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doi:10.2204/iodp.proc.308.102.2006

Downhole measurements

In situ temperature and pressure measurements

Several downhole tools were deployed during Expedition 308 to measure in situ temperature, formation pressure, and hydraulic conductivity. Temperature and pressure data are critical for constraining sediment properties and flow fields. In addition, temperature affects sediment diagenesis and microbial activity.

Temperature, pressure, and hydraulic conductivity were measured in situ with a newly developed temperature/dual pressure (T2P) probe and the Davis-Villinger Temperature-Pressure Probe (DVTPP). In situ temperature measurements were also made using the APC temperature (APCT) tool.

Temperature/dual pressure probe

A narrow-diameter penetration probe with one temperature and two pressure sensors (T2P) was deployed during Expedition 308 to evaluate in situ fluid pressure, hydraulic conductivity, and temperature in low-permeability sediments. The T2P measures pressure and temperature at the tool tip and pressure 21 cm up-probe from the tip measurement (Fig. F10). The needle probe diameter is 0.6 cm at the tip and 3.6 cm at the upper pressure port. A 3.6 cm diameter drive tube extends 100 cm above the shaft pressure sensor where the drive tube connects with the data acquisition unit. All deployment data are stored in memory in the data acquisition unit.

The T2P interfaces with the colleted delivery system (CDS) used to deploy other downhole tools such as the DVTPP. The CDS is lowered by wireline and engages in the bottom-hole assembly (BHA). Once the CDS is engaged in the BHA, the drill string is used to push the needle shaft and drive the tube into the formation. This penetration causes pressure and temperature increases. After penetration into the formation, the drill string is raised 3–4 m uphole and the CDS telescopes. The telescope action leaves the T2P in the formation and effectively decouples the probe from the drill string. The T2P remains in the formation to measure pressure and temperature. Dissipation of pressure and temperature is used to determine in situ pressure, hydraulic conductivity, and temperature. After adequate time for dissipation (30–90 min), the wireline pulls the CDS to its extended position and then pulls the T2P out of the formation. The CDS and T2P are pulled to the surface, where the data are downloaded from the data acquisition unit.

The slim design of the T2P facilitates rapid, high-quality measurement of in situ conditions in low-permeability sediment. The design minimizes pressure and temperature pulses generated during penetration. The two pressure sensors have different dissipation rates because they are at different diameters on the tool. Comparison of the dissipation curves allows equilibrium pressure to be interpreted with less recorded dissipation than if one sensor was used.

Davis-Villinger Temperature-Pressure Probe

The DVTPP is a modified version of the Davis-Villinger Temperature Probe (DVTP) (see Davis et al., 1997; Pribnow et al., 2000; Graber et al., 2002). The DVTPP provides simultaneous measurement of formation temperature and pressure. The probe tip incorporates a single thermistor in an oil-filled needle and ports that allow hydraulic transmission of formation pressure to an internal Paroscientific pressure gauge. A standard data logger records the pressure and temperature data. Thermistor sensitivity is 0.02 K in an operating range of –5° to 20°C.

A typical DVTPP deployment consists of connecting the DVTPP to the CDS and lowering the tool string by wireline to the seafloor and then holding for 10 min. The drill bit is then raised 12 m off the bottom of the hole. Subsequently, the tool string is lowered until the CDS engages in the BHA, with the tip of the tool extending 1.1 m below the drill bit. The DVTPP is pushed into the sediment by lowering the drill bit, the CDS is telescoped to decouple the DVTPP and the drill string, and pressure is recorded for ~40 min. The tool string is then recovered via wireline. All data are stored in memory. If smooth pressure decay curves are recorded after penetration then theoretical extrapolations to in situ pore pressure are possible. Decay time is a function of the sediment permeability and the magnitude of the initial insertion pressure, which is a function of the taper angle and diameter of the tool (Whittle et al., 2001; Heeseman, 2002). Temperature decay of the frictional heat generated during insertion of the DVTPP can be used to interpret the formation temperature. Temperature decays faster than pressure.

