m,
such as sediments. In more resistive formations measurement error becomes
significant (>20%), and it is more suitable to use the DLL (Schlumberger,
1989b).Downhole logs are spatially continuous records of physical, chemical, and structural properties of the formation penetrated by a borehole. The logs are made using a variety of probes combined into several tool strings. These strings are lowered down the hole on a heave-compensated electrical wireline and then pulled up at constant speed to provide continuous measurements as a function of depth of several properties simultaneously. Logs can be used to interpret 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. Finally, 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 to calibrate 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.
Three logging tool strings were used during Leg 185: the triple combination tool (triple combo), the Formation MicroScanner (FMS)/sonic, and the geochemical logging tool. In addition to wireline logs, in situ temperature measurements were made with the Adara tool, which is located in the coring shoe of the APC during piston-coring operations.
The tool strings used on Leg 185 were
Each tool string includes a telemetry cartridge for communicating through the wireline with the logging laboratory on the drillship, and a natural gamma-ray sonde that is used to identify lithologic markers that provide a common reference for correlation and depth shifting between multiple logging runs. Logging runs are typically conducted at 250-275 m/hr, except for the GLT, which is run at ~150 m/hr.
The logging tools are briefly described below, and their operating principles, applications, and approximate vertical resolution are summarized in Table T6. Some of the principal data channels of the tools, their physical significance, and units of measure are listed in Table T7. 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 (1989a, 1989b, 1994), Serra (1984, 1986, 1989), and the LDEO-Borehole Research Group (BGR) Wireline Logging Services Guide (1994).
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 scintillation detector and five-window spectroscopy 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 scintillation detectors for a significantly improved tool precision. Spectral analysis in the HNGS filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy.
Although the NGT response is sensitive to borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud, these effects are routinely corrected for during processing at LDEO.
Formation density was determined from the density of electrons in the formation, which was measured with the hostile environment lithodensity 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. Gamma rays emitted by the source experience Compton scattering, which involves the transfer of energy from gamma rays to the electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is in turn related to bulk density. Porosity may also be derived from the bulk density, if the matrix density is known.
The HLDS 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 barn/e-; illite = 3.03 barn/e-; quartz = 1.81 barn/e-; and kaolinite = 1.49 barn/e-. PEF values can be used in combination with NGT curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values.
The depth of investigation of the lithodensity tool is on the order of tens of centimeters, depending on the density of the rock.
Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator that produces fast (14.4 MeV) neutrons, and five neutron detectors (four epithermal and one thermal), positioned at different spacings. The tool is pressed against the borehole wall by an eccentralizing bow spring. Emitted neutrons are slowed down by collisions. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greater energy loss occurs when the neutron strikes a nucleus nearly equal to its own mass, such as hydrogen, which is present mainly in the pore water. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times that 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 tools were used to
measure the formation electrical resistivity: the DIT-E and the DLL. The DIT-E
provides three measurements of electrical resistivity, each with a different
depth of investigation into the formation. Deep- and medium-penetration
measurements are made inductively using transmitter coils that are energized
with high-frequency alternating currents, creating time-varying magnetic fields
that induce secondary Foucault currents in the formation. The strength of these
induced ground currents is inversely proportional to the resistivity of the
formation through which they circulate, as are the secondary inductive fields
that they create. The amplitude and phase of the secondary magnetic fields,
measured with receiving coils, are used as a proxy for the formation resistivity.
Shallow-penetration measurements with a high vertical resolution are made with a
spherically focused laterolog. This measures the current necessary to maintain a
constant voltage drop across a small fixed interval. Because of the inductive
nature of the deep- and medium-penetration measurements, DIT-E logs are accurate
only for formations with resistivities less than ~100 m,
such as sediments. In more resistive formations measurement error becomes
significant (>20%), and it is more suitable to use the DLL (Schlumberger,
1989b).
The DLL provides two
measurements of formation electrical resistivity, labeled "deep" (LLd)
and "shallow" (LLs) on the basis of their respective depths of
investigation. In both devices, a current beam 61 cm thick is forced
horizontally into the formation by using focusing (also called bucking)
currents. For the deep measurement both focusing and measurement currents return
to a remote electrode on the surface; thus, the depth of investigation is
considerable, and the effect of borehole conductivity and of adjacent formations
is reduced. In the shallow laterolog, the return electrodes that measure the
bucking currents are located on the sonde, and, therefore, the current sheet
retains focus over a shorter distance than the deep laterolog. Fracture porosity
can be estimated from the separation between the deep and shallow measurements
based on the observation that the deep measurement is sensitive to the presence
of horizontal conductive fractures only, whereas the shallow measurement
responds to both horizontal and vertical conductive structures. The DLL has a
response range of 0.2-40,000 m.
The depth of investigation
of both the DIT-E and the DLL depends on the resistivity of the rock and on the
resistivity contrast between the zone invaded by drilling fluid and the
uninvaded zone. In formations with resistivity higher than 100 m
the average radial depth of investigation of the DIT-E is ~1.5 m for the deep
induction (IDPH), 76 cm for the medium induction (IMPH), and 38 cm for the
spherically focused log. These values drop by ~20% for a 0.1 m
formation resistivity. The depth of investigation of the DLL will vary with the
separation between the sonde and the remote current return at the surface.
