Downhole measurements are used to determine physical, chemical, and structural properties of formations penetrated by a drill hole. Using a variety of instruments, these measurements are rapidly collected to make continuous in-situ records as a function of depth after the hole has been drilled or reentered. Logs are essential for performing stratigraphic and lithologic characterizations, as well as for determining physical and chemical properties of the formation at a scale that links discrete laboratory measurements with regional geophysical studies. Especially where core recovery is incomplete, logging data may provide the only way to interpret structure and lithostratigraphy. This proved especially valuable during Leg 193 and will continue to be important in the postcruise studies.
Adhering to standard wireline logging techniques, after completion of coring operations the logging tools are combined and stacked together, lowered downhole and pulled up at constant speed while on recording mode. In a somewhat different approach known as logging while drilling (LWD), logging sensors are placed at close proximity to the drill bit and measurements are made while the hole is drilled. However, in this case no core is obtained and the LWD logs are interpreted and correlated to nearby wireline logs and core data. The LWD technique was used during Leg 193 to provide data from the top 60-100 m normally missing from wireline logs in cored holes. The first hard-rock application of both technologies to the same interval in ODP history was performed during Leg 193.
Overall, the quality of logging data is largely determined by the state of the borehole wall. Good contact with the borehole walls is essential for obtaining high resolution data. However, deep investigation measurements such as resistivity and sonic velocity are less sensitive to variations in hole diameter. High temperatures can also limit data quality. During Leg 193, we drilled into a hydrothermal environment that required monitoring of in-situ temperature before running other logs because most logging tools are limited to operational temperatures of 175°C. The logging tools used during Leg 193 are listed in Tables T11 and T12.
Two types of memory tools were used during Leg 193 to determine downhole temperature conditions. They are listed below. In addition, two glass maximum-reading mercury thermometers were on several occasions attached to the wireline cable head.
The core barrel temperature tool (CBTT) was developed by the Borehole Research Group of the Lamont-Doherty Earth Observatory (LDEO) for assessing temperature conditions while drilling and determining if the conditions were favorable for subsequent LWD operations in hydrothermal environments. Similar to the drill string accelerometer (DSA), the CBTT is another step in a series of measurements while coring that has been used in the past to characterize in situ drilling conditions (Plank, Ludden, Escutia, et al., 2000; Shipboard Scientific Party, 2001). The primary purpose of the CBTT was to measure and record the borehole temperatures while drilling. These measurements could then be correlated to pump rates used during coring operations to determine the feasibility of performing LWD operations in the high temperature conditions that were encountered in the Manus Basin. The CBTT contains a thermocouple and a battery operated electronics board encased in a single dewar inside the pressure case that was designed for the DSA (Fig. F14).
During Leg 193, the preliminary plan was to use the CBTT in every other core barrel at Sites 1188, 1189, and 1190. The CBTT would be deployed on an RCB core barrel for a mudline measurement and then on every other RCB core barrel run. On each run, the CBTT would begin data acquisition at a predetermined depth as programmed by the logging scientists. For ease of deployment, the CBTT was designed as a removable extension of the RCB core barrel. Using standard threaded connections, the CBTT was attached to the top of the core barrel by a Transocean Sedco Forex core technician prior to the core barrel deployment. Except for the connection and disconnection of the CBTT, coring activities were not affected by its presence. Upon CBTT/core barrel retrieval, the CBTT was disconnected and the data downloaded to the third-party data acquisition system in the Downhole Measurements Laboratory for immediate analysis. The data was then correlated to pumping rates to determine the necessary parameters for successful subsequent LWD operation at Sites 1188, 1189, and 1190. Oven tests performed at LDEO showed that a glycerin-filled dewar allowed the electronics to survive in a 250°C environment an additional 4 hr (Fig. F15). When high temperatures were expected or when temperature conditions were unknown, we planned to use a glycerin-filled dewar to avoid damaging the tool.
The ultra high temperature multisensor memory tool (UHT-MSM) is a slimhole probe running on the coring line. It was deployed for the first time during Leg 169 (Shipboard Scientific Party, 1998). The tool was developed for the University of Miami by Geophysical Research Corporation (GRC, 1994a, 1994b, 1996). The tool is shown schematically in Fig. F16. The UHT-MSM contains internal and ultra high external temperature measuring devices, a pressure gauge, a multisensor memory unit, and a dewar flask that acts as an insulator to maintain a stable temperature and cool-down rate for the tool (Fig. F16). The heat shield is aircraft-grade aluminum bound at both ends by brass heat sinks. The dewar flask can maintain an internal temperature suitable for tool operation for 4-5 hr at an external temperature of 400°C. Operations are possible up to 10 hr if the average temperature does not exceed 232°C (GRC, 1994a, 1994b) (Table T12).