Advanced Piston Corer Temperature tool

The APCT tool fits into the cutting shoe on the APC and is used to measure sediment temperatures during regular APC coring. The tool contains electronic components, including battery packs, a data logger, and a platinum resistance-temperature device that is calibrated from 0° to 30°C. Descriptions of the tool and of the principles behind data analysis can be found in Pribnow et al. (2000) and Graber et al. (2002). The thermal time constant of the cutting shoe assembly where the APCT tool is inserted is ~2–3 min. The only modification to normal APC procedures required to obtain temperature measurements is to hold the corer in place for ~10 min after cutting the core. During this time, the APCT tool logs temperature on an internal microprocessor. Following deployment, the data are downloaded for processing. The tool can be preprogrammed to record temperatures at a range of sampling rates. A sampling rate of 10 s was used during Expedition 308. A typical APCT measurement consists of a seafloor temperature record lasting 10 min for the first deployment in each borehole and 2 min during subsequent deployments. This is followed by frictional heating pulse when the piston is fired, and the resultant temperature decay is monitored for 10 min. A final frictional pulse is generated upon removal of the corer from the sediment.

Downhole logging

The downhole logging program during Expedition 308 was designed to

• Assess how pressure, stress, and geology control fluid migration on a passive margin;
• Establish reference geotechnical and petrophysical properties at a location where overpressure is not present as well when overpressure is present;
• Learn about factors controlling slope (in)stability;
• Determine major depositional events and timing of landslides; and
• Provide information about turbidite processes along the continental slope.

In addition, the downhole measurements plan was established to define structural and lithologic boundaries, establish site-to-site correlations, provide ties to seismic data, produce direct correlations with laboratory data, and identify conduits that may serve as pathways for fluid migration. Finally, downhole measurements complemented core measurements by filling gaps in stratigraphy, determined lithogical interpretations in intervals of poor core recovery, and provided the means for correlation with the seismic data.

Logging while drilling and measurement while drilling

During Expedition 308, four Schlumberger logging-while-drilling (LWD) and measurement-while-drilling (MWD) tools were deployed at each site in the Brazos-Trinity and Ursa Basins. LWD surveys have been successfully conducted during ODP Legs 156 (Shipley, Ogawa, Blum, et al., 1995), 170 (Kimura, Silver, Blum, et al., 1997), 171A (Moore, Klaus, et al., 1998), 174A (Austin, Christie-Blick, Malone, et al., 1998), 188 (O’Brien, Cooper, Richter, et al., 2001), 193 (Binns, Barriga, Miller, et al., 2002), 196 (Mikada, Becker, Moore, Klaus, et al., 2002), 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003), and 209 (Kelemen, Kikawa, Miller, et al., 2004).

LWD and MWD tools measure different parameters. LWD tools measure formation properties with instruments located in the drill collars above the drill bit. MWD tools are also located in the drill collars and measure downhole drilling parameters (e.g., weight on bit, torque, etc.). The difference between LWD and MWD tools is that LWD data are recorded in memory and retrieved when the tools reach the surface, whereas MWD data are transmitted through the drilling fluid within the drill pipe by means of a modulated pressure wave, or mud pulsing, and monitored in real time. MWD tools enable both LWD and MWD data to be transmitted uphole when the tools are used in conjunction. LWD is often used generically for LWD and MWD measurements.

LWD and MWD tools used during Expedition 308 included the GeoVision Resistivity (GVR) tool, PowerPulse MWD tool, Array Resistivity Compensated (ARC) tool, and Vision Density Neutron (VDN) tool. This was the first time the ARC tool was used within ODP/IODP. Figure F11 shows the configuration of the LWD/MWD BHA, and Table T1 lists the set of measurements recorded.

