- FF
=
-m,
Downhole logs are continuous in situ records of physical and structural properties of the formation penetrated by a borehole. The logs are made using a variety of probes combined into tool strings (Fig. F7). These strings can be lowered down the hole on a heave-compensated electrical wireline and then pulled up at constant speed to provide continuous measurements, as a function of depth, of several properties simultaneously. Logs can be used to interpret the structure, stratigraphy, lithology, and mineral 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.
Owing to unstable conditions in the only hole logged during Leg 191, only one tool string was run: the triple combination (triple combo), which measures resistivity, density, and porosity (Fig. F7). This tool consists of the accelerator porosity sonde (APS), the hostile environment natural gamma-ray sonde (HNGS), the high-temperature lithodensity tool (HLDT), and the phasor dual induction-spherically focused resistivity tool (DIT). The Lamont multisensor gamma-ray tool (MGT) (Fig. F8) and universal data telemetry module (UDTM) were included at the top of the tool string. A downhole cable switch in the UDTM allowed simultaneous deployment and cable line switching between the MGT and Schlumberger tool strings. The Lamont-Doherty Earth Observatory (LDEO) temperature/acceleration/pressure (TAP) tool was attached to the bottom of the tool string.
Each tool string includes a telemetry cartridge for communicating through the wireline with the logging laboratory on the drillship and a natural gamma-ray sonde that is used to identify lithologic markers, providing a common reference for correlation and depth shifting between multiple logging runs.
The principal data outputs of the standard logging tools, their physical significance, and units of measure are listed in Table T2. The logging tools are briefly described below, and their operating principles, applications, and approximate vertical resolution are summarized in Table T3. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and at the Borehole Research Group Web site (see the "Related Leg Data" contents list).
Three spectral gamma-ray tools were used to measure and classify natural radioactivity in the formation: the HNGS, natural gamma spectrometry tool (NGT), and MGT. The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million), the three elements whose isotopes dominate the natural gamma radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for significantly improved tool precision. Spectral analysis of the HNGS data filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. Because NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud, corrections are routinely made for these effects during processing at LDEO.
The newly developed LDEO MGT was tested as a part of the triple-combo tool string (Fig. F7). The major advantage of the new tool is improved vertical resolution, comparable with the resolution of MST core measurements. This is achieved by real-time stacking of NGR spectral data from four independent small-sized scintillation detectors positioned at 0.64-m spacing in the measurement module (Fig. F8). The tool provides 256-channel spectral analysis of each detector data in the 0.2- to 3.0-MeV energy range. The full spectra are later combined into five- or three-window spectral data for compatibility with the older tools. The total gamma (in API units) and concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million) are calculated in real time either from spectral data of individual detectors or from stacked data. The tool also includes an accelerometer for precise depth correction in the process of data stacking.
Formation density was determined from the density of electrons in the formation, which was measured with the HLDT. The sonde contains a radioactive cesium (137Cs) gamma-ray source (622 keV) and far and near gamma-ray detectors mounted on a shielded skid that is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source experience Compton scattering, which involves the transfer of energy from gamma rays to the electrons in the formation by 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 HLDT also measures the photoelectric effect factor (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. Photoelectric absorption is strongly dependent on the atomic number of the constituents of the formation; it varies according to the chemical composition and is essentially independent of porosity. For example, the PEF of pure calcite = 5.08, illite = 3.03, quartz = 1.81, and kaolinite = 1.49 barn/e-. PEF values can be used in combination with NGT curves to identify different types of clay minerals. The PEF values, therefore, can give an indication of the chemical composition of the rock.
Coupling between the tool and borehole wall is essential for good HLDT logs. Poor contact results in underestimation of density values.
Formation porosity was measured with the APS. 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 by collisions. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus nearly equal to its own mass, such as in the case of hydrogen, which is mainly present in the pore water. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times that act as a measure of formation porosity. However, as hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.
