DOWNHOLE LOGGING

The downhole logging program during Leg 204 was specifically designed to obtain the data needed to assess the presence and concentration of gas hydrates on Hydrate Ridge. Several LWD and wireline logging devices were deployed, as described below. Not all tool strings were run in each hole; refer to individual site chapters for details of tool strings deployed at each site.

During Leg 204, four Anadrill LWD and measurement-while-drilling (MWD) tools were deployed at eight of the nine sites cored and drilled on southern Hydrate Ridge. These tools were provided by Schlumberger-Anadrill services under contract with the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG). LWD surveys were successfully conducted during seven 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), and Leg 196 (Mikada, Becker, Moore, Klaus, et al., 2002).

LWD and MWD tools measure different parameters. LWD tools measure in situ formation properties with instruments that are located in the drill collars immediately 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 into downhole computer 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 (see below). MWD tools enable both LWD and MWD data to be transmitted uphole when the tools are used in conjunction. The term LWD is often used more generically to cover both LWD- and MWD-type measurements.

The LWD and MWD tools used during Leg 204 include the resistivity-at-the-bit (RAB) tool, the power pulse MWD tool, the Nuclear Magnetic Resonance (NMR-MRP) tool, and the Vision Neutron Density (VND) tool. This was the first time the NMR-MRP tool was used during an ODP leg. Figure F17 shows the configuration of the LWD/MWD BHA, and Tables T4 and T5 list the set of measurements recorded.

LWD measurements are made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. Fluid invasion into the borehole wall is 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 erasable/programmable read-only memory chips to store logging 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. After drilling, the LWD tools are retrieved and the data downloaded from each tool through an RS232 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) into depth-measurement data files. The resulting depth-measurement data are transferred to the processing systems in the downhole measurements laboratory (DHML) on board the JOIDES Resolution for reduction and interpretation.

Unlike wireline tools which record data vs. depth, LWD tools record data vs. time. The Anadrill Integrated Drilling and Logging (IDEAL) system 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 (Fig. F18):

1. Position of the traveling block in the derrick,
2. Heave of the vessel by the action of waves/swells and tides, and
3. Action of the motion compensator.

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 is used to determine the exact depth of the drill bit and rate of penetration. The system configuration is illustrated in Figure F18 and is further described below:

1. Drilling line is spooled on the drawworks. From the drawworks, the drilling line extends to the crown blocks, which are located at the very 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. From the driller's console, the driller controls the operation of the drawworks, which, via the pulley system described above, controls the position of the traveling block in the derrick.
2. On the JOIDES Resolution, the heave motion compensator is suspended from the traveling block. The top drive is then attached to the motion compensator. The motion compensator uses pistons that are pressure charged and are thus able to provide a buffer against the 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.
3. 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.

A hookload sensor is used to measure the weight of the load on the drill string and can be used to detect whether the drill string is disconnected from the traveling block and held fast at the rig floor ("in-slips") or not. 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 very distinguishable. The heave of the ship will still continue to affect the bit depth whether the drill string is in-slips or out-of-slips.

On the JOIDES Resolution, the ability to measure the vessel's heave is addressed in two ways. The rig instrumentation system used by the driller measures and records the heave of the ship and the motion of the cylinder of the active compensator, among many other parameters, at the rig floor. The motion compensator cylinder either extends or retracts to compensate for ship heave that is detected by fixed accelerometers. Both the heave value and cylinder position measurement are transmitted to the Anadrill 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. The depth-filtering technique has significantly improved the quality of RAB image logs from previous ODP holes.

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 scintillation counter that provides a total gamma ray measurement (Fig. F19). 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 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 1-in (2.5 cm) electrode is located 3 ft (91 cm) from the bottom of the tool and provides a focused lateral resistivity measurement (R_RING) with a vertical resolution of 2 in (5 cm). The characteristics of R_RING are independent of where the RAB tool is placed in the BHA, and its depth of investigation is ~7 in (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 ~1 in (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 ~1, 3, and 5 in (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. Furthermore, these measurements are acquired with an ~6° resolution as the RAB tool rotates. The vertical resolution for each resistivity measurement is shown in Table T4.

