Previous   |    Next

doi:10.2204/iodp.proc.335.102.2012

Downhole logging

Downhole logs are measurements of physical, chemical, and structural properties of the formation surrounding a borehole that are made after completion of drilling. Data are continuous with depth (at vertical sampling intervals ranging from 2.5 mm to 15 cm) and are measured in situ. Sampling is intermediate between laboratory measurements on core samples and geophysical surveys and provides a necessary link for the integrated understanding of physical properties on all scales.

Logs can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the formation. They also provide information on the status and size of the borehole and on possible deviations or deformations induced by drilling, formation stress, or instability, such as breakouts or fractures. When core recovery is incomplete or disturbed, log data may provide the only way to characterize the formation. These data can be used to determine the actual thickness of individual units or lithologies when contacts are not recovered, to constrain the depth of features in the cores, or to identify intervals such as breccias that are often not recovered. When core recovery is good, log and core data complement one another and may be interpreted together. In particular, the imaging tools provide oriented images of the borehole wall than can help reorient the recovered core within the geographic reference frame.

Operations

Logs are recorded with a variety of tools combined into several tool strings, which are run down the hole after completion of drilling operations. Only one tool string was successfully deployed during Expedition 335: the triple combination, or “triple combo” (porosity, density, and resistivity) (Fig. F28; Table T14). The Formation MicroScanner (FMS)-sonic (electrical images of the borehole wall and sonic velocity) and the Ultrasonic Borehole Imager (UBI; borehole images) were scheduled to run, but a failure during the FMS-sonic deployment prevented us from recording these data.

Initially, a temperature log was also planned to record formation equilibrium temperature before drilling. A vertical seismic profile was also scheduled at the end of the expedition to tie the newly drilled section to the seismic surveys and the seismic Layer 2/3 boundary. The efforts necessary to reach the bottom of the hole at the beginning of the expedition and the limited new penetration precluded the deployment of these tools.

During hole cleaning operations, high-viscosity drilling fluids were regularly pumped into the hole to flush out debris. At the end of the last magnetic fishing run, the hole was circulated with seawater, and the fluid below ~1250 mbsf was displaced by freshwater. Freshwater (i.e., resistive water) was used to reduce the contrast in resistivity between the highly resistive gabbroic rocks and the borehole fluid and to enhance the quality of the images (Blackman et al., 2006; Shipboard Scientific Party, 1999b).

Each tool string deployment is a logging “run,” starting with the assembly of the tool string and the necessary calibrations. The tool string is sent down to the bottom of the hole while recording a partial set of data and then pulled up at a constant speed, typically 250–300 m/h, to record the main data. During each run, the tool string can be lowered down and pulled up the hole several times for quality control or to try to increase the circumferencial coverage of the imaging tools (see “Logged properties and tool measurement principles”). Each lowering or hauling-up of the tool string while collecting data constitutes a “pass.” During each pass, incoming data are recorded and monitored in real time on the surface MAXIS system. A logging run is complete once the tool string has been brought to the rig floor and disassembled.

Logged properties and tool measurement principles

The logs recorded during Expedition 335 are listed in Table T15. 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). A complete online list of acronyms for the Schlumberger tools and measurement curves is available at www.slb.com/modules/mnemonics/index.aspx.

Borehole temperature

Each tool string deployed during Expedition 335 included the Modular Temperature Tool (MTT) to measure borehole fluid temperature. The MTT was designed at the Lamont-Doherty Earth Observatory (LDEO) to be able to resolve centimeter-scale temperature variations at typical logging speeds of 250–300 m/h. It uses two temperature sensors: a fast responding thermocouple and a highly accurate resistance temperature detector (RTD). The sonde also contains an accelerometer for depth correction and is combined with a specially designed cartridge to allow data transmission through the Schlumberger string and wireline.

One additional temperature measurement was made during each logging run by a sensor included in the Logging Equipment Head with Tension and Mud Temperature (LEH-MT) cable head and processed by the Enhanced Digital Telemetry Cartridge (EDTC) (see “Auxiliary logging equipment”).

