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doi:10.2204/iodp.proc.336.102.2012

Downhole logging

Downhole logs are used to determine the physical, chemical, and structural properties of the formation penetrated by drilling. The data are rapidly collected, continuous with depth, and measured in situ; they can then be interpreted in terms of stratigraphy, lithology, mineralogy, magnetic characteristics, 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 site 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.

During Expedition 336, downhole measurements were taken in Holes 395A, U1382A, and U1383C.

Wireline logging

During wireline logging, measurements were made with a variety of Schlumberger logging tools and the third-party Deep Exploration Biosphere Investigative tool (DEBI-t). These tools were combined into a number tool strings that were run down the hole after coring operations were completed. Five wireline tool strings were used during Expedition 336 (Fig. F15; Table T5):

1. Microbiology combination: gamma ray, temperature, and deep UV–induced fluorescence;
2. Adapted microbiology combination I (AMC I): total gamma ray, density, electrical resistivity, and deep UV–induced fluorescence;
3. Formation MicroScanner (FMS)-Hostile Environment Natural Gamma Ray Sonde (HNGS): spectral and total gamma ray and microresistivity images of the borehole wall;
4. Adapted microbiology combination II (AMC II): total and spectral gamma ray, density, and deep UV–induced fluorescence; and
5. FMS-sonic: spectral and total gamma ray, microresistivity images of the borehole wall, and compressional and shear wave velocities.

Each of these tool strings also contained a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system on the drillship. The microbiology tool strings included a number of Lamont telemetry cartridges (MultiFunction Telemetry Module [MFTM], EFTB-Lamont Interface Cartridge [ELIC], and Schlumberger-Lamont Telemetry Adapter [SLTA]; see below), which allow the DEBI-t and Lamont Modular Temperature tool (MTT) to be run in-line with the Schlumberger tools and acquisition system.

In the case of taking downhole measurements in Hole 395A, the hole was not prepared for logging. Here, the logging string was run immediately after the old CORK was removed. The aim was to log the hole and measure the fluorescence of biomass on the borehole wall; hence, no logging preparation was done because it would have seriously disturbed the borehole.

In preparation for logging at newly drilled sites, the boreholes were cased for CORK installation to 102 and 60.41 m DSF in Holes U1382A and U1383C, respectively. Immediately before logging, the holes were fully prepared for CORK installation; several wiper trips were made until no tight spots were encountered, viscous drilling fluid (sepiolite) was circulated through the drill pipe to the bottom of the hole when significant fill was found, and finally, the hole was displaced with seawater (for detailed information, please see “Operations” in the “Site U1382” and “Site U1383” chapters [Expedition 336 Scientists, 2012c, 2012d]). The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs (where applicable). A wireline heave compensator (WHC) was employed when appropriate to minimize the effect of ship heave on tool position in the borehole (see “Wireline heave compensator”). 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 200–400 m/h, to provide continuous measurements as a function of depth of several properties simultaneously.

Logged formation properties and tool measurement principles

The logged properties, and the methods by which the tools measure them, are briefly described below. The main logs taken by the tools are listed in Table T6. More detailed information on individual tools and their geological applications may be found in Serra (1984, 1986, 1989), Schlumberger (1989, 1994), Rider (1996), Goldberg (1997), Lovell et al. (1998), Ellis and Singer (2007), and Robinson et al. (2008). A complete list of acronyms for Schlumberger tools and measurement curves is available at www.slb.com/​modules/​mnemonics/​index.aspx.

Borehole temperature

The microbiology tool string included the MTT to measure borehole fluid temperature. The Lamont MTT was designed to be able to resolve centimeter-scale temperature variations at typical logging speeds of 250–300 m/h. It uses a highly accurate resistance-temperature device measurement accuracy of ±0.5°C over a 0°–250°C range and contains an accelerometer for depth correction. The MTT is combined with a specially designed cartridge to allow data transmission through the Schlumberger string and wireline.

One additional temperature measurement was made during some of the logging runs by a sensor included in the logging equipment head-mud temperature (LEH-MT) cablehead and processed by the Enhanced Digital Telemetry Cartridge (EDTC) (see below).

Generally, because logs are recorded shortly after coring, the borehole temperature is highly perturbed by the large amounts of seawater and mud circulated during the drilling process.

