Downhole logs are used to directly determine the physical, chemical, and structural properties of formations penetrated by drilling. Where core recovery is incomplete, logging data may serve as a proxy for physical properties and sedimentological and petrological data. These data complement the discrete measurements obtained from cores and offer advantages over core-based analyses in that they are collected rapidly and represent continuous, in situ measurements of the formation. Logs also provide a link between core and the seismic measurements. Sonic velocity logs improve traveltime-to-depth conversion. During Leg 179, four standard logging strings were deployed at Site 1105.
Logging tools are joined together in tool strings (Fig. F13) and are run sequentially into the hole on a seven-conductor cable. Tool strings deployed during Leg 179 included the dual induction/array sonic/gamma ray/temperature tool string, Formation MicroScanner/gamma ray tool string, density/porosity/gamma tool string, and the borehole compensated sonic/gamma tool string (Fig. F13). Every Schlumberger tool and tool measurement used during Leg 179 has an associated three- or four-letter acronym. These are shown in Table T2. Specifications for each tool are presented in Table T3.
The principles of operation and uses of the tools are described in detail in Serra (1984, 1986), Timur and Toksöz (1985), Ellis (1987), Rider (1996), in the "Explanatory Notes" chapter of the Leg 176 Initial Reports volume (Shipboard Scientific Party, 1999), and briefly below.
The dual induction tool (DIT) provides three different measurements of electrical resistivity, each of which penetrates the formation to a different depth and has a different vertical resolution (Table T2; Fig. F13). Values are recorded every 0.1524 m. Water content and salinity are the most significant factors controlling the resistivity of rocks. Resistivity is therefore primarily related to the inverse square root of porosity (Archie, 1942). The other main factors influencing the resistivity of a formation include the concentration of hydrous and metallic minerals, hydrocarbons and gas hydrates, and the abundance, distribution, and geometry (tortuosity) of interconnected pore spaces. The DIT is a valuable tool in defining lithologic boundaries.
The array sonic digital tool (SDT) is aimed at maximizing the information obtained from measured sonic waveforms by acquiring a digitized full-sonic waveform downhole. This is achieved by using two transmitters and receivers with a 1-m spacing in addition to a linear array of eight receivers spaced at 15 cm (Fig. F13). The addition of a linear array in place of two discrete receivers is the main change in the SDT from earlier tools. The digitally recorded full-wave form is used postcruise to determine shear-wave (S-wave) and Stoneley wave velocities in addition to the real time compressional wave (P-wave) velocity. Standard vertical resolution is 60 cm, although special array processing can produce a 15-cm resolution.
The natural-gamma spectrometry tool (NGT) measures the natural radioactivity of the formation using a NaI scintillation crystal mounted inside the tool. In formations, gamma rays are emitted by the radioactive isotope 40K and by the radioactive isotopes of the U and Th decay series. Measurements are analyzed by dividing the incident gamma-ray signature into five discrete energy windows, which correspond to the main spectral peaks for each element. The total counts recorded in each window, for a specified depth in the well, are inverted to give the elemental abundances of K (wt%), U (ppm), and Th (ppm). The NGT also provides a measure of the total gamma-ray signature (SGR or K + U + Th) and a uranium-free measurement (CGR or Th + K). Values are recorded every 0.1524 m, and the vertical resolution of the NGT is on the order of 46 cm (Table T2). The natural gamma-ray measurement is commonly used to estimate the clay or shale content because there is a relatively high abundance of radioactive elements in clay minerals. There are rock matrices, however, for which the radioactivity ranges from moderate to extremely high values because of the presence of volcanic ash, potassic feldspar, or other radioactive minerals.
The Lamont-Doherty Earth Observatory (LDEO) temperature-logging tool (TLT) is a self-contained, high-precision, low-temperature tool for recording borehole temperature. Because drilling and circulation operations disturb the temperature conditions in the borehole, the data recorded by the TLT are unlikely to match equilibrated formation temperatures. Nevertheless, the spatial temperature gradient is useful in identifying abrupt gradient changes that commonly indicate localized fluid seepages from the formation.