LWD provides measurements before the sediments experience the adverse effects of continued drilling or coring operations. Fluid invasion into the borehole wall is reduced relative to wireline logging because of the shorter elapsed time between drilling and logging.

The LWD tools are battery powered and use erasable/programmable read-only memory chips to store data until they are downloaded. The LWD tools take measurements at evenly spaced time intervals and are synchronized with a system on the drilling rig that monitors time and drilling depth. LWD data are downloaded from each tool through an RS-232 serial link to a laptop computer. 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). The resulting depth-measurement data are transferred in the downhole measurements laboratory (DHML) on board the JOIDES Resolution for reduction and interpretation.

Depth-tracking systems

LWD tools record data versus time. The Schlumberger Integrated Drilling and Logging (IDEAL) system records the time and depth of the drill string below the rig floor. As shown in Figure F12, LWD operations aboard the JOIDES Resolution require accurate and precise depth tracking and the ability to independently measure and evaluate the movement of (1) the position of the traveling block in the derrick, (2) the heave of the vessel by the action of waves/​swells and tides, and (3) the action of the motion compensator.

Motion compensator and drawworks encoders

The length of the drill string (combined lengths of the BHA and the drill pipe) and the position of the top drive in the derrick are used to determine the depth of the drill bit and rate of penetration. The system configuration is illustrated in Figure F12 and is further described below.

Drilling line is spooled on the drawworks. From the drawworks, the drilling line extends to the crown blocks, which are located at the top of the derrick, and then down to the traveling block. The drilling line is passed several times, usually six or eight times, between the traveling blocks and the crown blocks and then fastened to a fixed point called the dead-man anchor. The driller controls the drawworks, which, via the pulley system, control the position of the traveling block in the derrick.

Aboard the JOIDES Resolution, the heave motion compensator is suspended from the traveling block. The top drive is attached to the motion compensator. The motion compensator uses pressure-charged pistons to provide a buffer against waves and swell. As the vessel rises, the pressure on the pistons increases and they extend to keep the bit on bottom, whereas when the vessel drops, the pistons retract and diffuse any extra weight from being stacked on the bit.

The drill string is connected to the top drive; therefore, movement of the top drive needs to be measured to provide the drill string depth.

To measure the movement of the traveling blocks, a drawworks encoder (DWE) is mounted on the shaft of the drawworks. One revolution of the drawworks will pay out a certain amount of drilling line and, in turn, move the traveling blocks a certain distance. Calibration of the movement of the traveling block to the revolutions of the drawworks is required.

Hookload sensor

A hookload sensor measures the weight of the load on the drill string and can detect whether the drill string is disconnected from the traveling block and held fast at the rig floor (“in-slips”). When drilling ahead, the string is “out-of-slips.” When the drill string is in-slips, motion from the blocks or motion compensator will not have any effect on the depth of the bit (i.e., it will remain stationary) and the DWE information does not augment the recorded bit depth. When the drill string is out-of-slips, the DWE information augments the recorded bit depth. The difference in hookload weight between in-slips and out-of-slips is distinguishable. The heave of the ship will continue to affect the bit depth whether the drill string is in-slips or out-of-slips.

Heave motion sensors

On the JOIDES Resolution, vessel heave is measured in two ways. The rig instrumentation system measures and records the heave of the ship and the motion of the cylinder of the active compensator on the rig floor. The motion compensator cylinder extends and retracts to compensate for ship heave, which is detected by fixed accelerometers. Heave and cylinder position are transmitted to the Schlumberger recording system via the Wellsite Information Transfer System (WITS) line. Software filtering may be used to smooth the time-depth file by applying a weighted average to the time-depth data based on the observed amplitude and period of ship heave. Depth-filtering has improved the quality of GVR image logs from previous ODP holes.

