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doi:10.2204/iodp.proc.341.102.2014

Downhole logging

Downhole logs are measurements of physical, chemical, and structural properties of the formation surrounding a borehole that are made after completion of drilling. The data are rapidly collected, continuous with depth (at vertical sampling intervals ranging from 2.5 to 150 mm), and measured in situ. These equate to vertical resolutions that are intermediate between laboratory measurements on core samples and geophysical surveys. Downhole logs are thus useful in calibrating the interpretation of geophysical survey data and provide a necessary link for the integrated understanding of physical properties at a range of spatial scales.

Downhole logs can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. They also provide information on the condition, shape, and size of the borehole and on possible deformation induced by drilling and/or formation stress. Where core recovery is incomplete or cored material is disturbed, log data may help to characterize the borehole section and fill in between sampled intervals. Where core recovery is good, log and core data complement one another and may be interpreted jointly.

Logging operations

During wireline logging operations, logs are recorded with a variety of logging tools combined into several tool strings, which are run down the hole after completion of drilling operations. Three primary tool strings were initially planned for Expedition 341 (Fig. F23; Table T4): the triple combo string (spectral and natural gamma ray, porosity, density, resistivity, and magnetic susceptibility), the Formation MicroScanner (FMS)-sonic string (spectral and natural gamma ray, sonic velocity, and resistivity images), and the Versatile Seismic Imager (VSI) string (vertical seismic profile [VSP] and gamma ray). A fourth tool string, the Magnetic Susceptibility Sonde (MSS) string (magnetic susceptibility and spectral and natural gamma ray) was also deployed at one site (U1417). These tool strings may be modified in response to expected borehole conditions or tool performance. In addition, because of concerns about borehole condition and time limitations near the end of the expedition, a modified tool string was designed for Sites U1420 and U1421 to record the highest priority measurements needed to meet primary science objectives. This Sonic-induction string (natural gamma ray, sonic velocity, and resistivity) is shown in Figure F24, and individual tools are listed in Table T4.

In preparation for logging, boreholes are flushed of debris by circulating viscous drilling fluid and filled with seawater or 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. Heavier logging fluid (sepiolite mud mixed with seawater, weighted with barite) may be used where borehole conditions call for additional stabilization. The BHA is pulled up to a depth of 80–100 m DSF, depending on the stability of the hole. Each tool string is then lowered downhole by a seven-conductor wireline cable during sequential deployments. Each tool string deployment is a logging “run,” starting with the assembly of the tool string and the necessary calibrations. A tool string is then sent down to the bottom of the hole while recording a partial set of data and then, except for the VSI string, is pulled up at a constant speed, typically 250–500 m/h, to record the primary data. The VSI string is held stationary at regularly spaced depths while shooting the seismic source. During each run, tool strings can be lowered down and pulled up the hole several times to evaluate reproducibility or to try to improve the quality of the data. Each lowering or hauling-up of the tool string while collecting data constitutes a “pass.” During each pass, incoming data are recorded and monitored in real time on the surface minimum configuration multitasking acquisition and imaging system (MAXIS). A logging run is complete once the tool string has been returned to the rig floor and disassembled. A wireline heave compensator (WHC) was employed to minimize the effect of ship’s heave on the tool string’s position in the borehole.

Logged properties and tool measurement principles

The main logging measurements recorded during Expedition 341 are listed in Table T5. More detailed descriptions of individual tools and their geological applications may be found in Ellis and Singer (2007), Goldberg (1997), Rider (1996), Lovell et al. (1998), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989). A complete list of acronyms for the Schlumberger tools and measurement curves is available at www.apps.slb.com/cmd/.

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) was used to measure natural gamma radioactivity in the formation. It uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The radioactive isotopes of these three elements dominate NGR emissions in most rocks and sediments. 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 HNGS also provides a measure of the total gamma ray emission (HSGR) and uranium-free or computed gamma ray emission (HCGR) that are measured in American Petroleum Institute units (gAPI). The HNGS response is influenced by the borehole diameter; therefore, HNGS data are corrected for borehole diameter variations during acquisition.

