The downhole logging program during Leg 209 was specifically designed for determining the orientation of faults and fractures as well as characterizing deformation and alteration features of the upper mantle and lower crustal rock sequences drilled north and south of the 15° 20'N fracture zone. Several wireline and logging-while-drilling (LWD) tools were deployed, as described below.
Individual logging tools were joined together into tool strings so that several measurements can be made during each logging run. The tool strings were lowered to the bottom of the borehole on a wireline cable, and data were logged as the tool string was pulled back up the hole. Repeat runs were made to improve coverage and confirm the accuracy of log data. Not all tool strings were run in each hole; refer to individual site chapters for details of logging strings deployed at each site. During Leg 209, the following logging strings were successfully deployed in Holes 1272A and 1275D (Fig. F15; Table T11):
The properties measured by each tool, sampling intervals, and vertical resolutions are summarized in Table T11. Explanations of tool name acronyms and their measurement units are summarized in Table T12. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the LDEO–Borehole Research Group (BRG) Wireline Logging Services Guide (WLSG) (2001).
The HNGS measures natural gamma radiation from isotopes of potassium, thorium, and uranium and uses a five-window spectroscopic analysis to determine concentrations of radioactive potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The HNGS also provides a measure of the total gamma ray emission (SGR) and uranium-free or computed gamma ray emission (CGR) that are measured in American Petroleum Institute units (gAPI). The HNGS response is influenced by the borehole diameter.
Processing of HNGS data at LDEO-BRG corrects for borehole diameter variations. The SGT uses a sodium iodide scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of potassium, uranium, and thorium concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging strings. It is included in all tool strings (except the triple combo, where the HNGS is used) to provide a reference log to correlate depth between different logging runs.
The HLDS consists of 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 both Compton scattering and photoelectric absorption. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is related to bulk density. Porosity may also be derived from this 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. As the PEF depends on the atomic number of the elements in the formation, it is essentially independent of porosity. Thus, the PEF varies according to the chemical composition of the formation. Some examples of PEF values are provided in Table T13. The PEF values can be used in combination with HNGS curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for high-quality HLDS logs. Poor contact results in underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.
The APS consists of a minitron neutron generator that produces fast neutrons (14.4 MeV) and five neutron detectors (four epithermal and one thermal) positioned at different spacings along the tool. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted high-energy (fast) neutrons are slowed down by collisions with atoms. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The largest energy loss occurs when the neutron strikes a nucleus of equal mass, such as hydrogen, which is mainly present in pore water. Once neutrons degrade to thermal energies (0.025 eV), they may be captured by the nuclei of silicon, chlorine, boron, and other elements, with the associated emission of a gamma ray. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and the neutron arrival times, which act as a measure of formation porosity. However, hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, so the raw porosity value is often an overestimate.
The DIT-E provides three different measurements of electrical resistivity, each with a different depth of penetration into the formation. Two induction devices (deep and medium resistivity) transmit 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 measured by the receiving coils. The measured conductivities are then converted to resistivity. A third device, a spherically focused resistivity instrument, gives higher vertical resolution, as it measures the current necessary to maintain a constant voltage drop across a fixed interval.
The TAP is a "dual-application" logging tool (i.e., it can operate either as a wireline tool or as a memory tool using the same sensors). Data acquisition electronics are dependent on the purpose and required precision of logging data. During Leg 209, the TAP was deployed as a memory tool in low-resolution mode; data were stored in the tool and downloaded after the logging run was completed. Temperatures determined using the TAP do not necessarily represent in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole.
The monopole source of the DSI generates compressional, shear, and Stoneley waves into hard formations. The configuration of the DSI also allows recording of cross-dipole waveforms. In many cases the dipole sources can also provide 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 velocity data from the DSI together with the formation density can be used to generate a synthetic seismogram.
The FMS produces high-resolution images of borehole wall microresistivity that can be used for detailed lithostratigraphic or structural interpretation. This tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole walls. Good contact with the borehole wall is necessary for acquiring good-quality data. Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. Coverage may be increased by a second run. The vertical resolution of FMS images is ~5 mm. The resistivity measurements are converted to color or gray-scale images for display. In site chapters in this volume, local contrasts in FMS images were improved by applying dynamic normalization to the FMS data. A linear gain is applied, which keeps a constant mean and standard deviation within a sliding window of 1 m. FMS images are oriented to magnetic north using the GPIT, assuming that the GPIT can locate magnetic north in magnetite-rich environments (see "General Purpose Inclinometer Tool" below for more details). This method allows the dip and strike of interpreted geological features intersecting the hole to be measured from processed FMS images.
