Downhole logs represent continuous measurements of the drilled formations as a function of depth. The advantage of downhole logging is the ability to record, concurrently, petrophysical as well as structural information of several properties. Operating on an intermediate scale between core measurements and borehole geophysics, downhole logs are characterized by fast data acquisition over large depth ranges under in situ conditions. During Leg 203, the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) performed open-hole measurements using the two ODP standard tool strings (triple combination [triple combo] and FMS-sonic) and two specialty tools (Well Seismic Tool [WST] and cement bond tool [CBT]). The first three of the above-mentioned tools were deployed in Hole 1243B, whereas the last tool was deployed in Hole 1243A. Additional data for borehole deviation in Hole 1234A were gathered by the General Purpose Inclinometer Tool (GPIT) of the FMS-sonic tool string. Generally, all measurements were performed while the tools were pulled upward. That is, the tools were lowered down the hole on a heave-compensated electrical wireline and then pulled up at constant speed of ~250-300 m/hr. The sampling interval of log data ranges from 15 cm for standard tools to 2.5 mm for high-resolution tools, such as the FMS.
The main target of downhole measurements during Leg 203 is the reconstruction of the drilled lithologic units. This includes a reorientation of core data on the basis of high-resolution images provided by the FMS. Measurements of porosity, P-wave velocity, and density allow the depiction of zones of weakness, which may subsequently influence borehole stability. A quick-view estimation of these logging data provides depth ranges, and, to a certain extent, lateral extension of these zones. Measurements of gamma ray activity, including its spectral proportions of potassium, thorium, and uranium, address issues concerning evolution of alteration assemblages within the drilled basaltic sequences. The WST provides a check shot vertical seismic profile (VSP) survey to estimate a depth-traveltime plot and to calibrate the sonic log.
The following tool strings used during Leg 203 were deployed in five logging runs. Runs one through three were performed in Hole 1243B, and runs four and five covered Hole 1243A.
All tools were provided by Schlumberger, except the TAP, which was run by LDEO-BRG. Each tool string contains a telemetry cartridge providing communication along a 9000-m, double-armored, seven-conductor wireline with the Schlumberger Minimum Configuration Maxis (MCM) computer van on the drillship. The 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 ship's heave on the tool position in the borehole. It responds to the ship's heave by hydraulically moving the compensator's heave to decouple the movement of the ship from the movement of the tool string in the borehole (Goldberg, 1990).
In preparation for logging, the borehole is usually flushed of debris by circulating viscous drilling fluid (sepiolite mud mixed with seawater; approximate density = 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 of between 50 and 100 mbsf, followed by a second run down to the bottom of the hole to repeat compensation of borehole irregularities. The hole is subsequently filled with additional sepiolite mud, and the pipe is raised to 50-70 mbsf, where it is kept to prevent hole collapse during logging.
The operating principles, applications, and approximate vertical resolution of the tools are summarized in Table T3. The principal data provided by the tools, their physical significance, and units of measure are listed in Table T4. More information on individual tools and their geological applications may be found in Desbrandes (1985), Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994a, 1994b), Serra (1984, 1986, 1989), and the LDEO-BRG Wireline Logging Services Guide (Borehole Research Group, 1990). The basic principles of the tools and type of measurements are summarized below.
Two spectral gamma ray tools were used to measure and classify natural radioactivity in the formation, the HNGS and the NGT. The NGT uses a sodium iodide (NaI) scintillation detector and five predefined energy windows to determine concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million), the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate (BGO) scintillation detectors and 256-window spectroscopy for a significantly improved tool precision. Gamma rays emitted by the formation pass the scintillator crystal in the tool and are converted to "light flashes" (Fig. F9). Subsequently, these flashes are converted to electrons through the photoelectric effect (PEF) and multiplied in the photomultiplier, producing distinctive energy peaks for the individual isotopes measured in millions of electron volts (MeV). The windows, described above, are designed to separate these energy peaks of the detected radioactive elements at 1.46 MeV for 40K, 2.62 MeV for 232Th, and 1.76 MeV for 238U. Energies below 500 MeV are filtered out by the HNGS tool, eliminating sensitivity to bentonite or potassium chloride in the drilling mud and improving measurement accuracy. In contrast, NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or potassium chloride present in the drilling mud. Corrections are routinely made for these effects during processing at LDEO-BRG.