The Enhanced Digital Telemetry Cartridge (EDTC) was used primarily to communicate data from the tool strings in the borehole to the surface. The EDTC also includes a sodium iodide scintillation detector that measures the total natural gamma ray emissions. It is not a spectral tool, but it provides high-resolution total gamma radiation for each pass, which allows precise depth match processing between logging runs and passes. The inclusion of a gamma ray tool (either HNGS or EDTC) in every tool string allows for use of gamma ray data for depth correlation between logging strings and passes.

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde. This 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 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 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. The raw porosity value is often an overestimate because hydrogen atoms bound in minerals such as clays or contained in hydrocarbons also contribute to the measurement.

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 (∑_f) is also measured by the tool.

Density

Formation density was measured with the Hostile Environment Litho-Density Sonde (HLDS). The HLDS normally consists of a radioactive cesium (¹³⁷Cs) gamma ray source (661 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 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 density is known.

The HLDS also measures the photoelectric effect (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach <150 keV after being repeatedly scattered by electrons in the formation. The PEF is determined by comparing counts from the far detector in the high-energy region, where only Compton scattering occurs, with those in the low energy region, where count rates depend on both reactions. The far detector is used because it has a greater depth of investigation (tens of centimeters). The response of the short-spaced detector, mostly influenced by mudcake (minimally present in riserless drilling because seawater-based mud is used) and borehole rugosity, is used to correct the density measurement for these effects. 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 b/e^–, illite = 3.03 b/e^–, quartz = 1.81 b/e^–, and kaolinite = 1.49 b/e^–.

Good contact between the tool and borehole wall is essential for high-quality HLDS logs. Poor contact results in underestimation of density values. Both the density correction and caliper measurements of borehole diameter are used to check the contact quality.

Electrical resistivity

The High-Resolution Laterolog Array (HRLA) provides six electrical resistivity measurements with different depths of investigation (including the borehole, or mud resistivity, and five measurements of formation resistivity with increasing penetration into the formation). This sonde sends a focused current into the formation and measures the intensity necessary to maintain a constant drop in voltage across a fixed interval, providing a direct resistivity measurement. The array has one central (source) electrode and six electrodes above and below it, which serve alternatively as focusing and returning current electrodes. By rapidly changing the role of these electrodes, a simultaneous resistivity measurement at six penetration depths is achieved. The tool is designed to ensure that all signals are measured at exactly the same time and tool position and to reduce the sensitivity to “shoulder bed” effects when crossing sharp beds thinner than the electrode spacing. The design of the HRLA, which eliminates the need for a surface reference electrode, improves formation resistivity evaluation compared to the traditional dual induction method. The HRLA is run centralized in the borehole for optimal results, so knuckle joints are used to centralize the HRLA while allowing the eccentralized density and porosity tools to maintain good contact with the borehole wall (Fig. F23).

The Phasor Dual Induction–Spherically Focused Resistivity Tool (DIT) was used rather than the HRLA to measure electrical resistivity in boreholes where unstable hole conditions presented greater risk to logging tools. The DIT provides three measures of resistivity at different depths of investigation into the formation. The two induction devices (deep and medium depths of penetration) transmit high-frequency alternating current through coil transmitters, creating a magnetic field that induces a secondary current in the formation. These currents produce a new inductive signal, proportional to the conductivity of the formation, which is measured by the receiving coils. Measured conductivities are then converted to resistivity (in ohm-meters). Spherically focused resistivity is measured by an electrode device that sends a current into the formation while maintaining a constant voltage drop. The amount of current necessary to keep a constant voltage gives a direct measure of resistivity. This device uses electrodes to focus the current flow into the formation so that equipotential surfaces are spherical and has a shallower depth of investigation and a higher vertical resolution than induction measurements.

Typically, igneous minerals found in crustal rocks, 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 pore water.

Magnetic susceptibility

The MSS, a wireline tool designed by Lamont-Doherty Earth Observatory (LDEO), measures the ease with which particular formations are magnetized when subjected to a magnetic field. The ease of magnetization, or susceptibility, is ultimately related to the concentration and composition (size, shape, and mineralogy) of magnetizable material within the formation. 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 can aid significantly in paleoclimatic and paleoceanographic studies, where construction of an accurate and complete stratigraphic framework is critical to reconstructing past climatic changes.

