m
(error = >20% at 150 m).After coring was completed, combinations of sensors were lowered downhole to measure the physical and chemical properties of formations penetrated by the borehole. Interpretation of these continuous in situ measurements can yield a lithostratigraphic, structural, geophysical, and geochemical characterization of the hole. The downhole tools run during Leg 180 are listed in Table T7; they were combined as represented in Figure F15 and described below.
The natural gamma ray is acquired (by either HNGS or NGT) on all combinations to provide a common basis for depth correlations. The FMS and the UBI provide high-resolution continuous images of sedimentary or structural features and hole geometry, and help place the core pieces back in depth and orientation.
Throughout the acquisition of downhole measurements, data are transmitted to the surface by the seven-conductor logging cable, displayed in real time, and simultaneously recorded. The TLT is an exception to this because its data are recorded internally and downloaded after logging.
The logging data became quickly available aboard the ship and were depth shifted as explained in the site chapters to produce the logging section of each site report. Further processing conducted onshore includes a differential depth shift and yields the data included on the accompanying LDEO CD-ROM.
Table T8 lists the main measurements and their approximate vertical resolution and sampling interval, as well as their depth of investigation. These tools and their applications are briefly described in the following sections according to the physical properties they measure. More detailed information can be found in Hearst and Nelson (1985), Serra (1984), Timur and Toksöz (1985), Ellis (1987), and Schlumberger (1989).
The DIT provides three
different measurements of electrical resistivity, each with a different depth of
investigation. Two induction devices (deep and medium resistivity) send
high-frequency alternating currents through transmitter coils, creating magnetic
fields that induce secondary (Foucault) currents in the formation. These
ground-loop currents produce new inductive signals proportional to the
conductivity of the formation, which are recorded by the receiving coils. The
third device (shallow resistivity) is the spherically focused resistivity that
measures the current necessary to maintain a constant voltage drop across a
fixed interval. This tool becomes inaccurate for formation resistivity exceeding
150 m
(error = >20% at 150 m).
Bulk formation resistivity is mainly controlled by fluid resistivity and by porosity. To a first-order approximation, it responds to the inverse square root of porosity (Archie, 1942). Fluid resistivity depends mainly on water salinity and hydrocarbon content. Additional factors affecting resistivity include clay content, temperature, hydrous and metallic minerals concentration, presence of vesicles, and the geometry of interconnected pore spaces.
The array sonic digital tool has three components. A sonic digital cartridge (SDC) hosts electronics. An SLS-C hosts two broadband (5-18 kHz) acoustic transmitters 2 ft (61 cm) apart and two receivers, 3 and 5 ft (91.4 and 152.4 cm), respectively, above the upper transmitter. An SLR hosts an array of eight receivers located 8-11.5 ft (243.8-350.5 cm) above the upper transmitter.
This setup allows recording of the full waveforms of sound waves that travel along the borehole wall over various source-receiver distances. Compressional wave velocity is determined in real time by a threshold-measuring technique that attempts to detect the first arrival. Occasionally, this technique fails, and either the threshold is exceeded by noise or the amplitude of the first compressional arrival is too small. The latter of these effects is known as cycle skipping and creates spurious spikes on the sonic log. Such problems may often be eliminated by reprocessing the data. Further processing of the full waveforms after logging may also determine shear and Stoneley wave velocities in relatively fast formations.
Compressional wave velocity is primarily controlled by porosity, matrix density, and elastic properties. In sedimentary rocks decreasing porosity and increasing lithification by diagenesis increases velocity.
The NGT measures the natural gamma radiation of the formation by a scintillation detector (Lock and Hoyer, 1971). The gamma-ray energy spectrum is divided into five discrete windows, and count rates are recorded for each.
The more recent HNGS provides the same measurements with larger dual detectors of higher quality. The result is vastly improved statistical accuracy, as well as faster logging speed. This tool is additionally capable of high-temperature operation (up to 235°C), from which it derives its name.
Most natural gamma rays are emitted by the radioactive isotope 40K and radioactive isotopes of the U and Th decay series, each at a characteristic energy level. Processing of the windowed counts therefore gives elemental abundances of K, U, and Th. Generally, K and Th are most abundant in clays, and the uranium-corrected gamma-ray log (CGR, HCGR) can be used to estimate the clay content of the formation. This was not the case, however, in the Woodlark Basin, where sandy formations often correspond to high gamma-ray counts as described in the site chapters.
