Downhole logs are used to determine in-situ physical and chemical properties of formations adjacent to the borehole. Continuous, insitu measurements provide stratigraphic, lithologic, geophysical, and mineralogic characterizations of a site. After coring is completed at a hole, a tool string (a combination of several sensors) is lowered down the hole on a conductor cable, and each sensor continuously monitors some property of the adjacent formation. Data typically are recorded at 15-cm intervals, though this figure depends on the sensor. The depths of investigation into the formation are also sensor-dependent.
Three Schlumberger logging tool strings were used during Leg 149 (Fig. 12): the geophysical combination (Quad-combo), the geochemical combination (GLT), and the Formation MicroScanner (FMS) combination. The Lamont-Doherty Earth Observatory (LDEO) temperature tool (TLT) was attached to the base of all tool strings to obtain downhole formation/fluid temperatures. The natural gamma-ray tool (NGT) was run as part of each tool string to correlate depths between logging runs.
The geophysical tool string used during Leg 149 (Fig. 12A) consisted of the dipole shear imager (DSI) for measuring sonic velocities; the high-temperature lithodensity tool (HLDT) for measuring formation bulk density and photoelectric effect (PEF); the phasor induction tool (DIT) for measuring electrical resistivity; and the natural gamma-ray tool (NGT) for measuring the natural radioactivity of the formation and for correlating between logging runs. A caliper on the HLDT indicated hole diameter.
The geochemical combination used during Leg 149 (Fig. 12C) consisted of the NGT tool, the aluminum clay tool (ACT), and the gamma-ray spectrometry tool (GST). This tool combination measures the relative concentrations of Si, Ca, Al, Fe, S, H, Cl, K, U, and Th.
The FMS tool string used during Leg 149 (Fig. 12B) consisted of an array sonic tool (SDT) and the FMS tool, which was coupled to a general purpose inclinometer tool (GPIT). The FMS and GPIT provided a spatially oriented microresistivity image of the borehole wall, as well as a caliper measurement. The SDT provided an alternative measurement of sonic velocities for comparison with the DSI tool.
For Leg 149, Schlumberger deployed their new Maxis 500 digital logging unit in conjunction with the "old" Offshore Service Unit (OSU). The "new" Maxis 500 system was capable, during Leg 149, of processing data from only the geophysical and FMS tool strings; the GLT was operated by the OSU unit. Data from the Maxis 500 was generated in a new data format (digital log information standard [DLIS]). For Leg 149, this was converted to log information standard (LIS) format for compatibility with shipboard and shore-based logging software.
A brief description of logging tools run during Leg 149 is given in the following sections. A detailed description of logging tool principles and applications is provided in Ellis (1987), Schlumberger (1989), Serra (1984), and Timur and Toksz (1985). The specifics of each tool are summarized in Table 7, and the approximate vertical resolutions of the tools are given in Table 8. The HLDT, DIT, NGT, ACT, GST, FMS, and GPIT tools are described in the "Explanatory Notes" chapter of the Leg 144 Initial Reports volume Shipboard Scientific Party, 1993b). The sonic tools, DSI and SDT, and the temperature tool, TLT, are described below.
Sonic tools measure compressional-wave traveltimes between a transmitter and receiver. This provides a direct measure of vertical traveltime in the adjacent formation (the interval traveltime [delta T]), which is used to calculate the porosity of the formation and sonic velocity. The product of velocity and density logs is an impedance log. A synthetic seismogram may be produced from the impedance log for comparison with seismic reflection profiles across the site.
The SDT-C sonic tool maximizes the information obtained by digitizing the complete seismic waveform. The tool has two transmitters and receivers with a 1-m spacing in addition to a linear array of eight receivers spaced at 15 cm, with a transmitter-receiver distance starting at 3.33 m (see Fig. 13A). The addition of a linear array in place of two discrete receivers is the main change from the earlier SDT tool. The digitally recorded full waveform is used later to deterine shear-wave and Stoneley-wave velocities in addition to the compressional velocity obtained immediately. The tools standard vertical resolution is 60 cm, although special array processing can improve resolution to 15 cm.
