f)
is also measured by the tool.Downhole logs are used to determine physical, chemical, and structural properties of the formation penetrated by a 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 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 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. Wireline logging was planned for all three sites, with logging while drilling (LWD) planned for the upper sections of the shelf and fan sites.
During wireline logging, the logs are made with a variety of Schlumberger logging tools combined into several "tool strings," which are run down the hole after coring operations are complete. Four wireline tool strings were used during Leg 188: the triple combination (triple combo) (resistivity, density, and porosity); the Formation MicroScanner (FMS)-sonic (resistivity image of the borehole wall and sonic velocities); the geological high-resolution magnetic tool (GHMT)-sonic (magnetic field strength, magnetic susceptibility, and sonic velocities) and the FMS alone (Fig. F14; Table T3).
Each tool string also contains a telemetry cartridge for communicating through the wireline to the Schlumberger Minimum Configuration Maxis (MCM) unit on the drillship and a natural gamma radiation tool that 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; approximate weight = 8.8 lb/gal or 1.11 g/cm3) through the drill pipe to the bottom of the hole. The bottom-hole assembly (BHA) was pulled up to a depth of between 30 and 100 mbsf, then run down to the bottom of the hole again to ream borehole irregularities. The hole was subsequently filled with more sepiolite mud, and the pipe was raised to 30-100 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 compensator (WHC) was employed to minimize the effect of ship's heave on the tool position in the borehole (Goldberg, 1990). 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 measurements as a function of depth of several properties simultaneously.
LWD measures in situ formation properties with instruments that are located in the drill collars immediately above the drill bit (Fig. F15). Measurements are made shortly after the hole is cut and before it is adversely affected by continued drilling or coring operations. Fluid invasion into the borehole wall is also reduced relative to wireline logging because of the shorter time between drilling and measurement. LWD has been successfully conducted during four previous ODP legs (Leg 156: Shipley, Ogawa, Blum, et al., 1995; Leg 170: Silver, Kimura, Blum, et al., 1998; Leg 171A: Moore, Klaus, et al., 1998; and Leg 174A: Austin, Christie-Blick, Malone, et al., 1998). The key difference between LWD and measurement while drilling (MWD) is that whereas LWD data are recorded in memory and downloaded when the tools reach the surface, MWD data are transmitted up the pipe by means of a pressure wave (mud pulsing) at 3 bits/s and monitored in real time. The term LWD is also used more generically to cover both LWD and MWD.
LWD operations were planned for the shelf and fan sites in the upper sections where core recovery had been poor on previous Antarctic ODP legs and that were too shallow for wireline logs to be made. A 40-m LWD hole was drilled at the shelf site, and a 261.1-m LWD hole was drilled at the fan site. Coring is not possible with an LWD BHA.
Two Schlumberger-Anadrill tools were used, the compensated dual resistivity tool (CDR; resistivity and spectral gamma ray) and the Power Pulse MWD tool. Figure F15 shows the configuration of the LWD BHA, and Table T4 lists the main set of measurements. A more detailed description of the LWD tools and their applications for ODP may be found in Moore, Klaus, et al. (1998), Schlumberger (1993), and Desbrandes (1994).
The LWD equipment is battery powered and uses erasable/programmable read-only memory chips (EPROM) to store the logging data until it is downloaded. The CDR takes measurements at evenly spaced time intervals and is synchronized with a system on the rig that monitors time and drilling depth. After drilling, the LWD tools were brought back to the drill floor and the data were downloaded from each tool through an RS232 serial link to a personal computer. The data were put onto a depth scale.
In addition to the CDR, the Power Pulse MWD tool was also run. The MWD tool makes several measurements, including downhole weight on bit and downhole torque, that are transmitted to the surface and not recorded in memory.
The weight on drawworks, drilling parameters, and pressure at the rig floor was digitally recorded using the Fusion instrumentation system. Together with the Anadrill rig-floor instrumentation and the downhole weight-on-bit information, the quality of the rig-floor data could be assessed as well as the efficacy of the passive heave compensation on the actual weight on bit.
LWD logs were provided by Schlumberger-Anadrill Drilling Services under contract with the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG).
The logged properties, and the methods that the tools use to measure them, are briefly described below. The operating principles, applications, and approximate vertical resolution of the tools are summarized in Table T3. Some of the principal data channels of the tools, their physical significance, and measurement units are listed in Table T5.More detailed information on individual tools and their geological applications may be found in Ellis (1988), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989).
Two wireline spectral gamma-ray tools were used to measure and classify natural radioactivity in the formation: the natural gamma-ray tool (NGT) and the hostile environment natural gamma-ray sonde (HNGS). The NGT uses a sodium-iodide scintillation detector and five-window spectroscopy to determine concentrations of K (percent), Th (parts per million), and U (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 a significantly improved tool precision. The HNGS filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl 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 KCl present in the drilling mud, corrections for these effects are routinely made during processing at LDEO.
Formation density was determined with the hostile environment lithodensity 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 (PEF). Photoelectric absorption of the gamma rays occurs when they 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 calcite = 5.08 barn/e-; illite = 3.03 barn/e-; quartz = 1.81 barn/e-; and kaolinite = 1.49 barn/e-. Good contact 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 that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one thermal) positioned at different spacings from the minitron. The measurement principle involves counting neutrons that arrive at the detectors after being slowed by neutron absorbers surrounding the tool. 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 heavier elements without losing much energy). If the hydrogen (i.e., water) concentration is small, as in low-porosity formations, neutrons can travel farther before being captured and the count rates increase at the detector. The opposite effect occurs when the water content is high. However, because hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.
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.