For both tools, gamma ray values are measured in American Petroleum Institute units. These units are derived from the primary Schlumberger calibration test facility in Houston, Texas, where a calibration standard is used to normalize each tool.
Formation density is estimated from the density of electrons in the formation, which are 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 arm. Coupling between the tool and borehole wall is essential for good HLDS logging. Poor contact may occur when the borehole diameter is greater than the length of the caliper (e.g., for borehole diameters >48 cm), which results in underestimation of density values. Gamma rays emitted by the source experience Compton scattering (Fig. F10), which involves the transfer of energy from gamma rays to the electrons in the formation via elastic collision (further information is provided by Rider [1996] and Schlumberger [1989, 1994a, 1994b]). 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 density is known.
The HLDS also measures the PEF caused by absorption of low-energy gamma rays (Fig. F10). Photoelectric absorption occurs when gamma rays reach an energy level <150 keV after being repeatedly scattered by electrons in the formation. Photoelectric absorption is strongly dependent on the atomic number of the constituents of 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-. In combination with the NGT log, PEF values can be used to identify different types of clay minerals. Hence, the PEF values can give an indication of the chemical composition of the rock. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values.
Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator, which produces fast neutrons (14.4 MeV), and five neutron detectors (four epithermal and one thermal) positioned at different intervals (spacing) from the minitron (Fig. F11). The tool is pressed against the borehole wall by a bow spring. Emitted neutrons are slowed by successive collisions with atomic nucleii in the formation (especially H), comparable to a billiard-ball effect. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus nearly equal to its own mass (such as H), which is mainly present in the pore water. After successive collisions, the speed of the thermal neutrons is so low that they are captured and absorbed by the formation's elements, thus, emitting a gamma ray in turn.
This provides the basis for two principal types of porosity measurements covered by contemporary neutron tools. Depending on the energy level, we can distinguish between (1) neutron-neutron and (2) neutron-gamma interactions. The former measures epithermal and thermal neutrons with an energy value of ~0.25 eV (Fig. F12). Neutrons with energy values lower than this are subject to diffusion and, finally, capturing by the elements in the formation (e.g., Cl, Si, and B). In turn, gamma rays are emitted by these elements, which can be measured by the second device (neutron-gamma). The APS tool used during Leg 203 is a neutron-neutron device. Its detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times, which act as a measure of formation porosity. However, as hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity value is often an overestimate.
Two types of resistivity measurement tools are available on board: the Dual Laterolog (DLL) and the DIT. The former has a response range of 0.2-40,000 
m and measures the direct resistivity of a formation. It is mostly applied in medium- to high-resistivity formations in igneous environments (e.g., oceanic basalts and gabbros). The DIT, with a response range of 0.2-2,000 m, is a conductivity-sensitive device and is most commonly applied in low- to medium-resistivity formations such as sediments. However, it has also produced very good results in ocean-crust basalts.
The DIT has a deep induction phasor-processed device (IDPH), a medium induction phasor-processed device (IMPH), a spherically focused resistivity measurement (SFLU), and a spontaneous potential (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 (Fig. F13). These induced currents create, in turn, a magnetic field that induces new currents in the receiver coils, producing a voltage. The currents are proportional to the conductivity of the formation, as is the voltage. 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 IDPH and IMPH measurements may be erroneous, but the error can be greatly reduced by downhole calibration. In such cases, SFLU measurements produce good results, similar to the DLL device (Shipboard Scientific Party, 1998). The resistivity of a rock is controlled largely by the porosity of the rock, the properties of the pore fluid(s), and the connectivity of the pores.
Spontaneous potentials can originate from a variety of causes, electrochemical, electrothermal, and electrokinetic streaming potentials, and membrane potentials as a result 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 (Fig. F14).
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP. When attached to the bottom of the triple combo string, the TAP operates in an autonomous mode with data stored in built-in memory. A two-component thermistor, for different temperature ranges, is mounted near the bottom of the tool in the slotted protective cover. The time constant of the thermistor assembly in water is ~0.4 s. The tool includes a pressure transducer (0-10,000 psi), which is used to activate the tool at a specified depth and perform pressure measurements. The TAP also incorporates a high-sensitivity vertical accelerometer, providing data for analyzing the effects of heave on a deployed tool string, and an internal temperature sensor for monitoring the temperature inside the electronic cartridge. Temperature and pressure data are recorded once per second, and accelerometer data can be recorded at a 4- or 8-Hz sampling rate.