The DSI consists of one dual-frequency (14 and 1 kHz) monopole transmitter and two pairs of dipole transmitters (2.2 kHz). The dipole transmitters create a "uni-directional" flexing of the borehole wall that excites shear waves, in contrast to more common "omnidirectional" monopole sources. The receiver consists of an array of eight receiver groups with a 15-cm spacing. Within each receiver group, four receivers are in line with the dipole transmitters (Fig. 13B). Like the SDT described above, the DSI can measure compressional- and Stoneley-wave velocities, but has the added advantage that it can measure shear-wave velocity directly in "soft" formations (where the shear-wave velocity is less than the velocity of the drilling fluid, such as a poorly lithified sediment). Besides the conventional first-motion detection methods, DSI uses digital correlation analysis between signals from the receiver array to determine wave propagation velocities. Full waveforms are digitally recorded to allow post-cruise extraction of additional information, such as seismic attenuation. The ratio of compressional- to shear-wave velocities provides information about porosity and lithology. Observation of the reflection, attenuation, and dispersion of Stoneley waves provide useful information about permeability and fracture aperture identification in the immediate vicinity of the borehole.
The TLT is a self-contained temperature recording tool that can be attached to any Schlumberger tool string. Data from two thermistors and a pressure transducer are collected at a pre-determined rate of one sample per 0.5 to 5.0 s and stored within the tool. Following the logging run, data are transferred from the tool to a shipboard computer for analysis. A fast-response, lower accuracy, thermistor is able to detect sudden temperature excursions such as might be caused by fluid flow from the formation. A slow-response, higher accuracy, thermistor can be used to estimate borehole fluid temperature. If the history of drilling-fluid circulation in the hole and at least two temperature logs are available (Jaeger, 1961), one can estimate the post-drilling equilibrium geotherm. Conversion to depth is based on pressure recordings from the pressure transducer and on correlation with winch time "wire-out" records. During Leg 149, to save time in downloading, we used two temperature tools alternately, one tool (used on the geophysical and geochemical strings) had both fast-and slow-response thermistors, while the second contained only a slow-response thermistor. New Macintosh-based software was implemented for processing the temperature data.
The quality of log data may be seriously degraded by rapid changes in the diameter of a hole and in sections where the borehole diameter is greatly increased or has been washed out. The result of these effects is to impair logging by causing "bridging" or "tool sticking" and to increase the volume of fluid between the formation and the logging tool. Deep investigation devices, such as resistivity and velocity tools, are least sensitive to borehole effect. Nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma-rays) are more sensitive because of their shallower depth of investigation and because of the effect of increased volume of drilling fluid on attenuation of neutrons and gamma rays. Corrections can be applied to the original data to reduce these effects. However, one cannot correct for very large washouts.
By using the NGT on each string, data can be depth-correlated between logging runs. Logs from different tool strings, however, may still have minor depth mismatches caused by either cable stretching or ship's heave during recording. Small errors in depth-matching can impair the multilog analyses in zones of rapidly varying lithology.Ship's heave is minimized by a hydraulic wireline heave compensator designed to adjust for ship motion during logging operations. Precise depth-matching of logs to cores is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section within the cored interval.
During each logging run, incoming data were observed in real time on a monitor in the Maxis logging unit and simultaneously were recorded on disk. After logging, data were processed and reformatted with the Terralog log-interpretation software package. FMS data were processed on board the ship using Schlumberger "Logos" software. Preliminary log interpretation was conducted aboard the ship. Further FMS and geochemical log processing and the final depth-shifting and data quality control were undertaken after the leg at the Borehole Research Laboratory of LDEO.
The CD-ROM in the back of this volume is a "data only" CD-ROM that contains both depth-shifted and processed logging data that has been provided by the Borehole Research Group at Lamont-Doherty Earth Observatory, as well as shipboard Gamma-Ray Attenuation Porosity Evaluation (GRAPE), index properties, and magnetic susceptibility data of cores collected on board JOIDES Resolution during Legs 149, 150, and 150X (land-based portion of Leg 150). Also included on this CD-ROM is the Macintosh image viewing application NIH image. CD-ROM production was done by the Borehole Research Group at Lamont-Doherty Earth Observatory, wireline logging operator for ODP.
The INDEX file contains a summary of all the files loaded on the CD-ROM. The software documentation file in the GENERAL INFORMATION directory contains information on which software packages work best to import portable bit map (PBM-8-bit binary) raster files. It also includes network sources for the graphics software and data compression information. The README file gives information about whom to contact with any questions about the production of or data on the CD-ROM.
All of the ASCII files (basic logging and dipmeter files) are TAB
delimited for compatibility with most spreadsheet and data base programs. Holes that have long logging runs are often divided into TOP, MIDDLE, and BOTTOM directories. Were the data collected continuously or were two or more sections of data spliced together, the files would be in the SPLICED directory.
In the FMS-PBM format subdirectory are two subdirectories: 1:1 ratio with maximum 10-m-long image raster files and 1:10 ratio with maximum 100-m-long image raster files. The image raster files are named according to their depth interval. The raster documentation files contain image file parameter information necessary for use with most graphic software packages.