Downhole logging tools are used to determine physical, chemical, and structural properties of the formation penetrated by a borehole. Data are rapidly collected, continuous with depth, and, most importantly, are measured in situ. Logs may be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the 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. Wireline logging was scheduled for four of the five sites cored during Leg 207.
Data are obtained by a variety of Schlumberger and Lamont-Doherty Earth Observatory (LDEO) logging tools combined into several tool strings and deployed in a hole after coring operations are completed. The tool strings are deployed by lowering them to the bottom of the hole and recording data as they are pulled (at constant rate) up the hole. Heave compensation is applied to eliminate or minimize ship motion while logging to ensure a constant rate of ascent of the tool string. Repeat runs or partial runs are undertaken for all tools for data quality control. Three tool strings were run during Leg 207, although not all were run at every site. Details are given in individual site chapters. The three tool strings that were run are the following:
Natural gamma radiation 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 depths are taken from the wireline cable depths. Further tool details are given in Figure F11 and Tables T14 and T15.
Each tool string contains a telemetry cartridge facilitating communication between the tools along the wireline (seven-conductor cable) and the Schlumberger minimum configuration multitasking acquisition and imaging system (MAXIS) (MCM) unit located on the ship. Ship heave motion is a further complication in the acquisition of quality wireline logging data. To overcome this problem, 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 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, 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 (DHML).
The logs acquired by the wireline tools are listed in Table T15. 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, HNGS, and MGT. The SGT uses a scintillation detector to measure the total gamma ray radiation originating in the formation and provides data for depth matching to other tool string passes. The HNGS uses two bismuth germanate scintillation detectors for high tool precision. It also filters out gamma ray energies <500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy.
The MGT was developed by LDEO-Borehole Research Group (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 potassium, thorium, and uranium radioisotopes or their equivalents. The spectral data from individual modules are sampled 4 times/s 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, 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 processing may correct for borehole size and tool sticking using the acceleration data.
Formation density was determined with the HLDT. This tool contains a radioactive cesium (137Cs) gamma ray source (622 keV) and far- and near-field gamma ray detectors mounted on a shielded skid, which is pressed against the borehole wall. 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 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 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. Because 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 photoelectric absorption cross-section index (Pe) 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. This 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. 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 keV), the neutrons are captured by the nuclei of chlorine, silicon, boron, and other elements, resulting in a gamma ray emission. This neutron capture cross section (f) is also measured by the tool.
The DIT provides three measures of electrical resistivity, each with a different depth of investigation into the formation. 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 that is measured by the receiving coils. The measured conductivities are then inverted 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, via 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, with the recorded data stored in 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 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 LSS utilizes the "depth-derived" borehole compensation principle. Two transmitters spaced 2 ft (0.61 m) apart are located 8 ft (2.44 m) below two receivers that are also 2 ft (0.61 m) apart. Hole size compensation is obtained by memorizing the first delay time reading and averaging it with a second reading measured after the tool has been pulled up to a fixed distance along the borehole. Because of the long spacing (10–12 ft; 3.04–3.66 m) between the transmitters and receivers, the tool has a wide investigation depth. The depth of the investigation is important, as drilling operations may damage the formation along the borehole wall, which will alter the acoustic velocity. Full waveforms are always recorded for each receiver. The LSS has been found to provide a reliable measurement of sonic traveltimes and has proved especially useful in the slower formations often encountered during paleoceanographic legs.
The FMS provides images of the borehole wall derived from high-resolution electrical resistivity. The tool has four orthogonal arms with pads, each containing 16 button electrodes, that are pressed against the borehole wall during the recording (Fig. F11). The electrodes are arranged in two diagonally offset rows of eight 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 geologic structures and lithologies along 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 tool has added a new dimension to wireline logging (Luthi, 1990; Salimullah and Stow, 1992; 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 orientation to be measured. The maximum extension of the caliper arms is 15 in, so in holes or parts of holes with 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. Acceleration and orientation data provide a means of correcting FMS images for irregular tool motion and allow 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, due to 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. Investigation measurements with greater formation penetration 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. These procedures were followed in all logging operations during Leg 207.
The depth of the wireline-logged measurement is determined from the length of the logging cable payed 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). The coring depth (drillers depth) is determined from the known length of the bottom-hole assembly 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, tidal changes, drill pipe stretch in the case of drill pipe depth, cable stretch (~1 m/km), cable slip in the case of logging depth, and incomplete heave compensation. To minimize the wireline tool motion caused by ship heave, the WHC adjusts for rig motion during wireline logging operations. 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 ray, can be correlated with the equivalent downhole logs using the software program SAGAN, which allows shifting of the core depths onto the logging depth scale. Precise core-log depth matching is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section in the cored interval. Where complete core recovery (via a composite depth splice) is achieved, core depths and densities can be corrected by correlation with the logging data (e.g., Hagelberg et al., 1992).
Logs from different wireline tool strings will have slight depth mismatches. Distinctive features recorded by the natural gamma tools run on every tool string (except the WST) provide relative depth offsets and a means of depth shifting for correlation between logging runs.
Data for each logging run are recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging in each hole, data processing by the shipboard Schlumberger engineer is carried out and data are transferred to the DHML and transmitted via satellite to LDEO-BRG for onshore processing. Data processing at LDEO-BRG consists of (1) depth shifting all logs relative to a common datum (i.e., mbsf), (2) making corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO-BRG, logging data are transmitted back to the ship, providing near–real time data processing. Further postcruise processing of the logging data from the FMS is performed at LDEO-BRG. Postcruise-processed logging data in ASCII are available directly from the LDEO-BRG 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 207 and other ODP legs. The processing includes additional depth adjustments as necessary to remove depth offsets between data from different logging runs and corrections specific to certain tools and logs. Documentation for the logs (with an assessment of logging quality), and conversion of the data to a widely accessible format are then performed. 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, and FMS images are available as GIF files.
The aim of creating a synthetic seismogram is to provide a means of matching the reflections expected from the formation (measured physical properties from logging and core sources) with those in the seismic data. This allows the seismic data to be interpreted in terms of measured formation properties; for example, lithologic or chronologic boundaries can be picked out as specific reflectors. If a synthetic seismogram can be generated for a number of sites, these data provide the basis for producing a regional seismic stratigraphy.
Velocity and density data are required to produce synthetic seismograms. These data are provided by and valid in downhole logging below the drill pipe, typically at ~60–70 mbsf. Core data were corrected for rebound (increased length with reduced density [e.g., Hamilton, 1976]) and temperature (increased temperature) changes and spliced onto the top of the logging data in order to provide full-depth velocity and density data sets. These full-depth data sets were then imported into the IESX module of the Schlumberger GeoQuest program GeoFrame to integrate with the seismic reflection data. Synthetic seismograms were generated with in-house software that convolves the velocity and density data with an Ormsby wavelet. This wavelet was designed to match the frequency content of the seismic data.