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 circulation. The data recorded under such circumstances may differ significantly from thermal equilibrium for 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.
Sonic velocities were measured with the DSI. The DSI tool employs a combination of monopole and dipole transducers (Fig. F15) to make accurate measurements of sonic wave velocities in a wide variety of lithologies (Schlumberger, 1995). The tool measures the transit times between sonic transmitters and an array of eight receivers. It averages replicate measurements, providing a direct measurement of sound velocity through the formation that is relatively free from environmental effects, such as formation damage and enlarged borehole (Schlumberger, 1989). Along with the monopole transmitters found on most sonic tools, the DSI has two crossed dipole transmitters. Thus, the DSI can be used to determine S-wave velocity in all types of formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at the S-wave velocity and is the most reliable way to estimate a shear velocity log. The omnidirectional source generates P-, S-, and Stoneley waves in hard formations at the borehole/formation interface. The configuration of the DSI also allows recording of cross-line dipole waveforms, which can be used to estimate S-wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source (80 Hz) enables Stoneley waveforms to be acquired as well. These "guided" waves are associated with the solid/fluid boundary at the borehole wall, and their amplitude decays exponentially away from the boundary in both the fluid and the formation.
In addition, information such as P-, S-, and Stoneley wave amplitudes, S-wave polarization, and Poisson's ratio can be extracted postcruise to provide information about lithology, porosity, and anisotropy. Amplitude processing and stacking of Stoneley wave reflections can also be used to identify fractures, fracture permeability, and aperture in the vicinity of the borehole. The tool configuration and data processing procedures are described in detail in the Leg 174B Initial Reports volume (Becker, Malone, et al., 1998).
Downhole magnetic field and acceleration measurements were performed with the GPIT. The GPIT is usually used in combination with the FMS, but it can be attached to other tools or run autonomously. The primary purpose of this sonde, 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 based on cable length alone, as the wireline is subject to both stretching and ship heave. Acceleration data are also used in processing of FMS data to correct the images for irregular tool motion. A third application of the acceleration data is the measurement of the hole deviation (in degrees). The magnetic measurements of inclination may be affected by the magnetization of the casing and/or drill pipe. Furthermore, local magnetic anomalies, generated by high remnant magnetization of basalts in the basement section of a borehole, can interfere with the determination of tool orientation. However, these magnetic measurements can be useful for constraining the magnetic stratigraphy of the basement section.
The FMS provides high-resolution electrical resistivity based images of borehole walls. The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or "buttons," which are pressed against the borehole wall during recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced ~2.5 mm apart (Fig. F16). A focused current is emitted from each button of the four pads into the formation with a return electrode near the top of the tool (Fig. F17). Array buttons on each of the pads measure current intensity variations. The FMS image is sensitive to structures within ~25 cm of the borehole wall and has a vertical resolution of 5 mm with a coverage of 25% of the borehole wall for a borehole diameter of 9
in (25.1 cm) (i.e., RCB bit size). FMS logging commonly includes two passes, which merge the images together to improve borehole wall coverage. To produce reliable FMS images, however, the pads must be pressed firmly against the borehole wall. The maximum extension of the caliper arms is 15.0 in (38.1 cm). In holes with a diameter >15 in (38.1 cm), the pad contact will be inconsistent and the FMS images can be blurred. The maximum borehole deviation where good data can be obtained with this tool is 10°. Irregular borehole walls also adversely affect the images as contact with the wall is poor. FMS images are oriented to local magnetic north using the GPIT (see "Accelerometry and Magnetic Field
Measurement"). Processing transforms these measurements of microresistivity variations into continuous, spatially oriented, and high-resolution images that display geologic structures behind the borehole walls. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation. This allows the dip and azimuth of geological features intersecting the hole to be measured from processed FMS images. FMS images are particularly useful for mapping structural features, dip, detailed core-log correlation, and positioning of core sections where there is poor recovery. The FMS images can be used to compare logs visually with core to ascertain the orientations of bedding, fracture patterns, and sedimentary structures. FMS images have proved particularly valuable in the interpretation of sedimentary, as well as igneous structures, in previous ODP legs and determination of volcanic sequences (Ayadi et al., 1998, Brewer et al., 1999). Detailed interpretations of FMS images in combination with other log data and core imaging will be conducted postcruise.