A single-coil sensor provides high-resolution measurements (~10 cm vertical resolution) with a shallow depth of investigation (~3 cm penetration into the borehole wall). A dual-coil sensor provides lower resolution measurements (~40 cm vertical resolution), with greater depth of investigation (~20 cm penetration into the borehole wall), and because of its more robust nature acts as a quality control for the high-resolution measurements. The MSS can be run as a component in a Schlumberger tool string, using a specially developed data translation cartridge. For quality control and environmental correction, the MSS also measures internal tool temperature and z-axis acceleration.

Acoustic velocity

The Dipole Shear Sonic Imager (DSI) measures the transit times between sonic transmitters and an array of eight receivers. The recorded waveforms are then used to calculate the sonic velocity of the formation. The omnidirectional monopole transmitter emits high-frequency (5–15 kHz) pulses to extract the compressional wave velocity of the formation, as well as the shear wave velocity when it is faster than the S-wave velocity of the borehole fluid. It combines replicate measurements, thus providing a measurement of compressional wave velocity through sediment that is relatively free from the effects of formation damage and an enlarged borehole (Schlumberger, 1989). The same transmitter can be fired in sequence at a lower frequency (0.5–1 kHz) to generate Stoneley waves that are sensitive to fractures and variations in permeability. Along with the monopole transmitters found on most sonic tools, the DSI 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, where shear wave velocity is slower than the S-wave velocity in the borehole fluid. Such slow formations are typically encountered in deep-ocean drilling. The two shear wave velocities measured from the two orthogonal dipole transmitters can be used to identify sonic anisotropy associated with the local stress regime.

Resistivity images

The FMS produces high-resolution images of borehole wall microresistivity that can be used for detailed lithostratigraphic and structural interpretation. The tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole walls. 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 based on their conductivity. 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. In addition, when the corresponding planar features can be identified in the recovered core samples, individual core pieces can be reoriented with respect to true north.

The maximum extension of the FMS caliper arms is 15 inches (~38 cm). In boreholes with a diameter larger than this maximum, the pad contact at the end of the caliper will be inconsistent, and the FMS images may appear out of focus and overestimate conductivity. Irregular borehole walls will also adversely affect the images if contact with the wall is poor. Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. Coverage may be increased by a second run. The vertical resolution of FMS images is ~5 mm.

Acceleration and inclinometry

The General Purpose Inclinometry Tool (GPIT) was included in the FMS-sonic tool string to calculate tool acceleration and orientation during logging. Tool orientation is defined by three parameters: tool deviation, tool azimuth, and relative bearing. The GPIT utilizes a three-axis inclinometer and a three-axis fluxgate magnetometer to record the orientation of the FMS as the magnetometer records the magnetic field components (Fx, Fy, and Fz). 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. Corrections for cable stretching and/or ship heave using GPIT acceleration data (Ax, Ay, and Az) allow precise determinations of log depths.

Vertical seismic profile

In a VSP experiment, a borehole seismic tool is anchored against the borehole wall at regularly spaced intervals and records acoustic waves generated by a seismic source positioned just below the sea surface. The main purpose of this experiment is to provide a direct measurement of the time necessary for seismic waves to travel from the surface to a given depth and to tie the observations in the well, recorded as a function of depth, to the reflections in the seismic survey data, recorded as a function of time.

The VSI used for the VSP is a three-axis geophone accelerometer that is anchored to the borehole wall by a caliper arm prior to recording. The orientation of the horizontal components, x and y, varies with sensor rotation during logging, but tool orientation is recorded. During Expedition 341, the VSI was anchored against the borehole wall at ~50 m intervals, and 5–10 recordings were typically taken at each station. The recorded waveforms were stacked, and a one-way traveltime was determined from the first breaks for each station. The seismic source used was a Sercel G-gun parallel cluster composed of two 250 inch³ air guns separated by 1 m. It was positioned by one of the ship cranes off the port side of the ship at a total horizontal offset from the top of the wellhead of ~30 m and maintained at a fixed water depth (typically between 2 and 7 m).

