f) is also measured by the tool.
Downhole logging tools are used to determine physical, chemical, and structural properties of the formation penetrated by drilling. Data are rapidly collected, continuous with depth, and, most importantly, measured in situ. Logs may be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is good, logging and core data are complementary and should be integrated and interpreted jointly, with logging data providing in situ ground truth for core data. Where core recovery is incomplete or disturbed, logging data may provide the only means to characterize the borehole section.
Downhole logs record formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. They are critical for calibrating geophysical survey data (e.g., through synthetic seismograms), providing the necessary link for the integration of core depth domain to seismic time domain data. Through logs, data collected at the borehole scale can be extended to a regional scale using geophysical surveys. Wireline logging was scheduled for three of the six sites cored during Leg 208.
Data are obtained by a variety of Schlumberger and LDEO logging tools, combined into several tool strings, which are deployed after coring operations in a hole are completed. The tool strings are deployed by lowering them to the bottom of the hole and recording data as they are pulled (at a constant rate) up the hole. Repeat runs, or short runs over critical intervals, are undertaken for data quality control. Three tool strings were run during Leg 208, although not all were run at every site. Details are given in individual site chapters. The three tool strings are
NGR tools are included on both the triple combo and FMS-sonic tool strings to provide a common reference for correlation and depth shifting between multiple logging runs. WST-3 depths are taken from the wireline cable depths. Further tool details are given in Figure F6 and Tables T8 and T9.
Each tool string contains a telemetry cartridge that facilitates communication along the wireline (seven-conductor cable) between the tools and the Schlumberger minimum configuration MAXIS (MCM) unit located on the ship. Ship heave motion further complicates the acquisition of quality wireline logging data. To overcome this, the wireline is fed over the wireline heave compensator (WHC). As the ship heaves in the swell, an accelerometer located near the ship's center of gravity measures the movement and feeds the data, in real time, to the WHC. The WHC responds to the ship's heave by adding or removing cable slack to decouple the movement of the ship from the tool string (Goldberg, 1990). During each logging run, incoming data are recorded and monitored in real time on the MCM logging computer. Tool strings are pulled up at a constant speed to provide continuous measurements as a function of depth. 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 using the real-time specialized acquisition system in the downhole measurements laboratory (DHML).
The logs acquired by the tools are listed in Table T9. 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 SGT, the HNGS, and the MGT. The SGT uses a scintillation detector to measure the total gamma radiation originating in the formation and provides data for depth matching to other log runs. The HNGS uses two bismuth germanate scintillation detectors for high 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.
The MGT was developed by LDEO-BRG to improve the vertical resolution of NGR logs by using an array of four short detector modules with ~2-ft 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 2–3 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 radiation (higher-energy gamma radiation 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 processing may correct for borehole size and tool sticking by using the acceleration data.
Formation density was determined with the HLDS. 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 wall. Gamma radiation emitted by the source undergoes Compton scattering, which involves the transfer of energy from gamma radiation to the electrons in the formation by way of elastic collisions. The amount of scattered gamma radiation that reaches the detectors is directly related to the density of electrons in the formation that is in turn related to bulk density. Porosity may also be derived from this bulk density if the matrix (grain) density is known. The HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of gamma radiation occurs when it reaches <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 (Gardner and Dumanoir, 1980). 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 the acquisition of quality HLDS logs; poor contact results in an underestimation of density values.
Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator (which 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 that 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. Because hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often overestimated. Upon reaching thermal energies of 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 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. 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 (using Archie's law) and fluid salinity.
Downhole temperature, acceleration, and pressure were measured with the TAP tool. It was attached to the bottom of the triple combo tool string and run in memory mode with the data stored in the built-in memory. After the logging run was complete, the TAP tool was removed from the Schlumberger tools and returned to the DHML, where the data were downloaded.
The tool has a dual-temperature measurement system for identification of both rapid temperature fluctuations and temperature gradients. A thin fast-response thermistor detects small, abrupt changes in temperature, and the thicker 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 the seafloor. A three-axis accelerometer measures tool movement downhole, which provides data for analyzing the effects of heave on the tool string. The long-term accumulation and analysis of these data, under varying cable lengths and heave conditions, will lead to enhanced performance of the WHC. Also, the acceleration log can aid in deconvolving heave effects postcruise, and it has proven at times to provide critical data. The temperature record must be interpreted with caution because 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. The data recorded under such circumstances may differ significantly from the thermal equilibrium of that environment. 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 P-wave velocity, even in the slow formations typically encountered on paleoceanographic sediment legs.