The WST is a single-axis check-shot tool used for zero-offset VSPs. The tool consists of a single geophone, pressed against the borehole wall, with a sampling interval of 1 ms. It is used to record the acoustic waves generated by a seismic source located near the sea surface. A GI gun (Sodera 210-in3 Harmonic) was used as a seismic source (Fig. F8). The gun, which produces waves with a frequency content exceeding 200 Hz, was suspended by a buoy at a depth of 2-3 meters below sea level, offset 55.8 m from the hole on the portside of the ship. Two hydrophones (one Schlumberger and one ODP hydrophone) were used (Fig. F8) to record the break signal. The Schlumberger hydrophone was suspended 2 m below the gun, and the other hydrophone was attached directly to the GI gun. Several test shots (~30-40 shots) were fired while the tool was lowered down to the bottom of the hole (BOH) and on the BOH itself. During these tests, it turned out that the first break of the lower hydrophone was poor. Instead, data obtained from the upper hydrophone gave a very good break signal, although the data were of limited use for source behavior because of the proximity of the hydrophone to the surface.
During operations in Hole 1243B, the WST was clamped against the borehole wall at intervals of 10 m, and the gun fired 10 times at intervals of 20 s. The resulting waveforms were stacked. A traveltime was determined from the first break in each trace. In general, the acoustic velocities determined from the sonic tool differ significantly from the seismic velocities because of frequency dispersion (e.g., the DSI operates at 10-20 kHz vs. 50-100 Hz in seismic data) and because the sound is forced to travel along the borehole wall, a path that is quite different from the one taken by the gun signal generated during a seismic reflection survey. One of the applications of data from the WST is a plot of depth vs. traveltime determined from check shots. This plot is used to calibrate the sonic logs and determine accurate drilling depths and their relative position with respect to targets on seismic reflection profiles. The seismic reflection survey conducted at the beginning of Leg 203, however, used a small water gun source and did not provide useful data within the basement. Thus, direct comparisons of WST measurements and these seismic data are not possible.
The quality of log data can be seriously degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. If the borehole 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). Resistivity and velocity measurements are the least sensitive to borehole effects. Nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays) are mainly affected by borehole fluid and/or drilling mud signal attenuation. Corrections can be applied to the original data to reduce the effects of these conditions and, generally, any departure from the conditions under which the tool was calibrated.
The depth of the wireline logging measurements is determined from the length of the logging cable paid out at the winch on the ship. Small errors in depth matching can distort the logging results in zones of rapidly changing lithology. Logs from different tool strings may have depth mismatches caused by either cable stretch or ship heave during recording. To minimize the effects of ship heave, a hydraulic WHC adjusts for rig motion during logging operations. Distinctive features recorded by the NGT, run on the FMS-sonic tool string (Hole 1243B) and on the CBT (Hole 1243A), provide calibration points and relative depth offsets among the logging runs and can be correlated with distinctive lithologic contacts observed in the core recovery or drilling penetration (e.g., pillow basalt-breccia transitions). 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), as well as 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.
Data for each logging run are recorded, stored digitally, and monitored in real time using the MCM. Measurements are performed and recorded on downhole and uphole logging runs. Whereas the downward measurements are taken for reference, tools are only partly operational because calipers have to be closed going downward, the upward runs are the actual logging runs. The tool string is pulled up at constant speed to provide continuous measurements as a function of depth of several properties simultaneously. The TAP is deployed as an autonomous recording tool. The preparation and data processing of the TAP are done in the Downhole Measurements Laboratory (DHML) using a specialized acquisition system. On completion of logging, data from the Schlumberger tools are transferred to the DHML for preliminary interpretation. Basic processing is then carried out to provide scientists with a comprehensive quality-controlled downhole log data set that can be used for comparison and integration with other data collected during Leg 203. Extended processing is usually carried out onshore at LDEO after the data are transmitted by satellite from the ship. These analyses include 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 GeoQuest's software package, GeoFrame, is used for most of the processing. Further postcruise processing of FMS log data is performed, and data are available 1 month after the cruise.
Processed acoustic, caliper, density, gamma ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available directly from the LDEO-BRG Internet site at: www.ldeo.columbia.edu/BRG/ODP/DATABASE. A summary of logging highlights is also posted on the LDEO-BRG Web site at the end of each leg.