Two RAB tool-collar configurations that were designed for use during Leg 204 give slightly different resistivity responses depending on the size of the measuring button sleeve and the hole diameter. The RAB tool used in series in the 6-in LWD/MWD BHA is called the GeoVision Resistivity (GVR), the most recent upgrade of the tool (Fig. F19). The diameter of its measuring button sleeve is 23.3 cm (9¹/₈ in), and the diameter of the three-cone rotary bit used during Leg 204 is 25 cm (9⁷/₈ in). 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 of <100, as expected for all Leg 204 sites, a resistivity correction factor of up to 4% may be applied to each of the GVR measurements. Estimated correction factors for the GVR tool are given in Table T5 (Schlumberger, 2001, unpubl. data). Because of its limited depth of penetration into the formation (see below), the correction factor for the shallow button resistivity is greatest.

A RAB-8 tool collar configuration for use with a larger 8-in-diameter BHA was specially designed for ODP. The U.S. Department of Energy provided partial funding support for the modified RAB-8 tool deployment during Leg 204. The design is intended to be run in conjunction with a modified motor-driven core barrel core-liner system, thus allowing for RAB measurements to be made while coring (Fig. F20).

ODP Leg 204 represents the first ever attempt to core and record LWD data simultaneously. The RAB-8 tool is deployed alone in the BHA, and the standard 25 cm (9⁷/₈ in) diameter rotary bit is used. The diameter of the measuring button sleeve for the RAB-8 tool is 24.1 cm (9 in); thus, the standoff is half that for the RAB-6 and the resistivity correction factors are negligible for all of the RAB-8 measurements, including the shallow button resistivity.

All logging data are collected at a minimum vertical 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 (46 MB) of the LWD tool. During Leg 204, we used a data acquisition sampling rate of 5 s for high-resolution GVR images. The RAB-8 tool was programmed with a sampling rate of 10 s for data acquisition. The maximum ROP allowed to produce one sample per 6-in interval is given by the equation

ROP (in meters per hour) = 548/sample rate. (7)

This relationship gives 110 m/hr maximum ROP for the GVR and 55 m/hr for the RAB-8 tool.

For Leg 204, the target ROP was 25-50 m/hr, roughly 25%-50% of the maximum allowable for the GVR and RAB-8 tools. These reduced rates improve the vertical resolution of the resistivity images to 5-10 cm per rotation. Under this configuration, the GVR tool has enough memory to record up to 80 hr of data and the RAB-8 tool can record as long as 30 hr. This would be sufficient, under normal operating conditions, to complete the scheduled LWD operations at the Leg 204 drill sites.

For the bit resistivity measurements, a lower transmitter (T₂) produces a current and a monitoring electrode (M₀) located directly below the ring electrode measures the current returning to the collar (Fig. F19). When connected directly to the bit, the GVR tool uses the lower few inches of the tool as well as the bit as a measurement electrode. The resultant resistivity measurement is termed RBIT, and its depth of investigation is ~12 in (30.48 cm).

The upper and lower transmitters (T₁ and T₂) 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 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 then measured and related to the resistivity of the formation. Because the ring electrode is narrow (~4 cm), the result is a measurement (R_RING) 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 (R_BUTTON) can be acquired azimuthally as the tool rotates within 56 sectors to produce a borehole image.

During Leg 204, Anadrill's MWD tool was deployed in combination with the LWD tools (Fig. F17). The MWD tool had previously been deployed during Leg 188 (O'Brien, Cooper, Richter, et al., 2001) and Leg 196 (Mikada, Becker, Moore, Klaus, et al., 2002). During Leg 204, the MWD tool was deployed with the LWD tools at all sites, except on the test run with the RAB coring tool. MWD tools measure downhole drilling parameters and consist of sensors located in the drill collar, immediately above the RAB tool in the BHA (Fig. F17).

The MWD data are transmitted by means of a pressure wave (mud pulsing) through the fluid within the drill pipe. The 6-in (17 cm) power-pulse MWD tool operates by generating a continuous mud-wave transmission within the drilling fluid and by changing the phase of this signal (frequency modulation) to transmit relevant bit words representing information from various sensors. Figure F21 illustrates the MWD mud pulse system and a representative pressure wave. Two pressure sensors attached to standpipe on the rig floor and on the gooseneck on the crown block are used to measure the pressure wave acting on the drilling fluid when information is transmitted up the drill pipe by the MWD tool. With the MWD mud pulsing systems, pulse rates range from 1 to 6 bps, depending primarily on water depth and mud density. Pulse rates of 6 bps were achieved during Leg 204.