GeoVision Resistivity tool

The GVR tool provides formation resistivity and electrical images of the borehole wall, similar to the Formation MicroScanner (FMS), but with complete coverage of the borehole walls and lower vertical and horizontal resolution. In addition, the GVR tool contains a scintillation counter that provides a total gamma ray measurement (Fig. F13). Because a caliper log is not available without other LWD measurements, the influence of the shape of the borehole on the log responses cannot be directly estimated.

The GVR tool is connected directly above the drill bit and uses the lower portion of the tool and the bit as a measuring electrode. This provides a bit resistivity measurement with a vertical resolution just a few centimeters longer than the length of the bit. A 2.5 cm electrode located 91 cm from the bottom of the tool provides a focused lateral resistivity measurement (RES_RING) with a vertical resolution of 5 cm. The characteristics of RES_RING are independent of where the GVR tool is placed in the BHA; its depth of investigation is ~18 cm. Button electrodes provide shallow-, medium-, and deep-focused resistivity measurements and azimuthally oriented images. These images reveal information about 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 GVR tool to render staggered depths of investigation of ~2.5, 7.6, and 12.7 cm. The tool's orientation system uses Earth's magnetic field as a reference to determine tool position with respect to the borehole as the drill string rotates, thus allowing azimuthal resistivity and gamma ray measurements. Measurements are acquired with a ~6° azimuthal resolution. The vertical resolution for each resistivity measurement is shown in Figure F13.

The diameter of the GVR measuring button sleeve is 23.3 cm, and the diameter of the three-cone rotary bit used during Expedition 308 is 25 cm. This results in a 1.7 cm gap, or “standoff,” between the resistivity buttons and the formation. The standoff causes the formation resistivity to be underestimated slightly, depending on the ratio between the formation and borehole fluid resistivity. For a resistivity ratio <100, as expected for Expedition 308 sites, a resistivity correction factor of as much as 4% may be applied to each GVR measurement. Estimated correction factors for the GVR tool are given in Table T2 (Schlumberger, unpubl. data [2001]). Because of its limited depth of penetration into the formation, the correction factor for shallow button resistivity cannot be constrained.

GVR programming

All data are collected at a minimum vertical density of 15 cm whenever possible; hence, a balance must be determined between rate of penetration (ROP) and sampling rate. This relationship depends on the recording rate, the number of data channels to record, and the memory capacity (46 MB) of the LWD tool. During Expedition 308, we used sampling rate of 5 s for high-resolution GVR images. The maximum ROP allowed to produce 1 sample/15 cm interval is given as follows:

This relationship gives 110 m/h maximum ROP for the GVR and 18 m/h minimum. For Expedition 308, the target ROP was 25 m/h, ~23% of the maximum ROP. This rate improves the vertical resolution of the resistivity images to 5–10 cm per rotation. Under this configuration, the GVR tool has enough memory to record 150 h of data. This was sufficient to complete the scheduled LWD operations at the Expedition 308 drill sites.

Bit resistivity measurements

For the bit resistivity measurements, a lower transmitter (T2) produced a current and a monitoring electrode (M0) located directly below the ring electrode measured the current returning to the collar (Fig. F13). When connected directly to the bit, the GVR tool used the lower few inches of the tool as well as the bit as a measurement electrode. The bit resistivity measurement (RES_BIT) had a depth of investigation of 81 cm.

Ring resistivity measurements

The upper and lower transmitters (T1 and T2, respectively) produce currents in the collar that meet at the ring electrode. The sum of these currents is 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 GVR tool uses a cylindrical focusing technique that takes measurements in the central (M0) and lower (M2) 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 measured and related to the resistivity of the formation. The narrow ring electrode (~4 cm) provides a resistivity measurement (RES_RING) with 5 cm vertical resolution.

Button resistivity measurements

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 formation. The buttons are 4 cm in diameter, and the shallow-, medium-, and deep-resistivity measurements (RES_BS, RES_BM, and RES_BD, respectively) can be acquired azimuthally as the tool rotates within 56 sectors to produce a borehole image.

