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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 (Table T13). These tool strings are lowered downhole on a heave-compensated electrical wireline and raised at a constant speed (typically 250–300 m/h) to provide continuous simultaneous measurements of various properties as a function of depth. During Expeditions 304 and 305, a main objective of the wireline logging program was to orient faults, fractures, deformation features, and any obvious petrologic boundaries using borehole imaging techniques. Borehole images may then help orient core pieces or sections if the core recovery is sufficiently high. In addition to defining structural features, the logging program also attempts to establish lithologic or physical properties boundaries, as interpreted from logging tool response characteristics as a function of depth, determine serpentinization and/or alteration patterns in basalts and gabbroic and ultramatic rocks, and produce direct correlations with discrete laboratory measurements on the recovered core.

Tool string configurations and geophysical measurements

Individual logging tools were joined together into tool strings so that several measurements could be made during each logging run (Table T13). The 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 precision of logging data. During Expeditions 304 and 305, up to five different tool strings were deployed (Fig. F23):

  • The triple combination 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), and the Accelerator Porosity Sonde (APS) (the Lamont-Doherty Earth Observatory [LDEO] high-resolution Temperature/Acceleration/Pressure [TAP] tool is attached at the bottom of this tool string);
  • The Formation MicroScanner (FMS)-sonic tool string, which consists of the FMS, the General Purpose Inclinometer Tool (GPIT), the Scintillation Gamma Ray Tool (SGT), and the Dipole Sonic Imager (DSI) tool
  • The Ultrasonic Borehole Imager (UBI) tool string, which also includes the GPIT and the SGT
  • The three-component Well Seismic Tool (WST-3)
  • The Goettingen Borehole Magnetometer (GBM; third-party magnetometer)

The properties measured by each tool, the sample intervals used, and the vertical resolutions are summarized in Tables T13 and T14. Explanations of tool name acronyms and their measurement units are summarized in Table T14. 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 (LDEO-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 the NGR 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 to the HNGS log account for variability in borehole size and borehole potassium concentrations. All of these effects are corrected at LDEO-BRG during the expedition. The HNGS also provides a measure of the total gamma ray emission (in American Petroleum Institute [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 natural gamma ray 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 and UBI tool strings to provide a reference log to correlate depth between different logging runs.


Density is 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. F23). Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering 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 number of electrons in the formation, which is a function of the bulk density. The HLDS measures the photoelectric effect factor (PEFL) 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. Because the PEFL depends on the atomic number of the elements in the formation, it is less sensitive to the porosity than the density measurement is. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values.

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 spacing along the tool. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted high-energy (fast) neutrons are slowed by collisions with 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 nucleus of equal mass, such as hydrogen. 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; these are combined to estimate formation porosity. However, in igneous and altered rocks, in addition to water in pore spaces or fractures, bound hydrogen may also be present in alteration minerals such as clays; therefore, neutron logs may overestimate the porosity. The pulsing of the neutron source provides the measurement of the thermal neutron capture cross section (E) in capture units (cu). 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 (Serra 1984).

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 sea 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; therefore, the current sheet retains focus over a shorter distance than the LLD. Because of high resistivity expected in an igneous environment, the DLL is recommended over the Dual Induction Tool (DIT), as the DLL tool response ranges from 0.2 to 40,000 Ω·m, whereas the DIT response range is 0.2–2,000 Ω·m. 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. Rock-forming minerals are mostly silicates having a high resistivity (106–1014 Ω·m), but when the rock matrix contains conductive minerals (clays and Ti-Fe oxide in gabbros), electrical conduction via electronic processes may be appreciable, as oxides reach resistivity of 10–6 Ω·m (Olhoeft, 1981; Guéguen and Palciauskas, 1992).


The TAP tool is deployed in low-resolution memory mode (4 Hz for accelerometry data and 1 Hz for temperature and pressure), with the data being stored in the tool and then downloaded after the logging run is completed. Temperatures determined using the TAP tool are not necessarily the in situ formation temperatures because water circulation during drilling can disturb temperature conditions in the borehole. From the downhole 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 cross-line dipole waveforms. In hard rocks, the dipole sources can result in a better or equivalent estimate of shear wave velocity to 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 the 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 is differenced or summed appropriately to produce inline and cross-line 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. In most instances, the shear wave data should be reprocessed postcruise to correct for dispersion, which is caused by the variation of sound velocity with frequency.

High-resolution electrical images

The FMS provides high-resolution electrical-resistivity-based images of borehole walls (Fig. F24). 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 layers and fracture patterns. FMS images are particularly useful for mapping structural features, dip determination, detailed core-logging 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 logging data and core imaging will be carried out postcruise.

Preliminary structural analysis of the FMS images was completed on board. 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 was known, we extracted the azimuth and dip of each plane. 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 completed with Geoframe (version, Schlumberger software 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 at the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined (Fig. F25). A 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 in the borehole, the shape of the borehole, and the roughness of the borehole wall. Changes in the 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 the 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. The UBI orientated 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).

Accelerometry and magnetic field measurement

Downhole magnetic field measurements were made with the GPIT. The GPIT is included in the FMS and UBI tool string 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 logging depths.

