Downhole logs can be used to determine physical, chemical, and structural properties of formations penetrated by a drill hole. Wireline log data are rapidly collected using a variety of instruments to make continuous in situ measurements as a function of depth below the sea-floor after the hole has been drilled or reentered. Logs are essential to determine the borehole stratigraphy at a scale that links laboratory measurements on core samples with regional geophysical studies. After processing, logs also provide information regarding the physical state of the borehole and the character of the formations penetrated.
The principles, operation, and uses of the tools are described in Serra (1984, 1986) and Rider (1996). Standard ODP logging tools, applications, and principles for marine geology and geophysics are described by Goldberg (1997) and in previous ODP Initial Reports (e.g. Eberli, Swart, Malone, et al., 1997). They are briefly summarized below, and the tool string configurations are shown in Figure F10.
The natural gamma-ray tool (NGT) and hostile environment natural gamma-ray sonde (HNGS) measure the natural gamma radiation from isotopes of potassium, thorium, and uranium in the formation surrounding the tool.
The accelerator porosity sonde (APS) emits fast neutrons, which are slowed by hydrogen in the formation, and the energy of rebounded neutrons is measured. In sediments, most hydrogen is in the pore water and, hence, porosity may be derived. However, in igneous and metamorphic rocks most of the tool response is due to hydrogen in fluids filling fractures and bound in alteration minerals.
The hostile environment litho-density tool (HLDT) emits high-energy gamma rays, which are scattered by the electrons in the formation. The electron density (and hence bulk density) is derived from the energy of the returning gamma rays. Porosity may also be derived from bulk density, if the matrix density is known. In addition, the photoelectric effect (PEF) is measured, and this varies according to the chemical composition of the formation. The HLDT, APS, and HNGS were first used by ODP during Leg 166 (Eberli, Swart, Malone, et al., 1997).
The dual laterolog (DLL) tool measures the formation resistivity at two different penetration depths by measuring the intensity of a variable current flowing from an electrode to a remote return. This intensity is proportional to the formation's electrical conductivity. Two symmetrical guard electrodes emit focusing currents constraining the current beam to flow perpendicularly out into the formation and coaxially with the tool. The deep current (LLd) is focused in a path ~0.6 m wide that flows back to the surface while achieving deep penetration into the formation and reducing the borehole effect. For the shallow measurement (LLs), the same current electrode is used at a different frequency for measuring and focusing currents for the LLs return to electrodes located within the tool. This restricts the measurement to a shallower region. Definition of lithologic units, as well as estimation of total and fracture porosity, are often obtained from DLL measurements.
The Dipole Sonic Imager (DSI) employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1995). In addition to high-quality compressional wave velocity measurements, the DSI excites a flexural mode in the borehole that can be used to determine shear-wave velocity in all types of formations. The configuration of the DSI also allows recording of cross-line dipole waveforms that can be used to estimate shear-wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well.
The Formation MicroScanner (FMS) produces high-resolution images of the microresistivity character of the borehole wall. The tool is comprised of four orthogonal pads, each having 16-button electrodes that are pressed against the borehole wall (Serra, 1989). Roughly 30% of a 25-cm diameter borehole is imaged. The vertical resolution is ~5 mm, which allows features such as thin units, fractures, and veins to be imaged. A three-component magnetometer within the general-purpose inclinometry tool (GPIT) is part of the FMS tool configuration and allows the images to be oriented with respect to true geographical north. Hence, directional structure information can be obtained for unit boundaries, faults, and foliations.
Vertical seismic profile (VSP) experiments measure compressional-wave velocities in situ in the seafloor where it is intersected at the borehole from direct waves at seismic frequencies (10-100 Hz) and at seismic scales (hundreds of meters). The experiment provides results intermediate between small-scale measurements such as laboratory analysis of cores and sonic logs and large-scale measurements such as seismic refraction and reflection. During a VSP, a borehole seismometer is clamped successively at different depths in the borehole (Fig. F11; Hardage, 1983; Balch and Lee, 1984; Gal'perin, 1974). Seismic sources such as air guns and water guns are fired next to the ship, and the borehole seismometer records both the direct, downgoing waves and up-going waves reflected from changes in acoustic impedance below the receiver. Interval velocities are computed from the difference in arrival time of the direct wave between different receiver depths. An excellent review of VSP methodology as carried out on JOIDES Resolution is given in Shipboard Scientific Party (1989; pp. 182-199). Processing techniques can be applied to separate the upgoing and downgoing wavefields (Ross and Shaw, 1987; Christie et al., 1983; Kommedal and Tjostheim, 1989), which can then be analyzed for the attenuation properties of rock (Rutledge and Winkler, 1989; Swift and Stephen, 1992), for prediction of acoustic properties below the bottom of the hole (Swift et al., 1991) and for correlation with borehole lithology, wireline logs, and events on conventional seismic reflection and refraction profiles (Bolmer et al., 1992). Resolution of structure is limited by the signal-to-noise ratio, bandwidths of the source and receiver, and rock velocities. In marine use, the frequency of the source ranges from 5-30 Hz for explosives and large air guns to ~120 Hz for water guns. In rocks having velocities from 4 to 8 km/s, the finest scales resolvable (about one-quarter of a wavelength) with a broadband receiver under good signal-to-noise conditions are 8-200 m. Previous DSDP and ODP VSP experiments are described in Stephen (1979), Duennebier et al. (1987), and Shipboard Scientific Party (1995c).
