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Downhole measurements

The main objectives of the Expedition 329 downhole measurement program are to document crustal physical properties, define structural and lithologic boundaries as a function of depth, and identify alteration in the basaltic basement from the passage of fluids. In addition, wireline logging data allow us to delineate alteration patterns, fracture densities, and structural orientations and determine how these correlate with current and paleostress environments. Comparison of wireline logging data to laboratory analyses of discrete samples helped to document the physical and chemical nature of the drilled environments. In this way, they document important constraints on subseafloor habitability. For example, the wireline logs complement core measurements by documenting the thickness of lithologic units in intervals where core recovery is poor.

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 measured in situ; they can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, log data may provide the only way to characterize the borehole section; where core recovery is good, log and core data complement one another and may be interpreted jointly. Borehole logging and core-log integration are invaluable for reconstructing recovery gaps and estimating bulk geochemical and structural characteristics of deep basement drill sites.

Downhole logs are sensitive to formation properties on a scale that is intermediate between the scale of data obtained from laboratory measurements on core samples and the scale of data obtained from geophysical surveys. They are useful in calibrating the interpretation of geophysical survey data (e.g., through the use of synthetic seismograms) and they provide a necessary link for the integrated understanding of physical properties on all scales.

Wireline logging

During wireline logging, logs were made with a variety of Schlumberger logging tools combined into several tool strings, which were run down the hole after coring operations were completed. Two wireline tool strings were used during Expedition 329 (Fig. F12; Table T12), the triple combination (triple combo; gamma radiation, density, and electrical resistivity) and the Formation MicroScanner (FMS)-gamma (gamma radiation, and microresistivity image of the borehole wall).

In preparation for logging, the boreholes were flushed of debris by circulating with seawater through the drill pipe to the bottom of the hole. The BHA was pulled up. The tool strings were then lowered downhole by a seven-conductor wireline cable during sequential runs. A wireline heave compensator was employed to minimize the effect of ship’s heave on the tool position in the borehole. During each logging run, incoming data were recorded and monitored in real time on the MCM MAXIS logging computer. The tool strings were then pulled up at constant speed, typically 250–300 m/h, to provide continuous measurements as a function of depth of several properties simultaneously.

Logged sediment properties and tool measurement principles

The logged properties, and the methods that the tools use to measure them, are briefly described below. The main logs taken by the tools are listed in Table T13. More detailed information on individual tools and their geological applications may be found in Ellis and Singer (2007), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986, 1989).

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of 40K, 232Th, and 238U. The isotopes of these three elements dominate the natural radiation spectrum. The HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. The computation of the elemental abundances uses a least-squares method of extracting thorium, uranium, and potassium elemental concentrations from the spectral measurements.

Density and photoelectric effect

Formation density was determined with the Hostile Environment Litho-Density Sonde (HLDS). The sonde 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 by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source undergo Compton scattering, which involves the transfer of energy from gamma rays to the electrons in the formation through elastic collision. 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 HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of the gamma rays occurs when their energy is reduced below 150 keV after being repeatedly scattered by electrons in the formation. Because PEF depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.

Electrical resistivity

The phasor dual induction–spherically focused resistivity tool (DITE-SFL) was used to measure electrical resistivity. The DITE-SFL 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 (in units of ohm-meters). For the shallow penetration resistivity, the current necessary to maintain a constant drop in voltage across a fixed interval is measured; this is a direct measurement of resistivity. Typically, igneous minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals as well as ionic solutions like interstitial water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity (with Archie’s law) and fluid salinity.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity–based images of borehole walls. The tool has four orthogonal arms and pads, each containing 16 button electrodes that are pressed against the borehole wall during recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each. A focused current is emitted from the button electrodes into the formation, with a return electrode near the top of the tool. Resistivity of the formation at the button electrodes is derived from the intensity of current passing through the button electrodes. Processing transforms these measurements into oriented high-resolution images that reveal the geologic structures of the borehole wall. Further analysis can provide measurements of dip and direction (azimuth) of planar 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; the fact that the images are oriented means that fabric analysis can be carried out and bed orientations can be measured.

The maximum extension of the caliper arms is 38.1 cm (15 inches). In holes with a diameter >38.1 cm, the pad contact will be inconsistent and the FMS images may appear out of focus and too conductive. Irregular (rough) borehole walls will also adversely affect the images if contact with the wall is poor.

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 or ledges caused by layers of harder material, the logs from those tools that require good contact with the borehole wall (i.e., FMS and density) 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 conditioning are completed.

The quality of the depth determination depends on a series of factors. 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 drillers depth and the wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, and drill pipe stretch in the case of drillers depth. In the case of log depth, discrepancies occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. Tidal changes in sea level also have an effect. To minimize the wireline tool motion caused by ship heave, a new hydraulic wireline heave compensator adjusts for rig motion during wireline logging operations.

Logging data flow and log depth scales

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS 500 system. The initial logging data are referenced to the rig floor (wireline depth below rig floor). After logging was completed, the data were shifted to a seafloor reference (wireline depth below seafloor).

The data were transferred onshore to Lamont-Doherty Earth Observatory, where standardized data processing took place. The main part of the processing is depth matching to remove depth offsets between data from different logging runs, which results in a new depth scale, wireline matched depth below seafloor. Also, corrections are made to certain tools and logs, documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII for the conventional logs and GIF for the FMS images. Schlumberger Geo-Quest’s GeoFrame software package is used for most of the processing. The data were transferred back to the ship within a few days of logging and were made available (in ASCII and digital log information standard [DLIS] formats) through the shipboard IODP logging database.