In most rocks, electrical
conduction occurs primarily by ion transport through pore fluids and is strongly
dependent on porosity. Electrical resistivity data can therefore be used to
estimate formation porosity using Archie's Law (Archie, 1942) if the formation
does not contain clay. Archie's Law is expressed as FF
= a
-m,
where FF is the
formation factor (i.e., the ratio of the formation resistivity to that of the
pore fluids),
is the porosity, m is
known as the cementation factor and is dependent on the tortuosity and
connectivity of pore spaces, and a
is a constant that varies based upon rock type. For a first order
interpretation, a is
conventionally taken as 1 and m
as 2, but more rigorous values can be determined from resistivity and porosity
measurements on core samples (see "Physical
Properties"). For example, laboratory measurements from core
samples from DSDP Hole 504B led to values of a
= 10.0 and m = 1.0 in
the massive units of layers 2A and 2B, indicating that current conduction was
occurring in cracks and microcracks present at mineral scale throughout the rock
(Pezard, 1990). In the presence of chlorites, zeolites, and particularly
smectites as alteration products of mid-ocean-ridge basalt, surface conduction
through the Stern-Gouy layer may also play a significant role. Archie's Law will
overestimate the porosity in this case, and a more sophisticated conductivity
model should be used (Pezard, 1990).
The DIT-E also measures spontaneous potential (SP) fields. SPs can originate from a variety of causes—electrochemical, electrothermal, electrokinetic streaming potentials, and membrane potentials—related to differences in the mobility of ions in the pore and drilling fluids. The interpretation of SP logs remains problematic because of this multiplicity of sources.
Downhole temperature, acceleration, and pressure were measured with the LDEO high-resolution TAP tool. When attached to the bottom of the triple-combo string, the TAP 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 that should eventually lead to the fine tuning of the wireline heave compensator (WHC).
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. Packer tests in Hole 801C during Leg 144 indicated a region of high permeability beneath the hydrothermal mineralization. Temperature logs in such environments can help differentiate between convective and conductive heat transfer regimes.
The temperature record must be interpreted with caution, because 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, which is conventionally used on the FMS/sonic string, was not functional during Leg 185. The LSS was therefore used to measure elastic compressional wave velocity in the formation. The LSS provides long-spacing measurements through the "depth-derived" borehole compensation principle. Acoustic traveltime readings between two sources and two receivers are memorized at one depth and combined with a second set of readings made after the sonde has been pulled the appropriate distance along the borehole. The LSS records the full waveform for each source-receiver pair, in addition to its automatic determination of arrival time.
The depth of investigation for sonic tools depends on the spacing of the detectors and on the petrophysical characteristics of the rock, such as rock type, porosity, and alteration, but is of the order of tens of centimeters.
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," that are pressed against the borehole wall during the recording (Fig. F14). 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, which reflect the microresistivity 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.
The FMS image is sensitive to structure within ~25 cm of the borehole wall and has a vertical resolution of 5 mm with a coverage of 22% of the borehole wall on a given pass. FMS logging commonly includes two passes, the images of which are merged to improve borehole wall coverage. To produce reliable FMS images, however, the pads must be firmly pressed against the borehole wall. The maximum extension of the caliper arms is 15.0 in. In holes with a diameter >15 in, the pad contact will be inconsistent and the FMS images can be blurred. 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.
Downhole magnetic field measurements were made with the GPIT. 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 allows more precise determination of log depths than is possible on the basis of cable length alone because the wireline is subject to both stretching and ship heave. Acceleration data is also used in processing of FMS data to correct the images for irregular tool motion.
Local magnetic anomalies, generated by high remanent magnetization of the basalts in the basement section of a borehole, can interfere with the determination of tool orientation. However, these magnetic anomalies can be useful to infer the magnetic stratigraphy of the basement section.
The AACT measures the wet weight percent of aluminum in the formation. It is generally similar to the NGT, but its spectrometer includes two more windows for a more detailed analysis of the spectrum. Aluminum abundance is determined by neutron-induced gamma-ray spectroscopy using a 252Cf low-energy source (2 MeV) on the CNT-G mounted immediately above the AACT on the geochemical tool string. Aluminum activation occurs when thermal neutrons are captured by 27Al and produces 28Al. The latter decays with a half-life of 2.3 min, emitting 1.78-MeV gamma rays. The contribution to the gamma-ray spectrum from natural radiation is corrected using the counts from the NGT placed above the neutron source: when logging upward, the upper detector measures the natural radiation before activation, and the lower detector measures the induced radiation after activation. The naturally occurring component is then subtracted from the total measured after activation.
