DOWNHOLE MEASUREMENTS

Downhole logs are used to determine physical, chemical, and structural properties of the formation penetrated by a borehole. The 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 penetrated formation. Where core recovery is good, log and core data are complementary and should be integrated and interpreted jointly; where core recovery is incomplete or disturbed, log data may provide the only means to characterize the borehole section.

Downhole logs are sensitive to formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. They are useful in calibrating the interpretation of geophysical survey data (e.g., through the use of synthetic seismograms) and provide a necessary link for the integrated understanding of physical properties on all scales. Wireline logging was scheduled for three of the eight sites. Unfortunately, because of lack of time only two holes were logged (Holes 1218A and 1219A).

Wireline Logging

Measurements are obtained using a variety of Schlumberger and LDEO logging tools combined into several tool strings, which are deployed in the hole after coring operations are completed. Standard time estimates had to be revised during this leg as a result of the installation of a new wireline that had to be "seasoned," which doubled run times into and out of the holes. Two wireline tool strings were used during Leg 199.

  1. The triple combination (triple combo) tool string consisting of resistivity (phasor dual induction tool [DIT]), bulk density (hostile environment litho-density tool [HLDT]), gamma ray (hostile environment natural gamma sonde [HNGS]), and porosity (accelerator porosity sonde [APS]) components, and two additional LDEO tools that measured high-resolution gamma ray (multisensor gamma ray tool [MGT]) and high-resolution temperature/acceleration/pressure tool (TAP tool); and
  2. The Formation MicroScanner (FMS)-sonic tool string consisting of microresistivity (FMS), sonic velocity (dipole shear sonic imager [DSI]), gamma ray (natural gamma ray tool [NGT]), and orientation/acceleration (general purpose inclinometer tool [GPIT]) components.

Natural gamma radiation tools were included on both tool strings to provide a common reference for correlation and depth shifting between multiple logging runs. Further tool details are given in Figure F10 and Table T8.

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 ship. The ship's heave motion is a further complication in the acquisition of quality wireline logging data. To overcome this complication, 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 toolstring (Goldberg, 1990).

During each logging run, incoming data are recorded and monitored in real time on the MCM logging computer. The 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).

Logged Sediment Properties and Tool Measurement Principles

The main 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).

Natural Radioactivity

Three wireline spectral gamma ray tools were used (one for each tool run) to measure and classify natural radioactivity in the formation: the NGT, the HNGS, and the MGT. The NGT uses a sodium iodide (NaI) scintillation detector and five-window spectroscopy to determine concentrations of K (percent), Th (parts per million), and U (parts per million), the three elements whose isotopes dominate the spectrum of natural radiation. The HNGS is similar to the NGT but 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-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 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 (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 processing may correct for borehole size and tool sticking using the acceleration data.

Density

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 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. 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.

Porosity

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. 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, Si, B, and other elements resulting in a gamma ray emission. This neutron capture cross section (Sf) is also measured by the tool.

Electrical Resistivity

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 that 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 (via Archie's Law) and fluid salinity.

Temperature, Acceleration, and Pressure

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 built-in memory. The data were downloaded in the DHML after the logging run was complete and the TAP tool was removed from the Schlumberger tools.

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 deployed 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.

Acoustic Velocity

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 measurements in addition to the compressional wave velocity measurements, even in the slow formations typically encountered during some ODP legs.

FMS

The FMS provides high-resolution electrical resistivity-derived images of borehole walls. The tool has four orthogonal arms with pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording (Fig. F10). 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 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. Further processing 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; Lovell et al., 1998; Salimullah and Stow, 1992). 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 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.

Accelerometry and Magnetic Field Measurement

Three-component 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, allowing the true dip and direction (azimuth) of structures to be determined.

Log Data Quality

The principal influence on log 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 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.

Log Depth Scales

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). The coring depth (driller's 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 driller's depth of recovered core and the wireline log 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 log depth. Tidal changes in sea level will also have an effect. In order 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 log data. Core measurements, such as susceptibility and density, can be correlated with the equivalent downhole logs using the Sagan program, which allows shifting of the core depths onto the log 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 within the cored interval.

Logs from different wireline tool strings will have slight depth mismatches. Distinctive features recorded by the natural gamma tool, run on every tool string, provide relative depth offsets and a means of depth shifting for correlation between logging runs.

Data Recording and Processing

Data for each logging run were recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging at each hole, data were transferred to the DHML for preliminary interpretation. 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 199 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 in ASCII format are available (see the "Related Leg Data" contents list).

Core-Log-Seismic Integration

The IESX seismic interpretation software package was used to display site survey seismic sections acquired prior to Leg 199. Velocity and density logs were used to create synthetic seismograms, which were overlaid on the seismic section and used to refine the depth-traveltime relation. In this way, lithostratigraphic units in the core are correlated with reflectors and sequences in the seismic section.

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