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doi:10.2204/iodp.proc.346.102.2015

Downhole measurements

Downhole logs are used to determine physical, chemical, and structural properties of the formation penetrated by a borehole. The data are rapidly collected, continuous with depth, and measured in situ; they can be interpreted in terms of the stratigraphy, lithology, mineralogy, magnetic characteristics, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, log data may provide the only way to characterize the borehole section. Where core recovery is good, log and core data complement one another and may be interpreted together.

Downhole logs measure formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys. They are useful in calibrating the interpretation of geophysical survey data (e.g., through the use of synthetic seismograms) and provide a necessary link for the integrated understanding of physical and chemical properties on all scales.

During Expedition 346, downhole measurements were taken in Holes U1423B, U1425B, U1427A, and U1430B.

Wireline logging

During wireline logging operations, the logs are recorded with a variety of Schlumberger logging tools combined into several tool strings, which are lowered into the hole after completion of coring operations. Two main tool strings were used during Expedition 346. The first tool string is a variation of the triple combination (triple combo) in which the porosity tool has been replaced by a Magnetic Susceptibility Sonde (MSS). We called this modified tool string the paleo combination (paleo combo), and it measured, from top to bottom, resistivity, NGR, density, caliper (borehole diameter), and magnetic susceptibility. The paleo combo was run first at each logged site. The second tool string is the Formation MicroScanner (FMS)-sonic, which provides sonic velocity, FMS resistivity images of the borehole wall, and caliper (Fig. F15; Table T12). In Hole U1430B, the tool strings were modified to maximize data acquisition in the lowest part of the hole. A short version of the paleo combo was run first to acquire NGR, density, and magnetic susceptibility. Another short tool string was run, including resistivity at the top and gamma radiation at the bottom. The FMS-sonic was run as the third tool string. This strategy was successful in acquiring data as deeply as possible. Each tool string also contains an enhanced digital telemetry cartridge (EDTC) for communicating through the wireline to the Schlumberger data acquisition system (MAXIS unit) on the drillship.

In preparation for logging, the boreholes were flushed of debris by circulating drilling fluid and then filled with seawater (Holes U1423B and U1425B) or seawater-based logging gel (sepiolite mud mixed with seawater and weighted with barite; approximate density = 10.5 lb/gal; Holes U1427A and U1430B) to help stabilize the borehole walls. The BHA was pulled up to ~80 m WSF to cover the unstable upper part of the holes. The tool strings were then lowered downhole on a seven-conductor wireline cable before being pulled up at constant speed, typically ~500 m/h, to provide continuous measurements of several properties simultaneously.

Each tool string deployment is termed a logging “run.” During each run, tool strings can be lowered down and pulled up the hole several times for control of repeatability or to improve the quality of the data. Each lowering or hauling up of the tool string while collecting data constitutes a “pass.” Incoming data were recorded and monitored in real time on the MCM MAXIS logging computer. A wireline heave compensator (WHC) was used to minimize the effect of ship’s heave on the tool position in the borehole (see below).

Logged sediment properties and tool measurement principles

The logged properties, and the principles used in the tools to measure them, are briefly described below. The main logs are listed in Table T13. More detailed information on individual tools and their geological applications may be found in Serra (1984, 1986, 1989), Schlumberger (1989, 1994), Rider (1996), Goldberg (1997), Lovell et al. (1998), and Ellis and Singer (2007). A complete online list of acronyms for the Schlumberger tools and measurement curves is at www.apps.slb.com/cmd/.

Natural gamma radioactivity

The Hostile Environment Natural Gamma Ray Sonde (HNGS) was used on both the paleo combo and FMS-sonic tool strings (except in Hole U1423B) to measure NGR in the formation. It uses two bismuth germanate scintillation detectors and five-window spectroscopy to determine concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million) from the characteristic gamma ray energies of isotopes in the ⁴⁰K, ²³²Th, and ²³⁸U radioactive decay series. The radioactive isotopes of these three elements dominate the natural radiation spectrum. The computation of the elemental abundances uses a least-squares method of extracting U, Th, and K elemental concentrations from the spectral measurements. 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 HNGS also provides a measure of the total spectral gamma ray (HSGR) emission and uranium-free or computed gamma ray (HCGR) emission that are measured in American Petroleum Institute units (gAPI). The HNGS response is influenced by the borehole diameter, and therefore the HNGS data are corrected for borehole diameter variations during acquisition.

An additional NGR sensor was housed in the EDTC, which was used primarily to communicate data to the surface. It includes a sodium iodide scintillation detector that measures the total NGR emission of the formation. It is not a spectral tool (does not provide U, Th, and K concentrations), but it provides high-resolution total gamma ray for each pass.

The inclusion of a HNGS sonde in every tool string allows use of the gamma ray data for precise depth match processing between logging strings and passes and for core-log integration.

