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doi:10.2204/iodp.proc.309312.102.2006

Downhole measurements

Downhole wireline logs are spatially continuous records of the in situ physical, chemical, and structural properties of the formation penetrated by a borehole. They provide information on a scale that is intermediate between laboratory measurements on core samples and geophysical surveys. The logs are recorded rapidly using a variety of probes or sondes combined into tool strings (Fig. F19). These tool strings are lowered downhole on a heave-compensated electrical wireline and raised at a constant speed (typically 250–400 m/h) to provide continuous simultaneous measurements of the various properties as a function of depth, with a vertical sampling interval ranging from 2.5 mm to 15 cm. During Expedition 309/312, wireline logging provided continuous, in situ measurements of geophysical properties of drilled basalts, dikes, and gabbros. A main objective of the wireline logging program was to orient faults, fractures, deformation features, and any obvious petrologic boundaries using borehole imaging techniques. Downhole measurements will be used in conjunction with whole-core images from the DMT Core Scanner to reorient veins, fractures, and other features back into the geographic reference frame. Borehole images then help orient core pieces or sections where core recovery is sufficiently high. Core recovery during drilling of igneous basement is often incomplete and biased, with weaker rock types preferentially lost. In contrast, wireline logging provides continuous data across all intervals, including those with low recovery. In addition to defining structural features, the logging program will also attempt to establish lithologic or physical property boundaries, as interpreted from logging tool response characteristics as a function of depth; determine alteration patterns in basalts, sheeted dikes, and the upper plutonic section; and produce direct correlations with discrete laboratory measurements on recovered core.

Tool string configurations

Individual logging tools were joined together into tool strings so that several measurements could be made during each logging run (Table T16). Tool strings were lowered to the bottom of the borehole on a wireline cable, and data were logged as the tool string was pulled back up the hole. Repeat runs were made to improve coverage and document the accuracy of log data. Several different tool strings were deployed during Expedition 309/312, and their principal configurations are demonstrated in Figures F19 and F20:

• The triple combination (triple combo) tool string, which consists of the Hostile Environment Spectral Gamma Ray Sonde (HNGS), the Dual LateroLog (DLL) tool, the Hostile Environment Litho-Density Sonde (HLDS), the Accelerator Porosity Sonde (APS), and the Lamont-Doherty Earth Observatory (LDEO) Temperature/Acceleration/Pressure (TAP) tool. During Expedition 312, the Environment Measurement Sonde (EMS) was also included to the tool string;
• The FMS-sonic tool string, which consists of the FMS, the GPIT, the Scintillation Gamma Ray Tool (SGT), and the Dipole Sonic Imager (DSI) tool;
• The UBI tool string, which also includes the GPIT and the SGT; and
• The Well Seismic Tool (WST) and Versatile Seismic Imager (VSI).

Explanations of tool name acronyms and their measurement units are summarized in Table T16. Parameters measured by each tool, sample intervals used, and vertical resolution are summarized in Table T17. 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-Borehole Research Group (BRG) Wireline Logging Services Guide (1994).

Natural gamma radiation

Two gamma ray tools were used to measure and characterize natural radioactivity in the formation: the HNGS and the SGT. The HNGS measures natural gamma radiation from isotopes of potassium, thorium, and uranium using five-window spectroscopy to determine concentrations of radioactive potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. Corrections are made to account for variability in borehole size and borehole potassium concentrations during processing of HNGS data at LDEO-BRG. The HNGS also measures total gamma ray emission (in gAPI units) and the uranium-free or computed gamma ray (in gAPI units). The SGT uses a sodium iodide (NaI) scintillation detector to measure the total NGR emission, combining the spectral contributions of potassium, uranium, and thorium 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, UBI, and VSI tool strings to provide a reference log to correlate depth between different logging runs.

Density

Density was measured with the HLDS, which consists of 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 (Fig. F19). Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering involves the transfer of energy from gamma rays to electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is a function of the bulk density. The HLDS measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma ray energies drop to <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 (Gardener and Dumanoir, 1980). Thus, the PEF varies according to the chemical composition of the formation. Some examples of PEF values are: pure pyrite = 16.97, calcite = 5.08, potassium feldspar = 2.86, and quartz = 1.81 b/e^– (barn = 10^–24 cm²). 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 contact quality.

