f) is also measured by the tool.
Downhole logging began in the 1920s as "electrical coring" (Schlumberger et al., 1934), which measured the spontaneous potential (SP) between the surface and the sonde at depth, and it now encompasses a wide range of sensors. It is a method of measurements performed in a drill hole to determine the in situ physical, chemical, and structural properties of formations penetrated by the hole. Logs can be acquired after the hole is drilled (wireline logging) or while/after drilling (logging while drilling [LWD] or measurement while drilling). Wireline logging is most effective in stable formations and where the hole is nearly vertical, whereas LWD can be performed while drilling where formations are unstable or after drilling where the hole has deviated far from the vertical. Wireline logging was used during ODP Leg 205, whereas LWD was done in the area during Leg 170 in 1996 (Kimura, Silver, Blum, et al., 1997).
The combinations of wireline logs on the JOIDES Resolution are collected at a speed of 250-300 m/hr, using a variety of instruments that make continuous in situ measurements as a function of depth. The sampling interval of log data ranges from 2.5 mm to 15 cm in depth. The data are rapidly collected, quasicontinuous with depth, and measured in situ; they can be interpreted in terms of the stratigraphy, lithology, mineralogy, 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 jointly. Downhole logs are sensitive to 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 properties on all scales. After processing, logs are typically displayed as curves or images of physical and chemical properties of the formations intersected by the borehole. Image logs, in particular, serve to illustrate the physical shape and state of the hole and the character of the formations penetrated. Log data have been used for an ever-growing number and range of applications, such as characterizing formation fluids and measuring the in situ temperature and stress conditions.
Log data quality is largely determined by the state of the borehole wall and individual logging tools. There is also a small but significant difference between drill pipe depth and logging depth that should be taken into account when using the logs for correlation with core and log measurements. In addition, logs from different wireline tool strings may have slight depth mismatches. Therefore, after data acquisition, basic hole condition correction and depth match are generally required. With further processing, log data are combined with core and seismic data to achieve integrated characterization, analysis, and interpretation of both formation fluids and subsurface environments.
During ODP Leg 205, Hole 1253A was drilled, cored, and logged. The logs were acquired using a variety of Schlumberger and Lamont-Doherty Earth Observatory-Borehole Research Group (LDEO-BRG) logging tools that are combined into two "tool strings." These tool strings are listed as follows:
Each tool string also contains a telemetry cartridge facilitating communication from the tools along a double-armored seven-conductor wireline cable to the Schlumberger's Minimum Configuration Maxis (MCM) computer van on the drill ship. The 9000-m-long logging cable connects the MCM to the tool string through the logging winch and LDEO-BRG wireline heave compensator (WHC).
The WHC is employed to minimize the effect of the ship's heave on the tool position in the borehole. The logging winch is located aft of the pipe racker. The 160-m-long logging cable fairlead runs from the winch forward to the drill floor, through a sheave back to the heave compensator located alongside the logging winch, then forward to another sheave on the rig floor, up to the crown block on the top of the derrick, and then down into the drill string. 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 hydraulically moving the compensator sheave to decouple the movement of the ship from the desired movement of the tool string in the borehole (Goldberg, 1990).
In preparation for logging, the boreholes are usually flushed of debris by circulating a "pill" of viscous drilling fluid (sepiolite mud mixed with seawater; approximate weight = 8.8 lb/gal or 1.11 g/cm3) through the drill pipe to the bottom of the hole. The bottom-hole assembly (BHA) is pulled up to a depth between 50 and 100 mbsf then run down to the bottom of the hole again to ream borehole irregularities. The hole is subsequently filled with more sepiolite mud, and the pipe is raised to ~50-70 mbsf and kept there to prevent hole collapse during logging. The tool strings are then lowered downhole during sequential runs. During each logging run, incoming data are recorded and monitored in real time on the MCM computer. The tool strings are then pulled up at constant speed to provide continuous measurements as a function of depth of several properties simultaneously. After the logs are acquired, the data are transferred to the downhole measurements laboratory (DHML) and also to LDEO-BRG for processing using a high-speed satellite data link. The LDEO-BRG TAP tool is usually deployed as a memory tool. Its preparation and data processing of the LDEO-BRG TAP tool are done in the DHML using a specialized acquisition system.