Because logs are recorded shortly after coring, borehole temperature is highly perturbed by the large amounts of seawater circulated during the drilling process. However, temperature changes observed over several hours between successive logging runs provide a measure of the thermal rebound of the borehole fluid toward the formation temperature. The rate of thermal rebound can be modeled to estimate equilibrium temperature profiles (Bullard, 1947; Lachenbruch and Brewer, 1959).

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) measures the natural radioactivity in the formation. It uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of K, Th, and U, whose radioactive signals dominate the natural radiation spectrum. The HNGS filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy.

The EDTC used to communicate data to the surface includes a sodium iodide scintillation detector to measure the total natural gamma ray emission. It is not a spectral tool, but it provides high-resolution total gamma ray for each pass that allows for precise depth match processing between logging runs and passes.

Density

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 an eccentralizing arm. Gamma rays emitted by the source undergo Compton scattering, where gamma rays are scattered by electrons in the formation. The number and energy 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 be derived from this bulk density if the matrix (grain) density is known. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.

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 encountered, it varies with the chemical composition of the minerals present and can be used for the identification of some minerals (Bartetzko et al., 2003; Blackman et al., 2006).

Porosity

Formation porosity was measured with the Accelerator Porosity Sonde (APS). The sonde includes a minitron neutron generator that produces fast (14.4 MeV) neutrons, along with five neutron detectors (four epithermal and one thermal) positioned at different distances from the minitron. The tool’s detectors count neutrons that arrive 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. In contrast, neutrons simply bounce off heavier elements without losing much energy. If the hydrogen content is high, as in high-porosity formations, many neutrons are slowed and captured by the formation and the count rate will be low. When hydrogen content is low, fewer neutrons are absorbed and more reach the detector. However, in igneous and hydrothermally altered rocks, hydrogen may be present in alteration minerals, which can result in an overestimation of porosity values in these rocks.

Upon reaching thermal energies (0.025 eV), neutrons are captured by the nuclei of Cl, Si, B, and other elements, resulting in a gamma ray emission. This neutron capture cross section (∑_f) is also measured by the tool and allows us to identify such elements.

Electrical resistivity

The High-Resolution Laterolog Array (HRLA) tool provides six resistivity measurements with different depths of investigation (including the borehole, or mud resistivity, and five measurements of formation resistivity with increasing penetration into the formation). The sonde 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 measurement. The array 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 same tool position and to reduce the sensitivity to “shoulder bed” effects when crossing sharp beds thinner than the electrode spacing. The design, which eliminates the need for surface reference electrodes, improves formation resistivity evaluation compared to the traditional dual laterolog sonde that was used in earlier expeditions to Hole 1256D.

Typically, igneous minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals as well as ionic solutions like pore water are conductors. In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and thus is strongly dependent on porosity. Electrical resistivity, therefore, can be used to evaluate alteration, porosity, and fluid salinity.

Acoustic velocity

The Dipole Sonic Imager (DSI) generates acoustic pulses from various sonic transmitters and records the full waveforms with an array of eight receivers. The waveforms are then used to calculate the sonic velocity in the formation. The omnidirectional monopole transmitter emits high-frequency (5–15 kHz) pulses to extract the compressional velocity (V_P) of the formation, as well as the shear velocity (V_S) when it is faster than the sound velocity in the borehole fluid. The same transmitter can be fired in alternance at a lower frequency (0.5–1 kHz) to generate Stoneley waves that are sensitive to fractures and variations in permeability (Hornby et al., 1989). The DSI also has two orthogonal dipole transmitters (0.5–3.5 kHz), which allow the measurement of shear wave velocity in “slow” formations, where V_S is slower than the velocity in the borehole fluid. However, in formations such as the basement penetrated in Hole 1256D, V_S can be measured from the monopole waveforms. These higher frequency waveforms usually provide a sharper shear arrival and more accurate estimate of V_S than either of the dipole sources.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity images of the borehole wall. The tool has four orthogonal arms and pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording. A focused current is emitted from the button electrodes into the formation, with a return electrode near the top of the tool. The “buttons” are arranged in two diagonally offset rows of eight electrodes each (see insert in Fig. F28), and the intensity of the current passing through each one is recorded at a vertical sampling rate of 2.54 mm (inch). By combining these measurements with the acceleration and orientation data recorded by the GPIT (see “Accelerometry and magnetic field measurement”), processing transforms the raw data into oriented high-resolution images that reveal the geologic structures of the borehole wall. Features such as fractures, breccias, pillows, or flows can be resolved. The images are oriented to magnetic north, so that the dip and direction (azimuth) of features in the formation can be measured. Because the pads cover only ~25% of the borehole wall, two full passes are usually recorded along the entire open hole in order to try to increase the azimuthal coverage of the images.

