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doi:10.2204/iodp.proc.327.102.2011

Downhole measurements

Downhole measurements are used to determine the physical, chemical, and structural properties of the formation penetrated by a borehole. Wireline logging data are collected rapidly and continuously with depth and are 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, these data may provide the only way to characterize the continuous borehole section. Where core recovery is good, log and core data complement one another and may be interpreted jointly. Wireline logs measure formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and those obtained during geophysical surveys. They are useful in calibrating the interpretation of geophysical survey data and provide a necessary link for the integrated understanding of physical properties on all scales.

Sediment temperature measurements are used to assess thermal conditions at depth, both within sediments and in the underlying volcanic crust. These sediment temperature measurements cannot be made remotely from a surface ship but require drill string deployment of tools within sediment at the bottom of a borehole. The same tools can be deployed in the open hole, but measurements there may be disturbed by flowing water or the thermal influence of recent drilling and other operations.

Wireline logging

Data are recorded during wireline logging operations by a variety of Schlumberger and third-party tools. These tools can be combined in many configurations and run into the borehole after drilling or coring operations are completed. A single wireline tool string was used to log Hole U1362A during Expedition 327 (Fig. F12; Table T4). The tool string contained a telemetry cartridge for communicating through the wireline to the Schlumberger data acquisition system on the drillship.

The wireline tools used and their measurement principles are briefly described below. The main measurements are listed in Table T4. 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), and Serra (1984, 1986). Acronyms and units for wireline measurements are listed in Table T5. An online list of acronyms for Schlumberger tools and measurement curves is available in the Documents section of the Expedition 327 log database (brg.ldeo.columbia.edu/data/iodp-usio/exp327/U1362A/). In the following sections, tools deployed during Expedition 327 are described, working upward from the bottom of the tool string.

Ultrasonic Borehole Imager

The Schlumberger Ultrasonic Borehole Imager (UBI) features a high-resolution transducer that provides acoustic images of the borehole wall. The transducer emits ultrasonic pulses at a frequency of 250 or 500 kHz (low and high resolution, respectively); these pulses are reflected at the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined. The continuous rotation of the transducer and the upward motion of the tool produce a complete map of the borehole wall. The amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the UBI tool relative to the center of the borehole, the shape of the borehole, and the roughness of the borehole wall. Changes in borehole wall roughness (e.g., at fractures intersecting the borehole) are responsible for the modulation of the reflected signal; this makes it possible for fractures or other variations in the character of the drilled rocks to be recognized in the amplitude image. The recorded traveltime image gives detailed information about the shape of the borehole and allows calculation of a borehole caliper value from each recorded traveltime.

Amplitude and traveltime are recorded together with a reference to magnetic north by means of the General Purpose Inclinometry Tool (GPIT), permitting image orientation. If features (e.g., fractures) recognized in the core are observed in the UBI images, core orientation is possible. UBI-oriented images can also be used to measure stress in the borehole through identification of borehole breakouts and slip along fault surfaces penetrated by the borehole (Paillet and Kim, 1987).

General Purpose Inclinometry Tool

Three-axis acceleration and magnetic field measurements were made with the Schlumberger GPIT. The primary purpose of this tool is to determine the acceleration and orientation of the UBI tool during logging. Images are corrected for irregular tool motion caused by the drillship’s heave and can be oriented for accurate determination of the dip and direction of features. The GPIT’s vertical acceleration data are also used to calculate downhole tool motion, which in turn is used to evaluate the wireline heave compensator (WHC).

Modular Temperature Tool

The Lamont-Doherty Earth Observatory (LDEO) Modular Temperature Tool (MTT) measures borehole fluid temperature using a resistance-temperature device. Along with acceleration, this temperature is sent in real time to the surface using Schlumberger telemetry.

Hostile Environment Litho-Density Sonde

Formation density was determined with the Schlumberger 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 via 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. The use of drilling mud containing barite can significantly affect PEF measurements.

Hostile Environment Natural Gamma Ray Sonde

The Schlumberger Hostile Environment Natural Gamma Ray Sonde (HNGS) uses two bismuth germanate scintillation detectors and five-window spectroscopy to provide measurements of total gamma ray and to determine the concentrations of isotopes that dominate the natural radiation spectrum: ⁴⁰K, ²³²Th, and ²³⁸U.

Logging equipment head

The Schlumberger logging equipment head (LEH), or cable head, measures tension at the top of the wireline tool string, which helps diagnose difficulties running the tool string up or down the borehole or when exiting or entering the drill string or casing. A spontaneous potential (SP) electrode was installed in the LEH to record a qualitative SP log of the borehole. Developed for wells drilled into sediment formations on land, the SP method is traditionally a measure of the slight electric potential difference between an electrode in the logging tool and one grounded at the surface. This potential difference normally results from the flow of conductive water through formations and the charge separation of clay near the borehole wall. The absolute value of the SP curve is meaningless: only the curve deflection is of interest. During Expedition 327, SP measurements did not use a surface electrode or fish; instead SP is a measure of the potential difference between the LEH electrode and the wireline cable grounded to the drillship. The SP measurement is further impaired by stray current at the rig and by the comparable salinities of the drilling fluid (seawater) and formation water.