The measurement principles and applications of the logging tools are described below. The acronyms and measurement units can be found in Table T7, and principles, applications, and approximate vertical resolution of the tools are summarized in Table T8. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the LDEO-BRG Wireline Logging Services Guide (LDEO-BRG, 2000).
Two spectral gamma ray tools are usually used to measure and classify natural radioactivity in the formation: the SGT and the HNGS. The SGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of potassium (K, in weight percent), thorium (Th, in parts per million), and uranium (U, in parts per million), the three elements whose isotopes dominate the natural radiation spectrum. High K and Th values indicate greater clay concentrations, and increased U values often indicate the presence of organic matter. The HNGS is similar to the SGT, but it uses two bismuth germanate scintillation detectors for a significantly improved tool precision.
The SGT response is sensitive to the borehole diameter, and these diameter effects are corrected during postcruise processing.
Formation density is determined from the density of electrons in the formation, which is measured with the HLDT. The tool 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. Gamma rays emitted by the source undergo Compton scattering, which involves the transfer of energy from gamma rays to the electrons in the formation via elastic collisions. The number of scattered gamma rays that reach the detectors is directly related 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 HLDT also measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach <150 keV after being repeatedly scattered by electrons in the formation. Because PEF depends on the atomic number of the elements in the formation, it varies according to the chemical composition and is essentially independent of porosity. For example, the PEF of pure calcite = 5.08 b/e-; illite = 3.03 b/e-; quartz = 1.81 b/e-; and kaolinite = 1.49 b/e-. PEF values can be used in combination with SGT curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDT logs. Poor contact results in underestimation of density values. Poor contact may occur when the borehole diameter is greater than the length of the caliper (e.g., for borehole diameters >48 cm).
The depth of investigation into the formation of the lithodensity tool is of the order of tens of centimeters, depending on the density of the rock.
Formation porosity, usually called neutron porosity, is 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 intervals 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, which 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. However, because hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.
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 (f) is also measured by the tool.
The DIT, a conductivity-sensitive device, is most accurate in low- to medium-resistivity formations and is commonly used to measure the formation electrical resistivity.
The DIT has a deep-reading induction device (IDPH), a medium-reading induction device (IMPH), a spherically focused log device (SFLU), and an SP device. The two induction devices transmit high-frequency alternating currents through transmitter coils, creating time-varying magnetic fields that induce currents in the formation. These induced currents create, again, a magnetic field that induces new currents in the receiver coils producing a voltage. These induced currents are proportional to the conductivity of the formation so the voltage is proportional to the resistivity of the formation. The SFLU is a shallow-penetration galvanic device that measures the current necessary to maintain a constant voltage drop across a fixed interval of the formation. In high-resistivity formations (>100 
m), both inductive IDPH and IMPH measurements may be erroneous but the error can be greatly reduced by downhole calibration if a massive formation exists of exceedingly high resistivity.
SPs can originate from a variety of causes: electrochemical, electrothermal, electrokinetic streaming potentials, and membrane potentials because of differences in the mobility of ions in the pore and drilling fluids. SP may be useful to infer fluid flow zones and formation permeability.
Downhole temperature, acceleration, and pressure are measured with the LDEO TAP tool. When attached to the bottom of the triple combo tool string, the LDEO TAP tool is run in an autonomous mode, with data stored in built-in memory. Two thermistors are mounted near the bottom of the tool to detect borehole fluid temperatures at different rates. A thin fast-response thermistor is able to detect small abrupt changes in temperature. A thicker slow-response thermistor is used to estimate temperature gradients and thermal regimes more accurately. The pressure transducer is included to activate the tool at a specified depth. A three-axis accelerometer measures tool movement downhole, providing data for analyzing the effects of heave on a deployed tool string, which should eventually lead to a finer adjustment of the wireline heave compensator.
The borehole temperature record provides information on the thermal regime of the surrounding formation. The vertical heat flow can be estimated from the vertical temperature gradient combined with measurements of the thermal conductivity from core samples.
The temperature record must be interpreted with caution, as the amount of time elapsed between the end of drilling and the logging operation is generally not sufficient to allow the borehole to recover thermally from the influence of drilling fluid circulations. The data recorded under such circumstances may differ significantly from the thermal equilibrium of that environment. Nevertheless, from the spatial temperature gradient 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.
