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Downhole measurements

Downhole logging tools are used to determine physical, chemical, and structural properties of the formation penetrated by drilling. Data are rapidly collected, continuous with depth, and, most importantly, are measured in situ. Logs may be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is good, logging and core data are complementary and should be integrated and interpreted jointly, with logging data providing in situ ground truth for core data. Where core recovery is incomplete or disturbed, logging data may provide the only means to characterize the borehole section.

Downhole logs record formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. They are critical for calibrating geophysical survey data (e.g., through synthetic seismograms), providing the necessary link for the integration of core depth domain to seismic time domain data. Through logs, data collected at the borehole scale can be extended to a regional scale using geophysical surveys, a crucial point for paleoenvironmental (climatic and oceanographic) reconstructions.

Wireline logging was conducted at all three sites cored during Expedition 307. To accommodate for the length of the tool string, logged holes were deepened by ~30–50 m with respect to targeted depth. Downhole logging operations began after the hole had been cored and flushed with a viscous drilling mud. The drilling assembly was then pulled up to ~90–60 mbsf, and the logging tools were passed through the drill pipe into the open hole. The logging tools are joined together into tool strings so that compatible tools are run together. Each tool string was lowered separately to the base of the hole and then measurement took place as the tool string was raised at a near-constant cable speed between 275 and 500 m/h (see the “Downhole measurements” sections in the individual site chapters). A wireline heave compensator (WHC) was used to minimize the effect of the ship’s heave on the tool position in the borehole (Goldberg, 1990). Further information on the procedures and wireline tools used during Expedition 307 can be found at​TOOLS_LABS/​index.html.

Wireline logging tool strings

Data were obtained by a variety of Schlumberger and LDEO logging tools, which were combined into the following tool strings (Fig. F8; Table T6):

  1. The triple combination (triple combo) tool string consists of the Hostile Environment Gamma Ray Sonde (HNGS), Accelerator Porosity Sonde (APS), Hostile Environment Litho-Density Sonde (HLDS), Dual Induction Tool (DIT), Environmental Measurements Sonde (EMS), and the LDEO Temperature/​Acceleration/​Pressure (TAP) tool.
  2. The Formation MicroScanner (FMS)-sonic tool string consists of the Scintillation Gamma Ray Tool (SGT), Dipole Sonic Imager (DSI), General Purpose Inclinometer Tool (GPIT), and the microresistivity FMS.
  3. The Well Seismic Tool (WST) consists of one geophone pressed against the borehole wall that is used to record the acoustic waves generated by an air gun located near the sea surface, offset by ~10 m from the ship. No marine mammals were observed during or immediately prior to use of the air gun. The WST was only run in Hole U1317D.

Principles and uses of the logging tools

The properties measured by each tool, sampling intervals, and vertical resolutions are described and summarized in Table T6. Explanations of tool name acronyms and their measurement units are summarized in Table T7. 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).

Data acquisition

Each tool string contains a telemetry cartridge facilitating communication between the tools and the Schlumberger Minimum Configuration Multitasking (MCM) acquisition and imaging system unit located on the ship along the wireline (seven-conductor cable). Ship heave motion is a further complication in the acquisition of quality wireline logging data. To overcome this, the wireline is fed over the WHC. As the ship heaves in the swell, an accelerometer located near the ship’s center of gravity measures the movement and feeds the data, in real time, to the WHC. The WHC responds to the ship’s heave by adding or removing cable slack to decouple the movement of the ship from the tool string (Goldberg, 1990). During each logging run, incoming data are recorded and monitored in real time on the MCM logging computer.

Natural radioactivity

Two wireline spectral gamma ray tools, the HNGS and the SGT, were used during Expedition 307 to measure and classify natural radioactivity in the formation and to provide a common reference for correlation and depth shifting between multiple logging runs. The HNGS uses two bismuth germanate scintillation detectors for high tool precision. It measures the natural gamma radiation from K, Th, and U isotopes and uses a 256-window spectroscopic analysis to determine concentrations of radioactive 40K (in weight percent), 232Th (in parts per million), and 238U (in parts per million).

The SGT uses a sodium iodide scintillation detector to measure the total natural gamma ray emissions from 40K, 238U, and 232Th in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging runs. It is included in the FMS-sonic tool string to provide a reference log to correlate depth between different logging runs.

In a general manner, the radius of investigation depends on several factors: hole size, mud density, formation bulk density (denser formations display a slightly lower radioactivity), and the energy of the gamma rays (a higher-energy gamma ray can reach the detector from deeper in the formation). The HNGS data are corrected for hole size during logging.