The drilling parameters transmitted by mud pulse to the Anadrill surface recording system during Leg 204 include downhole weight on bit (DWOB), downhole torque on bit (DTOR), bit bounce, and tool stick slip. These measurements are made using paired strain gauges near the base of the MWD collar. Table T6 lists the set of measurements recorded using the MWD tool, which are transmitted to the surface. DWOB and DTOR information are also transmitted to the driller via the surface rig instrumentation system transmission protocol using the WITS standard. The comparison of MWD drilling parameter data, rig instrumentation system data, and ship-heave information recorded synchronously during Leg 204 was used to improve drilling control and to assess the quality of the recorded LWD data.

In addition to the drilling parameters listed in Table T6, the mud pulse system also transmitted some geophysical data from LWD tools to the surface. Measurement parameters from each LWD collar are updated at rates corresponding to 15-cm to 1.5-m depth intervals, depending on the initialized values and ROP of the tool. These parameters are used to verify the operational status of each tool downhole and provide real-time logs. In contrast to these real-time data, the downhole memory in the LWD tools is set to record data at a minimum rate of one sample per 15 cm.

The U.S. Department of Energy provided funding support to deploy Anadrill's NMR-MRP tool during Leg 204. The basic technology behind this tool is similar to modern wireline nuclear magnetic resonance technology (e.g., Kleinberg et al., 2003; Horkowitz et al., 2002), which is based on measurement of the relaxation time of the magnetically induced precession of polarized protons. A combination of bar magnets and directional antennas are used to focus a pulsed polarizing field into the formation (Fig. F22). The NMR-MRP tool measures the relaxation time of polarized molecules in the formation, which provides information related to the formation porosity. By exploiting the nature of the chemical bonds within pore fluids, for hydrogen in particular, the NMR-MRP tool can provide estimates of the total porosity and bound fluid volume and thus be useful to determine whether water, gas, or gas hydrates are present in the formation.

During Leg 204, the NMR-MRP tool acquired formation and engineering information in memory and transmitted some data to the surface via MWD. The relaxation-time spectra were recorded downhole, and total porosity estimates were transmitted to the surface in real time. These spectra were stacked in postprocessing to improve the measurement precision. The signal investigates a 15-cm cylindrical volume of the borehole, and for a 9⁷/₈-in bit size, the depth of investigation of the measurement is ~5 cm into the formation. Lateral tool motion may reduce NMR-MRP data quality in some circumstances. Therefore, accelerometers and magnetometers contained in the downhole tool are used to evaluate data quality and determine the maximum relaxation times that can be resolved. Data were also acquired while sliding (not rotating) the tool over short intervals to compare measurements with and without the effect of lateral vibration while drilling. Both real-time and memory data from the NMR-MRP tool were transmitted to LDEO-BRG and Schlumberger via Inmarsat B for reprocessing and data-quality assessment.

The VND tool is similar in principle to the Azimuthal Density Neutron (ADN) tool (Anadrill-Schlumberger, 1993; Mikada, Becker, Moore, Klaus, et al., 2002). The density section of the tool uses a 1.7-Ci ¹³⁷Cs gamma ray source in conjunction with two gain-stabilized scintillation detectors to provide a borehole-compensated density measurement. The detectors are located 5 and 12 in (12.7 and 30.48 cm) below the source (Fig. F23). The number of Compton scattering collisions (change in gamma ray energy by interaction with the formation electrons) is related to the formation density. Returns of low-energy gamma rays are converted to a photoelectric effect value, measured in barns per electron. The photoelectric effect value depends on electron density and hence responds to bulk density and lithology (Anadrill-Schlumberger, 1993). It is particularly sensitive to low-density, high-porosity zones.