Well Seismic Tool

The WST-3 is used to produce a zero-offset vertical seismic profile and/or check shots in the borehole. The WST-3 consists of a three-axis geophone used to record the full waveform of acoustic waves generated by a seismic source positioned just below the sea surface. During Expedition 305, an 80 inch3 generator-injector air gun, positioned at a water depth of ~2 m with a borehole offset of 50 m on the port side of the JOIDES Resolution, was used as the seismic source. The WST-3 was clamped against the borehole wall at 30 to 50 m intervals, and the air gun was typically fired between 5 and 15 times 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, 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) difference 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.

Goettingen Borehole Magnetometer

The GBM tool was designed and developed in 1989 by the Geophysical Institute of the University of Goettingen, Germany (Fig. F26). The tool consists of three fluxgate sensors that log the horizontal (x and y) and vertical (z) components of the magnetic flux density. The tool is equipped with three angular rate sensors to monitor the spin history around the horizontal axes x and y, as well as the vertical tool axis z, and records inclinometer variations around the x- and y-axes during a logging run. The tool connects directly to the Schlumberger cable head. The housing is made of low-magnetic monel and is not affected by pressure and temperature up to 70 MPa at 100°C. A 300 lb low-magnetic sinker bar was added on top of the magnetometer for faster deployment.

The three LITEF miniature fiber-optic rate sensors provide angular rate output. They have a small volume and low weight and require little power (2 VA). Free from gravity-induced errors and with no moving parts, the sensor is insensitive to shock and vibration. Each rate sensor is an unconventional gyro because it does not have a spinning wheel. It detects and measures angular rates by measuring the frequency difference between two contrarotating light beams. The light source is a superluminescent diode. Its broad spectrum provides light with short coherence length to keep the undesirable backscattering effects in the optical path to sufficiently low levels. The beam is polarized, split, and phase modulated. The output light travels through a 110 m long fiber coil. The light travels to the detector, which converts the light into an electronic output signal. When the gyro is at rest, the two beams have identical frequencies. When the gyro is subjected to an angular turning rate around an axis perpendicular to the plane of the two beams, one beam then has a greater optical path length and the other beam has a shorter optical path length. Therefore, the two resonant frequencies change and the frequency differential is measured by optical means, resulting in a digital output. Readings are output at 1 Hz. The angular rate is a function of time sampled with 5 Hz and the accumulated angle. The angular rate measured by the sensor is influenced by the Earth’s rotation, which depends on the latitude (Φ) and varies from 15.04°/h at the poles to 0°/h at the equator. From equator to pole, Earth’s measured rotation increases by sin (ϕ). To obtain the rotation rate about an inertial system, the effect of Earth’s rotation must be eliminated. If the rotation rate around each axis is known, the orientation of the tool can be derived as a function of depth from the rotation history. The maximum operation temperature for the fiber optic gyros is given at 71°C.

Logging operations

In preparation for logging, the borehole is flushed with freshwater. The tool strings are then lowered downhole during sequential runs. The tool strings are pulled uphole at constant speed (typically at 250–300 m/h) to provide continuous measurements as a function of depth of several properties simultaneously. Each tool string also contains a telemetry cartridge, facilitating communication from the tools along a double-armored seven-conductor wireline cable to the computer van on the drillship.

Two wireline heave compensation systems were used during Expeditions 304 and 305. The LDEO-BRG wireline heave compensator (WHC) is employed to minimize the effect of ship’s heave on the tool position in the borehole. As the ship heaves, an accelerometer located near the ship’s center of gravity senses the movement and feeds the accelerometry data, in real time, to the WHC. The WHC responds by hydraulically moving the compensator’s heave to decouple the movement of the ship from the tool string in the borehole. The second heave compensation system operates via a hydraulic pump, which controls the motion of a wireline drum. This system also uses data from ship three-axis accelerometers to correct for tool motion at the drum. Ultrasonic transducers are used to measure the radius of the drum to calculate tangential velocity and obtain an estimate of the tool speed downhole. The data are processed in the acquisition system and correlated with the compensated depth.

Data for each wireline logging run were recorded, stored digitally, and monitored in real time using the Schlumberger MAXIS 500 system located in the new Offshore Service Unit-F-model Modular Configuration MAXIS Electrical Capstan Capable (OSU-FMEC) winch unit. The OSU-FMEC is a full backup system to the main Schlumberger minimum configuration MAXIS system previously used for ODP logging operations. During Expedition 304, the OSU-FMEC unit was used as the primary acquisition system because we were using both heave compensator units.

Wireline logging data quality

Logging data quality may be seriously degraded by changes in the hole diameter. Deep-investigation measurements such as resistivity and sonic velocity are less sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive to borehole diameter variability because of their shallower depth of investigation and the effect of drilling fluid volume on neutron porosity and GRA density. Corrections can be applied to the original data in order to reduce these effects. 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.

Logging data flow and processing

Data for each wireline logging run were recorded and stored digitally and monitored in real time using the Schlumberger MAXIS 500 system. After logging was completed in each hole, 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 software package. Logging data were also transmitted to shore for processing. Shore-based 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. Data in ASCII are available directly from the IODP-USIO, Science Services, LDEO Web site at​DATA/​IODP/​index.html. GBM logging data were recorded in real time, and raw data were sent to Goettingen for shore-based postcruise processing.