Neutron porosity

The APS consists of a minitron neutron generator that produces fast neutrons (14.4 MeV) and five neutron detectors (four epithermal and one thermal) positioned at different spacings along the tool. The tool is pressed against the borehole wall by an eccentralizing bow-spring (Fig. F19). Emitted high-energy (fast) neutrons are slowed by collisions with other atoms, and the amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. Significant energy loss occurs when the neutron strikes a hydrogen nucleus of equal mass, which is mainly present in pore water. Degrading to thermal energies (0.025 eV), the neutrons are captured by the nuclei of silicon, chlorine, boron, and other elements, resulting in a gamma ray emission. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and the neutron arrival times, which act as a measure of formation porosity. Hydrogen bonds in minerals such as clays, however, also contribute to the measurement, so the raw porosity value is often an overestimate. In sediments, hydrogen is mainly present in pore water, so the neutron log is essentially a measure of porosity, assuming pore fluid saturation. In igneous and hydrothermally altered rocks, hydrogen may also be present in alteration minerals such as clays; therefore, neutron logs may not give accurate estimates of porosity in these rocks. The pulsing of the neutron source provides the measurement of the thermal neutron cross section in capture units. This is a useful indicator for the presence of elements of high thermal neutron capture cross section such as boron, chloride, and rare earth elements.

Electrical resistivity

The DLL tool provides two resistivity measurements with different depths of investigation: deep and shallow. In both devices, a 61 cm thick current beam is forced horizontally into the formation by using focusing (also called bucking) currents. Two monitoring electrodes are part of the loop that adjusts the focusing currents so that there is no current flow in the borehole between the two electrodes. For the deep laterolog (LLD) measurement, both measuring and focusing currents return to a remote electrode on the surface; this configuration greatly improves the depth of investigations and reduces the effect of borehole and adjacent formation conductivity. In the shallow laterolog (LLS) measurement, the return electrodes that measure the focusing currents are located on the sonde, and therefore the current sheet retains focus over a shorter distance than the LLD. Fracture porosity can be estimated from the separation between the LLD and LLS measurements, based on the observation that the former is sensitive to the presence of horizontal conductive fractures only whereas the latter responds to both horizontal and vertical conductive structures. Because the solid constituents of rocks are essentially infinitely resistive relative to the pore fluids, resistivity is controlled mainly by the nature of the pore fluids, porosity, and permeability. In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and is strongly dependent on porosity.

Temperature/Acceleration/Pressure

The TAP tool was deployed in low-resolution memory mode (4 Hz for accelerometry data and 1 Hz for temperature and pressure) with data being stored in the tool and then downloaded after the logging run was completed. The EMS tool, used during Expedition 312, uses a platinum resistor to measure the borehole fluid temperature. Temperatures determined using these tools are not necessarily in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole. From the spatial temperature gradient, however, abrupt temperature changes can be identified that may represent localized fluid flow into the borehole, indicating fluid pathways, fracturing, and/or changes in permeability at lithologic boundaries.

Acoustic velocities

The DSI tool employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of formations. The omnidirectional source generates compressional, shear, and Stoneley waves in hard formations. The configuration of the DSI tool also allows recording of both inline and crossline dipole waveforms. In hard rocks, dipole sources can result in a better or equivalent estimate of shear wave velocity than that from a monopole source. These combined modes can be used to estimate shear wave splitting caused by preferred mineral and/or structural orientation in consolidated formations. A low-frequency (80 Hz) source enables Stoneley waveforms to be generated as well. The DSI tool measures transit times between sonic transmitters and an array of eight receiver groups with 15 cm spacing along the tool, each consisting of four orthogonal elements that are aligned with the dipole transmitters. During acquisition, the output from these 32 individual elements are differenced or summed appropriately to produce inline and crossline dipole signals or monopole-equivalent (compressional and Stoneley) waveforms, depending on the operation modes. Preliminary processing of DSI data estimates monopole and dipole mode velocities using waveform correlation of the digital signals recorded at each receiver.