The gamma ray source and detectors are positioned behind holes in the fin of a full gauge 9-in (25.08 cm) clamp-on stabilizer (Fig. F23). This geometry forces the sensors against the borehole wall, thereby reducing the effects of borehole irregularities and drilling. The vertical resolution of the density and photoelectric effect measurements is ~15 and 5 cm, respectively. For measurement of tool standoff and estimated borehole size, a 670-kHz ultrasonic caliper is available on the VND tool. The ultrasonic sensor is aligned with and located just below the density detectors. In this position, the sensor can also be used as a quality control 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 1-in drill collar. Neutrons are detected in near- and far-spacing detector banks, located 12 and 24 in (30.48 and 60.96 cm), respectively, above the source. The vertical resolution of the tool under optimum conditions is ~34 cm. The neutron logs are affected to some extent by the lithology of the matrix rock because the neutron porosity unit is calibrated for a 100% limestone environment. 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 salinities, lithology, and other environmental factors will also affect neutron porosities, and these parameters must be estimated for each borehole during neutron log processing (Schlumberger, 1994).

In near-vertical drill holes, the VND tool does not collect quadrant azimuthal data. Data output from the VND tool includes apparent neutron porosity (i.e., the tool does not distinguish between pore water and lattice-bound water), formation bulk density, and photoelectric effect (PEF). The density logs graphically presented here have been "rotationally processed" to show the average density that the tool reads while it is rotating. In addition, the VND tool outputs a differential 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 <1 in 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).

Figure F24 shows onboard flow of the LWD/MWD data during Leg 204. Surface drilling parameters and MWD data were directly transmitted to the Schlumberger IDEAL system. A laptop personal computer was used to download and transfer the LWD data from all the tools on the rig floor to the IDEAL system for depth-time correlation. The log data were then distributed to the shipboard party via the workstation in the DHML. RAB image data were processed and converted to graphic format using GeoFrame (see "Logging Data Flow and Processing") and custom codes on the DHML workstation prior to distribution to the shipboard party.

Drill String Acceleration Tool

To evaluate downhole motion and pressure during coring, a device was designed by the LDEO-BRG to measure acceleration on various coring tools used by ODP (Guerin and Goldberg, 2002a). The DSA is a memory probe that records pressure and three-component acceleration and is attached to the top of a core barrel (Fig. F25). The true vertical displacement at the bit may be estimated by time integration of the acceleration data. In addition, drilling vibration data and downhole pipe motion data, information about core barrel landing, pressure up, and other operational events are recorded by the DSA tool. To date, the DSA tool has been used with the APC, RCB, FPC, and HRC.

Specifications and Operations

The principal components of the DSA tool are a pressure sensor and two accelerometers. The pressure sensor measures pressure in the drill pipe every second, which is used to trigger the recording of acceleration data at a preprogrammed depth. One accelerometer is a vertical-component high-sensitivity transducer, recording motion along the axis of the drill string. The other is a high-frequency, three-axis transducer to record bit vibrations. The DSA tool runs as a self-contained memory device with a battery life allowing 9 hr of operations and enough memory to record ~1.5 hr of data at a 100-Hz sampling frequency. Before deployment, the DSA tool is connected through a serial port to a data acquisition PC for initialization and the initial recording depth, typically 200 m above coring depth, is defined by the LDEO logging staff scientist. After recovery, the DSA tool is reconnected to a PC and the data are downloaded in ASCII files ready for immediate analysis. The first processing step is the conversion of the raw data into acceleration, using calibration coefficients provided by the accelerometer manufacturers. The sensors, electronics board, memory, and batteries are enclosed in a 1.2-m-long stainless steel pressure case and can operate under temperature and pressure up to 85°C and 75 MPa, respectively.

Continuous coring in ODP holes is performed by recovering the core barrel at regular intervals, typically every 9.5 m. The procedure is the same for the APC in soft sediments and for the RCB in harder formations (Fig. F25). The empty core barrel is lowered with a wireline or allowed to fall freely to the bottom of the drill string, advances 9.5 m during coring, and then is retrieved by a wireline that latches to the top of the core barrel. After reaching the rig floor, the core is removed for analysis and an empty core barrel is dropped in the drill string to continue the coring process. The DSA tool was designed to minimize any impact on coring operations. The initialized tool is attached by threaded collars to the top of the core barrel (Fig. F25), and the DSA tool/core barrel assembly is lowered to the bottom of the drill string. The top of the DSA tool can receive the normal core retrieval tool, and the entire assembly is recovered together. Only a few minutes are required to connect and disassemble the DSA tool/core barrel connection, which usually do not add to the net time on the rig floor for core retrieval. DSA data are typically available to assess the coring process and characterize formation attributes before cores have degassed and reached thermal equilibrium.