The GPIT is included in the FMS-sonic tool string to calculate tool acceleration and orientation during logging. Tool orientation is defined by three parameters: tool deviation, tool azimuth, and relative bearing. The GPIT utilizes a three-axis inclinometer and a three-axis fluxgate magnetometer to record the orientation of the FMS images as the magnetometer records the magnetic field components (Fx, Fy , and Fz). Corrections for cable stretching and/or ship heave using acceleration data (Ax, Ay , and Az) allow precise determinations of log depths. A hydraulic wireline heave compensator designed to adjust for rig motion during logging operations minimizes ship heave.
In magnetic-rich rocks such as serpentinites or oxide-rich gabbros, there is some uncertainty to the orientation of the GPIT. We compared downhole GPIT data with shipboard determinations of remanent magnetization and magnetic susceptibility. In this way, we were able to evaluate whether assumptions about remanence (no tectonic rotation around a vertical axis since magnetization), relative abundance of magnetic minerals, and GPIT measurements (magnetometer can determine the direction of magnetic north) yielded consistent results (see the "Site 1272" and "Site 1275" chapters).
Data for each wireline logging run were recorded and stored digitally and monitored in real time using the Schlumberger MAXIS 500 system. After logging was completed in each hole, data were transferred to the shipboard downhole measurements laboratory (DHML) for preliminary processing and interpretation. FMS image data were interpreted using Schlumberger's Geoframe software package (version 3.8).
Logging data were also transmitted to LDEO-BRG using a satellite high-speed data link for processing soon after each hole was logged. Data processing at LDEO-BRG consists of
Once processed at LDEO-BRG, logging data were transmitted back to the ship, providing near–real time data processing. Processed data were then replotted on board (see the "Downhole Measurements" sections in each site chapter). Further postcruise processing of the logging data from the FMS is performed at LDEO-BRG. Postcruise-processed data (in ASCII) are available directly from the LDEO-BRG Internet World Wide Web site at www.ldeo.columbia.edu/BRG/ODP/DATABASE. A summary of "logging highlights" is posted on the LDEO-BRG Web site at the end of each leg.
Logging data quality may be seriously degraded by changes in hole diameter and in sections where the borehole diameter greatly decreases or is washed out. Deep-investigation measurements such as resistivity and sonic velocity are least sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data in order to reduce these effects. HNGS and SGT data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have depth mismatches caused by either cable stretch or ship heave during recording.
During Leg 209, two LWD resistivity tools were available: the Resistivity-at-the-Bit (RAB) and Resistivity-at-the-Bit while Coring (RAB-C) systems. Only the RAB-C was used for Hole 1275C. Schlumberger Drilling and Measurements provided these tools under contract with LDEO-BRG. LWD surveys have been successfully conducted during eight previous ODP legs: Leg 156 (Shipley, Ogawa, Blum, et al., 1995), Leg 170 (Kimura, Silver, Blum, et al., 1997), Leg 171A (Moore, Klaus, et al., 1998), Leg 174A (Austin, Christie-Blick, Malone, et al., 1998), Leg 188 (O'Brien, Cooper, Richter, et al., 2001), Leg 193 (Binns, Barriga, Miller, et al., 2002), Leg 196 (Mikada, Becker, Moore, Klaus, et al., 2002), and Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003).
LWD tools measure in situ formation properties with instruments that are located in the drill collars immediately above the drill bit. LWD data were recorded into downhole computer memory and retrieved when the tools reach the surface. Measurements were made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. Fluid invasion into the borehole wall was also reduced relative to wireline logging because of the shorter elapsed time between drilling and taking measurements.
The LWD equipment is battery powered and uses nonvolatile memory chips to store logging data until they are downloaded. The LWD tools took measurements at evenly spaced time intervals and were synchronized with a system on the drilling rig that monitors time and drilling depth. After drilling, the LWD tools were retrieved and the data were downloaded from each tool using a DB-25 485 cable serial link to a processing computer in the DHML. Synchronization of the uphole and downhole clocks allows merging of the time-depth data (from the surface system) and the downhole time-measurement data (from the tools) into depth-measurement data files. The resulting depth-measurement data were transferred to the processing systems in the DHML for reduction and interpretation.
Unlike wireline tools, which record data vs. depth, LWD tools record data vs. time. Schlumberger's Drilling and Measurements Integrated Drilling Evaluation and Logging system (IDEAL) records the time and the depth of the drill string below the rig floor. LWD operations aboard the JOIDES Resolution require accurate and precise depth tracking and the ability to independently measure and evaluate the movement of the following:
RAB tools provide resistivity measurements of the formation and electrical images of the borehole wall, similar to the Formation MicroScanner but with complete coverage of the borehole walls and lower vertical and horizontal resolution. In addition, the RAB tool contains a thallium-doped sodium iodine scintillation detector that provides a total gamma ray measurement (Fig. F16). Because a caliper log is not available without other LWD measurements, the influence of the shape of the borehole on the log responses can only be estimated by a qualitative comparison of the shallow, medium, and deep button images.
