f.Downhole logs are used to determine physical, chemical, and structural properties of formations penetrated by drilling. The data are rapidly collected, are continuous with depth, and measure in situ properties; they can be used to interpret the formation's stratigraphy, lithology, and mineralogy. In intervals of no core recovery, logs provide the only means to describe the formation. Where XCB core recovery is poor, logs will often supply superior physical and chemical measurements of the interval compared to those of the core; where core recovery is good, log data complement core data. Logs also provide a link between core and seismic sections: sonic velocity logs improve depth to traveltime conversions, and synthetic seismograms may be compared directly to the seismic sections.
Logging tools were joined together in "tool strings" (Fig. F6), so that several measurements could be made during each logging run. The tool strings were lowered to the bottom of the borehole on a wireline cable, and data were logged as the tool string was pulled back up the hole. Repeat runs were made in some holes to improve coverage and confirm the accuracy of log data. During logging runs, a wireline heave compensator was employed to minimize the effect of ship's heave (caused by sea swell) on the motion and position of the tool string in the borehole. Not all tool strings were run in each hole; individual site chapters give details of logging strings deployed at each site. Examples of the use of downhole logs for paleoceanographic objectives are given in Lyle, Koizumi, Richter, et al. (1997).
Schlumberger tools and tool measurements are identified by three- or four-letter acronyms (Table T4). Often the acronym has no convenient translation, and the acronym itself becomes the name of the tool or measurement. In the site chapters we have tried to avoid the use of acronyms if practical. For example, "density" instead of "RHOM" is used for the bulk density log. The GHMT (geological high-resolution magnetic tool) is always called the GHMT, and the FMS (Formation Micro-Scanner) is always called the FMS. The advantage of using the acronyms is that the tool, units, and measurement methods are completely specified by the three- or four-letter acronyms.
During Leg 184, we deployed the following three logging strings (Fig. F6; Table T5):
Log data from the NGT placed at the top of all tool strings provide a common basis for correlation of several logging runs and for depth shifting all logs. A telemetry unit is placed at the very top of each string.
Brief descriptions of individual logging tools used during Leg 184, including their geological applications and data-quality controls, are given in the following text. The properties of the formation measured by each tool, the sample intervals used, and the precision of the measurements made (including the vertical resolution and depth of investigation into the formation wall at typical logging speeds) are summarized in Table T5. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994a, 1994b, 1995), Serra (1984, 1986, 1989), Timur and Toksöz (1985), and the LDEO-Borehole Research Group (BRG) Wireline Logging Services Guide (1994).
The HNGS and NGT measure the natural gamma radiation from isotopes of potassium, thorium, and uranium in the sediment surrounding the tool. High K and Th values indicate greater clay concentrations, and increased U values often indicate the presence of organic matter. The NGT and HNGS also provide a measure of the total or spectral gamma-ray signature (in American Petroleum Institute gamma radiation units [gAPI]) and uranium-free or computed gamma ray (gAPI units).
The APS emits fast
neutrons, which are slowed by hydrogen in the formation, and the energy of the
rebounded neutrons is measured at detectors spaced along the tool. Most hydrogen
is in the pore water, hence porosity may be derived. Hydrogen bound in minerals
such as clays also contributes to the measurement, however, so that the raw
porosity value is often overestimated. The neutrons slowed to thermal energies
are captured by nuclei, especially those of chlorine and the heavier elements;
this effect is measured by the APS as the neutron capture cross section, f.
The HLDS emits high-energy gamma rays, which are scattered in the formation. The bulk density is derived from the energy of the returning gamma rays. Porosity may also be derived from this bulk density, if the matrix density is known. In addition, the HLDS measures the photoelectric effect (PEF; absorption of low-energy gamma rays), which varies according to the chemical composition of the sediment (e.g., PEF of pure calcite = 5.08, illite = 3.03, quartz = 1.81, and kaolinite = 1.49 barn/e-). The HNGS, APS, and HLDS together comprise the integrated porosity-lithology tool.
The DIT measures the formation electrical resistivity at three different penetration depths: by electromagnetic induction for the deep and medium resistivity, and by current balancing for the shallow resistivity. Porosity, fluid salinity, clay content, grain size, and gas hydrate content all contribute to the resistivity.
The full specifications of the TAP tool are described in Table T6. Temperature (recorded by high-precision thermistors) and pressure are measured every second. Tool acceleration is recorded four times per second. Data are recorded as a function of time and correlated with depth based on a synchronized time-wireline cable depth record and on pressure recordings.
The DSI employs a combination of monopole and dipole transducers (see Fig. F7) to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1995). It replaces the more commonly used array sonic tool. In addition to robust and high-quality determination of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to determine shear-wave velocity in all types of formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at shear-wave velocity and is the most reliable means to estimate a shear velocity log. The configuration of the DSI also allows recording of cross-line dipole waveforms. These modes can be used to estimate shear-wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well.
