1. WSTP geometry, structure, and recording capabilities
The original Uyeda temperature tool had a thin, stainless-steel probe. The second-generation tool had a slightly thicker probe tip which extended past the end of a pore-fluid filter block. The newest generation (Leg 139) WSTP includes a temperature-probe tip which is slightly longer and wider overall, but still ends with the same 1.3-cm diameter (Fig. II-2). Because the filter assembly was also made longer to improve fluid-sampling performance while tool length was kept constant, the new WSTP probe tip extends just 3.5 cm past the end of the sampling filter. During Leg 139, a 2.7-cm-diameter, solid sleeve was fabricated to fit over the temperature probe for use when the tool was to be run without fluid sampling (Fig. II-2). This sleeve is run instead of the filter assembly, to reduce the time constant of the probe and improve the geometry of the measurement so that 1) the frictional heating (or cooling) pulse associated with insertion of the probe can be assumed to approximate more closely a line source, and 2) insertion of the instrument is less likely to fracture semilithified sediments. In either configuration, the probe has a time constant in sediments on the order of 2-3 minutes, and it must be held in position for at least 10 minutes to obtain enough data to reliably extrapolate to in-situ sediment temperature.
The temperature-measuring systems of the WSTP and original Uyeda tool are similar in concept but differ primarily in their electronic capabilities. The Uyeda recorder was limited to storing 128 values of thermistor resistance, and could be programmed to read these values at 1- or 2-minute intervals during stations that lasted 2 or 4 hours, respectively. The second-generation WSTP recorder sampled 16,000 values from three channels (temperature plus two pressure transducers) at an interval of up to about 5 seconds. Unfortunately this recorder proved to be unreliable. The present WSTP digital recording package is referred to as the "double-Current data-logger" (DCDL). The DCDL records a single data channel (i.e., temperature from a single thermistor), and can store up to 2048 values sampled at a fixed time interval of 4.369 s, using a 12-bit A/D converter. For Leg 139, when two thermistors were run in the WSTP, two DCDLs were mounted into a single frame and run in tandem, with each recorder monitoring a separate circuit (Fig. III-2). The DCDL has also proven unreliable on recent legs and adaptation of a commercial data logger is now under way.
Because Leg 139 included operations immediately adjacent to high-temperature hydrothermal vents, it was necessary to "harden" the WSTP for use in high-temperature and reactive fluids. Part of this upgrade included extending the temperature range of the data logger and sensing thermistors. DCDL components were upgraded to military specifications (125°C operating temperature), and two thermistor circuits were run with overlapping ranges (Fig. III-2). High temperature pass-throughs were designed for the WSTP's lower bulkhead (to allow the thermistor cables to reach the data logger), and Kalrez o-rings were used for the pressure case. Additional WSTP modifications included complete rewiring with high-temperature conductors and connectors. These upgrades allow measurements of in-situ temperatures up to 200°C as long as there is a high-temperature thermistor installed in the probe tip and the electronics are cooled by continuous circulation around the tool during penetration.
2. WSTP preparation and deployment
Once the WSTP is prepared for general use during a leg (by ODP marine specialists), collecting temperature data is relatively simple. It is advisable to check the thermistor calibration and electronic circuits throughout the leg by verifying the bottom-water reading of the thermistor recorder circuit and by substituting a decade box for the thermistor before and after stations; the DCDL data-dumping software now available on the downhole-tools PC provides a dedicated "Calibration" function to assist with this operation. A fully charged battery pack should be used for each run, particularly if extensive instrument testing or calibration is conducted immediately before deployment and/or if a long station is planned.
As described in the WSTP cookbook and excerpted in Chapter II, the WSTP can be used in several deployment modes. The instructions directed toward obtaining a good water sample generally apply to measuring temperatures as well. The standard deployment methods described in Chapter II include latched-in (RCB) and colleted (APC/XCB) delivery operation, each requiring about 1-3 hours between cores (depending on water depth). Normally, the tool is lowered on the coring line and latched into the bit held just above the bottom of the hole. As the tool is lowered to the bit, a 5- to 10-minute pause to check recorder performance and water temperatures should be made just above the mud line, where the temperatures inside the pipe should be close to (but possibly slightly warmer than) the stable bottom-water temperature. Once the tool is lowered and latched into the bit, the pipe is then lowered, pushing the probe to the measurement depth in undisturbed sediments ahead of the hole. As described previously, the driller should continue circulation at least until the probe is pushed into the sediments, to keep the bottom of the hole clear of fill. The driller can also "clean up" the bottom of the hole with a few seconds of rotation and pumping, if necessary, before the probe is inserted. Calculations made following Leg 110 indicate that so long as the WSTP is inserted 60 cm or more below the bottom of the hole, within 2-4 hours after drilling stops, the thermal effects of circulation on sediment temperature measurements are negligible (Fisher and Hounslow, 1990).
