Previous   |    Next

doi:10.2204/iodp.proc.340.102.2013

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. The 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 penetrated formation. They also provide information on the condition, shape, and size of the borehole and on possible deformations induced by drilling or formation stress. When core recovery is incomplete or disturbed, log data may provide the only way to characterize the formation. Data can be used to determine the actual thickness of individual units or lithologies when contacts are not recovered, to pinpoint the depth of features in cores with incomplete recovery, or to identify intervals that were not recovered.

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. Four tool strings planned for deployment were used during Expedition 340: a triple combination, or triple combo, string (spectral and natural gamma radiation, density, and resistivity); a modified triple combo string (spectral and natural gamma radiation, density, resistivity, and magnetic susceptibility); a Formation MicroScanner (FMS)-sonic string (gamma ray, sonic velocity, and resistivity images); and the Versatile Seismic Imager (VSI) string; vertical seismic profile and gamma ray) (Fig. F11; Table T4).

In preparation for logging, the boreholes were flushed of debris by circulating a “slug” of viscous drilling fluid (sepiolite mud mixed with seawater; approximate density = 8.8 ppg or 1.055 g/cm³; if barite-weighted, density = 10.5 ppg or 1.258 g/cm³) through the drill pipe to the bottom of the hole. The BHA was pulled up to ~80 mbsf. Tool strings were then lowered downhole by a seven-conductor wireline cable during sequential deployments. Each tool string deployment is a logging “run,” starting with the assembly of the tool string and the necessary calibrations. The tool string is then sent down to the bottom of the hole while recording a partial set of data and, except for the VSI, is pulled up at a constant speed, typically 250–500 m/h, to record the main data. The VSI is held stationary at regularly spaced depths while shooting the seismic source and then pulled up between stations. During each run, tool strings can be lowered down and pulled up the hole several times for control of repeatability or to try to improve the quality of the data. Each lowering or raising 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 MCM MAXIS system. A logging run is complete once the tool string has been brought to the rig floor and disassembled. A wireline heave compensator (WHC) was employed to minimize the effect of ship’s heave on the tool string’s position in the borehole (see “Wireline heave compensator”).

Logged properties and tool measurement principles

The main logs recorded during Expedition 340 are listed in Table T5. 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, 1989). A complete list of acronyms for the Schlumberger tools and measurement curves is available at www.apps.slb.com/​cmd/.

Natural radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) was used on the triple combo tool string to measure natural radioactivity in the formation. It uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of K, Th, and U. The radioactive isotopes of these three elements 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 Enhanced Digital Telemetry Cartridge (EDTC; see “Telemetry cartridges”), which is used primarily to communicate data to the surface, includes a sodium iodide scintillation detector to measure the total natural gamma ray emission. The EDTC is not a spectral tool, but it provides high-resolution total gamma ray for each pass, which 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 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 also 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. For example, the PEF of calcite is 5.08 b/e^– and the PEF of quartz is 1.81 b/e^–.

Electrical resistivity

The High-Resolution Laterolog Array (HRLA) 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 tool sends a focused current beam 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 alternately 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. Typically, igneous minerals found in crustal rocks, calcite, silica, and hydrocarbons are electrical insulators, whereas ionic solutions like pore water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity (via Archie’s law) for a given salinity and resistivity of the pore water.

Magnetic Susceptibility Sonde

The Magnetic Susceptibility Sonde (MSS), a wireline tool designed by the Lamont-Doherty Earth Observatory (LDEO), measures the ease with which formations are magnetized when subjected to Earth’s magnetic field. The ease of magnetization is ultimately related to the concentration and composition (size, shape, and mineralogy) of magnetizable material within the formation. This measurement provides one of the best methods for investigating stratigraphic changes in mineralogy and lithology because the measurement is quick, repeatable, and nondestructive and because different lithologies often have strongly contrasting susceptibilities. The sensor used during Expedition 340 was a three-coil sensor providing deeper reading measurements, with a vertical resolution of ~40 cm. The MSS was run as a component of a Schlumberger tool string, using a specially developed data translation cartridge. For quality control and environmental correction, the MSS also measures internal tool temperature, z-axis acceleration, and low-resolution borehole conductivity.

Acoustic velocity

The Dipole Shear Sonic Imager (DSI) measures the transit times between sonic transmitters and an array of eight receivers. It combines replicate measurements, thus providing a measurement of compressional wave velocity (V_P) through sediment that is relatively free from the effects of formation damage and of an enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, it also has two cross-dipole transmitters, which allow an additional measurement of shear wave velocity (V_s). Dipole measurements are necessary to measure shear velocities in “slow” formations, where V_s is slower than the velocity in the borehole fluid. Such slow formations are typically encountered in deep ocean drilling.

Formation MicroScanner

The FMS provides high-resolution electrical resistivity images of borehole walls. The tool has four orthogonal arms and pads, each containing 16 button electrodes that are pressed against the borehole wall during the recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each. A focused current is emitted from the button electrodes into the formation, with a return electrode near the top of the tool. Resistivity of the formation at the button electrodes is derived from the intensity of current passing through the button electrodes. Processing transforms these measurements into oriented high-resolution images that reveal the geologic structures of the borehole wall. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved; the images are oriented to magnetic north so that fabric analysis can be carried out and the dip and direction (azimuth) of planar features in the formation can be measured.

