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doi:10.2204/iodp.proc.339.102.2013

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 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.

Downhole logs measure 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

During wireline logging operations, the logs are recorded with Schlumberger logging tools combined into tool strings, which are lowered into the hole after completion of coring operations. Three tool strings were used during Expedition 339: the triple combination (triple combo), which measures natural gamma radiation, porosity, density, and resistivity; the Formation MicroSanner (FMS)-sonic, which provides FMS resistivity images of the borehole wall and sonic velocities; and the Versatile Seismic Imager (VSI) for the vertical seismic profile (VSP) (Fig. F15; Table T5). Each tool string also contains a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system (MAXIS unit) on the drillship. The triple combo was run in different configurations in order to test the High-Resolution Laterolog Array (HRLA), which was run in sedimentary formations for the first time in IODP.

In preparation for logging, the boreholes were flushed of debris by circulating a high-viscosity mud (sepiolite) sweep and filled with heavy mud (attapulgite weighted with barite; approximate density = 10.5 lb/gal) to help stabilize the borehole walls. The BHA was pulled up to between 84 and 102 m WSF to cover the unstable upper part of the hole. The tool strings were then lowered downhole on a seven-conductor wireline cable before being pulled up at constant speed, typically 275 or 550 m/h, to provide continuous measurements of several properties simultaneously. A wireline heave compensator (WHC) was used to minimize the effect of ship’s heave on the tool position in the borehole (see below). During each logging run, incoming data were recorded and monitored in real time on the Schlumberger MCM MAXIS logging computer.

Logged sediment properties and tool measurement principles

The logged properties, and the principles used in the tools to measure them, are briefly described in this section. The main logs are listed in Table T6. More detailed information on individual tools and their geological applications may be found in Serra (1984, 1986, 1989), Schlumberger (1989, 1994), Rider (1996), Goldberg (1997), Lovell et al. (1998), and Ellis and Singer (2007). A complete online list of acronyms for the Schlumberger tools and measurement curves is at www.slb.com/modules/mnemonics/index.aspx.

Natural gamma radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) was used on the triple combo tool string to measure and classify NGR in the formation. It has two bismuth germanate scintillation detectors and uses five-window spectroscopy to determine concentrations of K, U, and Th from the characteristic gamma ray energies of isotopes in the ⁴⁰K, ²³²Th, and ²³⁸U radioactive decay series.

An additional NGR sensor is housed in the Enhanced Digital Telemetry Cartridge (EDTC), run on all tool strings. Its sodium iodide scintillation detector measures the total NGR emission of the formation, with no spectral information.

The inclusion of a gamma ray sonde in every tool string allows use of the NGR data for depth correlation between logging strings and passes and for core-log integration. Although the EDTC NGR measurement has less sensitivity than the HNGS measurement, it was sufficiently accurate to be used for depth matching because of the generally high NGR values and the abundance of distinctive features in the logs at the Expedition 339 sites.

Density and photoelectric factor

Formation density was measured 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 decentralizing arm. Gamma rays emitted by the source undergo Compton scattering, in which gamma rays are scattered by electrons in the formation. The number of scattered gamma rays that reach the detectors is proportional 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 factor (PEF). Photoelectric absorption of the gamma rays occurs when their energy falls below 150 keV as a result of being repeatedly scattered by electrons in the formation. Because PEF is higher for elements with a higher atomic number, it also varies according to the mineral composition and can be used for the identification of some minerals. For example, the PEF of calcite is 5.08 b/e^–, illite is 3.03 b/e^–, quartz is 1.81 b/e^–, and kaolinite is 1.49 b/e^–. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde (APS) in two holes (U1387C and U1389E). It was not run in the other holes because the often-wide borehole and porous sediments were not ideally suited for the APS porosity measurement. The sonde includes a minitron neutron generator that produces fast (14.4 MeV) neutrons and five neutron detectors (four epithermal and one thermal) positioned at different distances from the minitron. The tool’s detectors count neutrons that arrive at the detectors after being scattered and slowed by collisions with atomic nuclei in the formation.

The highest energy loss occurs when neutrons collide with hydrogen nuclei, which have practically the same mass as the neutron (the neutrons bounce off of heavier elements without losing much energy). If the hydrogen (i.e., water) 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 in high-porosity formations where 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 (Σ_f) is also measured by the tool.

Electrical resistivity

This expedition continues the transition to the HRLA resistivity tool from the reliable but dated Phasor Dual Induction/Spherically Focused Resistivity tool (DIT), which is being retired. The HRLA was first used during IODP Expedition 335 in Hole 1256D, but had not been run in low-resistivity IODP sediment formations until this expedition.

