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doi:10.2204/iodp.proc.303306.110.2006

Downhole measurements

Downhole logging provides continuous in situ geophysical measurements 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 logging data are often the most reliable source of information. Where core recovery is good, core data can be correlated with logging 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/h (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 string position in the borehole (Goldberg, 1990). Further information on the procedures and wireline tools used during Expedition 306 can be found at iodp.ldeo.columbia.edu/​TOOLS_LABS/​index.html and www.ldeo.columbia.edu/​BRG/​ODP/​LOGGING/​index.html.

Logging tools and tool strings

During Expedition 306, the following logging tool strings were deployed (Fig. F11; Table T4):

1. The triple combination tool string, which consists of the Hostile Environment Gamma Ray Sonde (HNGS), the Hostile Environment Litho-Density Sonde (HLDS), the Digital Dual Induction Tool model E (DIT-E), and two Lamont-Doherty Earth Observatory–Borehole Research Group (LDEO-BRG) tools, the Multi-Sensor Spectral Gamma Ray Tool (MGT), and the Temperature/Acceleration/Pressure (TAP) tool.

2. The Formation MicroScanner (FMS)-sonic tool string, which consists of the Scintillation Gamma Ray Tool (SGT), the Dipole Sonic Imager (DSI), the General Purpose Inclinometer Tool (GPIT), and the FMS.

Principles and uses of the logging tools

The properties measured by each tool, sampling intervals, and vertical resolutions are summarized in Table T4. Explanations of tool name acronyms and their measurement units are summarized in Table T5. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the LDEO-BRG Wireline Logging Services Guide (1994).

Natural radioactivity

Three wireline spectral gamma ray tools were used during Expedition 306 to measure and classify natural radioactivity in the formation: the HNGS, the SGT, and the LDEO-BRG MGT. The HNGS measures the natural gamma radiation from isotopes of K, Th, and U and uses a five-window spectroscopic analysis to determine concentrations of radioactive ⁴⁰K (in weight percent), ²³²Th (in parts per million), and ²³⁸U (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The spectral analysis filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or potassium chloride (KCl) in the drilling mud and improving measurement accuracy. The HNGS also provides a measure of the total standard gamma ray emission and uranium-free or computed gamma ray emission that are measured in gAPI. The HNGS response is influenced by the borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud. KCl may be added to the drilling mud to prevent freshwater clays from swelling and forming obstructions. All of these effects are corrected during processing of HNGS data at LDEO-BRG.

The SGT uses a sodium iodide scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of ⁴⁰K, ²³⁸U, and ²³²Th concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging strings. It is included in the FMS-sonic tool string to provide a reference log to correlate and depth between different logging runs. In the FMS-sonic tool string, the SGT is placed between the two tools, providing correlation data to a deeper level in the hole.

The MGT was developed by LDEO-BRG 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 inch × 4 inch NaI detector, a programmable 256-channel amplitude analyzer, and a 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.

Density

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 that is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering involves the ricochet of gamma rays off electrons in the formation via elastic collision, transferring energy to the electron in the process. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is related to bulk density. Porosity may also be derived from this bulk density if the matrix density is known.

The HLDS also measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma radiation. Photoelectric absorption occurs when gamma radiation 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 is essentially independent of porosity. Thus, the PEF varies according to the chemical composition of the formation. PEF values can be used in combination with HNGS curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.

Porosity

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 (3_f) is also measured by the tool.

Electrical resistivity

The DIT-E was used to measure electrical resistivity. The DIT-E provides three measures of electrical resistivity, each with a different depth of penetration. 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 (by Archie’s law) if fluid salinity is known.

Temperature, acceleration, and pressure

Downhole temperature, acceleration, and pressure were measured with the LDEO-BRG 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.

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 cross-dipole transmitters, providing shear wave velocity measurement in addition to the compressional wave velocity, even in the slow formations typically encountered during IODP expeditions.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity–derived images of the borehole (~30% of a 25 cm diameter borehole on each pass). The vertical resolution of FMS images is ~5 mm, allowing features such as clasts, thin beds, and bioturbation to be imaged. The resistivity measurements are converted to color or grayscale images for display. The tool uses four orthogonal imaging pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording. 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 inches, 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.

In site chapters in this volume, local contrasts in FMS images were improved by applying dynamic and static normalizations to the FMS data. A linear gain is applied, which keeps a constant mean and standard deviation within a sliding window of ~1 m or the entire logged interval, respectively. FMS images are oriented to magnetic north using the GPIT. This method allows the dip and strike of geological features intersecting the hole to be measured from processed FMS images.

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.

Wireline logging data quality

Logging data quality may be seriously degraded by changes in the hole diameter and in sections where the borehole diameter greatly decreases or is washed out. Deep-investigation measurements such as resistivity and sonic velocity are least sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data to reduce these effects. HNGS and SGT data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have depth mismatches caused by either cable stretch or ship heave during recording.

Logging depth scales

The depth of the wireline-logged measurement is determined from the length of the logging cable paid 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 drillers depth and the wireline logging depth occur because of incomplete heave compensation, tidal changes, and cable stretch (~1 m/km) in the case of logging depth. The small differences between drill pipe depth and logging depth, and the even more significant discrepancy between IODP curation depth and logging depth, should be taken into account when using the logs for correlation between core and logging 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 logging depth scale.

Logging data flow and processing

After logging was completed in each hole, data were transferred to the shipboard downhole measurements laboratory for preliminary processing and interpretation. FMS image data were interpreted using Schlumberger’s Geoframe (version 4.0.4.1) software package. Logging data were also transmitted onshore for processing soon after each hole was logged. Onshore data processing consisted of (1) depth-shifting all logs relative to a common datum (i.e., in mbsf), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed onshore, the data were transmitted back to the ship, providing final processed logging results during the expedition. Processed data were then replotted onboard (see “Downhole measurements” section in each site chapter). Postcruise-processed data in ASCII format are available directly from the IODP–USIO, Science Services, LDEO Internet World Wide Web site at iodp.ldeo.columbia.edu/​DATA/​IODP/​index.html.

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