Array resistivity compensated tool

The ARC tool is capable of multidepth borehole-compensated real-time and memory resistivity and gamma ray measurements at 2 MHz and 400 kHz (Fig. F14). The measured resistivity utilizes electromagnetic wave propagation in the formation. It has five transmitters and two receivers, giving five phase and five attenuation resistivities for each frequency. Each measurement is compensated for borehole rugosity using mixed borehole compensation. It also contains a plateau gamma ray detector for correlation. The gamma ray measurement point for the ARC tool is 7.6 cm behind the resistivity measurement point. Also included in the collar is the annular pressure-while-drilling (APWD) sensor, located 69 cm ahead of the resistivity measure point. The APWD sensor measures borehole annulus pressure and temperature.

Measurement-While-Drilling (PowerPulse) tool

The Schlumberger MWD PowerPulse tool (Fig. F15) was deployed in combination with LWD tools. The MWD tool had previously been deployed during ODP Legs 188 (O’Brien, Cooper, Richter, et al., 2001), 196 (Mikada, Becker, Moore, Klaus, et al., 2002), and 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003). MWD tools measure downhole drilling parameters, immediately above the ARC tool (Fig. F11).

MWD data are transmitted by means of a pressure wave (mud pulsing) through the fluid within the drill pipe. The 17.15 cm diameter MWD PowerPulse tool generates a continuous mud-wave transmission within the drilling fluid and changes the phase of this signal (frequency modulation) to transmit relevant bits representing the sensor data (Fig. F16). Pressure sensors attached to a standpipe on the rig floor and on the gooseneck on the crown block measure the pressure wave pulsed by the MWD tool. Pulse rates of 12 bps were achieved during Expedition 308.

Parameters transmitted by mud pulses included downhole weight on bit (DWOB), downhole torque on bit (DTOR), torsional vibration, lateral shock, and tool stick-slip (Table T1). Measurements are made using paired strain gauges, accelerometers, and lateral shock near the base of the MWD collar. Comparison of MWD drilling parameter data, rig instrumentation system data, and ship-heave information was used to improve drilling control and to assess the quality of the recorded LWD data.

The mud pulse system also transmitted annular pressure, temperature, equivalent circulating density, gamma radiation, resistivity, density, and porosity from the LWD tools. Measurement parameters from each LWD collar were updated at rates corresponding to 15 cm to 1.5 m depth intervals, depending on the initialized values and ROP of the tool. These data confirmed the operational status of each tool and provided real-time logs for identifying lithologic contacts and potential shallow-water flow or overpressured zones.

Vision Density Neutron tool

The density section of the VDN tool uses a 1.7 Ci (Curie = 3.7 × 10¹⁰ Bq), ¹³⁷Cs gamma ray source in conjunction with two gain-stabilized scintillation detectors to provide borehole-compensated density. The detectors are located 12.7 and 30.48 cm below the source (Fig. F17). The number of Compton scattering collisions (change in gamma ray energy by interaction with the formation electrons) is related to formation density. Returns of low-energy gamma rays are converted to a photoelectric effect (PEF) value, measured in barns per electron. PEF depends on electron density and hence responds to bulk density and lithology (Anadrill-Schlumberger, 1993). It is particularly sensitive to low-density and high-porosity zones.