The HRLA provides six resistivity measurements with different depths of investigation, including the borehole mud resistivity and five measurements of formation resistivity with increasing penetration into the formation. The tool sends a focused current into the formation and measures the intensity necessary to maintain a constant drop in voltage across a fixed interval, providing direct resistivity measurements. The tool has one central (source) electrode and six electrodes above and below it, which serve alternatively as focusing and returning current electrodes. By rapidly changing the role of these electrodes, a simultaneous resistivity measurement at six penetration depths is achieved. The tool is designed to ensure that all signals are measured at exactly the same time and tool position, and to reduce the sensitivity to “shoulder bed” effects when crossing sharp beds thinner than the electrode spacing. The design of the HRLA, which eliminates the need for a surface reference electrode, improves formation resistivity evaluation compared to the traditional dual induction. The HRLA needs to be run centralized in the borehole for optimal results, so bowspring centralizers are used to keep the HRLA in the center of the borehole, while knuckle joints allow the density and porosity tools to be eccentralized and maintain good contact with the borehole wall (Fig. F15).

The DIT was used to measure electrical resistivity in Hole U1386C as a quality control check because the HRLA was untested in the low-resistivity formations commonly logged in IODP. The DIT provides three measures of electrical resistivity, each with a different depth of investigation into the formation (Table T6). 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 ohm-meters). The spherically focused resistivity is measured by an electrode device that sends a current into the formation maintaining a constant voltage drop. The amount of current necessary to keep the voltage gives a direct measure of resistivity. This device uses several electrodes to focus the current flow into the formation so that equipotential surfaces are spherical and has a higher vertical resolution than the induction measurements.

Calcite, silica, and hydrocarbons are electrical insulators, whereas ionic solutions like interstitial water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity for a given salinity and resistivity of the interstitial water. Clay surface conduction also contributes to the resistivity values, but at high porosities this is a relatively minor effect.

Acoustic velocity

The Dipole Shear Sonic Imager measures the transit times between sonic transmitters and an array of eight receivers. It combines replicate measurements, thus providing a direct measurement of sound velocity through formations that is relatively free from the effects of formation damage and an enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it also has two crossed-dipole transmitters, which allow the measurement of shear wave velocity in addition to compressional wave velocity. Dipole measurements are necessary to measure shear velocities in slow formations whose shear velocity is less than the velocity of sound in the borehole fluid. Such slow formations are typically encountered in deep-ocean drilling.

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 logging. 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.

The development of the FMS added a new dimension to wireline logging (Luthi, 1990; Salimullah and Stow, 1992; Lovell et al., 1998). Features such as bedding, stratification, fracturing, slump folding, and bioturbation can be resolved. Because the images are oriented to magnetic north, further analysis can be carried out to provide measurement of the dip and direction (azimuth) of planar features in the formation. In addition, when the corresponding planar features can be identified in the recovered core samples, individual core pieces can be reoriented with respect to true north.

The maximum extension of the caliper arms is 40.6 cm (16 inches). In holes with a diameter greater than this maximum (relatively common during Expedition 339), the pad contact at the end of the caliper arms 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. Standard procedure is to make two full passes up the borehole with the FMS to maximize the chance of getting full borehole coverage with the pads.

Vertical seismic profile

In a VSP experiment, a borehole seismic tool is anchored against the borehole wall at regularly spaced intervals and records the full waveform of elastic waves generated by a seismic source positioned just below the sea surface. These “check shot” measurements relate depth in the hole to traveltime in reflection seismic lines. The VSI used here contains a three-axis geophone. In the planned VSP survey, the VSI was anchored against the borehole wall at approximately 25 m station intervals (where possible), with 5–10 air gun shots typically taken at each station. The recorded waveforms were stacked and a one-way traveltime was determined from the median of the first breaks for each station. The seismic source used was a Sercel G. Gun Parallel Cluster, composed of two 250 in³ air guns separated by 1 m. It was positioned on the port side of the JOIDES Resolution at a water depth of ~7 meters below sea level (mbsl) with a borehole offset of ~45 m.

Precautions were taken to protect marine mammals. If there were no mammals in or approaching the safety radius (940 m for water depths >1000 m, 1850 m for water depths between 100 and 1000 m), air gun operations commenced using a ramp-up, or “soft start” procedure (gradually increasing the operational pressure and air gun firing interval) to provide time for undetected animals to respond to the sounds and vacate the area. Once the air guns were at full power, the check shot survey proceeded. Marine mammal observations continued during the check shot survey and if a mammal entered the safety radius, the survey was suspended.