The HLDS uses a chemical gamma-ray source (0.66 MeV, 137Cs) to induce a back-scattered flux of gamma rays that is measured at fixed distances from the source. It also records a photoelectric effect (PEFL). The radioactive source and two detectors are located in a skid pressed against the borehole wall by a spring-loaded caliper arm.
Attenuation of these induced gamma rays is mainly caused by Compton scattering and is thus controlled by the density of electrons in the formation. Formation density (RHOM) is extrapolated by assuming that the atomic weight of most rock-forming elements is approximately twice the atomic number. This density is converted to density porosity (DPHI) using the formula
where dw is the seawater density taken as 1.03 g·cm-3, and dm is the grain density taken as 2.71 g·cm-3, which corresponds to a limestone matrix.
Photoelectric absorption occurs at low energy levels (<150 keV) and depends on the energy of the incident gamma ray, the atomic cross section, and the nature of the atom. The PEFL measurement is almost independent of porosity and, therefore, can be used directly as a matrix lithology indicator.
Excessive roughness of the borehole wall allows drilling fluid between the skid and the formation and induces inaccuracy in the density measurement. Delta-rho (DRH) is a correction that attempts to compensate this effect; its magnitude is a good indicator of hole rugosity and measurement quality.
The APS contains an accelerator that uses high voltage to produce 14-MeV fast neutrons from tritium. Five detectors then measure the population of formation-backscattered neutrons.
The neutrons must be reduced to the epithermal energy level (0.4-10 eV) by scattering before they can be detected. Because the scattering cross section for hydrogen is about 100 times larger than that of any other common element in the crust, most energy dissipation is related to hydrogen content. The measured neutron population is closely related to porosity because hydrogen occurs primarily in pore fluids. However, the log will overestimate porosity in hydrous minerals because hydrogen also occurs as bound water in hydrous minerals such as clay. There is also an effect caused by the borehole fluid, but it is minimized by using a bowspring to push the tool against the formation and by applying an empirical correction using caliper data. The theoretical vertical resolution is 0.25 m (Table T8), but it can be degraded by poor contact with the formation in enlarged boreholes.
When the neutrons reach thermal energy levels (0.025 eV) they are captured and absorbed by atomic nuclei such as hydrogen, chlorine, silicon, and boron. Sigma (SIGF) is a measurement of the rate at which this absorption takes place; it is controlled primarily by the salinity of the formation fluid and the porosity.
The lithologic analysis primarily uses four measurements: natural gamma ray, neutron porosity, DPHI, and PEFL.
Generally, natural gamma-ray magnitude is proportional to clay content. However, the relationship between near-array porosity (APLC) and DPHI can also be used to examine clay content as described below, if it is certain that the formation does not contain significant amounts of dolomite.
Both porosity measurements, DPHI and APLC, are based on the assumption that the matrix is pure calcium carbonate. In this situation, both curves will read true porosity and will overlay perfectly. When the matrix is other than this, an error is induced on the output porosity. Fortunately, the induced errors act oppositely on the two curves. For instance, in a pure sand matrix DPHI will be too high and APLC will be too low, resulting in a total separation of about 0.05. In dolomite or clay, the effect is reversed; DPHI reads too low, whereas APLC reads too high. The separation in some clays can be as large as 0.40.
The PEFL measurement brings additional information. A pure limestone matrix reads about 5.2 barns/e-, whereas a pure sandstone reads 1.8 barns/e-. Clays tend to be intermediate, as are dolomites. The presence of iron-bearing minerals may cause the PEFL to spike to values as high as 17.
It should be noted that very few formations are pure enough in matrix to achieve the log responses described above. What typically appears on the log is a volumetric average of the constituents of the formation. Reconstructing the original volumes of these constituents is not just a tricky job--it's an art!
