f), which is a function of the porosity and is also measured by the Accelerator Porosity Sonde.
Downhole logs are used to determine physical, compositional, and structural properties of the formation surrounding the 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 is crucial for characterizing the borehole section. Where core recovery is good, log and core data complement one another and may be interpreted jointly. Logs are sensitive to formation properties on a scale intermediate between laboratory measurements on core samples and geophysical surveys. Logging during Leg 201 helped characterize physical properties to define constraints on the downhole microbial communities. It was also useful in delineating the possible hydrologic conduits and the flow regime that can affect the deep biosphere.
Logs are recorded with a variety of Schlumberger logging tools combined into several tool strings that are run down the hole after coring operations are complete. Two wireline tool strings were used during Leg 201: the triple combination (triple combo) tool string (resistivity, density, and porosity) and the Formation MicroScanner (FMS)-sonic tool string (resistivity image of the borehole wall and sonic velocities) (Fig. F14; Table T21).
Each tool string contains a telemetry cartridge for communicating through the wireline with the minimum configuration MAXIS (MCM) unit on the drill ship and a natural gamma radiation tool, which provides a common reference for correlation and depth shifting between multiple logging runs. Logging runs are typically conducted at 250-275 m/hr.
In preparation for logging, the boreholes were flushed of debris by circulating a pill of viscous drilling fluid (sepiolite mud mixed with seawater; weight = ~8.8 lb/gal or 1.11 kg/dm3) through the drill pipe to the bottom of the hole. The BHA was pulled up to a depth of between 60 and 100 mbsf then run down to the bottom of the hole again to ream borehole irregularities. The hole was subsequently filled with a sepiolite mud pill, and the pipe was raised to 60-100 mbsf and kept there to prevent hole collapse during logging. The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs. A wireline heave motion compensator (WHC) was employed to minimize the effect of ship's heave on the tool position in the borehole. During each logging run, incoming data were recorded and monitored in real time on the MCM logging computer. The tool strings were then pulled up at constant speed to provide continuous measurement as a function of depth of several properties simultaneously.
The properties measured by the tools and the methods used by the tools to measure them are briefly described below. The operating principles, applications, and approximate vertical resolution of the tools are summarized in Table T21. Some of the principal data channels of the tools, their physical significance, and units of measure are listed in Table T22. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1998), and Serra (1984, 1986).
Two wireline spectral gamma ray tools were used to measure and classify natural radioactivity in the formation: the Natural Gamma Ray Spectrometry Tool (NGT), and the Hostile Environment Gamma Ray Sonde (HNGS). The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million), the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for significantly improved tool precision. Spectral analysis in the HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or potassium chloride in the drilling mud and improving measurement accuracy. Although the NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or potassium chloride present in the drilling mud, these effects are routinely corrected for during processing at Lamont-Doherty Earth Observatory (LDEO).
Formation density was determined from the number of electrons in the formation, which is measured 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 via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number 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 factor (PEF). Photoelectric absorption occurs when gamma rays reach <150 keV after being repeatedly scattered by electrons in the formation. As 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 is essentially independent of porosity. For example, the PEF of pure calcite = 5.08 b/e-, illite = 3.03 b/e-, quartz = 1.81 b/e-, and kaolinite = 1.49 b/e-. PEF values can be used in combination with NGT curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.
Formation porosity was measured with the Accelerator Porosity Sonde. The sonde incorporates a minitron neutron generator, which produces fast (14.4 MeV) neutrons, and five neutron detectors (four epithermal and one thermal), positioned at different spacings. The tool is pressed against the borehole wall by an eccentralizing bowspring. Emitted neutrons are slowed down by collisions inside the formation. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The highest energy loss occurs when a neutron strikes a hydrogen nucleus, which has practically the same mass (neutrons simply bounce off heavier elements without losing much energy). The neutron detectors record the number of neutrons arriving at various distances from the source. The lower the porosity and, accordingly, the lower the hydrogen content, the farther the neutron will be able to travel and vice versa. However, as hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is commonly an overestimate.
Upon reaching thermal energies (0.025 eV) after multiple collisions, the neutrons are captured by the nuclei of chlorine, silicon, boron, and other elements, resulting in a gamma ray emission. The capacity of the formation to capture these neutrons is the neutron capture cross section (f), which is a function of the porosity and is also measured by the Accelerator Porosity Sonde.
The Phasor Dual Induction-Spherically Focused Resistivity (DIT-E) tool was used to measure electrical resistivity. The DIT-E provides three measures of electrical resistivity, each with a different depth of investigation into the formation: a deep-reading induction (IDPH), a medium-reading induction (IMPH), and a spherically focused log (SFLU). The two induction devices transmit high-frequency alternating currents through transmitter coils, creating time-varying magnetic fields that induce currents in the formation. These induced currents form loops around the borehole, creating a magnetic field that induces new currents in the receiver coils, producing a voltage. These induced currents are proportional to the conductivity of the formation, as is the voltage. The measured conductivities are then converted to resistivity. The SFLU is a shallow-penetration galvanic device that measures the current necessary to maintain a constant voltage drop across a fixed interval of the formation. It is a direct measurement of resistivity, with a higher resolution than the induction devices but more sensitive to borehole conditions. Sand grains and hydrocarbons are electrical insulators, whereas ionic solutions and clays are good conductors. Electrical resistivity can therefore be used to evaluate fluid salinity, water saturation, porosity, and the characteristics of the pore structure.