Wireline log data quality

The principal influence on log data quality is the diameter of the borehole and the condition of the borehole wall. If the borehole diameter is large or if it is rugose or variable over short intervals because of washouts during drilling or ledges caused by layers of harder material, the HLDS logs may be degraded. Deep investigation measurements, such as gamma ray, that do not require contact with the borehole wall are generally less sensitive to borehole conditions. Very narrow (“bridged”) sections also cause irregular log results. Borehole quality is improved by minimizing fluid circulation while drilling, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning are completed.

The quality of depth determination depends on a series of factors. The depth of the logging measurements is determined from the length of the logging cable played out at the winch on the ship and from calculation of cable stretch. The seafloor is identified on the natural gamma log by the abrupt reduction in gamma ray count at the water/sediment interface (mudline). Discrepancies between driller’s depth and wireline log depth can occur because of core expansion, incomplete core recovery, tidal variation, incomplete heave compensation, and drill pipe stretch in the case of driller’s depth. In the case of log depth, error is introduced because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. To minimize wireline tool motion caused by ship heave, a hydraulic WHC adjusts for rig motion during wireline logging operations.

Logging data flow and processing

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS system. Immediately after logging, field prints were made and shared with the science party. Simultaneously, the data were transferred on shore to the Borehole Research Group at LDEO for standardized data processing. There, data were depth shifted, corrections were made to the UBI images, documentation for the logs was prepared, and the data were converted to ASCII for the conventional logs and GIF for the UBI images. The Schlumberger GeoFrame reservoir characterization software package was used for most of the processing. The data were transferred back to the ship within a few days of logging and made available (in ASCII and digital log interchange standard [DLIS] formats) through the shipboard IODP log database.

The initial logging data are referenced to the rig floor (WRF). After logging was completed, the data were shifted to a seafloor reference (WSF) on the basis of the step in gamma radiation and caliper deflection at the casing shoe, and ultimately all data were shifted to a single reference pass, yielding the WMSF scale.

In situ temperature measurements

The Expedition 327 downhole measurements program included deployment of three kinds of temperature tools that were used to help determine heat flow within sediments and assess the thermal state of the borehole. Measurements within sediments should provide an indication of predrilling thermal conditions because the temperature tools penetrate ahead of the bit into undisturbed material. In contrast, measurements in boreholes can be strongly influenced by drilling operations, particularly if made soon after drilling. Borehole thermal data in basement are potentially useful, nevertheless, because they can indicate formation intervals that are likely to be hydrogeologically active. In addition, temperature measured in a sealed borehole, such as one isolated by a CORK, provide useful information on ambient borehole conditions and the thermal state of the surrounding formation.

Advanced piston corer temperature tool

The third-generation advanced piston corer temperature tool (APCT-3) is an instrumented version of the coring shoe that is run during APC coring. It is deployed in soft sediments to obtain formation temperatures to determine the geothermal gradient.

The APCT-3 does not require a separate tool run but is deployed on an APC inner core barrel and provides an in situ temperature measurement while adding 15–20 min to the core barrel run. The tool is lowered with the core barrel down the drill string by coring line and then held just above the seafloor for 10 min to measure bottom water temperature. The shoe is lowered into the bit and hydraulically stroked into the sediment (~9.5 m ahead of the bit for a normal deployment) and remains stationary while temperature data are recorded for 7–8 min. The APC inner core barrel is retrieved, the instrumented shoe is removed, and the data are downloaded into a computer. Processing requires extrapolating the frictional decay curve to determine in situ temperature by fitting the data to a theoretical model (Heesemann et al., 2006).

Sediment Temperature tool

The Sediment Temperature (SET) tool is designed to take heat flow measurements in semiconsolidated sediments that are too stiff for the APCT-3. Coring must be interrupted to take a SET temperature measurement: the tool is lowered into the borehole on a dedicated coring line, generally with a brief pause just above the seafloor to measure the bottom water temperature. The SET is typically run with the colleted delivery system, which latches into the bottom-hole assembly (BHA). The tool’s probe extends 1.4 m below the bit and is pushed into undisturbed bottom sediment by the driller. The colleted delivery system allows the probe to be disengaged from the BHA, which helps to reduce disturbance from drill string movement while the probe is in the sediment. The probe is typically held in the sediment for 10 min and then returned to the surface, where its data are downloaded for analysis. As with the APCT-3, SET data are analyzed by fitting observations to an idealized model incorporating tool geometry and sediment properties.

Miniaturized temperature data logger

After the original instrument string was removed from the CORK in Hole U1301B, two custom-modified ANTARES type 1857 miniaturized temperature data loggers (MTLs) were encased in a perforated protective pipe below a sinker bar and lowered on the coring line into the borehole. The coring line was stopped for ~5 min at 5 m intervals in the uppermost 50 m of the hole and for 5 min every 25 m thereafter to register a downhole temperature profile. The MTLs recorded temperature with a resolution of 0.001°C. The loggers were calibrated in 2009 and were accurate to 0.002°C.

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