the rate of thermal neutron decay in the formation can be measured and used to differentiate between hydrocarbons in the pore space and salt water. Sigma is measured in capture units (cu). Higher values of Σf equate to a greater ability to capture thermal neutrons.

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, which is measured by the receiving coils. The measured conductivities are then converted to resistivity (in ohm-meters). For the shallow penetration resistivity, the current necessary to maintain a constant drop in voltage across a fixed interval is measured. This is a direct measurement of resistivity. Typically, igneous minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals as well as ionic solutions like pore water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity using Archie's law (Archie, 1942, 1950) and fluid salinity.

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 direct measurement of sound 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, the DSI also has two crossed-dipole transmitters that allow the measurement of orthogonal shear wave velocities in addition to the compressional wave velocity, even in the slow formations typically encountered during IODP expeditions.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity–based images of borehole walls. The tool has 4 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 8 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. Further analysis can provide measurements of dip and direction (azimuth) of planar features in the formation.

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; the fact that the images are oriented means that fabric analysis can be carried out and bed orientations can be measured.

The maximum extension of the caliper arms is 38.1 cm (15 inches). In holes with a diameter larger than 38.1 cm, 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.

Ultrasonic Borehole Imager

The UBI features a high-resolution transducer that provides acoustic images of the borehole wall. The transducer emits ultrasonic pulses at a frequency of 250 or 500 kHz (low and high resolution, respectively), which are reflected at the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined. A continuous rotation of the transducer and the upward motion of the tool produce a complete map of the borehole wall. The amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the UBI tool in the borehole, the shape of the borehole, and the roughness of the borehole wall. Changes in the borehole wall roughness (e.g., at fractures intersecting the borehole) are responsible for the modulation of the reflected signal; therefore, fractures or other variations in the character of the drilled rocks can be recognized in the amplitude image. The recorded traveltime image gives detailed information about the shape of the borehole, which allows calculation of one caliper value of the borehole from each recorded traveltime. Amplitude and traveltime are recorded together with a reference to magnetic north by means of a General Purpose Inclinometer Tool (GPIT), permitting the orientation of images. If features (e.g., fractures) recognized in the core are observed in the UBI images, orientation of the core is possible. The UBI orientated images can also be used to measure stress in the borehole through identification of borehole breakouts and slip along fault surfaces penetrated by the borehole (i.e., Paillet and Kim, 1987). In an isotropic, linearly elastic rock formation that is subjected to an anisotropic stress field, drilling a subvertical borehole causes breakouts in the direction of the minimum principal horizontal stress (Bell and Gough, 1983).

Borehole inclination and magnetic field measurement

Three-component acceleration and magnetic field measurements were 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 and UBI tool strings during logging. Thus, the FMS and UBI images can be corrected for irregular tool motion caused by the vessel's heave, which allows for more accurate determination of the dip and direction (azimuth) of features.

Log data quality

The principal influence on log data quality is the condition of the borehole wall. If the borehole diameter is variable over short intervals resulting from washouts during drilling or ledges caused by layers of harder material, the logs from those tools that require good contact with the borehole wall (i.e., FMS, density, and porosity 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 depth determination depends on a series of 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 and a calculation of cable stretch. 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 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 (Goldberg, 1997).

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 rig floor (wireline depth below rig floor [WRF]). After logging was completed, the data were shifted to a seafloor reference (wireline depth below seafloor [WSF]) based on the step in gamma radiation at the sediment/water interface. These data were made available to the science party.

The data were transferred onshore to Lamont-Doherty Earth Observatory, where standardized data processing took place. The main part of the processing is depth matching to remove depth offsets between data from different logging runs, which results in a new depth scale, wireline matched depth below seafloor (WMSF). 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 for the conventional logs and GIF for the FMS images. Schlumberger Geo-Quest'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

Measurements such as NGR and density are taken both downhole and on cores. They can be correlated using Correlator software, which allows shifting of the core depths onto the wireline depth.

Core-log-seismic integration

GeoFrame's IESX seismic interpretation software package was used during Expedition 324 to display site survey seismic sections acquired precruise. Velocity and density logs, together with the equivalent measurements made on core in the physical property laboratory, were used to create synthetic seismograms. The depth-traveltime relation was adjusted until the features in the synthetic seismogram matched the features in the seismic section. In this way, lithostratigraphic units in the core are correlated with reflectors and sequences in the seismic section.

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