Natural radioactivity

The HNGS, a spectral gamma ray tool, uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of ⁴⁰K, ²³²Th, and ²³⁸U in the formation. These isotopes and the radioactive decay products of U and Th dominate the natural radiation spectrum. The HNGS filters out gamma ray energies of <500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. The computation of the elemental abundances uses a least-squares method of extracting thorium, uranium, and potassium elemental concentrations from the spectral measurements.

The EDTC (see below) used to communicate data to the surface includes a sodium iodide scintillation detector to measure total natural gamma ray emission. It is not a spectral tool, but it provides high-resolution total gamma ray intensities for each pass, which allows for precise depth-match processing between logging runs and passes.

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 decentralizing arm. Gamma rays emitted by the source undergo Compton scattering, which involves the transfer of energy from the gamma rays to the electrons in the formation via 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 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 therefore can be used to identify some minerals (for example, the PEF of calcite = 5.08 barns per electron [b/e^–] and quartz = 1.81 b/e^–). Good contact between the tool and borehole wall is essential for good HLDS logs. Poor contact typically results in underestimation of density values.

Electrical resistivity

The High-Resolution Laterolog Array (HRLA) tool provides six 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). The 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 Tool (the sonde conventionally used during IODP expeditions before the first usage of the HRLA during Expedition 335, Hole 1256D).

Typically, silicate minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals as well as ionic solutions like pore water are conductors. In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and thus is strongly dependent on porosity. Electrical resistivity, therefore, can be used to evaluate porosity and fluid salinity.

Acoustic velocity

The Dipole Shear 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 formations 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 crossed-dipole transmitters, which allow measurement of shear wave velocity in addition to compressional 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 (e.g., the sediments overlying igneous basement at Expedition 336 sites; however, these sediments were in the cased hole section and were not measured).

Fluorescence

The DEBI-t was designed and built specifically for Expedition 336 to better our understanding of the nature of microbial communities harbored in young ridge flanks, to understand their role in ocean crust weathering, and to elucidate whether these deep-seated microbial communities are acquired or indigenous. The DEBI-t was supported by the University of Southern California and developed by Photon Systems, Inc., Caltech’s Jet Propulsion Laboratory, and Lamont-Doherty Earth Observatory (LDEO). This unique tool is based on deep UV (<250 nm) laser detection of biological material and is designed here to assess the relative bioload of the borehole wall and aid in the effective targeting and deployment of the microbiology observatories. The DEBI-t utilizes a 224 nm excitation source and detects fluorescence between 280 and 380 nm. It is equipped with a pinhole camera that records video of the borehole in order to provide spatial context to the fluorescence information (see Table T7 for DEBI-t specifications).

The original microbiology combination tool string obtains a number of parameters in addition to native fluorescence, including three-axis downhole acceleration, three-axis magnetic field, temperature, and total and spectral (Th, U, and K) gamma ray measurements. The DEBI-t operates with a “fire and forget” methodology, in which the control software directs the instrument to fire the laser, collect data, and transmit information uphole as soon as power is supplied. Power is supplied via a 24 V power supply in the LDEO MFTM. The MFTM also acts as part of the communications interface between the DEBI-t and the Schlumberger wireline tools. During logging operations, the DEBI-t transmits real-time clock, laser power, and detector status to provide information regarding the health of the system. Transmission of these data is initiated by a request from the MFTM. There is no real-time shipboard control over any of the instrument parameters. However, the ability to monitor instrument status in real time allows us to power cycle the DEBI-t while still in the borehole, should the need arise. The transmitted data are logged against depth and used to depth-match the data recorded to the memory card in the DEBI-t, which is logged against time.

The DEBI-t is rated to operate at a maximum pressure of 10,000 psi, and the electronics and optics in analogous systems have been operated successfully at temperatures between 0° and >40°C. Standard procedure is to log down through the seafloor to the bottom of the open hole, log back up, log all the way down to total depth, and then finally log all the way back up through the seafloor.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity–based 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. 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; Salimullah and Stow, 1992; Lovell et al., 1998). Features such as vesicles, veins, fractures, and volcaniclastic breccia can be resolved, and the fact that the images are oriented means that fabric analysis can be carried out and structural feature (e.g., fracture) orientations can be measured. If the same features in these high-resolution electrical images can be identified in the recovered core samples, individual core pieces can be reoriented with respect to true north.