The chemical composition of the formation was measured with the GST. This is an induced gamma-ray device that measures some of the major element constituents of sedimentary and igneous rocks. The GST contains a pulsed source (minitron) of 14-MeV neutrons and a sodium iodide scintillation detector. Through scattering interactions with the atoms in the rock surrounding the borehole, the neutrons are progressively slowed until they reach thermal energy levels at which they can be captured by nuclei in the formation. When this occurs, the capturing nucleus emits a gamma ray of characteristic energy. Comparison of the spectra recorded downhole with a library of standard spectra on the ship provides an estimate of the elemental composition of the formation. The spectra are dominated by characteristic sets of gamma rays from six elements: Ca, Si, Fe, Cl, H, and S. As their sum is always unity, the results do not reflect the actual elemental composition. Instead, ratios (or yields) of these elements are used in interpreting the lithology and porosity of the formation and the salinity of the formation fluid. Shore-based processing is necessary to compute the absolute dry-weight fractions of the major oxides.
A boron sleeve surrounds the GST and increases the signal-to-noise ratio by shielding the path of fast neutrons from the borehole fluid, thus reducing the reading of iron from the tool housing. The measurement accuracy is dependent on logging speed, hole size, and porosity. The GST can be run inside the logging pipe, but corrections for the effect of the pipe are not always reliable. This through-pipe data, however, can be used from a qualitative point of view and offers useful information about the formation geochemistry in holes that otherwise could not be logged because of well-bore instability.
The quality of log data may be seriously degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. Resistivity and velocity measurements are the least sensitive to borehole effects, whereas the nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays) are most sensitive because of the large attenuation by borehole fluid. 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.
Logs from different tool strings may have depth mismatches, caused by either cable stretch or ship heave during recording. Small errors in depth matching can distort the logging results in zones of rapidly changing lithology. 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 every log tool string, provide correlation and relative depth offsets among the logging runs and can be calibrated to distinctive lithologic contacts observed in the core recovery or drilling penetration (e.g., basement contacts). In Hole 801C, which is cased into basement, the point of entry into the bottom of the casing is also obvious on many of the tools and is a useful correlation point. 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.
Raw count rates for six elements (Ca, Si, Fe, S, Cl, and H) are obtained in real time by the Schlumberger data acquisition software. In addition to these six elements, postcruise reprocessing inverts the gamma-ray spectrum at each depth for titanium, gadolinium, and potassium. Though gadolinium is present in concentrations of only a few parts per million, its neutron capture cross section is so large that it can account for 10%-30% of the total gamma spectrum. Inclusion of these additional elements improves the quality of the overall inversion, particularly improving the accuracy of calculated calcium abundance by converting sources of unaccounted variance to signals. However, the determined potassium concentrations are less accurate than those from the NGT, and the hydrogen concentrations are less accurate than those from the neutron tool.
When both the geophysical (triple combo) and geochemical (GLT) Schlumberger tool strings are run, additional reprocessing of geochemical logs is possible. The relative abundances of Ca, Si, Fe, Ti, Al, K, S, Th, U, and Gd are used to calculate a log of predicted photoelectric effect. The difference between this log and the actual log of photoelectric effect can be attributed to the only two major elements not directly measured, Mg and Na. Major elements are converted from volume percent to weight percent using logs of total porosity (bound water + pore water) and density. Major elements are expressed in terms of oxide dry weight percent, based on the assumption that oxygen is 50% of the total dry weight.
If the GLT data are available but not enough log types are run to permit complete solution for oxide weight percentage, one further processing step is made. Omitting chlorine and hydrogen, the yields of the other geochemical tool elements (Ca, Si, Fe, Ti, S, K, and Gd) are summed, and each is expressed as a fraction of this total yield. This procedure corrects for porosity and count-rate variations. Although the absolute abundance of each element is not determined, downhole variations in relative abundance are indicated.
The precision and reliability of the various logging measurements are governed by the resolutions of the various tools and the condition of the drill hole. Vertical resolutions of the various logging tools is generally ~46 cm, with several exceptions (Table T6).
Core-log integration during Leg 185 involved comparing lithologies in cores with the responses of the various logs within the corresponding drilled intervals. After calibrating the logs with the core recovery on a small scale, the logs were then used to interpret details of the sequences that were not recovered during coring.
The primary logging tools used in the core-to-log integration were the FMS high-resolution microresistivity image of the hole, the NGR spectrum, and the geochemical logs.
Temperature measurements were taken at Site 1149 during Leg 185 to determine the amount of heat carried by the subducting sediments into the Izu-Bonin Trench. The discrete in situ measurements were made with the Adara tool, which is located in the coring shoe of the APC during piston-coring operations. The components include a platinum temperature sensor and a data logger. The platinum resistance temperature device is calibrated for temperatures ranging from -20° to 100°C, with a resolution of 0.01°C. In operation, the adapted coring shoe is mounted on a regular APC core barrel and lowered down the pipe by wireline. The tool is typically held for 5-10 min at the mudline to equilibrate with bottom-water temperatures and then lowered to the bottom of the drill string. Standard APC coring techniques are used, with the core barrel being fired out through the drill bit using hydraulic pressure. The Adara tool (and the APC corer) remains in the sediment for 10-15 min to obtain a temperature record. This provides a sufficiently long transient record for reliable extrapolation back to the steady-state temperature. The nominal accuracy of the temperature measurement is ~0.1°C.