Procedures and Data

Downhole logs reveal the physical, chemical, and structural properties of formations penetrated by a borehole. A variety of geophysical tools make rapid, continuous, in situ measurements as a function of depth after the hole has been drilled that can be used to interpret the lithology of the penetrated formation. 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 the borehole section. Geophysical well logs can aid in characterizing sedimentary sequences and integrating core and seismic reflection data. Individual logging tools are joined together into tool strings (Fig. F26) so that several measurements can be made during each logging run (Table T7). The tool strings are lowered to the bottom of the borehole on a wireline cable, and data are logged as the tool string is pulled back up the hole. Repeat runs are made to improve coverage and confirm the accuracy of log data. Not all tool strings are run in each hole; refer to individual site chapters for details of logging strings deployed at each site.

Logging Tools and Tool Strings

During ODP Leg 204, the following four different logging strings were deployed (Fig. F26; Table T7):

1. The triple combination (triple combo) string (resistivity, density, and porosity measurements), which consists of the Hostile Environment Gamma Ray Sonde (HNGS), the phasor dual induction-spherically focused resistivity (DIT-SFR) tool, the Hostile Environment Litho-Density Tool (HLDT), and the Accelerator Porosity Sonde (APS). The LDEO high-resolution Temperature/Acceleration/Pressure (TAP) tool was attached at the bottom of this tool string and the Inline Checkshot Tool (QSST) was added at the top.
2. The FMS-sonic tool string, which consists of the FMS, General Purpose Inclinometer Tool (GPIT), and Scintillation Gamma Ray (SGT) Tool, and the Dipole Sonic Imager (DSI).
3. The Well Seismic Tool (WST).
4. The Vertical Seismic Imager (VSI).

Leg 204 was the first time the QSST and the VSI have been run during an ODP leg.

The parameters measured by each tool, the sample intervals used, and the vertical resolution are summarized in Table T7. Explanations of tool name acronyms and their measurement units are summarized in Table T8. 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-BRG Wireline Logging Services Guide (2001).

Hostile Environment Spectral Gamma Ray Sonde and Scintillation Gamma Ray Tool

The HNGS measures the natural gamma radiation from isotopes of potassium, thorium, and uranium and uses a five-window spectroscopic analysis to determine concentrations of radioactive K (in weight percent), Th (in parts per million), and U (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The spectral analysis filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. The HNGS also provides a measure of the total gamma ray emission (American Petroleum Institute units [gAPI]), and the uranium-free or computed gamma ray emission (CGR) (in gAPI units). The HNGS response is influenced by the borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud. KCl may be added to the drilling mud to prevent freshwater clays from swelling and forming obstructions. All of these effects are corrected for during processing of HNGS data at LDEO-BRG.

The SGT tool uses a sodium iodide scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of K, U, and Th concentrations in the formation. The SGT tool 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 and depth between different logging runs. With the FMS-DSI tool string, the SGT tool is placed between the two tools, providing correlation data to a deeper level in the hole.

Hostile Environment Litho-Density Tool

The HLDT 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. Compton scattering involves the ricochet of gamma rays off electrons in the formation via elastic collision, transferring energy to the electron in the process. 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 HLDT also measures the 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 sediment. Some examples of PEF values are pure calcite = 5.08, illite = 3.03, quartz = 1.81, and kaolinite = 1.49 b/e^-. 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 good HLDT 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.

Accelerator Porosity Sonde

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, and 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.

Phasor Dual Induction-Spherically Focused Resistivity Tool

The DIT-SFR tool provides three different measurements of electrical resistivities, 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 that gives higher vertical resolution, measures the current necessary to maintain a constant voltage drop across a fixed interval.

High-Resolution Temperature/Acceleration/Pressure Tool

The TAP tool 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 204, the TAP tool 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 tool do not necessarily represent in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole. However, from the spatial temperature gradient, abrupt temperature changes can be identified that may correspond to contrasts in permeability at lithologic boundaries or may represent localized fluid flow into the borehole, indicating fluid pathways and fracturing.