In accordance with the requirements of the National Environmental Policy Act and the Endangered Species Act, all seismic activities were conducted during daytime and Protected Species Observers kept watch for protected species (marine mammals, sea turtles, and endangered marine species) for the duration of the zero-offset VSP. Any sighting of protected species within the exclusion zone of 940 m for deepwater depths (>1000 m) and 1410 m for intermediate water depths (100–1000 m) would interrupt the survey for 60 min after the last sighting or until the protected species were seen leaving the exclusion zone. Protected Species Observers began observations 1 h prior to the use of the seismic source, which started with a 30 min ramp-up procedure, gradually increasing the operational pressure and firing rate to provide time for undetected protected species to vacate the area. The same ramp-up procedure would be used when resuming activity after any interruption because of the sighting of protected species or whenever the gun was not fired for more than 30 min.

Auxiliary logging equipment

Cable head

The Schlumberger logging equipment head (or cable head) measures tension at the very top of the wireline tool string, which diagnoses difficulties in running the tool string up or down the borehole or when exiting or entering the drill string or casing. The logging equipment head-mud temperature (LEH-MT) used during Expedition 341 also includes a thermal probe to measure the borehole fluid temperature.

Telemetry cartridges

Telemetry cartridges are used in each tool string to allow the transmission of the data from the tools to the surface. The EDTC also includes a sodium iodide scintillation detector to measure the total natural gamma ray emission of the formation. This gamma ray log was used to match depths between different passes and runs. In addition, it includes an accelerometer that can be used in real time to evaluate the efficiency of the WHC.

Joints and adapters

Because the tool strings combine tools of different generations and with various designs, they include several adapters and joints between individual tools to allow communication, provide isolation, avoid interferences (mechanical and acoustic), terminate wiring, or position the tool properly in the borehole. Knuckle joints in particular were used to allow some of the tools such as the HRLA to remain centralized in the borehole, while the overlying HLDS was pressed against the borehole wall.

All these additions are included and contribute to the total length of the tool strings in Figures F23 and F24.

Wireline heave compensator

The WHC system, which was first used during Expedition 320T (February 2009), 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), 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 EDTC, respectively. An 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 monitoring of the efficiency of the compensator.

Logging data flow and depth scales

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS 500 system. These data were then copied to the shipboard processing stations for preliminary processing. Typically, the main pass of the triple combo is used as a reference to which other passes are interactively depth matched. The initial logging data are referenced to the rig floor (WRF). After depth matching, all the logging depths were shifted to a seafloor reference (WSF) based on the step in the gamma radiation at the sediment/water interface and preliminarily depth matched to remove offsets between different logging passes. Potential sources of error in depth matching include sea state, uncompensated heave, wireline stretch, and errors in the reference log used, and the magnitude of uncertainty is typically on the order or centimeters to meters (IODP Depth Scales Terminology, v. 2.0, 2010). These data were made available to the science party within a few days of their acquisition.

The downhole log data were also transferred onshore to LDEO for standardized data processing. The main part of the processing is rigorous depth matching to remove depth offsets between different logging passes, which results in a new depth scale: wireline log matched depth below seafloor (WMSF). Also, corrections are made to certain tools and logs (e.g., FMS images are corrected for tool acceleration), documentation for the logs (with an assessment of log quality) is prepared, and data are converted to ASCII for the conventional logs and to SEG-Y for the VSP data. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing. The standardized processed data are made available (in ASCII and DLIS formats) shortly after the expedition through the IODP logging database (iodp.ldeo.columbia.edu/DATA/index.html).

For a summary of borehole and downhole logging depth scales used on Expedition 341, see Table T6.

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 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, conditioning by 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 payed 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 DSF, CSF-A, and WMSF depths occur because of core expansion, incomplete core recovery, tides, or incomplete heave compensation for the drilling 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 also will have an effect. The hydraulic WHC was used to adjust the wireline length for rig motion during wireline logging operations, to minimize the wireline tool motion caused by ship heave.

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