The RAB tools are connected directly above the drill bit, and they use the lower portion of the tool and the bit as a measuring electrode. This allows the tool to provide a bit resistivity measurement with a vertical resolution just a few inches longer than the length of the bit. A 2.5-cm electrode is located 91 cm from the bottom of the tool and provides a focused lateral resistivity measurement (RRING) with a vertical resolution of 5 cm. The characteristics of RRING are independent of where the RAB tool is placed in the bottom-hole assembly (BHA), and its depth of investigation is ~18 cm. In addition, button electrodes provide shallow-, medium-, and deep-focused resistivity measurements as well as azimuthally oriented images. These images can then reveal information about formation structure and lithologic contacts. The button electrodes are ~2.5 cm in diameter and reside on a clamp-on sleeve. The buttons are longitudinally spaced along the RAB tool to render staggered depths of investigation of ~2.5, 7.6, and 12.7 cm. The tool's orientation system uses the Earth's magnetic field as a reference to determine the tool position with respect to the borehole as the drill string rotates, thus allowing both azimuthal resistivity and gamma ray measurements provided that azimuth can be measured correctly in serpentinized peridotites with a few weight percent magnetite. Resistivity measurements are acquired with ~6° resolution as the RAB tool rotates, and gamma ray profiles are acquired in quadrants and can only be referenced to the top of the hole. The sampling interval for each resistivity measurement is shown in Table T11.
Two RAB tool collar configurations were designed for use during Leg 209. The RAB tool used in the 63/4-in LWD BHA is called the GeoVision resistivity, which is the most recent upgrade of the tool, and was not used during Leg 209. A RAB-8 tool collar configuration for use with a larger 8-in-diameter BHA was deployed at Site 1275. The design is intended for use in conjunction with a modified motor-driven core barrel core-liner system, allowing RAB measurements to be made while coring (RAB-C; Fig. F16). Hole 1275C was the second attempt to core and record LWD data simultaneously (the first was during Leg 204). The RAB-8 tool was deployed alone in the BHA, and the standard 25-cm-diameter rotary bit was used. The diameter of the measuring button sleeve for the RAB-8 tool is 24.1 cm; thus, borehole size resistivity correction factors are negligible for all of the RAB-8 measurements, including the shallow button resistivity. The RAB-8 tool provides electrical images of the borehole wall, similar to the FMS, and allows for recovering cores with a 6.51 cm diameter. The vertical resolution of the images is lower than that of the FMS (Table T11) but the images have complete coverage of the borehole walls.
All logging data were collected at a minimum vertical sampling density of 15 cm whenever possible; hence, a balance must be determined between the rate of penetration (ROP) and the sampling rate. This relationship depends on the recording rate, the number of data channels to record, and the memory capacity of the LWD tool. The RAB-8 tool was programmed with a sampling rate of 10 s for data acquisition and a memory capacity of 5 MB. The maximum ROP allowed to produce one sample per 15.24 cm interval is given by the equation
This relationship gives 55 m/hr maximum ROP for the RAB-8 tool. During Leg 209, the target ROP was 1–5 m/hr, roughly 1%–5% of the maximum allowable for the RAB-8 tool. These reduced rates improve the maximum vertical resolution of the resistivity images to 3 cm. Under this configuration, the RAB-8 tool was able to record as long as 30 hr, which was sufficient to complete the LWD operations at Site 1275 (see "Downhole Measurements" in the "Site 1275" chapter).
For the bit resistivity measurements, a lower transmitter produces a current and a monitoring electrode located directly below the ring electrode measures the current returning to the collar. The resultant resistivity measurement is termed RBIT, and its depth of investigation is ~30.5 cm.
The upper and lower transmitters produce currents in the collar that meet at the ring electrode. The sum of these currents is then focused radially into the formation. These current patterns can become distorted depending on the strength of the fields produced by the transmitters and the formation around the collar. Therefore, the RAB-8 tool uses a cylindrical focusing technique that takes measurements in the central and lower monitor coils to reduce distortion and create an improved ring response. The ring electrode is held at the same potential as the collar to prevent interference with the current pattern. The current required for maintaining the ring at the required potential is then measured and related to the resistivity of the formation. Because the ring electrode is narrow (~4 cm), the result is a measurement (RRING) with 5-cm vertical resolution.