The FMS was introduced by Schlumberger in 1986 to measure the electrical resistivity of the borehole with an array of sensors sufficiently dense to produce high-resolution borehole resistivity images (Ekstrom et al., 1986). Because ODP operations require logging tools to pass through drill pipe (3.8 in), a modified, smaller diameter tool was specifically designed by Schlumberger and introduced on ODP Leg 126 (Shipboard Scientific Party, 1990a, 1990b; Pezard et al., 1990). The FMS used in ODP has four orthogonal pads that are pressed against the borehole wall, each with an array of 16 electrodes. The electrode spacing, together with a vertical sampling distance of 2.5 mm and processing that corrects the offset rows to one level, yields a vertical and horizontal resolution of 5 mm. However, the size threshold of a detectable feature depends on the contrast in conductivity and can be on the order of microns. The 16 conductivity traces from one pad are displayed side by side to produce an image. The four images (one from each pad) typically cover only about 25% of the borehole. The coverage can sometimes be increased by running the tool twice, but both images may overlap in elliptical holes. Proper pad contact with the formation is ensured only in holes with diameter smaller than 37 cm. The electrode currents probe the conductivity of the rock to a depth of a few centimeters into the borehole wall; therefore, they respond to such variations in physical and chemical properties of the rock as porosity or surface conduction when conductive clay minerals such as smectites are present.
Initial processing of the data into images was conducted aboard ship using proprietary Schlumberger GeoFrame software on a Sun workstation and included speed correction and depth shifting. Further processing and interpretation of the FMS images were performed postcruise and included a differential depth shift. Directional data that are recorded by the GPIT allow orientation of these images with respect to geographic north.
Applications of the FMS images include identification and mapping of fractures, faults, and bedding contacts, determination of strike and dip of structures, lithologic discrimination, detailed correlation of coring and logging depth, and orientation of cores (Serra, 1989; Pezard and Anderson, 1990; Pezard et al., 1992; MacLeod et al., 1992). The FMS can also be used to determine borehole geometry from the precise measurement of borehole diameter by its two orthogonal calipers.
The UBI, a Schlumberger evolutionary descendant of the borehole televiewer (BHTV; Zemanek, Caldwell, et al., 1970; Zemanek, Glenn, et al., 1970), produces an acoustic image of the borehole wall. The BHTV was first introduced to DSDP during Leg 83 (Newmark, Anderson, et al., 1985) but the UBI was first introduced to ODP during Leg 180. A focussing rotating transducer emits either 250- or 500-kHz ultrasonic pulses that are directed toward the borehole wall. Amplitude and traveltime of the reflected signals are then recorded either 140 or 180 times per rotation. The 7.5 revolutions per second allow upward movement of 3.6 cm·s-1 (logging speed of 130 m/hr) while sampling vertically every 5 mm. This, combined with a footprint of 4 mm (at 500 kHz) or 9 mm (at 250 kHz), allows the entire borehole wall to be scanned with a resolution of ~5 mm for hole diameters ~30 cm. Data are presented in the form of unwrapped images of the borehole wall both in amplitude and radius. Directional data recorded by the GPIT allow these images to be oriented geographically.
The amplitude of the reflected signal depends essentially on the roughness of the borehole wall, but also on the reflection coefficient of the fluid-rock interface, the position of the UBI tool in the borehole, and the shape of the borehole. Fractures or lithologic variations in the drilled rocks can easily be recognized in the amplitude image and thus help to reorient cores. The radial images give detailed information about the shape of the borehole and can be considered as very sophisticated caliper data. The radius data can thus be used to construct horizontal borehole cross sections. These data cover the borehole wall completely, whereas the FMS data cover the borehole wall only partially.
The GPIT, which is part of the FMS and UBI string, contains a three-component accelerometer from which the hole inclination (DEVI) and tool relative bearing (RB) are derived. The RB is the rotational orientation of the tool with respect to the high side of the hole. Additionally, the accelerometer data may be used to correct the log for irregular tool motion caused by sticking or ship heave.
The GPIT also contains a three-component magnetometer that measures the rotational orientation of the tool with respect to magnetic north (P1AZ). The local magnetic declination is entered in the logging software so that the images are oriented with respect to the geographic north. The P1AZ and RB measurements are combined to give the azimuth of a deviated hole (HAZI).
Borehole geometry studies have become possible with the appearance of oriented orthogonal caliper measurements provided by the dipmeter (Cox, 1970; Babcock, 1978; Hottman et al., 1979; Schafer, 1980; Gough and Bell, 1981), the later FMS (Shamir et al., 1988; Shipboard Scientific Party, 1990b; Pezard et al., 1992), and the higher resolution measurements of the borehole televiewer (Zemanek, Caldwell, et al., 1970; Zemanek, Glenn, et al., 1970). These studies have often revealed borehole elongations that can have multiple origins (Beaudemont et al., 1988; Guenot, 1989; Dart and Zoback, 1989).