Formation density was determined with the HLDS. The tool contains a radioactive 137Cs gamma ray source (662 keV) and far and near gamma ray detectors mounted on a shielded skid that is pressed against the borehole wall. Gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. Compton scattering involves the ricochet of gamma rays off electrons in the formation via elastic collision, transferring energy to the electron in the process. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which, in turn, is related to bulk density. Porosity may also be derived from this bulk density if the matrix (grain) density is known. As mentioned earlier, the HLDS also measures photoelectric absorption of the gamma rays which occurs when they reach <150 keV after being repeatedly scattered by electrons in the formation. Photoelectric absorption varies according to the chemical composition of the formation (Gardner and Dumanoir, 1980). For example, the photoelectric effect factor of calcite = 5.08 b/e, illite = 3.03 b/e, quartz = 1.81 b/e, and kaolinite = 1.49 b/e. Good contact between the tool and borehole wall is essential for acquisition of quality HLDS logs. Poor contact results in an underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.


Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator that produces fast neutrons (14.4 MeV) and five neutron detectors (four epithermal and one thermal) positioned at differing distances from the minitron. The measurement principle involves counting neutrons that arrive at the detectors after being slowed by neutron absorbers surrounding the tool. The highest energy loss occurs when neutrons collide with hydrogen nuclei that have practically the same mass as the neutron (the neutrons simply bounce off heavier elements without losing much energy). If the hydrogen (i.e., water) concentration is small, as in low-porosity formations, neutrons can travel farther before being captured and the count rates increase at the detector. The opposite effect occurs when the water content is high. Because hydrogen bound in minerals such as clays or hydrocarbons also contributes to the measurement, the raw porosity value is often overestimated. 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 (Σf) is also measured by the tool.

Electrical resistivity

The DIT was used to provide 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, which is measured by the receiving coil and is proportional to the conductivity of the formation. The measured conductivities are then converted to resistivity (measured in ohm-meters). For the shallow-penetration resistivity, the current necessary to maintain a constant voltage drop across a fixed interval is measured. This is a direct measurement of resistivity. Sand grains and hydrocarbons are electrical insulators, whereas ionic solutions (i.e., pore waters) and clays are conductors. Electrical resistivity can therefore be used to evaluate porosity (by Archie’s law) and fluid salinity.

Temperature, acceleration, and pressure

Downhole temperature, acceleration, and pressure were measured with the TAP tool. It was attached to the bottom of the triple combo tool string and run in memory mode with the data stored in built-in memory. After the logging run was complete, the TAP tool was removed from the tool string and returned to the downhole measurements laboratory where the data were downloaded.

The TAP tool has a dual-temperature measurement system for identification of both rapid temperature fluctuations and temperature gradients. A thin fast-response thermistor detects small, abrupt changes in temperature, and the thicker slow-response thermistor more accurately estimates temperature gradients and thermal regimes. A pressure transducer is used to activate the tool at a specified depth, typically 200 m above seafloor. A one-axis accelerometer measures tool movement downhole, which provides data for analyzing the effects of heave on the deployed tool string. The acceleration log can, in principle, be used to correct the logging depths for the effects of heave. The temperature record must be interpreted with caution because the elapsed time between the end of drilling and the logging operation is generally not sufficient to allow the borehole to reach thermal equilibrium following circulation of the drilling fluid. The fluid temperature recorded under such circumstances may differ significantly from the temperature of the formation. Nevertheless, it is possible to identify abrupt temperature changes that may represent localized fluid flow into the borehole indicative of fluid pathways and fracturing and/or breaks in the temperature gradient that may correspond to contrasts in permeability at lithologic boundaries. For comparison and calibration of the TAP tool, drilling mud temperature was also measured with the Schlumberger EMS.

Acoustic velocity

The DSI measures transit times between sonic transmitters and an array of eight receivers. It averages replicate measurements, thus providing a direct measurement of sound velocity through sediments that is relatively free from the effects of formation damage and borehole enlargement (Schlumberger, 1989). The tool contains the monopole transmitters found on most sonic tools, but also has two crossed-dipole transmitters, providing shear wave velocity measurement in addition to the compressional wave velocity, even in slow formations (~1600 m/s).

Formation MicroScanner

The FMS provides high-resolution electrical resistivity images of the borehole wall (~30% of a 25 cm diameter borehole can be imaged on each pass). The development of the FMS tool has added a new dimension to wireline logging (Luthi, 1990; Lovell et al., 1998). The vertical resolution of FMS images is ~5 mm, allowing features such as clasts, thin beds, bioturbation, and corals to be imaged. The tool has four orthogonal arms with pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording (Fig. F8). The electrodes are arranged in two diagonally offset rows of eight electrodes. A focused current is emitted from the button electrodes into the formation with a return electrode located near the top of the tool. The intensity of current passing through the button electrodes is measured. The maximum extension of the caliper arms is 15 inches, so in holes or parts of holes with a larger diameter, the pad contact will be inconsistent and the FMS images may appear out of focus and too conductive. Irregular borehole walls will also adversely affect the image quality if they lead to poor pad/wall contact. Processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous, spatially oriented, high-resolution images that map the geologic structures of the borehole wall. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved, and spatially oriented images allow fabric analysis and bed orientations to be measured. Local contrasts in FMS images were improved by applying a dynamic normalization to the FMS data. In such a normalization procedure, a linear gain is applied that keeps a constant mean and standard deviation within a sliding window of 2 m.

