ODP Technical Note 10


C. APC Temperature Tool

With the development of the hydraulic piston corer during the late stages of DSDP, R. Von Herzen developed a miniature recorder designed to fit in the cutting shoe and measure sediment temperatures as a core was taken (Horai and Von Herzen, 1985; Koehler and Von Herzen, 1986). The Von Herzen tool was extremely successful, but the eight instruments purchased by ODP in 1984 became damaged, one by one, until none was left by Leg 117. It took several years to develop a replacement tool, but this was finally accomplished in time for ODP Leg 139. In the winter of 1990-91, ODP contracted with Adara (a small geotechnical company in British Columbia) for the design and construction of 10 new tools. The Adara development included interface boxes (to allow the tools to communicate with an IBM-compatible PC), connector cables, deployment and data-analysis software, documentation, and assorted battery packs, retaining rings, and spare parts. Four of the tools were delivered in time for Leg 139, and the rest have been delivered since then. One tool remains at TAMU/ODP in College Station for training and testing purposes, and the other nine tools and all the necessary hardware and software are routinely available on board JOIDES Resolution as of Leg 146.

1. APC tool design and components
Like the original Von Herzen instrument, the Adara temperature tool (APC tool) is designed to measure sediment temperatures as a piston core is taken. Each tool comprises a series of circuit boards, battery packs, a temperature sensor, and assorted retaining hardware (Fig. III-4). The programmable recorder and battery packs are built into a cylindrical frame, designed to fit in the cutting shoe of the advanced hydraulic piston corer. At the base of the APC tool frame are two stainless-steel prongs which fit into holes at the bottom of the annular cavity in the cutting shoe. These prongs help to stabilize the instrument in the cavity, and one of the prongs also contains a platinum resistance temperature device (RTD) which is used to monitor formation temperature during a station. The wall thickness of the cutting shoe outside the annular cylinder is about 1.5 cm, giving it an effective time constant of roughly 2-3 minutes in sediments. Thus the APC must be held in place for at least 10 minutes (and preferably 15) to record enough data to reliably extrapolate to in-situ sediment temperature.

The APC tool is run with two battery packs, each of which contains seven ordinary "camera" batteries. The tool was originally designed to draw primary power off one of the battery packs, and clock power off the second. A diode has been added to the APC tools to allow the instruments to draw clock and main instrument power from both battery packs at the same time, which helps increase overall battery life and reduces the chances of running out of power during a station. In contrast to the WSTP, data storage in the APC tool recorder is nonvolatile, meaning that previously collected data are retained in memory even if power is lost during a station. The APC tools are calibrated over a temperature range of -20° to 100°C, and will operate at temperatures up to 125°C. The latter limit was verified during Leg 139 when an APC tool functioned perfectly until the recorded temperature reached 127°C! The tool might actually have operated to higher temperatures, but the battery packs began to deform, and the tool lost power. At higher temperatures there is also danger of damage to the tool's o-ring's, which could cause catastrophic flooding.

2. APC tool preparation and deployment
As noted above, the major advantage of the APC tool over the WSTP is that the APC tool is used to measure temperatures as a piston core is taken, with very little time required in addition to that needed to take the core. On the other hand, use of the APC tool is limited to depths at which the force required on pullout is within safe limits, generally the upper 120 m. This depth limit is somewhat less than the limit for regular APC coring, because the sediments will "grip" the corer more tightly during a 10-15-minute station to measure temperatures than during the much shorter time required simply to collect a core.

The APC tool is normally used on selected cores in a sequence of APC cores in the upper sediments, with allowable depths of measurement determined by the lengths of the cored intervals, usually 9.5 m. (If offset APC holes are cored at the same site, staggering of APC-tool deployments in two or more holes can increase sampling density.) The selection of temperature stations and preparation of the recorder must be carefully coordinated to avoid disrupting the coring routine, which can be surprisingly rapid-fire in a typical APC hole, particularly in shallow water. Interested scientists must consult in advance with the ODP lab officer and downhole-tools specialist to specify the cores on which the APC tool is to be run and the sampling sequence to be used. APC tools were run on successive cores during Legs 139 and 146 (i.e., Cores 3H, 4H, 5H, etc.); this mode of operation requires extremely careful planning and at least two tools and two dedicated personnel.

Each APC tool contains a microprocessor and can be programmed to operate with a variety of functions and sampling intervals. Programming is accomplished with a spreadsheet-like interface on a PC, and the test sequence is downloaded to a tool which has been preloaded into a coring shoe. The shoe is then made up to a core-barrel cross-over sub and handed to the ODL core techs for deployment.

If the APC tool and cutting shoe are ready at the appropriate time, the station will proceed like a normal piston core, with one obvious exception: after the corer is hydraulically fired into the sediment, it must be held in position to record enough data to allow the in-situ temperature to be determined. The corer is then retrieved in normal fashion. It may also be desirable to pause with the tool at mud line during tool retrieval to check the bottom-water temperature. When it is brought on deck, the cutting shoe is broken off and given to the downhole tools-scientist or specialist before the core is removed from the core barrel.

3. Data retrieval and processing
As with the WSTP, the downhole-tools marine specialist is responsible for dumping the raw data from the APC tool to the downhole-tools PC and uploading archived data to the VAX. The APC- tool software is easy to run and includes automatic conversion of recorded resistances to temperatures and plotting on the computer screen. Processing of the data to determine the equilibrium sediment temperature requires running a separate program on the PC. To determine the best estimate of true (equilibrium) sediment temperature, the measured temperatures must be fit to a function that describes the approach of the APC cutting shoe after penetration to true temperature. This function was formulated in two dimensions by K. Horai (in an unpublished manuscript titled "A theory of processing down-hole temperature data taken by the hydraulic piston corer (HPC) of the DSDP.") The data-reduction procedure is summarized by Horai and Von Herzen (1985). Copies of these papers are available in the shipboard reprint collection (see Appendix A). The procedure for data reduction is somewhat interactive and subjective, and must be conducted carefully, because (1) Horai's formula probably does not apply at very short times after penetration, and (2) the thermal properties of the cutting shoe introduce an ambiguity in assigning an effective time origin in treating the data.

Just before Leg 146, a new graphically interactive program was provided by Adara for processing data collected with the APC tool. This APC tool program is based on the original Horai programs which ran on the VAX. The new program is similar in format to the program developed at ODP for WSTP data processing, with the following differences: (1) the APC tool program runs under DOS rather than Windows, (2) the APC tool program does not allow direct access to the data (although the data can be accessed through DOS by exiting or shelling out of the processing program), and (3) the APC tool program does not provide the ability to make hard-copy of plots. The program is very fast, however, allowing shipboard scientists to determine equilibrium temperatures quite easily. Figure III-3 shows examples of excellent data recorded with the APC tool during Leg 139, and the fit of the data to a theoretical cooling curve, calculated with the revised programs presently available.

To In-Situ Temperature Measurements, Section D.
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