However, most attention during the last 20 yr has focused on the relationship between the anisotropy of the horizontal state of stress and a particular type of borehole elongation termed breakouts (Babcock, 1978).
Breakouts are characterized by seven conditions. They were defined by Babcock (1978) as zones (1) of elongation, where one diameter is significantly greater than the other (by at least 1 cm); (2) that begin and end abruptly; (3) that persist for a significant depth interval (larger than the pad height); (4) that slow or stop the natural clockwise rotation of the tool; and (5) where the smallest measured caliper is equal to the drill-bit size. Plumb and Hickman (1985) added that breakouts (6) are symmetrically elongated and (7) correspond to high electrical conductivity zones.
Breakouts are induced by remote anisotropic stress concentration around the borehole for three main reasons. First, the theoretical calculation of the state of stress around a circular hole in an elastic impervious medium (Kirsch, 1898; Hubbert and Willis, 1957; Hottman et al., 1979; Bell and Gough, 1979), a porous permeable medium (Haimson and Fairhurst, 1967) or a poroelastic medium (Detournay and Cheng, 1988) shows that the stress difference is greatest along the direction of the remote minimum horizontal principal stress, SHmin, which is the preferred site of shear failure initiation. Second, the orientation of breakouts remains consistent between different formations within a well and between wells on a basin scale. Third, the SHmin direction deduced from breakouts coincides with that deduced from other stress indicators, such as hydrofracture directions or large-scale geological features (Bell and Gough, 1979; Gough and Bell, 1981, 1982; Fordjor et al., 1983; Zoback et al., 1985; Hickman et al., 1985; Plumb and Hickman, 1985; Morin et al., 1989, 1990).
As a consequence, breakouts have been used to infer the remote direction of the minimum principal horizontal stress (Newmark et al., 1984; Newmark, Zoback, et al., 1985; Zoback et al., 1985; Hickman et al., 1985; Morin et al., 1989). Such an analysis, together with the packer hydrofracturing test, was a major component of the plan of Leg 180 toward characterizing the state of stress in the vicinity of an active normal fault.
The LDEO TLT is a self-contained tool that was attached to the base of the Schlumberger triple combo. Data from a fast and a slow thermistor and a pressure gauge are collected every second and are recorded by an internal Tattletale computer. The Schlumberger Maxis unit wireline depth vs. time is recorded independently by a surface computer. After logging, the data are transferred from the tool to a shipboard computer. The fast-response (0.5 s) thermistor, although less accurate (0.01°C resolution), is able to detect small abrupt temperature excursions caused by fluid flow from the formation. The slow-response (2.5 s) thermistor is more accurate (0.001°C resolution) and can be used to estimate the temperature gradient. Data are recorded as a function of time. Conversions to depth were done by correlating the time between the wireline time-depth data obtained from the Schlumberger Maxis unit and the time-temperature pressure data from the TLT. The depth shift applied to obtain mbsf takes into account the fact that the TLT sensors are 1.3 m below the wireline zero.
Because logging is conducted soon after water circulation during drilling, the water temperatures measured by the TLT are not in equilibrium with the formation temperatures; thus, it is common to observe a gradual warming of the TLT temperatures as logging proceeds. Therefore, the temperatures taken from the last logging run are the closest to formation temperature, but even these should be considered minimum estimates. The measured temperatures are therefore extrapolated to equilibrium formation temperature whenever possible, as described in the site chapters. The geothermal gradient obtained from these and other in situ temperature data, coupled with the conductivity measurements taken on core samples, yield an estimate of heat flow at the well.
Downhole data quality may be seriously degraded in excessively large diameter sections of the borehole or by rapid changes in the hole diameter. Electrical resistivity and velocity measurements are less sensitive to such borehole effects. The nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays) are the more seriously impaired because they rely on direct tool contact with the formation. However, processing can reduce these borehole effects to some extent.
Different logs may have small depth mismatches caused by cable stretch or ship heave during recording. To minimize such errors, a wireline hydraulic heave compensator adjusts for ship motion during logging. The natural gamma-ray tool is incorporated in all tool strings whenever possible to provide a common basis for log correlations.