Accelerometry and magnetic field measurements

Three-component acceleration and magnetic field measurements were made with the GPIT. The primary purpose of this tool is to determine the acceleration and orientation of the FMS-sonic tool string during logging. This provides a means of correcting the FMS images for rotation and irregular vertical tool motion, allowing the true dip and direction (azimuth) of structures to be determined.

Check shot survey

A check shot survey (or zero-offset vertical seismic profile) is a type of borehole seismic survey designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the ship using an air gun, and recording the resultant signal. The data can then be used to convert depth to traveltime and improve the absolute values of the sonic log. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.

Wireline logging and data quality

Logging data quality may be seriously degraded by changes in the 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 least sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data to reduce these effects. Natural gamma radiation data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have residual depth mismatches caused by ship heave during recording.

Logging depth scale

The depth of the wireline-logged measurement is determined from the length of the logging cable extended from the winch on the ship. When possible, the seafloor is identified on the natural gamma ray log by the abrupt reduction in gamma ray count at the water/​sediment boundary (mudline). The coring depth (drillers depth) is determined from the known length of the bottom-hole assembly and pipe stands, and the mudline, which is usually recovered in the first core from the hole.

Discrepancies between the drillers depth of recovered core and the wireline log depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, drill pipe stretch in the case of drill pipe depth, cable stretch (~1 m/km), and cable slip in the case of logging depth. Tidal changes in sea level will also have an effect. To minimize the wireline tool motion caused by ship heave, the WHC adjusts for rig motion during wireline logging operations. The small but significant differences between drill pipe depth and logging depth should be taken into account when using the logs for correlation between core and logging data. The depths of core data sets, such as density and natural gamma radiation, can be correlated with the equivalent downhole logs using programs such as Sagan, which allows mapping of the core depths onto the logging depth scale. Precise core-log depth matching may be difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section within the cored interval. Where complete core recovery (by a composite depth splice) (Hagelberg et al., 1992) is achieved, core depths can be corrected by correlation with the logging data.

Logs from different wireline tool strings will also have slight depth mismatches. Distinctive features recorded by the natural gamma tools (HNGS or SGT), run on every tool string (except the WST), provide relative depth offsets and a means of depth shifting for correlation between logging runs.

Logging data flow and processing

Data for each logging run were recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging in each hole, data acquisition preprocessing by the Schlumberger engineer is carried out; data are subsequently transferred to the downhole measurements laboratory and transmitted via satellite to LDEO-BRG for onshore processing. Data processing at LDEO-BRG consists of:

  1. Depth matching the logs to a common reference, usually the pass covering the greatest interval;
  2. Depth-shifting all logs relative to a common datum (i.e., seafloor);
  3. Making corrections specific to individual tools; and
  4. Quality control.

Once processed at LDEO-BRG, logging data were transmitted back to the ship, providing data processing in a matter of days. Processed data were then replotted on board (see the “Downhole measurements” sections in each site chapter). Further postcruise processing of the FMS data is performed at LDEO-BRG. Postcruise-processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data are available in ASCII, with FMS images as GIF files directly from the IODP U.S. Implementing Organization, Science Services-LDEO Internet World Wide Web site at​DATA/​IODP/​index.html. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing. A summary of “logging highlights” is posted on the LDEO-BRG Web site at the end of each expedition.

Core-log-seismic integration

The aim of creating a synthetic seismogram is to provide a means of matching the reflections expected from the formation (measured physical properties from logging and core sources) with those in the seismic data (Mayer et al., 1985). This allows the seismic data to be interpreted in terms of the measured formation properties. For example, lithologic or chronologic boundaries can be picked out as specific reflectors. If a synthetic seismogram can be done for the three sites, they provide the basis for fine tuning the regional seismic stratigraphy.

A synthetic seismic log is a presentation of the data contained in a sonic log in the form of a seismic trace. The high-frequency data of the sonic log are replayed at the low frequency of the seismic data. A seismic section is the result of acoustic reflections from subsurface strata. The reflections depend on the contrasts of the acoustic impedances (i.e., velocity × density) of the adjacent layers, that is, the reflection coefficient (R). When both a sonic and a density log are run in a well, the acoustic impedances of the layers logged can be calculated.

Velocity and density data are required to produce the synthetic seismograms and are only produced in downhole logging once the tool string has exited the drill pipe, typically at ~60–80 mbsf. Because of this, the core data, corrected for rebound (increased length with reduced density [e.g., Hamilton, 1976]) and temperature increase, has to be spliced onto the top of the logging data in order to provide full-depth velocity and density data sets. These full-depth data sets can then be imported into the IESX module of the Schlumberger GeoQuest program GeoFrame to calculate the synthetic seismograms.