High-resolution electrical images

The FMS provides high-resolution electrical-resistivity-based images of borehole walls (Fig. F21). The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or “buttons,” which are pressed against the borehole wall during recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each, spaced ~2.5 mm apart. A focused current is emitted from the four pads into the formation, with a return electrode near the top of the tool. Array buttons on each of the pads measure the current intensity variations. The FMS image is sensitive to structure within ~25 cm of the borehole wall and has a vertical resolution of 5 mm with coverage of 22% of the borehole wall on a given pass where the borehole is in gauge. FMS logging commonly includes two passes, the images of which are merged to improve borehole wall coverage. The pads must be firmly pressed against the borehole wall to produce reliable FMS images. In holes with a diameter >38 cm (15 inches), the pad contact will be inconsistent and the FMS images can be blurred. The maximum borehole deviation where good data can be recorded with this tool is 10° from vertical. Irregular borehole walls will also adversely affect the images, as contact with the wall is poor. FMS images are oriented to magnetic north using the GPIT. Processing transforms these measurements of the microresistivity variations of the formation into continuous, spatially oriented, and high-resolution images that mimic geologic structures behind the borehole walls. This allows the dip and azimuth of geologic features intersecting the hole to be measured from the processed FMS image. FMS images can be used to visually compare logs with core to ascertain the orientations of lithologic boundaries and fracture patterns. FMS images are particularly useful for mapping structural features, dip determination, detailed core-log correlation, positioning of core sections with poor recovery, and stress distribution. FMS images have proved to be particularly valuable in the interpretation of volcanic stratigraphy (Ayadi et al., 1998; Lovell et al., 1998; Brewer et al., 1999; Barr et al., 2002) and gabbroic structure (Haggas et al., 2001; Miller et al., 2003) during ODP legs.

Further interpretation of FMS images in combination with other log data and core imaging will be carried out postcruise. Conventionally, structural analysis of FMS images is achieved by fitting sinusoidal curves on the unwrapped borehole image. Each planar structure intersecting the borehole wall corresponds to a sinusoid on the FMS images and is indicated by a color distinction. As the borehole image orientation is known, we can extract for each plane its azimuth and dip. The plane azimuth is determined by picking the inflexion point of the sinusoid where the amplitude is half the peak value (H). The dip is calculated as tan^–1 (H/D), with D being the borehole diameter. FMS data processing and analysis were undertaken using Geoframe (version 4.0.4.2), a Schlumberger software program that allows interactive display and analysis of the oriented images.

Ultrasonic borehole images

The 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), which are reflected by the borehole wall and then received by the same transducer. Amplitude and traveltime of the reflected signal are then determined (Fig. F22). The continuous rotation of the transducer and the upward motion of the tool produce a complete map of the borehole wall. Amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the UBI tool in 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; therefore, fractures or other variations in the character of drilled rocks can be recognized in the amplitude image. The recorded traveltime image gives detailed information about the shape of the borehole, which allows calculation of one caliper value of the borehole from each recorded traveltime. Amplitude and traveltime are recorded together with a reference to magnetic north by means of a magnetometer (GPIT), permitting the orientation of images. If features (e.g., fractures) recognized in the core are observed in the UBI images, orientation of the core 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 (i.e., Paillet and Kim, 1987). In an isotropic, linearly elastic rock subjected to an anisotropic stress field, drilling a subvertical borehole causes breakouts in the direction of the minimum principal horizontal stress (Bell and Gough, 1983).

Magnetic field

Downhole magnetic field measurements were made with the GPIT. The GPIT is included in the FMS and UBI tool strings to calculate tool acceleration and orientation during logging. Tool orientation is defined by three parameters: tool deviation, tool azimuth, and relative bearing. The GPIT utilizes a three-axis inclinometer and a three-axis fluxgate magnetometer to record the orientation of the FMS and UBI images as the magnetometer records the magnetic field components (Fx, Fy, and Fz). Corrections for cable stretching, tool sticking, and/or ship heave using acceleration data (Ax, Ay, and Az) allow precise determinations of log depths.

Well Seismic Tool and Versatile Seismic Imager

Borehole seismic tools are used in wells to detect the acoustic signal generated at the surface. The WST and VSI are used to determine the time–depth relation and to produce a zero-offset vertical seismic profile and/or check shots in the borehole. The WST consists of a single geophone (Fig. F19) used to record the full waveform of acoustic waves generated by a seismic source positioned just below the sea surface. The VSI (Fig. F20) uses three-axis single sensor seismic hardware and software and advanced wireline telemetry for efficient data delivery from the borehole to the surface. Each sensor package delivers high-fidelity wavefields through the use of a three-axis geophone accelerometer, which are acoustically isolated from the main body of the tool (Fig. F20). The geophone accelerometer detects particle motion and provides a linear and flat response from 3 to 200 Hz. The VSI maximal operational temperature is 177°C or 350°F.