Log data quality

The main influence on log data quality is the condition of the borehole wall. Where the borehole diameter varies over short intervals because of washouts (wide borehole) or ledges made of layers of harder material, the logs from tools that require good contact with the borehole wall (i.e., the FMS, density, and porosity tools) may be degraded. Deep investigation measurements such as gamma ray, 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. During the expedition, increased drilling fluid circulation was required to prevent clay from gumming up the roller-cones on the RCB bit, which resulted in washing out (widening) the borehole in places. Conversely, holes drilled with the APC/XCB polycrystalline diamond compact drill bit (PDC; without roller cones) were less washed out and therefore in better condition for logging.

The quality of the wireline depth determination depends on several factors. The depth of the logging measurements is determined from the length of the cable played out from the winch on the ship. The seafloor is identified on the NGR log by the abrupt reduction in gamma ray count at the water/sediment boundary (mudline). Discrepancies between the drilling depth and the wireline log depth occur. In the case of drilling depth, discrepancies are because of core expansion, incomplete core recovery, or incomplete heave compensation. In the case of log depth, discrepancies between successive runs occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. Tidal changes in sea level affect both drilling and logging depths. To minimize the wireline tool motion caused by ship heave, a hydraulic WHC was used to adjust the wireline length to compensate vertical rig motion during wireline logging operations.

Wireline heave compensator

The current WHC system, which was first used during IODP Expedition 320T (February 2009), is designed to compensate for the vertical motion of the ship and maintain a steady motion of the logging tools. It uses vertical acceleration measurements made by a motion reference unit (MRU) located under the rig floor near the center of gravity of the ship, to calculate the vertical motion of the ship. It then adjusts the length of the wireline by varying the distance between two sets of pulleys through which the cable passes. Real-time measurements of uphole (surface) and downhole acceleration are made simultaneously by the MRU and the EDTC, respectively. A Lamont-Doherty Earth Observatory (LDEO)-developed software package allows these data to be analyzed and compared in real time, displaying the actual motion of the logging tool string and enabling monitoring of the efficiency of the compensator.

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 were referenced to the rig floor (WRF). After logging was completed, the data were shifted to a seafloor reference (WSF), which was based on the step in gamma radiation at the sediment/water interface.

The data were transferred on shore to LDEO, 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 log matched depth below seafloor (WMSF). Also, corrections are made to certain tools and logs (e.g., FMS imagery is corrected for tool acceleration, including “stick and slip” motion), 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 wireline log data processing. The data were transferred back to the ship within a few days of logging, and this processed data set was made available to the science party (in ASCII and DLIS formats) through the shipboard IODP logging database and shipboard servers.

In situ temperature measurements

During Expedition 339, in situ temperature measurements were made with the APCT-3. At least four in situ temperature measurements were made at each site using the APCT-3. The APCT-3 fits directly into the coring shoe of the APC and consists of a battery pack, data logger, and a platinum resistance-temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool is first stopped at the mudline for 5 min to thermally equilibrate with bottom water. However, the lowest temperature recorded during the run down was occasionally preferred to the average temperature at the mudline as an estimate of the bottom water temperature because it was more repeatable and bottom water is expected to have the lowest temperature in the profile. After the APC penetrated the sediment, it was held in place for about 6 min as the APCT-3 recorded the temperature of the cutting shoe every second (a longer equilibration time is preferable). When the APC is plunged into the formation, there is an instantaneous temperature rise from frictional heating. This heat gradually dissipates into the surrounding sediments as the temperature at the APCT-3 equilibrates toward the temperature of the sediments.

The equilibrium temperature of the sediments was estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). The synthetic thermal decay curve for the APCT-3 tool is a function of the geometry and thermal properties of the probe and the sediments (Bullard, 1954; Horai and Von Herzen, 1985). The equilibrium temperature must be estimated by applying a fitting procedure (Pribnow et al., 2000). However, where the APC has not achieved a full stroke or where ship heave pulls the APC up from full penetration, the temperature equilibration curve will be disturbed and temperature determination is more difficult. The nominal accuracy of the APCT-3 temperature measurements is ±0.05°C.

The APCT-3 tool temperature data were combined with measurements of thermal conductivity (see “Physical properties”) obtained from whole-core samples to obtain heat flow values. Heat flow was calculated according to the Bullard method, to be consistent with the synthesis of ODP heat flow data by Pribnow et al. (2000).

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