The button electrodes function the same way as the ring electrode. Each button is electrically isolated from the body of the collar but is maintained at the same potential to avoid interference with the current field. The amount of current required to maintain the button at the same potential is related to the resistivity of the mud and formation. The buttons are 4 cm in diameter, and the measurements (RBUTTON) can be acquired azimuthally as the tool rotates within 56 sectors to produce a borehole image.
Structural data were determined from RAB images using Schlumberger's GeoFrame software. GeoFrame presents RAB data as a planar "unwrapped" 360° resistivity image of the borehole with depth. The image orientation is referenced to north, which is measured by the magnetometers inside the tool, and the hole is assumed to be vertical. Horizontal features appear horizontal on the images, whereas planar, dipping features are sinusoidal in aspect. Sinusoids are interactively fitted to beds and fractures to determine their dip and azimuth, and the data are exported from GeoFrame for further analysis.
Methods of interpreting structure and bedding differ considerably between core analysis, wireline FMS images, and RAB image analysis. Resolution is considerably lower for RAB image interpretation (3 cm at best, compared with <1 mm within cores and 0.5 cm for FMS images), and therefore identified features are likely to be different in scale. For example, microfaults (width < 1 mm) and shear bands (width = 1–2 mm or up to 1 cm) can only be interpreted from core analysis and, sometimes, from FMS data. This should be considered when directly comparing reports. The RAB tool provides 360° coverage at a lower resolution; FMS provides higher-resolution data, but coverage is restricted to ~30% of the borehole wall. Fractures can be identified in RAB images by their anomalous resistivity or conductivity and from contrasting dip relative to surrounding foliation trends. Differentiating between fractures and foliation planes can be problematic, particularly if both are steeply dipping and have similar orientations.
During processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12 in) and/or irregular borehole diameter affects most recordings, particularly the HLDS, which requires eccentralization and good contact with the borehole wall. Hole deviation can also negatively affect the data; the FMS, for example, is not designed to be run in holes with >10° deviation, as the tool weight might cause the caliper to close.
FMS image processing is required to convert the electrical current in the formation, emitted by the FMS button electrodes, into a gray or color-scale image representative of the conductivity changes. This is achieved through two main processing phases: data restoration and image display. During the data restoration process, speed corrections, image equalization, button correction, emitter exciter (EMEX) voltage correction, and depth-shifting techniques are applied to the data.
Speed corrections use the data from the z-axis accelerometer to correct the vertical position of the data for variations in the speed of the tool (i.e., GPIT speed correction), including tool sticking and slipping. In addition, an image-based speed correction is also applied to the data. This correction checks the GPIT speed correction. If the correction is successful, the readings from the two rows of buttons on the pads will line up. If not, the readings will be offset from each other, creating a zigzag effect on the image.
Image equalization is the process whereby the average response of all the buttons of the tool are rendered approximately the same over large intervals, to correct for various tool and borehole effects that affect individual buttons differently. These effects include differences in the gain and offset of the pre-amplification circuits associated with each button and differences in contact with the borehole wall between buttons on a pad and between pads. If the measurements from a particular button are unreasonably different from adjacent buttons (e.g., "dead buttons") over a particular interval, they are declared faulty, and the defective trace is replaced by traces from adjacent good buttons. The button current response is controlled by the EMEX voltage, which is applied between the button electrode and the return electrode. The EMEX voltage is regulated to keep the current response within the operating range. The button response is divided by the EMEX voltage, and, as a result, the response corresponds more closely to the conductivity of the formation.
Each of the logging runs are depth-matched to a common scale by means of lining up distinctive features of the natural gamma log from each of the tool strings. If the reference-logging run is not the FMS tool string, the specified depth shifts are applied to the FMS images. The position of data located between picks is computed by linear interpolation.
Once the data are processed, both static and dynamic images are generated. In static normalization, a histogram equalization technique is used to obtain the maximum quality image. In this technique, the resistivity range of the entire interval of good data is computed and partitioned into 256 color levels. This type of normalization is best suited for large-scale resistivity variations. The image can be enhanced when it is desirable to highlight features in sections of the borehole where resistivity contrasts are relatively low when compared with the overall resistivity range in the section. This enhancement is called dynamic normalization, and by rescaling the color intensity over a smaller interval, the contrast between adjacent resistivity levels is enhanced. It is important to note that with dynamic normalization, resistivities in two distant sections of the hole cannot be directly compared with each other. A 2-m normalization interval was used.
The FMS images are displayed as an unwrapped borehole cylinder with a circumference derived from the bit size. Several passes can be oriented and merged together on the same presentation to give additional borehole coverage where the tool pads followed a different track during the second logging pass. A dipping plane in the borehole can be displayed as a sinusoid on the image and the amplitude of this sinusoid is proportional to the dip of the plane. The images are oriented with respect to north; hence, the strike of dipping features can also be determined.