The DIT measures the formation electrical resistivity and provides three different measurements of electrical resistivity based on multiple depths of investigation: deep induction, medium induction, and shallow spherically focused resistivity. Deep- and medium-penetration measurements are made inductively using transmitter coils that are energized with high-frequency alternating currents, creating time-varying magnetic fields that induce secondary Foucault currents in the formation. The strength of these induced ground currents is inversely proportional to the resistivity of the formation through which they circulate, as are the secondary inductive fields that they create. The amplitude and phase of the secondary magnetic fields, measured with receiving coils, are used as a proxy for the formation resistivity. Shallow penetration measurements with a high vertical resolution are made with a spherically focused laterolog. This measures the current necessary to maintain a constant voltage drop across a small fixed interval. Because the solid constituents of rocks are essentially infinitely resistive relative to the pore fluids, resistivity is controlled mainly by the nature of the pore fluids, porosity, and permeability.
In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and is strongly dependent on porosity. Electrical resistivity data can therefore be used to estimate formation porosity using Archie's Law (Archie, 1942) if the formation does not contain clay. Archie's Law is expressed as
-m,
where
= porosity,
= a constant that varies with rock type.
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP tool. When attached to the bottom of the triple-combo string, the TAP operates in an autonomous mode, with data stored in built-in memory. A two-component thermistor (for different temperature ranges) is mounted near the bottom of the tool in the slotted protective cover. The time constant of the thermistor assembly in the water is ~0.4 s. The tool also includes a pressure transducer (0-10,000 psi), which is used to activate the tool at a specified depth to perform pressure measurements. The TAP tool also incorporates a high-sensitivity vertical accelerometer, providing data for analyzing the effects of heave on a deployed tool string, and an internal temperature sensor for monitoring the temperature inside the electronics cartridge. Temperature and pressure data are recorded once per second, and accelerometer data can be recorded at 4- or 8-Hz sampling rate.
The borehole temperature record provides information on the thermal regime of the surrounding formation. The vertical heat flow can be estimated from the vertical temperature gradient combined with measurements of the thermal conductivity from core samples and conductive heat transfer regimes.
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 thermal equilibrium of that environment. 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 contacts.
The quality of log data may be seriously degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. Resistivity and velocity measurements are the least sensitive to borehole effects, whereas the nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays) are most sensitive because of the large attenuation by borehole fluid. Corrections can be applied to the original data to reduce the effects of these conditions, and generally, any departure from the conditions under which the tool was calibrated.
Logs from different tool strings may have depth mismatches, caused by either cable stretch or ship heave during recording. Small errors in depth matching can distort the logging results in zones of rapidly changing lithology. To minimize the effects of ship heave, a hydraulic wireline heave compensator adjusts for rig motion during logging operations. Distinctive features recorded by the NGT, run on every log tool string, provide correlation and relative depth offsets among the logging runs and can be calibrated to distinctive lithologic contacts observed in the core recovery or drilling penetration (e.g., basement contacts).
Data for each logging run were recorded, stored digitally, and monitored in real time using the Schlumberger minimum configuration maxis. On completion of logging at each hole, data were transferred to the downhole measurements laboratory for preliminary interpretation. Basic processing is then carried out in order to provide scientists with a comprehensive, quality controlled downhole log data set that can be used for comparison and integration with other data collected during each ODP leg. This processing is usually carried out onshore at LDEO after the data are transmitted by satellite from the ship. It 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 conventional logs). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing.
Processed acoustic, caliper, density, gamma-ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format will be available directly from the LDEO-BRG Web site (see the "Related Leg Data" contents list). A summary of logging highlights is also posted on the LDEO-BRG web site at the end of each leg.
The drill string acceleration (DSA) measurement tool was an experimental device deployed during Leg 191. The DSA tool attaches to the core barrel and records drill string acceleration in two frequency bands during the process of coring. The acquired data may be used for drill string vibration analysis, heave evaluation, and as a reference signal for seismic-while-drilling measurements. The DSA tool operates as a memory tool, recording the tool's acceleration, ambient pressure, and internal temperature. The tool uses two accelerometers: an axial (vertical) high-sensitivity accelerometer (HSA), and a three-axis low-sensitivity accelerometer. The DSA can operate either in heave mode (only the HSA signal is recorded) or in drill mode (both accelerometer signals are recorded).
The tool does not require connection to the logging cable and the data can be offloaded to a computer after the tool is retrieved from the hole.