The gamma ray source and detectors are positioned behind holes in the fin of a full-gauge 24.5 cm diameter clamp-on stabilizer (Fig. F17). This geometry forces the sensors against the borehole wall to reduce the effects of borehole irregularities. The vertical resolution of the density and PEF measurements is ~15 and 5 cm, respectively. For measurement of tool standoff, a 670 kHz ultrasonic caliper is available on the VDN tool. The ultrasonic sensor is aligned with and located just below the density detectors. The sensor can also be used as a quality control check for the density measurements. Neutron porosity measurements are obtained using fast neutrons emitted from a 10 Ci americium oxide–beryllium (AmBe) source. Hydrogen quantities in the formation largely control the rate at which the neutrons slow down to epithermal and thermal energies. The energy of the detected neutrons has an epithermal component because much of the incoming thermal neutron flux is absorbed as it passes through the 2.54 cm drill collar. Neutrons are detected in near- and far-spacing detector banks, located 30.48 and 60.96 cm above the source. The vertical resolution of the tool under optimum conditions is ~34 cm. The neutron logs are affected by the different lithologies because the tool is calibrated for 100% limestone. Neutron porosity logs are processed to eliminate the effects of borehole diameter, tool size, temperature, drilling mud hydrogen index (dependent on mud weight, pressure, and temperature), and mud. Formation fluid salinity, lithology, and other environmental factors also affect neutron porosity. These parameters must be estimated for each borehole during neutron log processing (Schlumberger, 1994).

Data output from the VDN tool includes apparent neutron porosity (i.e., the tool does not distinguish between pore water and lattice-bound water), formation bulk density, and PEF. Because of the lack of quadrant data, density logs have been “rotationally processed” to show the image-derived density (IDRO). The IDRO algorithm uses the bulk density image from the VDN tool to compute a single density. It identifies which sectors at each depth level provide the highest-quality density measurements and computes a density based on those sectors while the tool is rotating. The VDN tool also outputs a density caliper record based on the standard deviation of density measurements made at high sampling rates around the circumference of the borehole. The measured standard deviation is compared with that of an in-gauge borehole, and the difference is converted to the amount of borehole enlargement (Anadrill-Schlumberger, 1993). A standoff of <2.54 cm between the tool and the borehole wall indicates good borehole conditions, for which the density log values are considered to be accurate to ±0.015 g/cm³ (Anadrill-Schlumberger, 1993).

LWD/MWD data flow

MWD data were monitored in the onboard downhole measurements laboratory (DHML) and displayed in the drillers shack, operations superintendent's office, co-chiefs' office, and core laboratory.

The main parameters monitored were the equivalent circulating density relative to seafloor (ECD_rsf), annular pressure while drilling (APWD), annular pressure while drilling above hydrostatic (APWD*), resistivity measurements from the GVR, gamma ray measurements from the GVR, and the density caliper (DCAV). ECD_rsf was calculated as follows:

where

• ECD_rsf = equivalent circulating density relative to seafloor (lb/gal [ppg]);
• P_apwd = APWD reading (lb/in² [psi]);
• P_wsf = water pressure at seafloor (lb/in² [psi]);
• D_apwd = TVD (true vertical depth) of APWD sensor (ft);
• D_w = water depth (ft);
• RKB = distance from sea level to dual elevator stool (ft); and
• 0.0519 = conversion factor.

Hydrostatic pressure at the seafloor is

where ECD_sw = equivalent mud weight of seawater (8.65 lb/gal [ppg]).

Wireline logging tools

Wireline logs reveal the physical, chemical, and structural properties of formations. Where core recovery is good, core data are used to calibrate the geophysical signature of the rocks. In intervals of low core recovery or disturbed cores, log data may provide the only way to characterize sediments. Wireline logs also aid in linking core data and seismic reflection data. Individual logging tools are joined together into tool strings (Fig. F18); consequently, several measurements can be made during one logging run (Table T3). The tool strings are lowered to the bottom of the borehole on a wireline, and data are logged as the tool string is pulled back up the borehole. Repeat runs improve coverage and confirm the reproducibility of logging data.

Wireline logging tool strings

Three logging strings were deployed during Expedition 308 (Fig. F18; Table T3):

• The triple combination (triple combo) tool string consisting of the Hostile Environment Gamma Ray Sonde (HNGS), Phasor Dual Induction–Spherically Focused Resistivity (DIT-SFR) tool, Hostile Environment Litho-Density Tool (HLDT), and Accelerator Porosity Sonde (APS). The Lamont-Doherty Earth Observatory (LDEO) high-resolution Temperature/​Acceleration/​Pressure (TAP) tool was attached at the bottom of this tool string, and the Inline Check Shot Tool (QSST) was added at the top.
• The FMS-sonic tool string consisting of the FMS, General Purpose Inclinometer Tool (GPIT), Scintillation Gamma Ray Tool (SGT), and Dipole Sonic Imager (DSI).
• The Well Seismic Tool (WST).