The maximum extension of the caliper arms is 38.1 cm (15 inches). In holes with a diameter of >38.1 cm, the pad contact at the end of the caliper arms 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. Standard procedure is to make two full passes up the borehole with the FMS to maximize the chance of getting optimal borehole coverage with the pads.

Borehole inclination 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. This information allows the FMS images to be corrected for irregular tool motion and the dip and direction (azimuth) of features in the images to be determined. The GPIT can be run with other tools as part of other tool strings that can carry remanent or induced magnetization; therefore, its magnetic measurements can sometimes be affected. However, on the FMS-sonic tool string, the GPIT has greater nonmagnetic insulation from the other tools, which greatly reduces the effects on its magnetic measurements. On the microbiology combination tool string, nonmagnetic knuckle joints were placed either side of the GPIT to improve the tool’s insulation.

Auxiliary logging equipment

Cablehead

The Schlumberger LEH-MT and logging equipment head-q tension (LEH-QT) measures tension at the very top of the wireline tool string, which helps diagnose difficulties in running the tool string up or down the borehole or when exiting or entering the drill string or casing. Additionally, the LEH-MT cablehead includes a temperature sensor on its outside to measure borehole fluid temperature.

Telemetry cartridges

Telemetry cartridges are used in each tool string to allow the data to be transmitted from the tools to the surface. In addition, the EDTC includes a sodium iodide scintillation detector to measure the total natural gamma ray emission of the formation. (The EDTC also contains an accelerometer that provides data to optimize the WHC before logging begins and to acquire the best possible downhole data.) The temperature measurements made by the LEH-MT are also processed by the EDTC before being sent to the surface for real-time monitoring. Because non-Schlumberger tools were run during Expedition 336 logging, several telemetry cartridges were required to allow communication with the Schlumberger telemetry and permit wireline transmission of data collected to the Schlumberger acquisition unit. A Lamont MFTM and ELIC were placed above the DEBI-t to provide 24 V power to the tool and to allow this third-party tool to run on Schlumberger telemetry. The SLTA was placed above the Lamont MTT in order to facilitate running this tool in combination with Schlumberger tools.

Joints and adapters

Because the tool strings combine tools of different generations and with various designs, they include several adapters and isolation joints between individual tools to allow communication, avoid interferences (mechanical, acoustic, or electrical), or position the tool properly in the borehole. The knuckle joints in particular are used to isolate the GPIT tool from magnetized/​potentially magnetic portions of the surrounding tool string (microbiology combination tool string) or to decouple the remainder of the centralized tool string (AMC II) from the overlying HLDS sonde that is pressed against the borehole wall by a caliper arm and bowspring.

All of these additions are included and contribute to the total length of the tool strings in Figure F15.

Logging 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 some tools (e.g., FMS, density, and porosity tools) may be degraded. Deep (0.23–1.5 m) 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, performing a full wiper trip, 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 paid out at the winch on the ship. The seafloor is identified on the natural gamma ray log by the abrupt decrease in gamma ray count at the sediment/​water interface (“mudline”). Discrepancies between drilling and core depth and wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, and drill pipe stretch, in the case of drilling 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 WHC (see below) was used (when appropriate) to adjust the wireline length for rig motion during wireline logging operations.

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 the 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 the efficiency of the compensator to be monitored. In addition to an improved design and smaller footprint compared to the previous system, the location of the WHC with the winch unit on the starboard side of the derrick contributes to a significant reduction in the time necessary 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), which is based on the step in gamma radiation at the sediment/​water interface.

The data were transferred on shore to LDEO, 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. This processing 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 imagery is corrected for tool acceleration, including sonde “stick and slip”), documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII for conventional logs and GIF for FMS images. The Schlumberger Geo-Quest GeoFrame software package is used for most of the processing of all wireline logging data collected. The data were transferred back to the ship within a few days of logging, and the processed data set was made available to the science party (in ASCII and Digital Log Interchange Standard [DLIS] formats) through the shipboard IODP logging database and shipboard servers. This LDEO depth-matched data were used to depth-shift the fluorescence data written to memory in the DEBI-t on the ship.

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