In addition, the DIT-E measures the spontaneous potential (SP) of the formation. SP can originate from a variety of causes: electrochemical, electrothermal, electrokinetic streaming potentials, and membrane potentials, due to differences in the mobility of ions in the pore and drilling fluids. SP may be useful to infer fluid flow zones and formation permeability.
Downhole temperature, acceleration, and pressure were measured with the LDEO Temperature/Acceleration/Pressure (TAP) tool. Attached to the bottom of the triple combo tool string, the TAP is run in an autonomous mode, with data stored in built-in memory. Two thermistors with distinct responses are mounted near the bottom of the tool to detect borehole fluid temperatures. A thin, fast-response thermistor is able to detect small abrupt changes in temperature. A thicker slow-response thermistor is used to estimate temperature gradients and thermal regimes more accurately. The pressure transducer is included to activate the tool at a specified depth. A three-axis accelerometer measures tool movement downhole, providing data for analyzing the effects of heave on a deployed tool string.
The temperature record must be interpreted with caution, as the amount of time elapsed between the end of drilling and the logging operation is generally not sufficient to allow the borehole to recover thermally from the influence of drilling fluid circulation. The data recorded under such circumstances may differ significantly from the actual formation temperature. Nevertheless, from the spatial temperature gradient it is possible to identify abrupt temperature changes that may represent localized fluid flow into the borehole, indicative of fluid pathways and fracturing and/or breaks in the temperature gradient that may correspond to contrasts in permeability at lithologic boundaries.
The Dipole Sonic Shear Imager measures the transit times between sonic transmitters and an array of eight receivers. It averages replicate measurements at each depth, providing a direct measurement of sound velocity through sediments that is relatively free from the effects of formation damage and enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it also has two crossed dipole transmitters, which allow the measurement of shear and Stoneley wave velocities in addition to the compressional wave velocity, even in the slow formations typically encountered during ODP cruises. Stoneley waveforms can be indicators of fractured and/or permeable intervals
The FMS provides high-resolution electrical resistivity-based images of borehole walls. The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or buttons, which are pressed against the borehole wall during the recording (Fig. F14). The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced ~2.5 mm apart. A focused current is emitted from the four pads into the formation, with a return electrode near the top of the tool. Array buttons on each of the pads measure the current intensity variations. Processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous, spatially oriented, high-resolution images that mimic geologic structures behind the borehole walls. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved.
The maximum extension of the caliper arms is 15.0 in. In holes with a diameter >15 in, the pad contact will be inconsistent and the FMS images can be blurred. Irregular borehole walls will also adversely affect the images, as contact with the wall is poor.
Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometer Tool. 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.
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, clay swelling, or borehole wall collapse, 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 will also cause irregular log results. The quality of the borehole is improved by minimizing the circulation of drilling fluid, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning are completed.
The depth of the logging measurements is determined from the length of the logging cable payed 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). The coring depth (driller's depth) is determined from the known length of the BHA and pipe stands; the mudline is usually recovered in the first core from the hole.
Discrepancies between the driller's 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 drill pipe depth and incomplete heave compensation, cable stretch (~1 m/km), and cable slip in the case of log depth. Tidal changes in sea level will also have an effect. To minimize the wireline tool motion caused by ship heave, a hydraulic WHC adjusts for rig motion during wireline logging operations. The small but significant differences between drill pipe depth and logging depth should be taken into account when using the logs for correlation with core and log measurements. Core measurements such as susceptibility and density can be correlated to the equivalent downhole logs using the "Sagan" program, which allows shifting of the core depths onto the log depth scale. Precise core-log depth matching is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section within the cored interval.
Logs from different wireline tool strings will have slight depth mismatches. Distinctive features recorded by the natural gamma tool, run on every log tool string, provide correlation and relative depth offsets among the logging runs.
Data for each logging run were recorded, stored digitally, and monitored in real time using the Schlumberger MCM. On completion of logging in each hole, data were transferred to the downhole measurements laboratory for preliminary interpretation. Basic processing provides scientists with a comprehensive quality-controlled downhole log data set that can be used for comparison and integration with other data collected. This processing is carried out onshore at LDEO after the data are transmitted by satellite from the ship. Processing includes depth adjustments to remove depth offsets between data from different logging runs, corrections specific to certain tools and logs, documentation for the logs with an assessment of log quality, and conversion of the data to a widely accessible format (ASCII for the conventional logs and GIF for the FMS images). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing. Further postcruise processing of FMS data was performed at LDEO.
Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available directly from the LDEO-Borehole Research Group (BRG) internet web site at www.ldeo.columbia.edu/BRG/ODP/DATABASE. Access to logging data is restricted to Leg 201 participants for 12 months following the completion of the leg, and a password is required to access data during this period. Thereafter, access to this log data is openly available. A summary of logging highlights is also posted on the LDEO-BRG web site at the end of each leg.