Two combinations of wireline tools were used during Leg 193, the triple combination tool string (triple combo) and the Formation MicroScanner (FMS)/sonic tool strings (Fig. F17). The triple combo measures formation electrical resistivity, density, hydrogen content, and natural radioactivity. The FMS/sonic tool string provides electrical images of the borehole wall and measures acoustic velocity, magnetic field, and natural radioactivity. Natural radioactivity is measured with both tool strings because the data generated are used usually for depth matching between different logging runs. Both tool strings were modified for high-temperature operations by including a high-temperature cartridge that provided additional gamma-ray measurements and real time temperatures of the borehole fluids.
In the following sections, the basic principle of each tool is summarized. More information about the tools, the type of measurements, and their resolutions is available in Serra (1984), Ellis (1987), Schlumberger (1989), Rider (1996), and in Tables T11 and T12.
Three spectral gamma-ray tools were used to measure natural radioactivity in the formation: the natural gamma-ray tool (NGT), the hostile-environment natural gamma-ray sonde (HNGS), and the high-pressure, high-temperature telemetry gamma-ray cartridge (HTGC).
The NGT uses a sodium iodide (NaI) scintillation detector and 5-window spectroscopy to determine concentrations of potassium (40K), thorium (232Th), and uranium (238U), the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate (BGO) scintillation detectors and 256-window spectroscopy for a significantly improved tool precision. The HNGS derives its name from the fact that it is rated to a much higher temperature (260°C) than the NGT (175°C). The BGO crystals provide a significantly better spectral response than the NaI detectors because of their enhanced ability to stop gamma rays and convert their energy to full amplitude signals. This is caused by both their higher density, as compared to the NaI crystals, and their higher atomic number. However, the resolution of the peaks in the spectra obtained by the BGO detectors is not as good as that obtained by the NaI detectors, because the light emitted per unit of deposited gamma-ray energy is only ~12% of what is produced by the NaI crystals.
The HTGC has a gamma-ray detector that is made of a NaI crystal, a photo multiplier tube, a high-voltage generation circuit, and a preamplifier of the gamma-ray pulse all built in one package. The number of the output pulses from the detector is counted at a rate of 8 Hz, and the count is sent uphole. Housed in a heatsink and dewar flask that allows operations in high-temperature environments (260°C), this tool also has a 31-pin upper and lower head asset and is combinable with any standard 3.375-in tool without additional adaptors. The data acquisition software cannot distinguish between the tools, but the HTGC has the correct length and measure point and gamma-ray response for the flasked tool.
The gamma-ray values are measured in American Petroleum Institute (API) units. These units are derived from the primary Schlumberger calibration test facility in Houston, Texas, where a calibration standard is used to normalize each tool. Because the natural gamma-ray response is sensitive to borehole diameter and the weight and concentration of bentonite or potassium-chloride present in the drilling mud, corrections are routinely made for these effects during data processing at LDEO.
Formation density was measured from gamma-ray attenuation with the hostile environment litho-density sonde (HLDS). The sonde contains a radioactive cesium (137Cs) gamma-ray source (622 keV) and far and near gamma-ray detectors mounted on a shielded skid, which is pressed against the borehole wall by a hydraulically activated arm. Gamma rays emitted by the source experience Compton scattering, which involves the transfer of energy from gamma rays to electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is related to the density of electrons in the formation, which is in turn related to bulk density. Porosity may be derived from this bulk density if the matrix density is known. Photoelectric absorption occurs when gamma rays reach energies <150 keV after being repeatedly scattered by electrons in the formation. Because photoelectric absorption is strongly dependent on the mean atomic number of the elements in the formation, it varies according to chemical composition and is essentially independent of porosity.
The hydrogen content of the formation was measured with the accelerator porosity sonde. The sonde incorporates a minitron neutron generator, which produces fast (14.4 MeV) neutrons, and five neutron detectors (four epithermal and one thermal) that measure the number and arrival times of neutrons at different distances from the source. Neutrons emitted from the source are slowed by collisions with nuclei in the formation, experiencing an energy loss that depends on the relative mass of the nuclei with which the neutrons collide. Maximal energy loss occurs when a neutron strikes a hydrogen nucleus. In sediments, hydrogen is mainly present in pore water, so the neutron log is essentially a measure of porosity, assuming pore-fluid saturation. However, in igneous and hydrothermally altered rocks, hydrogen may also be present in alteration minerals such as clays; therefore, neutron logs may not give accurate estimates of porosity in these rocks.