Data quality is largely determined by the state of the borehole wall. If it is irregular, wide, or has many washouts, there are problems with the tools that require good contact with the wall (density, porosity, and FMS). However, deep investigative measurements, such as resistivity and sonic velocity, are less sensitive to variations in the hole diameter.
During Leg 176, Hole 735B was reentered and logged using two different wireline tool strings at the beginning of the leg and four tool strings at the end (Fig. F10), after coring and fishing operations were concluded. The wireline tool strings combined standard ODP tools with two new tools, the DSI and the Schlumberger BGKT three-component VSP tool. The wireline tools and the shipboard MAXIS unit, which recorded and monitored the log data acquisition in real time, were provided by Schlumberger. The Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) active wireline heave compensator was used during the logging operations to minimize the effects of the ship's motion during each of the logging runs. The LDEO-BRG memory temperature logging tool was also used to record in situ borehole temperatures. After the logs were acquired, the data were transferred to the shipboard Downhole Measurements Laboratory for preliminary interpretation using the Geoframe software package and also to the LDEO-BRG for processing using the SeaNET high-speed satellite data link.
The deployment of the DSI during Leg 176 consisted of two passes with the FMS-sonic tool string, where the DSI replaced the commonly used array sonic tool (SDT). Two passes are required to obtain consistent azimuth measurements for the cross-dipole mode and to acquire additional recording modes at normal logging speeds. Switching of recording modes was accomplished while the tool was downhole and in the same manner as it was performed during ODP Leg 174B (Becker, Malone, et al., 1998). The DSI was run first using the conventional P-wave first arrival, in-line dipole, and cross-dipole recording modes; then subsequently using the Stoneley-wave, high-frequency P-wave, S-wave, and cross-dipole recording modes.
Leg 176 was the first to use the Schlumberger three-component VSP tool (Fig. F10). This tool has been used for many years in the petroleum exploration industry and is supported by the Schlumberger logging engineer on board JOIDES Resolution.
To measure the far-field incident wavefield in the water column, we also deployed an over-the-side hydrophone at a depth of 200-300 m. This hydrophone is separate from the blast phone on the guns. The blast phone measures the initial blast of the gun and is used as the start time for seismic recording. The over-the-side hydrophone measures the true amplitude pressure waveform in the far field (at least a wavelength or two from the source). It includes the interference effect of the free surface above the guns. Monitoring during the VSP experiment is important for amplitude and waveform studies such as reflectivity modeling and attenuation analysis. Significant amplitude, waveform, and frequency changes can occur during the experiment because of small variations in depth or air pressure.
The Leg 176 VSP was planned (1) to provide seismic interval velocities with which the rock sequence intersected by the borehole could be compared, (2) to place the borehole results in their proper setting with respect to the seismically defined structure of the oceanic crust and mantle, (3) to estimate the degree of large-scale fracturing (>0.5 m) by comparing seismic interval velocities with velocities measured in the laboratory on core samples and measured in situ by sonic logging, (4) to correlate borehole lithology with the upgoing seismic reflected wavefield, (5) to predict structure and lithology changes below the drill hole, and (6) to estimate physical properties of rock on seismic scales by studying particle motion and downhole seismic attenuation.
The principal objectives of the core imaging project were (1) to provide a comprehensive set of digital core images, including both unrolled 360° and slabbed images, recorded using the DMT Digital Color Core-Scan system, (2) to identify and measure planar features on unrolled images for comparison to core structural analysis and integration with geographically oriented FMS images, and finally, (3) to match and reorient core images and structural data to true geographic north obtained from the GPIT on the FMS tool string.
The DMT Color CoreScan system is a portable core-imaging unit that was used for the first time on board JOIDES Resolution during Leg 173 (Whitmarsh, Beslier, Wallace, et al., 1998). Images were recorded on both slabbed and full-round core surfaces using a 24-bit, three-color (red, green, and blue) CCD line-scan camera with a resolution of 5184 pixels/m (131 pixels/in) and a spectral response between 400 and 700 nm. In the unrolled mode, whole-round core is rotated around its cylindrical axis with the camera line scan positioned parallel to the axis of rotation. Unrolled images as much as 1 m long are recorded in 33-cm sections that are inte-grated and light calibrated using the DMTGrab Software (DMT CoreScan Users Manual, 1996). Whole-round cores in the unrolled mode are scanned at a rate of ~1.20 min/m, which creates a 14-MB bitmap file. Split archive-half core images as long as 1 m are recorded in slabbed mode with the camera line scan traveling perpendicular to the vertical core axis. Scanning 1 m of split core takes ~45 s and creates a 10-MB bitmap file.
During Leg 176, all cores were scanned in the unrolled mode with the exception of those pieces that were not fully cylindrical and intervals of drilling breccia. Whole-round cores were scanned after the core was run through the MST. A vertical line marked on the core with a red grease pencil aided initial reorientation of the images back to the ODP reference frame. Unrolled images were obtained on contiguous core pieces whenever possible. The lengths of whole-round pieces unsuitable for scanning were measured so the allowance could be made for them when images were integrated into core barrel lengths using the DMT Software CoreLog (DMT CoreScan Users Manual, 1996). Initial structural analysis was performed; however, the majority of the structural analysis and core reorientation work was done after the cruise. During our sail to Hole 735B, split core (slabbed) images of the archive-half were obtained from the bottommost 50 m of the hole cored during Leg 118. Selected images of the working half were also obtained from new cores during Leg 176.