The Formation MicroScanner (FMS) produces high-resolution images of the resistivity of the borehole wall. The tool has four orthogonally oriented pads, each having 16-button electrodes that are pressed against the borehole wall (Serra, 1989). The tool works by emitting current from the four pads into the formation. The current intensity variations are measured by an array of receptors on each of the four pads. 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 formation fabric.
The hostile-environment natural gamma-ray sonde (HNGS) measures the natural gamma radiations from isotopes of K, Th, and U in the formation surrounding the tool. As opposed to the NGT, the HNGS uses larger bismuth germanate crystals, which detect a higher number of emitted photons. High K and Th values indicate greater clay concentrations.
The accelerator porosity sonde (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. Abundant hydrogen is in the pore water, hence porosity may be derived. However, hydrogen bound in minerals such as clays also contributes to the measurement. As a consequence, the true 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.
The litho-density sonde (HLDS) emits high-energy gamma rays, which are scattered by the electrons in the formation. The electron density, and hence 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 (absorption of low-energy gamma rays), which varies according to the chemical composition of the formation.
The borehole compensated sonic measures the traveltime of sound waves along the borehole wall between two transmitters and two receivers over distances 3 ft and 5 ft. The sonic velocity increases with compaction and lithification and will decrease in zones of higher porosity. An impedance log (density vs. velocity) can be used to generate synthetic seismograms for comparison with the seismic survey sections.
After coring operations ceased, the borehole was filled with freshwater gel mud, and the base of the drill pipe raised to ~33 mbsf. Each tool string was run sequentially in each hole, typically at logging speeds between 900 and 1800 ft/hr. After reaching the total depth of penetration, the tools were pulled upward at a constant rate to acquire the log data. The wireline heave compensator was used to minimize the effect of the ship's motion on the tool position.
Incoming data for each logging run were recorded and monitored in real time on the Schlumberger Maxis 500 logging computer. After logging, data were given preliminary interpretation. Schlumberger's GeoFrame software was used for processing the FMS images. The data were also transferred via high-speed data satellite link (Inmarsat B) to LDEO, where each logging run was shifted to a common depth scale and the NGT logs were recomputed. The processed data were then returned to the ship via the same satellite link and used in the site chapter report.
A two-ship vertical seismic profile (VSP) experiment was scheduled to be conducted by the Woods Hole Oceanographic Institution at Site 757 using their third-party three-component VSP tool. Because of the significant time shortage, the VSP experiment was canceled.
The principal influence on data quality is the state of the borehole wall. If the borehole width varies significantly over short intervals, or is more than 15 in wide, results from those tools (density, porosity, and FMS) that require good contact with the wall may be degraded. Very narrow sections will also cause irregular log results. The quality of the borehole is helped by minimizing the circulation of fluid during drilling and by logging a young hole.
Measurements that penetrate deeply into the formation, such as resistivity, are less sensitive to borehole conditions. Sonic velocity is more reliable in more compacted sediment or hard rocks. The maximum extent of the FMS pads is 15 in. Boreholes wider than this cannot be imaged adequately. Of the two natural-gamma tools, the HNGS has the more sensitive detector, and the data are corrected for borehole width in the tool itself; the NGT data requires shore-based reprocessing.
The depth of the logged measurements is calculated from the length of cable minus the cable length to the seafloor (seafloor is identified by the step reduction in gamma-ray activity at the sediment/water boundary). Differences between the core depth and the log depth are due to factors such as core expansion, incomplete core recovery, and nonrecovery of the mudline. Drill-pipe depth, as measured by the logs, may be slightly different as a result of incomplete heave compensation, cable stretch (~1 m/km), and cable slip. There is also an effect from tides. All of these factors should be taken into account when using the logs.