The electrical resistivity of the formation was measured with the dual induction tool-phasor (DIT-E). This sonde provides three measures of electrical resistivity based on different depths of investigation—shallow or spherically focused log (SFL), medium (IMPH), and deep (IDPH).
The DIT-E provides two inductive measurements, IDPH and IMPH. An alternating current of high frequency and constant intensity is sent through a transmitter coil. This current induces an alternating magnetic field, which again induces currents in the formation flowing in circular ground loops. These currents create, in turn, a magnetic field which induces a voltage in the receiver coil. Both the IDPH and IMPH are commonly used to evaluate the formation properties away from shallow borehole disturbances that could have been created by drilling processes.
The SFL provides a galvanic measurement of electrical resistivity with shallow penetration but high vertical resolution. A bucking current system establishes equipotential spheres. An independent survey current flows through the volume of investigation. Its intensity is proportional to the formation conductivity.
Sonic velocities were measured with the dipole shear imager (DSI). The DSI employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1995). In addition to compressional wave velocity measurements, the DSI excites a flexural mode in the borehole, which can be used to determine shear wave velocity in all types of formations. The configuration of the DSI also allows recording of cross-line dipole waveforms, which can be used to estimate shear wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well. These "guided" waves are associated with the solid/fluid boundary at the borehole wall and their amplitude exponentially decays away from the boundary in both the fluid and the formation.
The HTGC also has an external sensor that allows for real time temperature measurements of the borehole fluids. This sensor was located on the cablehead that was used on top of both the triple combo and the FMS/sonic tools strings. Two thermocouples were used to calibrate the cablehead temperature measurements. At 0°C there is a -1.1°C difference between the calibration standard and the cablehead measurements. At 106°C there is a 1.7°C difference (Fig. F18).
During Leg 193, the Anadrill resistivity-at-the-bit (RAB) tool was deployed. The tool provides resistivity measurements and electrical images of the borehole wall, similar to the FMS but with complete coverage of the borehole walls and slightly lower vertical resolution (Table T12). In addition, the RAB tool contains a scintillation detector giving a total gamma-ray measurement. A caliper log is not available from RAB measurements; thus, the shape of the borehole is not known and the influence of breakouts on the log responses cannot be estimated.
The RAB tool is connected directly to the drilling bit and uses the lower portion of the tool and the bit as a measuring electrode (Fig. F19). 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 1.5-in electrode is located 3 ft from the bottom of the tool and provides a focused lateral resistivity measurement (RRING) with a vertical resolution of 2 in. The characteristics of RRING are independent of where the RAB tool is placed in the bottom-hole assembly (BHA) and the RRING depth of investigation is ~22 in. In addition, button electrodes provide shallow, medium, and deep resistivity measurements as well as azimuthally oriented images. These images can then reveal information about formation structure and lithologic contacts. The ~1-in-diameter button electrodes reside on a clamp-on sleeve, which limits borehole wall standoff in an 8.5-in borehole to 0.188 in. The buttons are longitudinally spaced along the RAB tool to render staggered depths of investigation of ~11, 15, and 19 in. 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 measurements. Furthermore, these measurements are acquired as the RAB rotates, with an ~6° resolution.
The RAB collar configuration is intended to run in 8.5- and 9.875-in holes depending on the measuring button sleeve size. Although only an 8.25-in measuring sleeve was on board during Leg 193, both 8.5- and 9.875-in BHA configurations for the RAB tool were available (Fig. F20). Using an 8.5-in bit and 6.75-in collar configuration provides a shorter standoff between the resistivity buttons and the formation (~0.25 in) and higher resolution images; whereas the larger bit assembly provides more stability when beginning a hole in difficult drilling environments. During Leg 193, the 9.875-in assembly was used because of concerns in starting a hole with the smaller diameter collars.
For the RAB measurement, a lower transmitter produces a current and a monitoring electrode located directly below the ring electrode measures the current returning to the collar. When connected directly to the bit, the RAB tool uses its lower few inches and the bit as a measuring electrode. The resultant resistivity measurement is termed RBIT. When transmitted to the surface in real time, RBIT gives the most immediate information about the formation.
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 tool uses a cylindrical focusing technique that takes measurements in the central monitor coil and a lower monitor coil 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 (~1.5 in), the result is a measurement with a 2-in vertical resolution.
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 1.5-in-diameter buttons can acquire azimuthal measurements within 56 sectors to produce a borehole image.