Dipole Sonic Imager

The DSI employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of formations. In addition to a robust and high-quality measurement of V_P , the DSI uses the dipole source to generate a flexural mode in the borehole that can be used to estimate shear (S-) wave velocity even in highly unconsolidated formations. When the formation shear velocity is less than the sonic velocity of the borehole fluid, particularly in unconsolidated sediments, the flexural wave travels at the S-wave velocity and is the most reliable way to estimate a shear velocity log. Meanwhile, the monopole source generates P-, S-, 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 S-wave velocity in hard rocks better than or equivalent to the monopole source. These combined modes can be used to estimate S-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 (P- 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.

Formation MicroScanner Tool

The FMS produces high-resolution images of borehole wall microresistivity that can be used for detailed sedimentologic 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, allowing features such as burrows, thin beds, fractures, veins, and vesicles to be imaged. The resistivity measurements are converted to color or grayscale 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 (see below). This allows the dip and strike of geological features intersecting the hole to be measured from processed FMS images. FMS images can be used to visually compare logs with the core to ascertain the orientations of bedding, fracture patterns, and sedimentary structures and to identify stacking patterns. FMS images have proved particularly valuable in interpreting sedimentary structures, and they have been used to identify cyclical stacking patterns in carbonates, turbidite deposits, and facies changes (Serra, 1989).

General Purpose Inclinometer Tool

The GPIT is included in the FMS-sonic tool string to calculate tool acceleration and orientation during logging. The GPIT contains a triple-axis accelerometer and a triple-axis magnetometer. The GPIT records the orientation of the FMS images and allows more precise determination of log depths than can be determined from cable length, as the accelerometer data can be used to correct for cable stretching and/or ship heave.

Vertical Seismic Imager

The VSI is a borehole seismic wireline tool optimized for vertical seismic profiles (VSPs) and walkaway vertical seismic profiles (W-VSPs) in both cased and open hole and vertical and deviated wells. The VSI consists of multiple three-axis geophones in series separated by "hard wired" acoustically isolating spacers. A schematic illustration of the tool is given in Figure F27. The tool diameter is 3 in, with temperature and pressure ratings to 175°C and 20,000 psi, respectively.

During Leg 204, the VSI was nominally configured using three shuttles (each containing three geophones) spaced 15.12 m apart and combined with the SGT tool. In practice, after problems using three shuttles simultaneously, only one shuttle was used at a time. For vertical incidence VSP operations, the shuttles were mechanically clamped against the borehole wall and sources (e.g., generator-injector [GI] guns) on the JOIDES Resolution were fired between 5 and 15 times by control hardware in the Schlumberger multi-tasking acquisition and imaging system (MAXI)S unit. For constant-offset and W-VSP operations, sources were fired on another ship (e.g., Maurice Ewing) by Macha radio control from the Schlumberger MAXIS unit. The VSI tool was then unclamped and pulled uphole. Although a 7.5-m spacing had been planned, problems clamping the sensors resulted in variable station spacing. The VSI records the full seismic waveform for each firing. These waveform data are stacked by the MAXIS recording software and may be output in LDF (internal Schlumberger format) or SEG-Y formats.

Three types of VSP experiments (vertical incidence, constant-offset, and/or walkaway) were conducted during Leg 204 using the VSI and GI gun sources located on the JOIDES Resolution and the Maurice Ewing. The vertical incidence and constant offset surveys were conducted by alternating source firings between the JOIDES Resolution and the Maurice Ewing at each VSI depth station. These were followed by walkaway VSP experiments in which sources were fired from the Maurice Ewing alone as it moved along two crossing lines intersecting near the drill site. The U.S. Department of Energy provided partial funding support for this tool deployment during Leg 204.