The maximum extension of the FMS caliper arms is 15 inches. In holes with a diameter >15 inches, pad contact will be inconsistent and FMS images may appear out of focus and too conductive. Irregular (rough) borehole walls will also adversely affect the images if contact with the wall is poor.

Accelerometry and magnetic field measurement

Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometry Tool (GPIT). The primary purpose of this tool, which incorporates a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the FMS-sonic tool string during logging. Thus, FMS images can be corrected for irregular tool motion, and the dip and direction (azimuth) of features in the FMS images can be determined.

Vertical seismic profile

In a vertical seismic profile (VSP) experiment, the VSI is anchored against the borehole wall at regularly spaced intervals and records acoustic waves generated by a seismic source positioned just below the sea surface. The main purpose of this experiment is to provide a direct measurement of the time necessary for seismic waves to travel from the surface to a given depth, to tie observations in the well, recorded as a function of depth, to reflections in seismic surveys, recorded as a function of time.

The VSI sensor is a three-axis geophone accelerometer built into a “shuttle” that is connected to the tool sonde. During Expedition 340, the VSI was anchored against the borehole wall at ~25 m depth intervals, depending on the quality of the borehole, and 5–15 recordings were taken at each station. The recorded waveforms were stacked, and a one-way traveltime was determined from the first breaks for each station. The seismic source used was a Sercel G. Gun parallel cluster, composed of two 250 in³ air guns separated by 1 m. It was positioned by one of the ship cranes ~30 m to the port side of the ship, at a total horizontal offset from the top of the well head of ~50 m, and maintained at a water depth of ~7 m.

In accordance with the requirements of the National Environmental Policy Act and the Endangered Species Act, all seismic activities were conducted during daytime and protected species observers kept watch for protected species for the duration of the zero-offset VSP. Any sight of protected species within the exclusion zone of 940 m (defined for water depths >1000 m) would interrupt the survey for 60 min after the last sighting or until the protected species were seen leaving the exclusion zone. Protected species observers began observations 1 h prior to the use of the seismic source, which started with a 30 min ramp-up procedure, gradually increasing the operational pressure and firing rate to provide time for undetected protected species to vacate the area. The same ramp-up procedure would be used when resuming activity after any interruption due to the sighting of protected species or whenever the gun was not fired for >30 min.

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.

Telemetry cartridges

Telemetry cartridges are used in each tool string to allow the transmission of data from the tools to the surface. The EDTC also 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 depths between the different passes and runs. In addition, the EDTC includes an accelerometer, the data from which can be used in real time to evaluate the efficiency of the WHC.

Joints and adapters

Because the tool strings combine tools of different generations and various designs, they include several adapters and joints between individual tools to allow communication, provide isolation, avoid interferences (mechanical and acoustic), terminate wirings, or position the tool properly in the borehole. Knuckle joints in particular were used to allow some of the tools such as the HRLA to remain centralized in the borehole while the overlying HLDS was pressed against the borehole wall.

All these additions are included and contribute to the total length of the tool strings in Figure F11.

Log data quality

The principal factor in log data quality is the condition of the borehole wall. If the borehole diameter varies over short intervals because of washouts or ledges, logs from tools that require good contact with the borehole wall (i.e., the density tool) may be degraded. Deep investigation measurements such as 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 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, cable slip, or even tidal changes. Uncertainties in the depth of core samples occur because of incomplete core recovery or incomplete heave compensation. All of these factors generate some discrepency between core sample depths, logs, and individual logging passes. To minimize the effect of ship heave, a hydraulic WHC was used to adjust the wireline length for rig motion during wireline logging operations.

Wireline heave compensator

Evaluation of the WHC system continued during Expedition 340. The WHC 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 the EDTC, respectively. An 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 processing

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS 500 system. They were then copied to the shipboard processing stations for preliminary processing. Typically the main pass of the triple combo was used as a reference to which other passes were interactively depth matched. After depth matching, all logging depths were shifted to the seafloor based on a step in the gamma ray logs. These data were made available to the science party within a few days of their acquisition.

Downhole log data were also transferred onshore to LDEO for standardized data processing. The main part of the processing is depth matching to remove depth offsets between different logging passes, which results in a new depth scale: wireline log matched depth below seafloor (WMSF). Also, corrections are made to certain tools and logs (e.g., FMS images are corrected for tool acceleration), documentation for the logs (with an assessment of log quality) is prepared, and data are converted to ASCII for the conventional logs and to SEG-Y for the VSP data. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing.

Core-log-seismic integration

A depth-traveltime relationship must be determined at each site to correlate core and log data acquired in depth with seismic reflection measurements that are a function of traveltime. A direct measurement of the depth-traveltime relationship is given by the first arrival times in the VSP (see “Vertical seismic profile”).

A depth-traveltime relationship can also be estimated by constructing synthetic seismograms, which are computed from reflection coefficients obtained from contrasts in the V_P and density logs, to match the seismic traces closest to the borehole. Synthetic seismograms were calculated using the IESX seismic interpretation package (part of the Schlumberger GeoFrame software suite), which allows for interactively adjusting the depth-traveltime relationship until a good match is achieved between features in the synthetic seismogram and in the recorded seismic data. A calibrated depth-traveltime relationship allows for the most accurate correlation of the borehole log and core data with seismic reflection features.

Top of page   |    Previous   |    Next