Once pressed into the sediments, the WSTP is held stationary using the heave compensator (for latched-in, RCB deployment) or decoupled from the drill string (for colleted delivery, APC/XCB deployment). With RCB deployment, the heave compensator will be effective at maintaining the position of the bit and probe only when the amount of weight that will be held by the bit is greater than the sensitivity of the heave compensator, about 5000 lb. Thus measurements using the RCB BHA should be taken at depths only where sufficient weight can be maintained to hold the probe stationary. Unfortunately these depths may not be ideal for scientific purposes. In particular, it is difficult to measure temperatures with the WSTP deployed through an RCB BHA in very shallow sediments, which are often not competent enough to support the minimum weight required for heave compensation.
Keeping the probe stationary for about 10 minutes in undisturbed sediments is the primary consideration in obtaining good temperature data with the WSTP. The weight required to penetrate the sediments (for all deployments) and to hold the probe in place (for RCB deployments) will depend on sea conditions and the strength of the sediments, and will generally increase deeper in the section. If too little weight is applied, the probe may be allowed to move, degrading the data through two possible factors, additional frictional heating on probe movement and leakage of borehole fluids around the probe. On the other hand, if too much weight is placed on the bit, the probe may be slowly pushed ever deeper, resulting in continuous frictional heating, and the formation may even be fractured, allowing borehole fluids to leak around the probe and disturb the measurement.
When the measurement time has expired, the WSTP is pulled from the formation and retrieved with the coring line. Downhole-tools scientists may wish to have the WSTP pause again at mud line to check for recorder drift, although mud-line temperatures measured after a temperature station often are somewhat higher than those measured before a station, as warmer fluids can be swabbed up the pipe by the WSTP and core barrel.
3. WSTP data recovery, processing, and archiving
When the WSTP is recovered, it is extracted from its pressure case on the rig floor, carried up to the Downhole Tools Lab, and placed on the counter. The rig crew may need to be reminded to clean off the probe tip with a high-pressure water line before the tool is brought up to the lab. The tip is often packed with mud (particularly if a water sample was collected) and is very difficult to clean when dry.
WSTP data must be uploaded from the DCDL, converted from resistances to temperatures, and extrapolated to equilibrium to determine the in-situ temperature. Prior to Leg 139, downhole-tools scientists and marine specialists were required to learn the operating systems and file structures of at least two (and potentially four) different computers in order to download, convert, process, and store WSTP data. Each of these tasks is now considerably easier, and downhole-tools operators deal mainly with an IBM-compatible PC running DOS and Windows. Data are uploaded from the recorder to the PC, and automatically archived on the VAX. Conversion and processing are accomplished within a separate Windows program, with which scientists can also produce plots for their shipboard reports. Each of these specific tasks is described in the following sections.
Once the WSTP is returned to the lab, the stainless-steel shell is opened, and the DCDL is connected to the PC with a ribbon cable. The operator then runs the DCDL data-dumping program on the PC, which (1) uploads the data; (2) creates a header file of core, depth, and thermistor information; and (3) places archive copies of data and header files on the VAX. If the network is down, archive copies of these files are stored on a second hard drive on the PC. Working copies of the files also remain on the PC for processing. Data should be recovered from the DCDL as soon as possible after the WSTP is brought back on deck, as the data are stored in volatile memory and could be lost if the power supply to the recorder is interrupted.
Up until Leg 139, VAX-based programs were available to convert data from resistances to temperatures, and to provide a simple temperature-time plot of each station, but it was up to individual shipboard scientists to create their own programs and procedures for processing WSTP data. In 1989, K. Becker supplied the ODP with several PC-based programs (written in Fortran, with system calls to separate text-editing and graphics programs) which enabled scientists to interactively process pressure data collected during formation testing using a drill-string packer. During Leg 139, Becker modified one of these programs, along with a VAX program for analyzing APC-tool data, to produce two hybrid programs for processing WSTP and APC tool temperature data. These programs were similar in that they both matched real data to theoretical curves and extrapolated to infinite time to obtain "equilibrium" temperatures. During the fall of 1991 and spring of 1992, A. Fisher assisted M. Sun (ODP/TAMU) with developing an object oriented Windows program to support conversion and processing of WSTP temperature data. This new program is rooted in the methods used by the Leg 139 software, but is easier to use and features a fully graphical implementation, on-line help and advice, and an array of plotting options.
The downhole-tools scientist should be concerned with at least two stages of WSTP data processing: first, possibly correcting the raw data according to the results of calibrations before or after the station or at mud line during the station; and second, extrapolating the corrected temperatures to the in-situ value. A rigorous theory describes the approach of a cylindrical probe to in-situ temperature after penetration (Bullard, 1954; Jaeger, 1956), and the in-situ temperature should be determined by fitting the measured values to Bullard's (1954) F-function (tabulated by Lister, 1979; see also Hyndman et al., 1979). See the shipboard WSTP cookbook for more detailed instructions regarding data processing. Good-quality temperature vs. time records for a WSTP temperature run are shown in Figure III-3, along with examples illustrating the relationship between measured and extrapolated temperatures.