Tool acronyms, parameters measured sample intervals, and vertical resolution are summarized in Tables T3 and T4.

Wireline logging data flow and processing

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 DHML for preliminary processing and interpretation. FMS image data were interpreted using the Schlumberger GeoFrame (version 4.0.42) software package. Well seismic, sonic, and density data were used for calculation of synthetic seismograms to establish the seismic-to-borehole tie.

Logging data were also transmitted to LDEO Borehole Research Group (BRG) using a satellite high-speed data link for processing. Data processing at LDEO-BRG consisted of (1) depth-shifting all logs relative to a common datum (i.e., mbsf), (2) corrections specific to individual tools, and (3) quality control. Processed data were transmitted back to the ship. Further postcruise processing of the logging data from the FMS was performed at LDEO-BRG. Postcruise-processed data in ASCII are available from iodp.ldeo.columbia.edu/​DATA/​index.html. At the end of each expedition, a summary of “logging highlights” is posted on the USIO JOI Alliance Web site (iodp.ldeo.columbia.edu/​LOG_SUM/​index.html).

Wireline log data quality

Log data quality may be degraded where 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 shallow depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data to reduce these effects. The effects of very large washouts, however, cannot be corrected. HNGS and NGT data provide a depth correlation between logging runs. Logs from different tool strings may have minor depth mismatches caused by either cable stretch or ship heave during recording.

Interpreting structure from GVR and FMS images

Structural data were determined from unwrapped 360° GVR images using the Schlumberger GeoFrame software. The image orientation is referenced to north, as 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 unwrapped images. Sinusoids are interactively fitted to beds and fractures to determine dip and azimuth, and the data are exported from GeoFrame for further analysis.

Resolution for GVR images is 5–10 cm, compared to millimeters within cores and 0.5 cm for FMS images. This should be considered when directly comparing reports. The GVR tool provides 360° coverage; whereas the FMS provides higher-resolution data, but coverage is restricted to ~30% of the borehole. Fractures were identified within GVR images by their anomalous resistivity or conductivity and from contrasting dip relative to surrounding bedding trends. Differentiating between fractures and bedding planes can be problematic, particularly if both are steeply dipping with similar orientations.

During processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>30 cm) and/or irregular borehole diameter affects most recordings, particularly the HLDS, which requires centralization 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 into an image representative of the conductivity. This is achieved through two 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.

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.

Image equalization is the process whereby the average response of all the buttons is normalized over large intervals to correct for various tool and borehole effects. These effects include differences in the gain and offset of the preamplification 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-sonic 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. By rescaling the intensity over a smaller interval, the contrast between adjacent resistivity levels is enhanced. With dynamic normalization, resistivities in two distant sections of the hole cannot be directly compared. The interval used for the dynamic normalization was 2 m.

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.

Core-log-seismic correlation

We correlated core physical properties, wireline logs, LWD logs, and two-dimensional (2-D) and three-dimensional (3-D) seismic survey data in the Brazos-Trinity IV and Ursa Basins. To ensure accurate correlation of the data, it was important to ascertain the accuracy of the navigation of each survey, the hole deviation, the drill string position at the seafloor relative to the sea surface, the accuracy of the depth-converted seismic data, and the vertical and horizontal seismic resolution. Accurate correlation is critical to extend the study of the direct measurements of the subsurface physical properties away from the borehole using the seismic data. In order to correlate the 2-D/3-D seismic data with the LWD data, synthetic seismograms were constructed using densities and velocities for each site.

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