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 (Shipboard Scientific Party, 1998). In addition to a robust and high-quality measurement of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to estimate shear wave velocity even in highly unconsolidated formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at the shear wave velocity and is the most reliable way to estimate a shear velocity log. Meanwhile, the omnidirectional source generates compressional, shear, and Stoneley waves into hard formations. The configuration of the DSI also allows recording of both in-line and cross-line dipole waveforms. In many cases, the dipole sources could also result in estimates of shear wave velocity in hard rocks better than or equivalent to the 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 acquired as well.
The DSI measures the transit times between sonic transmitters and an array of eight receiver groups with 15-cm spacing, each consisting of four orthogonal elements that are aligned with the dipole transmitters. During acquisition, the output from these 32 individual elements are differenced or summed appropriately to produce in-line and cross-line dipole signals or monopole-equivalent (compressional and Stoneley) waveforms, depending on the operation modes. The detailed description of tool configuration and data processing are described in the Leg 174B Initial Reports volume (Shipboard Scientific Party, 1998).
The FMS provides high-resolution electrical resistivity-derived images of borehole walls. The tool has four orthogonal arms with 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 and are 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. Processing transforms these measurements of the resistivity variations of the formation into continuous, spatially oriented high-resolution images that mimic geologic structures behind the borehole walls. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation. FMS images are particularly useful for mapping structural features, dip determination, detailed core-log correlation, positioning of core sections with poor recovery, and analysis of depositional environments and stress distribution.
The FMS image is sensitive to structures within ~25 cm beyond the borehole wall and has a maximum vertical resolution of 5 mm with the coverage of 25% of borehole wall for a borehole diameter of 9
in (i.e., RCB bit size). FMS logging commonly includes two passes, the images of which are merged to improve the borehole wall coverage. To produce reliable FMS images, however, the pads must be firmly pressed against the borehole wall. The maximum borehole deviation where good data can be recorded with this tool is 10°. Irregular borehole walls will also adversely affect the images because contact with the wall is poor.
Three-component acceleration and magnetic field measurements are made with the 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. The acceleration data allow more precise determination of log depths than is possible on the basis of cable length alone, as the wireline is subject to both stretching and ship heave. This provides a means of correcting the FMS images for irregular tool motion, allowing the true dip and direction (azimuth) of structures to be determined.
The quality of several types of log data may be degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. If it is irregular, wide, or there are many washouts, there may be problems with those tools that require good contact with the wall (density, porosity, and FMS). Deep investigation measurements such as resistivity and sonic velocity are least sensitive to borehole conditions. Corrections can be applied to the original data to reduce the effects of these conditions and other departures from the conditions under which the tool has been calibrated.
The depth of the wireline logging measurements is determined from the length of the logging cable played out at the winch on the ship. The seafloor is identified on the natural gamma 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 BHA and pipe stands.
Discrepancies between the drillers depth and the wireline log depth occur because of core expansion, incomplete core recovery, drill pipe stretch in the case of drill pipe depth, cable stretch (~1 m/km), and cable slip in the case of log depth. Tidal changes in sea level, up to 1 m in the open ocean, will also have an effect. Precise core-log depth matching is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section within the cored interval.
Logs from different tool strings may also have depth mismatches, caused by either cable stretch or incomplete heave compensation during recording. Distinctive features recorded by the Natural Gamma Ray Spectrometry Tool, run on every tool string, provide relative depth offsets and thus a means of depth shifting for correlation between logging runs.
Data for each logging run were recorded, stored digitally, and monitored in real time using the MCM software. On completion of logging, data are loaded to the Schlumberger GeoFrame software system in the DHML for onboard preliminary interpretation. Basic processing is carried out postcruise to provide a quality-controlled downhole logging data set that can be used for comparison, integration, and correlation with other data collected during each ODP leg. The processing includes depth adjustments to remove depth offsets between data from different logging runs; corrections specific to certain tools and logs; documentation for the logs (with an assessment of log quality); and conversion of the data to a widely accessible format (ASCII for the conventional logs and GIF for the FMS images). Schlumberger's GeoFrame software package is used for most of the processing.
Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in are available in ASCII.