The FMS produces high-resolution images of the microresistivity of the borehole wall. The tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole wall (Serra, 1989). Roughly 30% of a 25-cm diameter borehole is imaged. The vertical resolution is ~5 mm, allowing features such as clasts, thin beds, fractures, and veins to be imaged. The images are oriented, so that both strike and dip can be obtained for the sediment fabric. The FMS images have proved particularly valuable in the interpretation of sedimentary structures during previous ODP legs and have been used to identify cyclical stacking patterns in carbonates (Eberli, Swart, Malone, et al., 1997), soft sediment slumping (Kroon, Norris, Klaus, et al., 1998), turbidite deposits (Lovell et al., 1998), cross-beds (Hiscott et al., 1992), and facies changes (Serra, 1989). Detailed interpretation of FMS images in combination with other log and core data will be carried out postcruise at LDEO-BRG, at Leicester University Borehole Research (LUBR), and at Laboratoire de Mesures en Forage (LMF), Cerege, Aix-en-Provence (France).
The GPIT was included in the FMS-sonic tool string to calculate tool acceleration and orientation during logging. The GPIT contains a triple-axis accelerometer and a triple-axis magnetometer. This tool records the orientation of the FMS images and allows more precise determination of log depths than can be determined from cable length, which may be stretched and/or affected by ship heave.
The GHMT normally consists of a high-sensitivity total magnetic field sensor (NMRS) coupled with a magnetic susceptibility sensor (SUMS). The NMRS measures the total magnetic field, using a proton precession magnetometer. A polarity stratigraphy can usually be determined by comparing the variations in total field and susceptibility downhole, after removing the background geomagnetic reference field and the effect of the pipe. The SUMS measures magnetic susceptibility by means of low-frequency induction in the surrounding sediment. It responds to magnetic minerals (mainly magnetite and "iron sulfides"), which are often contained in the detrital sediment fraction and can be a proxy for paleoenvironmental change. The GHMT measurements for magnetic susceptibility and earth conductivity are displayed in instrument units (IU).
Before logging, holes were flushed of debris by circulating heavy viscous drilling fluid (sepiolite mud and seawater) through the hole. The BHA was then pulled up to between 80 and 110 mbsf, then run down the hole again to ream out borehole irregularities and stabilize borehole walls. The hole was then filled with a sepiolite mud pill and the BHA raised to 80-110 mbsf.
Data from each logging run were recorded, stored digitally, and monitored in real time using the Schlumberger Multitask Acquisition and Imaging System 500. After logging at each hole, data were transferred to the shipboard downhole measurements laboratory for preliminary processing and interpretation. The FMS image data were interpreted using Schlumberger's Geoframe 3.1.4 software package. Sonic and density data were deciphered using GeoQuest's IESX software package to establish the seismic-to-borehole tie. Logs from the shipboard processed data were plotted as depth-related curves (or images), representing the physical and chemical properties of the strata penetrated.
Soon after each hole was logged, wireline log data were transmitted to LDEO-BRG for processing using a FFASTEST satellite high-speed data link. Data processing at LDEO-BRG includes (1) depth shifting all logs relative to a common datum (i.e., mbsf), (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. Log curves of LDEO-BRG-processed data were then replotted on board for refining interpretations (see "Wireline Logging," in the "Site 1143" chapter; "Wireline Logging," in the "Site 1144" chapter; "Wireline Logging," in the "Site 1146" chapter; and "Wireline Logging," in the "Site 1148" chapter). Further postcruise processing of the log data from the FMS and GHMT will be performed at LMF.
Postcruise processed acoustic, caliper, density, gamma-ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available directly from the LDEO-BRG Internet 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. Downhole logging aboard the JOIDES Resolution is provided by LDEO-BRG in conjunction with LUBR, LMF, the University of Aachen, the University of Tokyo, and Schlumberger well-logging services.
A major factor influencing the quality of log data is the condition of the borehole. If the borehole diameter is variable over short intervals (resulting from washouts during drilling, clay swelling, or borehole wall collapse), there may be data acquisition problems for tools that require good contact with the wall (i.e., FMS and density and porosity logging tools). Measurements such as resistivity and sonic velocity that penetrate deep into formation walls and do not require contact with the borehole wall are generally less sensitive to borehole conditions. The quality of boreholes was improved by minimizing the circulation of drilling fluid, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning were completed. Controls affecting data quality from each tool are discussed in the descriptions in "Log Depth Scales".
The depth of each logged measurement is calculated from the length of the logging cable, minus the cable length to the seafloor (the seafloor is identified by an abrupt reduction in gamma-ray counts at the water/sediment interface). Discrepancies between the drill-pipe depth, the log depth, and the total core length may occur because of core expansion, incomplete core recovery, 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 may also have an effect. Thus, small but significant differences may occur between drill-pipe depth and cable depth, which should be taken into account when using the logs. Sagan, a new software module for depth shifting spliced core data to the wireline log depth scale, was used for the second time (its first use was during Leg 182). Sagan was developed by Peter deMenocal and Ann Esmay of LDEO for ODP Logging Services Space. The beta version of the Sagan software package was used to convert core mcd into mbsf as measured by wireline logging (see "Composite Section").