The FMS-sonic tool string provides high-resolution electrical resistivity–derived images of the borehole wall. The tool has four orthogonal arms with pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording (Fig. F6). The electrodes are arranged in two diagonally offset rows of 8 electrodes. A focused current is emitted from the button electrodes into the formation, with a return electrode located 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 map the geologic structures of the borehole wall. Analysis of the processed FMS images can provide measurements of dip and direction (azimuth) of structural features in the formation.
The development of the FMS-sonic tool string has added a new dimension to wireline logging (Luthi, 1990; Lovell et al., 1998). 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. The maximum extension of the caliper arms is 15 in, so in holes or parts of holes with a larger diameter, 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 they lead to poor pad-wall contact. Acceleration and magnetic field measurements were made with the 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 and allows the true dip and direction (azimuth) of structures to be determined.
The principal influence on logging 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 ledges caused by layers of harder material, 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 logging 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. These procedures were followed in all logging operations during Leg 208.
The depth of the wireline-logged measurement is determined from the length of the logging cable extended from the winch on the ship. When possible, the seafloor is identified on the NGR log by the abrupt reduction in gamma ray count at the water/sediment boundary (mudline). The coring depth (drillers 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 drillers depth of recovered core and the wireline logging depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, drill pipe stretch in the case of drill pipe depth, cable stretch (~1 m/km), and cable slip in the case of logging depth. Tidal changes in sea level will also have an effect. To minimize the wireline tool motion caused by ship heave, the WHC 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 between core and logging data. The depths of core data sets, such as density and natural gamma radiation, can be correlated with the equivalent downhole logs using programs such as Sagan, which allow mapping of the core depths onto the logging depth scale. In zones where core recovery is low, precise core-log depth matching may be difficult because of the inherent ambiguity of placing the recovered section within the cored interval. Where complete core recovery (using a composite depth splice) (Hagelberg et al., 1992) is achieved, core depths and densities can be corrected by correlation with the logging data.
Logs from different wireline tool strings will have slight depth mismatches. Distinctive features recorded by the natural gamma tools (Fig. F6) run on every tool string (except the WST-3) provide relative depth offsets and a means of depth shifting for correlation between logging runs.
Data for each logging run were recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging in each hole, data acquisition processing by the Schlumberger engineer is completed; data were subsequently transferred to the DHML and transmitted via satellite to LDEO-BRG for shore-based processing. Data processing at LDEO-BRG consists of (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, logging data were transmitted back to the ship, providing near–real time data processing.
As a further check of data quality and log interpretation, wavelet analysis of the tool strings' accelerometer data and other standard logs may be undertaken. Continuous wavelet analysis allows rapid localization of cyclic patterns or discontinuities, both in space (depth) and scale (e.g., Torrence and Compo, 1998). In contrast to classical Fourier transform or windowed Fourier transform that decompose the original signal on the basis of an infinite periodic function depending on a unique parameter (space frequency), the wavelet transform allows a "depth-scale" representation that depends on a scale parameter and a translation parameter. The scale parameter (or dilatation factor) determines the characteristic frequencies at which the wavelet transform is computed, and the translation parameter allows local analysis. A reading of the wavelet power spectrum can be obtained by constructing a color diagram with the depth on the vertical axis and the scales (or the equivalent Fourier periods) on the horizontal axis, the modulus of the wavelet transform being represented by colored patches. This type of diagram is therefore comparable to an evolutionary power spectrum and can be revealing about the structure of a particular process. The main difference between the wavelet and the Fourier decomposition is in the support of the respective basis functions. The wavelet transform coefficients are influenced by local events, whereas the Fourier coefficients are influenced by the function on its entire domain. This makes the wavelet spectrum a better measure of the variance attributed to localized events. To facilitate the interpretation of this type of diagram, one can define a level above which a maximum in the wavelet spectrum is statistically significant. It has been shown that each point in the wavelet power spectrum has a c2 distribution with 2 degrees of freedom about the background spectrum (Torrence and Compo, 1998). The confidence level at each scale is therefore the product of the background spectrum and the desired significance level (for instance 95% confidence) from the c2 distribution. Here, the background spectrum is determined by calculating the depth average of the wavelet spectrum.
Further postcruise processing of the logging data from the FMS-sonic tool string is performed at LDEO-BRG. Postcruise-processed logging data are available directly from the LDEO-BRG World Wide Web site at www.ldeo.columbia.edu/BRG/ODP/DATABASE/. A summary of "logging highlights" is posted on the LDEO-BRG Web site at the end of each leg. Basic processing was conducted 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 208 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). Schlumberger GeoQuest's GeoFrame software package is used for most of the processing. Processed ASCII files of acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data are available, with FMS images as GIF files.
