f) is also measured by the tool.
Downhole logs provide continuous in situ geophysical parameters within a borehole. These measurements are used to assess the physical, chemical, and structural characteristics of formations penetrated by drilling and thus provide a means of reconstructing the stratigraphy, lithology, and mineralogy of a sequence.
Well logging is typically undertaken in the deepest hole drilled at any one site. Where core recovery is poor or disturbed, downhole logs are often the most reliable source of information; where core recovery is good, core data can be correlated with log data to refine stratigraphy and unit characterization.
Downhole logging operations begin after the hole has been cored and flushed with a viscous drilling fluid. The drilling assembly is then pulled up to ~70 mbsf, and the logging tools are passed through the drill pipe into the open hole. The logging tools are joined together into tool strings so that compatible tools are run together. Each tool string is lowered separately to the base of the hole, and then measurement takes place as the tool string is raised at a constant rate between 275 and 500 m/hr (see "Downhole Measurements" in the individual site chapters). A wireline heave compensator is used to minimize the effect of the ship's heave on the tool position in the borehole (Goldberg, 1990).
Two tool strings, composed of a variety of Schlumberger and Lamont-Doherty Earth Observatory (LDEO) tools, were deployed during Leg 202:
Each tool string contains a telemetry cartridge facilitating communication from the tools along the wireline (seven-conductor cable) to the Schlumberger Minimum Configuration Maxis (MCM) unit located on the drillship. A natural gamma radiation tool is also included on all tool strings in order to provide a common reference for correlation and depth shifting between multiple logging runs. Further tool details are found in Figure F13 and Table T9.
During each logging run, incoming data were recorded and monitored in real time on the MCM logging computer. The MGT is not a Schlumberger tool and cannot record data while the Schlumberger tools are active. Thus, the MGT requires separate passes for data acquisition, during which time control of the wireline transfers to the LDEO logger and data are recorded, in real time, on the specialized acquisition system in the downhole measurements laboratory.
The main logs acquired by the tools are listed in Table T10. A brief description of the measurement methods and the logged properties is given below. 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), and Serra (1984, 1986, 1989).
Three wireline spectral gamma ray tools were used to measure and classify natural radioactivity in the formation: the Natural Gamma Ray Spectrometry Tool (NGT), the Hostile Environment Gamma Ray Sonde (HNGS) and the LDEO MGT. 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 spectrum of natural radiation. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for significantly improved tool precision. It also 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.
The MGT was developed by the LDEO Borehole Research Group to improve the vertical resolution of natural gamma ray logs by using an array of four short detector modules with 60-cm spacing. Each module comprises a small 2 in x 4 in NaI detector, a programmable 256-channel amplitude analyzer, and an 241Am calibration source. The spectral data are subsequently recalculated to determine the concentration of K, Th, and U radioisotopes or their equivalents. The spectral data from individual modules are sampled four times per second and stacked in real time based on the logging speed. This approach increases vertical resolution by a factor of two to three over conventional tools, while preserving comparable counting efficiency and spectral resolution. The radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity), and the energy of the gamma rays (a higher-energy gamma ray can reach the detector from deeper in the formation). The MGT also includes an accelerometer channel to improve data stacking by the precise measurement of logging speed. Postcruise corrections for borehole size and tool sticking, based, respectively, on the caliper and acceleration data, are possible.
Formation density was determined with the HLDT. The tool 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. 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 collisions. The number of scattered gamma rays that reach the detectors is directly related to the density 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 (grain) density is known.
The HLDT 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 the PEF depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present. For example, the PEF of calcite = 5.08 b/e-; illite = 3.03 b/e-; quartz = 1.81 b/e-; and kaolinite = 1.49 b/e-. Good contact between the tool and borehole wall is essential for acquisition of quality HLDT logs; poor contact results in an underestimation of density values.
Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator that produces fast neutrons (14.4 MeV) and five neutron detectors (four epithermal and one thermal) positioned at differing intervals 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 hydrogen concentration is low, 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, B, Cd, and other rare earth and trace elements with large capture cross sections, resulting in a gamma ray emission. This neutron capture cross section (f) is also measured by the tool.
The DIT was used to measure electrical resistivity. The DIT provides three measures of electrical resistivity, each with a different depth of investigation. 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 (measured in 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 more conductive. Electrical resistivity can therefore be used to evaluate porosity (via Archie's Law) if fluid salinity is known.
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP tool run in memory mode. The tool uses fast- and slow-response thermistors to detect borehole fluid temperature at two different rates. The fast-response thermistor detects small, abrupt changes in temperature, whereas the slow-response thermistor more accurately estimates temperature gradients and thermal regimes. A pressure transducer is used to activate the tool at a specified depth, typically 200 m above seafloor. A three-axis accelerometer measures tool movement downhole, providing data for analyzing the effects of heave on a deployed tool string. The acceleration log can aid in deconvolving heave effects postcruise.
The elapsed time between the end of drilling and the logging operation is generally not sufficient to allow the borehole to reach thermal equilibrium following circulation of the drilling fluid. Nevertheless, 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 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 borehole enlargement (Schlumberger, 1989). The tool contains the monopole transmitters found on most sonic tools but also has two crossed dipole transmitters, providing shear wave velocity measurement in addition to the compressional wave velocity, even in the slow formations typically encountered during ODP legs.
The FMS provides high-resolution electrical resistivity-derived images of the borehole (~30% of a 25-cm diameter borehole on each pass). The tool uses four orthogonal imaging pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording (Fig. F13). 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 and converted to an image.
With the FMS tool, features such as bedding, fracturing, slump folding, and bioturbation can be resolved, and spatially oriented images allow fabric analysis and bed orientations to be measured (Luthi, 1990; Lovell et al., 1998; Salimullah and Stow, 1992).
The maximum extension of the caliper arms is 15 in, so in holes or parts of holes where the diameter is larger the pad contact will be inconsistent and the FMS images may appear out of focus and too conductive. Irregular borehole walls will also adversely affect the image quality if it leads to poor pad-wall contact.
Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometer Tool (GPIT). 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. This provides a means of correcting the FMS images for irregular tool motion, allowing the true dip and direction (azimuth) of structures to be determined.
The quality of the log data depends primarily on borehole conditions. Large-diameter or irregular boreholes can lead to problems with measurements requiring contact with the borehole wall (e.g., FMS, density, and porosity tools). 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 hole 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). Discrepancies between the driller's depth and the wireline log depth occur because of incomplete heave compensation, tidal changes, and cable stretch (~1 m/km) in the case of log depth. The small differences between drill pipe depth and logging depth, and the even more significant discrepancy between ODP curation depth and logging depth, should be taken into account when using the logs for correlation between core and log data. Core measurements such as susceptibility and density can be correlated with the equivalent downhole logs using the Sagan program, which allows linear shifting of the core depths onto the log depth scale.
Data are transferred to the downhole measurements laboratory for preliminary interpretation. Basic processing is carried out postcruise to provide scientists with a comprehensive quality-controlled downhole logging data set that can be used for comparison, integration, and correlation with other data collected during Leg 202 and other ODP legs. 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 and GIF for the FMS images). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing.
Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data are available in ASCII.