The phasor dual-induction/spherically focused resistivity tool (DIT) was used to measure electrical resistivity. The DIT provides three measures of electrical resistivity, each with a different depth of investigation into the formation. The two induction devices (deep and medium depths of penetration) transmit high-frequency alternating currents through transmitter coils, creating magnetic fields that induce secondary currents in the formation. These currents produce a new inductive signal, proportional to the conductivity of the formation, which is measured by the receiving coils. The measured conductivities are then converted to resistivity (in units of ohm-meters). For the shallow penetration resistivity, the current necessary to maintain a constant voltage drop across a fixed interval is measured; it is a direct measurement of resistivity. Sand grains and hydrocarbons are electrical insulators, whereas ionic solutions and clays are conductors. Electrical resistivity can therefore be used to evaluate porosity, fluid salinity, and the characteristics of the pore structure.
The CDR LWD tool broadcasts a 2-MHz electromagnetic wave and measures the phase shift and the attenuation of the wave between two receivers. These quantities are transformed into two independent resistivities that provide the two depths of investigation. The phase shift is transformed into a shallow resistivity; the attenuation is transformed into a deep resistivity.
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution temperature/acceleration/pressure (TAP) tool. When attached to the bottom of the triple combo 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 WHC.
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 boundaries.
The dipole shear sonic imager measures the transit times between sonic transmitters and an array of eight receivers. It averages replicate measurements, thus 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 wave velocity in addition to the compressional wave velocity, even in the slow formations typically encountered during ODP legs.
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 the recording (Fig. F16). The electrodes are arranged in two diagonally offset rows of eight electrodes each (Fig. F16). A focused current is emitted from the button electrodes into the formation, with a return electrode near the top of the tool. The intensity of current passing through the button electrodes is measured. Processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous spatially oriented high-resolution images that mimic the geologic structures of the borehole wall. Further processing 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; Lovell et al., 1998; Salimullah and Stow, 1992). It has enabled the formation to be viewed in its complete state and often to be grouped into facies assemblages. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved; the fact that the images are oriented means that fabric analysis can be carried out and bed orientations can be measured.
The maximum extension of the caliper arms is 15 in. In holes with a diameter larger than 15 in, the pad contact will be inconsistent, and the FMS images may 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 susceptibility measurement sonde (SUMS) measures magnetic susceptibility by means of low-frequency induction in the surrounding sediment. It responds primarily to magnetic minerals (mainly magnetites, hematite, and iron sulfides), which are typically contained in the detrital sediment fraction.
The nuclear resonance magnometer sonde (NMRS) measures the total magnetic field using a proton precession magnetometer. The data from the SUMS and the NMRS tools can be used to construct a polarity stratigraphy, using the method outlined below.
The total magnetic field (B) measured in the borehole depends on position (p) and time (t) (Pozzi et al., 1988):
where Bf(p) = Bfi(p) + Bfr(p).
Br(p)
is the Earth's main magnetic field, generated in the Earth's liquid outer core.
The field intensity is ~50,000 nT for Prydz Bay. Ba(p)
is the magnetic field caused by the BHA (up to ~2000 nT, decaying away from the
BHA) and crustal heterogeneities. Bt(p,t)
is the time-varying field (e.g., magnetic storms). Two passes of the GHMT are
run to check that this is negligible. Bfi(p)
is the field produced in the borehole by the induced magnetization (Ji)
of the sediment that is parallel to B(p,t)
and proportional to the magnetic susceptibility (
):
Ji =
B(p,t) ·
· Bfi(p)
is then given by Bfi(p)
= (Ji
/2) · (1 - 3 sin2
I), where
I is the
inclination of the Earth's field at the site. Bfr(p)
is the field produced in the borehole by the remanent magnetization (Jr)
of the sediment, whose polarity we aim to determine. Jr
is either parallel (normal polarity) or anti-parallel (reversed polarity) to B(p,t)
if the site has not moved significantly (relative to the magnetic poles) since
sediment deposition. We find Bfr(p)
by subtracting Br(p),
Ba(p),
and Bfi(p)
from the total field measurement B(p,t).
Under favorable conditions, a magnetostratigraphy is given simply by the sign of Bfr(p). Further processing, completed onshore, involves regression analysis of Bfr(p) vs. Bfi(p) downhole on intervals of various thickness. Correlation indicates normal polarity, and anticorrelation indicates reversed polarity.
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 while drilling, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning are completed.
The depth of the wireline-logged measurement is determined from the length of the logging cable played 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). In LWD, the logging depth is determined from the known length of the BHA and pipe stands, the position of the top drive, and the stroke of the heave compensator (see Moore, Klaus, et al., 1998). 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; 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 wireline heave compensator 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 with 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 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 MCM software. On completion of logging at each hole, data were transferred to the downhole measurements laboratory for preliminary interpretation. Basic processing was carried out postcruise to provide scientists with a comprehensive, quality-controlled downhole logging data set that can be used for comparison and integration with other data collected during each ODP leg. The 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; GIF for the FMS images). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing. Postcruise processing of FMS and GHMT log data was performed at the Laboratoire de Mesures en Forage, in Aix-en-Provence, France.
Processed acoustic, caliper, density, gamma-ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available (see the "Related Leg Data" contents list). A summary of logging highlights is also posted on the LDEO-BRG Web site (see "Related Leg Data" contents list) shortly after the end of each leg.
The IESX seismic interpretation software package was tested during Leg 188. It was used to display site-survey seismic sections acquired precruise as well as the seismic section acquired from the JOIDES Resolution during the cruise. Velocity and density logs were used to create synthetic seismograms, which were overlaid on the seismic section and used to refine the depth-traveltime relation. In this way, lithostratigraphic units in the core are correlated with reflectors and sequences in the seismic section.