Density and photoelectric factor

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 a hydraulically activated eccentralizing arm (Fig. F15). Gamma rays emitted by the source undergo Compton scattering, in which 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.

The HLDS also measures the photoelectric factor (PEF) caused by photoelectric absorption of low-energy gamma rays. Photoelectric absorption occurs when their energy falls below 150 keV as a result of being repeatedly scattered by electrons in the formation. The PEF is determined by comparing the counts from the far detector in the high-energy region, where only Compton scattering occurs, with those in the low-energy region, where the count rates depend on both reactions. Because the PEF depends on the atomic number of the elements in the formation, it also varies according to the chemical composition of the minerals present and can be used for the identification of some minerals. For example, the PEF 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 good HLDS logs; poor contact results in underestimation of density values. Both the density correction and caliper measurement of the hole are used to check the contact quality. In Holes U1427A and U1430B, the PEF log should be used with caution, especially in the washouts, because barium in the logging mud may have swamped the signal despite correction from mud effect.

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 tool 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 measurements. 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 tool position, and to reduce the sensitivity to “shoulder bed” effects when crossing sharp beds thinner than the electrode spacing. The design of the HRLA, which eliminates the need for a surface reference electrode, improves formation resistivity evaluation compared to the traditional dual induction. The HRLA needs to be run centralized in the borehole for optimal results, so knuckle joints were used to centralize the HRLA while allowing the density and magnetic susceptibility tools to maintain good contact with the borehole wall (Fig. F15).

Calcite, silica, and hydrocarbons are electrical insulators, whereas ionic solutions such as pore water are conductors. Electrical resistivity, therefore, can be used to evaluate porosity for a given salinity and resistivity of the pore water. Clay surface conduction also contributes to the resistivity values, but at high porosities this is a relatively minor effect.

Acoustic velocity

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

Formation MicroScanner

The FMS provides high-resolution electrical resistivity–based 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 logging. 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 the resistivity measurements into oriented high-resolution images that reveal the geologic structures of the borehole wall. Features such as bedding, stratification, fracturing, slump folding, and bioturbation can be resolved (Luthi, 1990; Pezard et al., 1990; Salimullah and Stow, 1992; Lovell et al., 1998). Because the images are oriented to magnetic north, further analysis can provide measurement of the dip and direction (azimuth) of planar features in the formation. In addition, when the corresponding planar features can be identified in the recovered core samples, individual core pieces can be reoriented with respect to true north.

Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. Standard procedure is to make two full passes up the borehole with the FMS to maximize the chance of getting full borehole coverage with the pads. The maximum extension of the caliper arms is 40.6 cm (16 inches). In holes with a diameter greater than this maximum, the pad contact at the end of the caliper arms will be inconsistent, and the 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.

Magnetic Susceptibility Sonde

The MSS is a nonstandard wireline tool designed by Lamont-Doherty Earth Observatory (LDEO). It measures the ease with which formations are magnetized when subjected to the 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. These measurements provide 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. High-resolution susceptibility measurements aid significantly in paleoclimatic and paleoceanographic studies, such as those of Expedition 346, where construction of an accurate and complete stratigraphic framework is critical to reconstruct past climatic changes.

A single-coil sensor was used during Expedition 346 to provide high-resolution measurements (~10 cm) that are shallow reading (~3 cm depth of horizontal investigation). A dual-coil sensor provided lower resolution (~40 cm), deeper reading (~20 cm depth of horizontal investigation) measurements and acted as a quality control for the high-resolution readings because of its more robust nature. The MSS was run as a component of the Schlumberger paleo combo tool string, using a specially developed data translation cartridge.

Magnetic susceptibility data from both the high-resolution and deep-reading sensors are plotted as uncalibrated units. The MSS reading responses are affected by borehole size. The deep-reading sensor electronics are also highly influenced by temperature: higher temperatures lead to higher susceptibility measurements. The acquired magnetic susceptibility data tend to be affected by a nonlinear long period temperature-related drift superimposed on signal variability. Preliminary processing was performed offshore to remove the temperature drift by calculating a least-squares polynomial fit to the data and subtracting the calculated trend from the data set. The residual components from both the high-resolution and deep readings should be an indication of the magnetic signal variability in the formation. When the magnetic susceptibility signal in the sediment is very low, the detection limits of the tool may be reached. For quality control and environmental correction, the MSS also measures internal tool temperature, z-axis acceleration, and low-resolution borehole conductivity.

Acceleration and inclinometry

The General Purpose Inclinometer Tool (GPIT) was included in the FMS-sonic 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 as the magnetometer records the magnetic field components (F_x, F_y, and F_z). Thus, the FMS images can be corrected for irregular tool motion, and the dip and direction (azimuth) of features in the FMS image can be determined. Corrections for cable stretching and/or ship heave using GPIT acceleration data (A_x, A_y, and A_z) allow precise determinations of log depths.