A generator-injector air gun, positioned at a water depth of ~7 m with a borehole offset of 50 m on the port side of the JOIDES Resolution, was used as the seismic source. The VSI was used during Expedition 312 and clamped against the borehole in 22 m intervals. The generator-injector air gun mode during the Expedition 312 vertical seismic profile experiment was set to “harmonic mode” with a chamber configuration of 150 and 105 inch³, respectively (see “Downhole measurements” in “Expedition 312” in the “Site 1256” chapter for details). The recorded waveforms were stacked during each operation, and a one-way traveltime was determined from the median of the first breaks for each station, thus providing check shots for calibration of the integrated transit time calculated from sonic logs. Check shot calibration is required for the core-seismic correlation because P-wave velocities derived from the sonic log may differ significantly from true formation velocity because of (1) frequency dispersion (the sonic tool operates at 10–20 kHz, but seismic data are in the 50–200 Hz range), (2) differences in travel paths between well seismic and surface seismic surveys, and (3) borehole effects caused by formation alterations (Schlumberger, 1989). In addition, sonic logs cannot be measured through pipe, so traveltime down to the uppermost logging point has to be estimated by other means.

In situ temperature measurements

During Expedition 309, temperature measurements were taken before coring recommenced in Hole 1256D to determine the in situ temperature at the bottom of the borehole. Two discrete in situ measurements were made with the APCT tool run in isolation into the borehole because borehole conditions exceeded the maximum range (60°C) of the temperature probe on the water-sampling temperature probe (WSTP) deployed during Expedition 309. The components of the APCT tool include a platinum temperature sensor and a battery-powered data logger. The platinum resistance-temperature device is calibrated for temperatures ranging from –20° to 100°C, with a resolution of 0.01°C (see Horai and Von Herzen, 1985). During operation, the APCT tool is mounted on a regular rotary core barrel and lowered down the pipe by wireline. The tool was held for 8 min at 4358 mbrf to equilibrate with near-bottom water temperatures and was then lowered to the bottom of the open hole (~4370 mbrf), where it was held for a further 5 min to measure the temperature. This provided a sufficiently long transient record to estimate steady-state temperature. The nominal accuracy of the temperature measurement is ±0.1°C.

Temperatures were also measured using the TAP tool (see “Temperature/​Acceleration/​Pressure”) that was run as part of the triple combo logging string following the second WSTP deployment. However, the TAP tool was run during Expedition 312 as part of the triple combo tool string and in combination with the EMS temperature and the DLL tools after drilling and coring ceased.

Logging operations

In preparation for logging, the borehole is flushed with fresh water. Tool strings are then lowered downhole during sequential runs. Tool strings are pulled uphole at constant speed (typically at 250–400 m/h). Each tool string also contains a telemetry cartridge, facilitating communication from the tools along a double-armored seven-conductor wireline cable to the Schlumberger Minimum Configuration Maxis (MCM) van on the drill ship. Data for each wireline-logging run are recorded, stored digitally, and monitored in real time using the Schlumberger MAXIS 500 system located in the Offshore Service Unit-F-model Modular Configuration MAXIS Electrical Capstan Capable (OSU-FMEC) winch unit. The logging cable connects the MCM to the tool string through the logging winch and LDEO-BRG wireline heave compensator (WHC). The WHC is employed to minimize the effect of ship’s heave on the tool position in the borehole. After the logs are acquired, data are transferred to the downhole measurements laboratory and also to LDEO-BRG.

Wireline log data quality

Logging data quality may be seriously degraded by changes in 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 less sensitive to borehole conditions. Nuclear measurements (density, neutron porosity, and both natural induced spectral gamma rays) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and GRA. Corrections can be applied to the original data in order to reduce these effects. Very large washouts, however, cannot be corrected for. HNGS and SGT data provide a depth correlation between logging runs, but logs from different tool strings may still have minor depth mismatches caused by either cable stretch or ship heave during recording. To minimize such errors, hydraulic heave compensator adjusts for ship motion in real time. Downhole data cannot be precisely matched with core data in zones where core recovery is low because of the inherently ambiguous placement of the recovered section within the interval cored.

Logging data flow and processing

Data for each wireline logging run were recorded, stored digitally, and monitored in real time using the Schlumberger MAXIS 500 system. After each logging phase was completed in Hole 1256D, data were transferred to the shipboard downhole measurements laboratory for preliminary processing and interpretation. FMS and UBI image data were interpreted using Schlumberger’s Geoframe (version 4.0.4.2) software package. Logging data were also transmitted to shore for processing. Onshore data processing consisted of (1) depth-shifting all logs relative to a common datum (i.e., in meters below seafloor), (2) corrections specific to individual tools, and (3) quality control and rejection of spurious values. Once processed onshore, data were transmitted back to the ship, providing final processed logging results during the expedition. Data in ASCII are available directly from the IODP–USIO Science Services, LDEO, Web site (iodp.ldeo.columbia.edu/​DATA/​IODP/​index.html). A summary of “logging highlights” is also posted on this Web site at the end of each expedition.

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