Ultrasonic borehole images

The UBI uses a rotating ultrasonic transducer to record acoustic images of the borehole wall (see insert in Fig. F28). 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. Both the amplitude and traveltime of the reflected signal are measured. 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. 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. Amplitude and traveltime are recorded with a reference to magnetic north by means of the GPIT (see “Accelerometry and magnetic field measurement”), permitting the orientation of the images and observed features. The full 360° coverage of the UBI images can also provide a measure of the local stress through identification of borehole breakouts in the direction of the minimum principal horizontal stress (Bell and Gough, 1983). Such subvertical features are more elusive to FMS images because of the partial azimuthal coverage provided by the four pads.

Accelerometry and magnetic field measurement

The primary purpose of the General-Purpose Inclinometry Tool (GPIT), which includes a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the imaging tools during wireline logging. Because of the failure to deploy the imaging tools during Expedition 335, the GPIT was only used to estimate downhole tool motion and evaluate the performance of the new heave compensator.

Auxiliary logging equipment

Cable head

The Schlumberger Logging Equipment Head (LEH, or cable head) measures tension at the very top of the wireline tool string, which diagnoses difficulties in running the tool string up or down the borehole or when exiting or entering the drill string or casing. The LEH-MT, used for the first time in IODP during Expedition 335, also includes a temperature sensor on its outside to measure the borehole fluid temperature, or “mud temperature.”

Telemetry cartridges

Telemetry cartridges are used in each tool string to allow the transmission of the data from the tools to the surface. In addition, the EDTC, also used for the first time in IODP during Expedition 335, includes a sodium iodide scintillation detector to measure the total natural gamma ray emission of the formation. This gamma ray log was used to match the depths between the different passes and runs. Temperature measurements from the LEH-MT are also processed by the EDTC before being sent to the surface for real-time monitoring.

Joints and adapters

Because the tool strings combine tools of different generations and with various designs, they include several adapters and isolation joints between individual tools to allow communication, avoid interferences (mechanical, acoustic, or electrical), or position the tool properly in the borehole. The knuckle joints in particular are used to allow some of the tools such as the HRLA to remain centralized in the borehole while the overlying HLDS and APS are pressed against the borehole wall. All of these additions are included and contribute to the total length of the tool strings in Figure F28.

Log data quality

The principal factor affecting log data quality is the condition of the borehole. If the borehole diameter is too large or varies over short intervals because of washouts and ledges made of layers of different materials, logs from tools that require good contact with the borehole wall (i.e., the FMS and density 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 logging 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. Uncertainties in logging depth occur because of ship heave, cable stretch, and cable slip. Similar uncertainties in coring depth occur because of incomplete recovery or ship heave. Tidal changes in sea level also have an effect, with an amplitude of >1 m at the time of logging Hole 1256D. All of these factors generate some discrepancy between core sample depths, logs, and individual logging passes. To minimize the effect of ship heave on logging depths, a new hydraulic wireline heave compensator (WHC) was used to adjust the wireline length for rig motion during wireline logging operations.

Wireline heave compensator

Expedition 335 continued evaluation of the new WHC system. The system 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 by the GPIT tool, respectively. A 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 evaluation 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 are referenced to the rig floor. After logging was completed, all logging depths were shifted to the seafloor. This is usually done by identifying the seafloor from a step in gamma ray, but in the case of Hole 1256D all depths were adjusted so that the bottom of the casing was at 269 mbsf. Using the gamma ray log, individual passes were interactively depth matched with one reference pass, in this case the main pass of the triple combo. The resulting data were made available to the science party within a day after their acquisition.

Downhole log data were also transferred onshore to LDEO for standardized data processing. In addition to a standardized depth matching, corrections are made to certain tools and logs, documentation for the logs (with an assessment of log quality) is prepared, and data are converted to ASCII for the conventional logs and to GIF for the FMS and UBI images. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing.

Top of page   |    Previous   |    Next