Inline Checkshot Tool

The Schlumberger QSST is a single-axis seismic checkshot tool that runs in-line with the triple combo tool string (Fig. F26). The QSST consists of a single hydrophone and does not utilize a clamping arm like the WST to force it into contact with the borehole wall. Seismic coupling is achieved by setting the tool on the hole bottom and allowing the tool to lean against the borehole wall, coupling passively to the formation. The QSST then records the vertically incident signals at the bottom of the hole that are generated by a seismic source on the JOIDES Resolution (e.g., GI guns) positioned just below the sea surface. The recorded signals enable a one-way traveltime to be determined from the surface to total depth, providing a check of V_P vs. depth for calibration of seismic profiles and correction of sonic logs. Off-bottom stations are not possible using the QSST. The rig time required to run this single-point checkshot survey is negligible since the QSST may be included during the triple combo logging run. Data from the QSST are provided by the Schlumberger multi-tasking acquisition and imaging system (MAXIS) unit in SEG-Y.

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 Schlumberger's GeoFrame 3.8 software package. Well seismic, sonic, and density data were used for calculation of synthetic seismograms with GeoQuest's IESX software package to establish the seismic to borehole tie.

Logging data were also transmitted to LDEO-BRG using a FFASTEST satellite high-speed data link for processing soon after each hole was logged. Data processing at LDEO-BRG consists of (1) depth-shifting all logs relative to a common datum (i.e., in mbsf), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO-BRG, log data were transmitted back to the ship, providing near real-time data processing. Processed data were then replotted on board (see the "Logging" section in each site chapter). Further postcruise processing of the log data from the FMS is performed at LDEO-BRG. Postcruise-processed data in ASCII format 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.

Downhole logging aboard the JOIDES Resolution is provided by LDEO-BRG in conjunction with Leicester University Borehole Research, the Laboratoire de Mesures en Forages Montpellier, University of Aachen, University of Tokyo, Schlumberger Well Logging Services, and Schlumberger Measurement While Drilling.

Logging data quality may be seriously degraded by changes in the 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 GRA. Corrections can be applied to the original data in order to reduce these effects. Very large washouts, however, cannot be corrected for. HNGS and NGT data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have minor depth mismatches caused by either cable stretch or ship heave during recording. Ship heave is minimized by a hydraulic wireline heave compensator designed to adjust for rig motion during logging operations.

With growing interest in natural gas hydrate, it is becoming increasingly important to be able to identify the presence of in situ gas hydrate and accurately assess the volume of gas hydrate and included free gas within gas hydrate accumulations. Numerous publications (Mathews, 1986; Collett, 1993, 1998a, 1998b, 2001; Goldberg, 1997; Guerin et al., 1999; Goldberg et al., 2000; Helgerud et al., 2000) have shown that downhole geophysical logs can yield information about the presence of gas hydrate.

Because gas hydrates are characterized by unique chemical compositions and distinct electrical resistivities, physical, and acoustic properties, it is possible to obtain gas hydrate saturation (percent of pore space occupied by gas hydrate) and sediment porosity data by characterizing the electrical resistivity, acoustic properties, and chemical composition of the pore-filling constituents within gas hydrate-bearing reservoirs. Two of the most difficult reservoir parameters to determine are porosity and the degree of gas hydrate saturation. Downhole logs often serve as a source of porosity and hydrocarbon saturation data. Most of the existing gas hydrate-log evaluation techniques are qualitative in nature and have been developed by the extrapolation of untested petroleum industry log evaluation procedures. To adequately test the utility of standard petroleum log evaluation techniques in gas hydrate-bearing reservoirs would require numerous laboratory and field measurements. However, only a limited number of gas hydrate occurrences have been sampled and surveyed with open-hole logging devices.

Reviewed below are downhole log measurements that together yield useful gas hydrate-reservoir information. The downhole measurements considered include gamma-gamma density, neutron porosity, electrical resistivity, acoustic transit time, and nuclear magnetic resonance. Most of these measurements are converted to porosity; however, because gas hydrate affects each measurement of porosity in a different fashion, the quantity of gas hydrate can be estimated by comparison of porosity measurements made using different techniques.

Gamma-Gamma Density Logs

Gamma density logs are primarily used to assess sediment porosities. The theoretical bulk density of a Structure I methane hydrate is ~0.9 g/cm³ (Sloan, 1998). Gas hydrate can cause a small but measurable effect on density-derived porosities. At relatively high porosity (>40%) and high gas hydrate saturation (>50%), the density log-derived porosities need to be corrected for the presence of gas hydrate (Collett, 1998b).