For quality control reasons, the minimum data density is one sample per 6-in interval; hence, a balance between rate of penetration (ROP) and LWD sampling rate must be determined. This relationship depends on the recording rate, the number of data channels to record, and the memory capacity of the LWD tool. The relationship between ROP and sample rate is as follows:
This equation defines the fastest ROP allowed at a given sample rate before the 1 sample per 6-in data density standard is breached. For Leg 193, the expected maximum ROP was 27 m/hr. Using this information the sample rate decided on was 20 s. The RAB was then programmed to record the following:
Under this configuration the RAB has enough memory to record 71 hr of data, which was sufficient time to complete the Leg 193 LWD operations. The abbreviations are used in the LDEO-BRG log data files (see "Related Leg Data").
A series of surface sensors were used during the Leg 193 LWD operations for depth calculations and heave corrections. They are described below.
The standard LWD recording mode configuration requires only a function of depth tracking. On a floating rig, this requires the ability to measure the movement of the following:
In an ideal situation, the heave and the motion compensator sensors will cancel each other out in that they will be equal and opposite actions. For example, a wave will cause the vessel to rise with respect to the seafloor, but to counteract this the motion compensator will work to keep the bit on bottom.
On riser vessels, guideline cables run from the moonpool area down to the blowout prevention system (BOP) on the seafloor. Each guideline is assembled on a system of pulleys, which are mounted on a compensating piston. As waves/swells (short term) and tide (long term) are acting on the vessel, the piston is constantly working to maintain a constant tension in the guideline cable by either retracting or extending. By attaching a sensor, which is able to measure the relative position of the piston, data on the vessel's vertical movement with respect to the seafloor can be determined. This sensor is termed the guideline tensionometer encoder (GTE).
On the JOIDES Resolution, the situation is complicated by the absence of a riser and a BOP system, hence no guidelines are available to be able to measure the vessel's heave. To substitute for this the LDEO BRG has developed a wireline heave compensator (WHC). The WHC is used during wireline logging operations by passing the wireline through a sheave on the compensator. This sheave is on a compensating piston that retracts to draw in the wireline cable during the vessel's downward movement and extends to pay out wireline cable during the vessel's upward movement. The vessel's movement is detected by accelerometers and a computer that activates the action of the compensating piston processes this information. A GTE sensor was mounted on the WHC in an attempt to integrate this information into the surface depth tracking system, and this sensor was also used to measure heave during wireline logging operations.
The length of the drill string (combined lengths of the BHA and the drill pipe) to the top drive and the position of the top drive in the derrick is what is used to determine the exact depth of the drill bit. On a floating rig, the system is as follows:
To measure the movement of the traveling blocks a drawworks encoder (DWE) is usually mounted on the shaft of the drawworks. One revolution of the drawworks will pay out a certain amount of drill line, and in turn move the traveling blocks a certain distance. Calibration of the movement of the traveling block to the revolution of the drawworks needs to be performed.
A motion compensator encoder (MCE) operates in exactly the same way as the GTE in that a length of thin wire on a spring-loaded retrieval system is attached to the top drive while the retrieval system and encoder section are attached to a fixed point above the compensating pistons. As the pistons are either extending or retracting to compensate for the heave of the ship, the encoder measures the amount of wire being paid out or being fed back into the retrieval system.
The combination of DWE and MCE sensors that provide LWD depth tracking capabilities can be incorporated into a single sensor that operates off a geolograph line. The geolograph line is a cable that is attached directly to the top drive. A cable retrieval system and encoding sensor is attached to a fixed point on or near the rig floor. The geolograph line then travels up and down the derrick with the top drive while the encoder measures the amount of line being paid out or retrieved. The geolograph system requires no calibration.
All the systems described above effectively work to provide similar information and are applicable for use on the JOIDES Resolution. Although these systems were tested during Leg 193, only the geolograph and GTE systems were used during LWD operations.
The hookload sensor is used to measure the weight of the load on the drill string to detect whether the drill string is "in slips" or "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 when the drill string is out of slips, the reverse is true. The difference in hookload weight between in slips and out of slips is quite distinguishable. The heave of the ship (measured by the GTE sensor) will still continue to affect the bit depth whether the drill string is in slips or out of slips.
The standpipe pressure sensor is used to measure the pressure acting on the drilling fluid in the standpipe. Although standpipe pressure is not a necessary measurement, it can be useful for the logging engineer. Also, with no mud logging service company on the JOIDES Resolution, the standpipe pressure sensor can provide pumping time information for the bit run.
Software filtering provides a smoothing of the time-depth file by taking data immediately before and after a given point and applying a weighted averaging algorithm to the depth file. The depth filtering technique has a marked effect on improving the quality of the logs and was applied to the Leg 193 RAB data.