m,
such as sediments. In more resistive formations, the measurement error becomes
significant (>20%) and it is more suitable to use the DLL (Schlumberger,
1989).Downhole logs are spatially continuous records of the in situ physical, chemical, and structural properties of the formation penetrated by a borehole. The logs are recorded rapidly using a variety of probes combined into tool strings. These strings are lowered downhole on a heave-compensated electrical wireline and pulled up at a constant speed to provide continuous simultaneous measurements of the various properties as a function of depth. Logs can be used to interpret 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 also sensitive to formation properties on a scale that is intermediate between laboratory measurements on core samples and geophysical surveys. The logs are used to calibrate the interpretation of geophysical survey data, for example, through the use of synthetic seismograms and to provide a necessary link for the integrated understanding of physical properties on all scales.
Two standard ODP logging tool strings were deployed during Leg 195: the triple combination (triple combo), and the Formation MicroScanner (FMS)/sonic combination. In addition to wireline logs, in situ temperature measurements were made with the Adara tool, which is located in the coring shoe of the APC during piston-coring operations, and the Davis-Villinger temperature probe, which is deployed on a wireline in between retrieval of RCB cores.
The tool strings used during Leg 195 were as follows:
Each tool string includes a telemetry cartridge, for communicating through the wireline with the downhole logging laboratory on the drillship, and a natural gamma ray sonde, which is used to identify lithologic markers, providing a common reference for correlation and depth shifting between multiple logging runs. Logging runs are typically conducted at 250-275 m/hr.
The logging tools are briefly described below, and their operating principles, applications, and approximate vertical resolution are summarized in Table T2. Some of the principal data channels of the tools, their physical significance, and units of measure are listed in Table T3. 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, 1994), Serra (1984, 1986, 1989), and the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) Guide to ODP Wireline Logging Services CD (2001) or on the World Wide Web at http://www.ldeo.columbia.edu/BRG/ODP/LOGGING/MANUAL/index.html.
Two spectral gamma ray tools were used to measure and classify natural radioactivity in the formation: the HNGS and the NGT. 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 NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud; these effects are routinely corrected for during processing at LDEO. 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 KCl in the drilling mud and improving measurement accuracy.
Formation density was determined from the density of electrons in the formation, which was measured with the 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 arm. Gamma rays emitted by the source undergo Compton scattering, which involves the transfer of energy from gamma rays to 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 the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when the energy of the gamma rays drops below 150 keV after being repeatedly scattered by electrons in the formation. As the PEF depends on the atomic number of the elements in the formation, it varies according to 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 b/e-. PEF values can be used in combination with NGT curves to identify different types of clay minerals. Coupling between the tool and the borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values. The radius of investigation into the formation of the lithodensity tool is on the order of tens of centimeters, depending on the density of the rock.
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) located at different spacings from the source. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted neutrons are slowed down 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 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 the neutron arrival times, which 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.
Two tools were used to
measure the formation electrical resistivity: the DIT-E and the DLL. The DIT-E
provides three measurements of electrical resistivity, each with a different
depth of investigation into the formation. 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 of the inductive
nature of the deep- and medium-penetration measurements, DIT-E logs are accurate
only for formations with resistivities less than ~100 m,
such as sediments. In more resistive formations, the measurement error becomes
significant (>20%) and it is more suitable to use the DLL (Schlumberger,
1989).
The DLL provides two
measures of formation electrical resistivity, labeled "deep" (LLd) and
"shallow" (LLs) on the basis of their respective depths of
investigation. In both devices, a current beam, 61 cm thick, is forced
horizontally into the formation using focusing (also called bucking) currents.
For the deep measurement, both focusing and measurement currents return to a
remote electrode on the surface; thus, the depth of investigation is
considerable, and the effects from borehole conductivity and adjacent formations
are reduced. In the shallow laterolog, the return electrodes that measure the
bucking currents are located on the sonde and, therefore, the current sheet
retains focus over a shorter distance than the deep laterolog. Fracture porosity
can be estimated from the separation between the deep and shallow measurements,
based on the observation that the former is sensitive to the presence of
horizontal conductive fractures only, whereas the latter responds to both
horizontal and vertical conductive structures. The DLL has a response range of
0.2-40,000 m.
The depth of investigation
of both the DIT-E and the DLL depends on the resistivity of the rock and on the
resistivity contrast between the zone invaded by drilling fluid and the
uninvaded zone. In formations with resistivities >100 m,
the average radial depth of investigation of the DIT-E is ~1.5 m for the deep
induction phasor-processed resistivity, 76 cm for the medium induction phasor-processed
resistivity, and 38 cm for the spherically focused resistivity log. These values
drop by ~20% for a 0.1-m
formation resistivity. The depth of investigation of the DLL will vary with the
separation between the sonde and the remote current return at the surface.
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
FF = a
-m,
where
= the porosity,
For a first-order interpretation, conventionally, a = 1 and m = 2, but more rigorous values can be determined from resistivity and porosity measurements on core samples (see "Physical Properties").
The DIT-E also measures spontaneous potential (SP) fields. Spontaneous potentials 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. The interpretation of SP logs remains problematic because of this multiplicity of sources.
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP tool. When 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 are mounted near the bottom of the tool to detect borehole fluid temperatures at different rates. 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, which should eventually lead to the fine tuning of the wireline heave compensator (WHC).
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. 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 equilibrium temperature 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 boundaries.