Neutron Porosity Logs

Neutron logs are also used to determine sediment porosities. Since Structure I methane hydrate and pure water have similar hydrogen concentrations, it can be generally assumed that neutron porosity logs, which are calibrated to pure water, are not significantly affected by the presence of gas hydrates. At high reservoir porosities, however, the neutron porosity log could overestimate porosities (Collett, 1998b).

Electrical Resistivity

Water content and pore water salinity are the most significant factors controlling the electrical resistivity of a formation. Other factors influencing resistivity of a formation include the concentration of hydrous and metallic minerals, volume of hydrocarbons including gas hydrates, and pore structure geometry. Gas hydrate-bearing sediments exhibit relatively high electrical resistivities in comparison to water-saturated units, which suggests that a downhole resistivity log can be used to identify and assess the concentration of gas hydrates in a sedimentary section. The relation between rock and pore fluid resistivity has been studied in numerous laboratory and field experiments. From these studies, relations among porosity, pore fluid resistivity, and rock resistivity have been found. Among these findings is the empirical relation established by Archie (Archie, 1942), which is used to estimate water saturations in gas-oil-water-matrix systems. Research has shown that the Archie Relation also appears to yield useful gas hydrate saturation data (reviewed by Collett, 2000).

Acoustic Transit Time

The velocity of P- and S-waves in a solid medium, such as gas hydrate-bearing sediment, is usually significantly greater than the velocity of P- and S-waves in water or gas-bearing sediments. Studies of downhole acoustic log data from both marine and permafrost associated gas hydrate accumulations have shown that the volume of gas hydrate in sediment can also be estimated by measuring interval velocities (Guerin et al., 1999; Helgerud et al., 2000; Collett, 2000). Analysis of sonic logging waveforms has also shown that the presence of gas hydrate can generate significant energy loss in monopole and dipole waveforms (Guerin and Goldberg, 2002b).

Nuclear Magnetic Resonance Logs

NMR logs use the electromagnetic properties of hydrogen molecules to analyze the nature of the chemical bonds within pore fluids. Relative to other pore-filling constituents, gas hydrates exhibit unique chemical structures and hydrogen concentrations. In theory, therefore, it should be possible to develop NMR well-log evaluation techniques that would yield accurate reservoir porosities and water saturations in gas hydrate-bearing sediments. Because of tool design limitations, gas hydrates cannot be directly detected with today's downhole NMR technology; however, they can be useful to yield very accurate gas hydrate saturation estimates. Because of the short transverse magnetization relaxation times (T2) of the water molecules in the clathrate, gas hydrates are not "seen" by the NMR tool and will be assumed to be part of the solid matrix. Thus, the NMR-calculated total porosity in a gas hydrate-bearing sediment should be lower than the actual porosity. With an independent source of accurate total porosity, such as density- or neutron-porosity-log measurements, it should be possible to accurately estimate gas hydrate saturations by comparing the apparent NMR-derived porosity to the total density-derived porosity.

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 (5-10 cm at best, compared with millimeters within cores and 0.5 cm for FMS images), and therefore, identified features are likely to be different in scale. For example, microfaults ("small faults;" <1 mm width) and shear bands (1-2 mm; up to 1 cm width) can only be identified in FMS data. This should be considered when directly comparing reports. The RAB tool provides 360° coverage at a lower resolution; the FMS provides higher-resolution data, but coverage is restricted to only ~30% of the borehole wall. Fractures were identified within RAB 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 and with similar orientations.

We correlated the results of a 3-D seismic survey acquired on the Thompson in June and July 2000 with the Leg 204 LWD and wireline log data. The correlation included core physical properties, wireline logs, and two-dimensional and 3-D seismic survey images collected during 14 yr of research at Hydrate Ridge. To ensure accurate correlation of the data, it was important to ascertain the accuracy of the navigation of each of the associated surveys, 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 3-D seismic data.

In order to correlate the 3-D seismic data with the LWD data, synthetic seismograms were constructed using the best densities and velocities for each site, which included data such as the LWD density, core density, wireline sonic and core P-wave velocity. The 256-ms long-source wavelet used at each site was calculated deterministically using 25 traces from the seafloor reflection in the vicinity of Hole 1245A, where the seafloor was reasonably flatlying and smooth.