Log data quality

The main influence on log data quality is the condition of the borehole wall. Where the borehole diameter varies over short intervals because of washouts or ledges made of layers of harder material, the logs from tools that require good contact with the borehole wall (i.e., the FMS, density, and shallow-reading magnetic susceptibility 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. “Bridged” sections, where borehole diameter is much below the bit size, also cause irregular log results. The quality of the borehole is improved by minimizing the circulation of drilling fluid while drilling, flushing the borehole to remove debris, and logging as soon as possible after drilling and hole conditioning are completed.

The quality of the wireline depth determination depends on several factors. The depth of the logging measurements is determined from the length of the cable payed out from the winch on the ship. The seafloor is identified on the NGR log by the abrupt reduction in gamma ray count at the water/sediment boundary (mudline). Discrepancies between the drilling depth and the wireline log depth may occur. In the case of drilling depth, discrepancies are because of core expansion, incomplete core recovery, or incomplete heave compensation. In the case of log depth, discrepancies between successive runs occur because of incomplete heave compensation, incomplete correction for cable stretch, and cable slip. In the case of very fine sediment in suspension, the mudline can be an elusive datum. Tidal changes in sea level affect both drilling and logging depths, although these were minimal in the studied region.

Wireline heave compensator

During wireline logging operations, the up-and-down motion of the ship causes a similar motion (heave) of the downhole logging tools. If the amplitude of this motion is large, depth discrepancies can be introduced into the logging data. The risk of damaging downhole instruments is also increased. A WHC system was thus designed to compensate for the vertical motion of the ship and maintain a steady motion of the logging tools to ensure high-quality logging data acquisition (Liu et al., 2013; Iturrino et al., 2013). The WHC uses a vertical accelerometer (motion reference unit [MRU]) positioned under the rig floor near the ship’s center of gravity to calculate the vertical motion of the ship with respect to the seafloor. It then adjusts the length of the wireline by varying the distance between two sets of pulleys through which the cable passes in order to minimize downhole tool motion. 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 monitoring of the efficiency of the compensator. During Expedition 346, the WHC was used during downhole logging acquisition at Site U1425.

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 were referenced to the rig floor (WRF). After logging was completed, the data were shifted to a seafloor reference (WSF), which was based on the step in gamma radiation at the sediment/water interface.

The data were transferred on shore to LDEO, where standardized data processing took place. The main part of the processing is depth matching to remove depth offsets between data from different logging runs, 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 imagery is corrected for tool acceleration, including sonde “stick and slip”), documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII for the conventional logs and GIF for the FMS images. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing of the wireline logging data collected. The data were transferred back to the ship within a few days of logging, and this processed data set was made available to the science party (in ASCII and DLIS formats) through the shipboard IODP logging database and shipboard servers.

Figures presented in the downhole logging sections often combine several depth scales (CSF-A, WSF, and WMSF) and mbsf depth is often used as a general depth term.

In situ temperature measurements

During Expedition 346, in situ temperature measurements were made with the APCT-3 in Hole A. Most commonly, at least four in situ temperature measurements were made at each site. The APCT-3 fits directly into the coring shoe of the APC and consists of a battery pack, a data logger, and a platinum resistance-temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool is first stopped at the mudline for 5 min to thermally equilibrate with bottom water. However, the lowest temperature recorded during the run down was occasionally preferred to the average temperature at the mudline as an estimate of the bottom water temperature because it was more repeatable, and bottom water is expected to have the lowest temperature in the profile. When the APC is plunged into the formation, there is an instantaneous temperature rise from frictional heating. This heat gradually dissipates into the surrounding sediment as the temperature at the APCT-3 equilibrates toward the temperature of the sediment. After the APC penetrated the sediment, it was held in place for 5 min as the APCT-3 recorded the temperature of the cutting shoe every second.

The equilibrium temperature of the sediment was estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). The synthetic thermal decay curve for the APCT-3 is a function of the geometry and thermal properties of the probe and the sediment (Bullard, 1954; Horai and Von Herzen, 1985). The equilibrium temperature was estimated by applying a fitting procedure (Pribnow et al., 2000). However, where the APC did not achieved a full stroke or where ship heave pulled the APC up from full penetration, the temperature equilibration curve was disturbed and temperature determination was less accurate. The nominal accuracy of the APCT-3 temperature measurements is ±0.05°C.

The APCT-3 temperature data were combined with measurements of thermal conductivity (see “Physical properties”) obtained from whole-core samples to obtain heat flow values. Heat flow was calculated according to the Bullard method, to be consistent with the synthesis of ODP heat flow data by Pribnow et al. (2000).

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