The DSI, which is conventionally used on the FMS/sonic string, was not functional during Leg 195, and the LSS was used to measure the elastic compressional wave velocity of the formation. The LSS provides long-spacing measurements through the "depth derived" borehole compensation principle. Acoustic traveltime readings between two acoustic sources, spaced 2 ft apart, and two receivers, also spaced 2 ft apart, are memorized at one depth and combined with a second set of readings made after the sonde has been pulled an appropriate distance up the borehole. The LSS records the full waveform for each source-receiver pair, in addition to its automatic determination of arrival time. The radial depth of investigation for sonic tools depends on the spacing of the detectors and on the petrophysical characteristics of the rock, such as rock type, porosity, and alteration, but is of the order of tens of centimeters. Velocity data, together with the formation density data, can be used to generate a synthetic seismogram.
The FMS provides high-resolution electrical resistivity-based images of borehole walls, which can be used for detailed lithologic or structural interpretation. The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or "buttons," which are pressed against the borehole wall during recording (Fig. F13). The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced ~2.5 mm apart (Fig. F13). 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 current intensity variations in the formation. Processing transforms these measurements, which reflect microresistivity variations in the formation into continuous, spatially oriented, high-resolution images that mimic geologic structures behind the borehole walls. FMS images are oriented with respect to magnetic north using the GPIT. This allows further processing that can provide measurements of dip amount and dip direction (azimuth) of planar features in the formation. FMS images are particularly useful for mapping structural features, dip determination, detailed core-log correlation, positioning of core sections with poor recovery, and analysis of depositional environments. For example, FMS images have been used for identifying sedimentation patterns in turbidite deposits (Lovell et al., 1998), facies changes (Serra, 1989), and volcanic stratigraphy (Brewer et al., 1998).
The FMS image is sensitive to structure within a radial depth of ~25 cm from the borehole wall and has a vertical resolution of 5 mm with coverage of ~22% of the borehole wall on a given pass. FMS logging commonly includes two passes, the images of which can be merged to improve borehole wall coverage. To produce reliable FMS images, the pads must be firmly pressed against the borehole wall. 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. The maximum borehole deviation for which good data can be recorded with this tool is 10°. Irregular borehole walls will also adversely affect the images, as contact with the wall is poor.
Downhole magnetic field measurements were made with the GPIT. The primary purpose of this sonde, 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. The acceleration data allow more precise determination of log depths than is possible on the basis of cable length alone, as the wireline is subject to both stretching and ship heave. Acceleration data are also used in processing of FMS data to correct the images for irregular tool motion.
Local magnetic anomalies, generated by high remanent magnetization of the basalts in the basement section of a borehole, can interfere with the determination of tool orientation. However, these magnetic anomalies can be useful for inferring the magnetic stratigraphy of the basement section.
The quality of log data may be seriously degraded if the hole diameter is excessively large or changes rapidly. 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 WHC adjusts for rig motion during logging operations. Distinctive features recorded by the NGT, run on every 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). 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. The precision and reliability of the various logging measurements are governed by the resolutions of the various tools and the condition of the drill hole. The vertical resolution of the various logging tools is generally ~46 cm, with several exceptions (Table T2).
Temperature measurements were taken at Site 1200 during Leg 195 to determine the in situ temperatures within the conduit of the serpentine mud volcano. The discrete in situ measurements were made with the Adara tool, which is located in the coring shoe of the APC during piston-coring operations. The components of the tool include a platinum temperature sensor and a data logger. The platinum resistance-temperature device is calibrated for temperatures ranging from -20° to 100°C, with a resolution of 0.01°C. In operation, the adapted coring shoe is mounted on a regular APC core barrel and lowered down the pipe by wireline. The tool is typically held for 5-10 min at the mudline to equilibrate with bottom-water temperatures and then is lowered to the bottom of the drill string. Standard APC coring techniques are used, with the core barrel being fired out through the drill bit using hydraulic pressure. The Adara tool (and the APC corer) remains in the sediment for 10-15 min to obtain a temperature record. This provides a sufficiently long transient record for reliable extrapolation back to the steady-state temperature. The nominal accuracy of the temperature measurement is ~0.1°C.
Data for each logging run were recorded, stored digitally, and monitored in real time using the Schlumberger MAXIS 500 system. After logging at each hole, data were transferred to the shipboard downhole measurements laboratory for preliminary processing and interpretation. FMS image data were processed and interpreted using Schlumberger's GeoFrame 3.7 software package. Logs from the shipboard processed data were plotted as depth-related curves or images representing the physical and chemical properties of the strata penetrated.
Log data were also transmitted to LDEO-BRG using a FFASTEST satellite high-speed data link for further processing soon after each hole was logged. Data processing at LDEO-BRG includes (1) depth-shifting all logs relative to a common datum (i.e., meters below seafloor), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO-BRG, log data were transmitted back to the ship, providing near real-time data processing. Log curves of LDEO-BRG processed data were then replotted on board for refining interpretations (see "Downhole Measurements" sections in each site chapter). Further postcruise processing of the log data from the FMS is performed at LDEO-BRG.
Postcruise, processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available directly from the LDEO-BRG World Wide Web site at http://www.ldeo.columbia.edu/BRG/ODP/DATABASE/DATA/search.html. Access to logging data is restricted to Leg 195 participants for 12 months following the completion of the leg, and a password is required to access data during this period. Thereafter, access to these log data is free. A summary of "logging highlights" is also posted on the LDEO-BRG web site at the end of each leg.
Downhole logging aboard JOIDES Resolution is provided by LDEO-BRG in conjunction with Leicester University Borehole Research, the Laboratoire de Mesures en Forage (Université de Montpellier 2), University of Aachen, University of Tokyo, and Schlumberger Well Logging Services.
