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doi:10.2204/iodp.proc.311.108.2006

Tool description

Like the earlier generation APCT tools, the APCT-3 tool fits in an annular cavity in the APC cutting shoe (Fig. F1A). Tool electronics are arranged on a cylindrical frame; sensor and registration prongs extend close to the end and outside edge of the cutting shoe. During standard deployment, the APC inner core barrel is run to bottom on the coring wireline at the bottom of the borehole. Pump pressure is then applied to the drill pipe, which acts as a hydraulic accumulator. When pressure is great enough, it severs shear pins and strokes the inner core barrel 9.5 m into the sediment, well beyond the thermal influence of drilling operations. Following penetration, the core barrel is decoupled from the drill string and the APCT-3 is left stationary for 7–10 min. This allows part of the thermal disturbance associated with frictional heating of the penetration to decay. Finally, the coring shoe and core barrel are extracted from the formation and returned to the deck, where communication with the APCT-3 tool can be established and data are extracted for analysis.

The APCT-3 temperature logger consists of a temperature sensor and two circuit boards mounted on a cylindrical supporting frame (Fig. F1B). There is sufficient space on the electronics frame to hold a second electronics set, although this space is currently not used. It is anticipated that scientists may wish to develop additional measurement capabilities in the future using this space. At the base of the frame are two prongs that fit into index holes at the bottom of the annular cavity in the cutting shoe. These prongs help keep the electronics in place during deployment, and one of them houses the temperature sensor.

The sensor element for the APCT-3 tools is an aged, glass-encapsulated thermistor (Model YSI 55032). This thermistor has a high temperature coefficient (~4% per degree change in temperature) across a wide usable temperature range from –80 to +100C (Fig. F2B) and can withstand temperatures as high as 200C for brief periods. The lead wires of the sensor are soldered to the circuit board that holds the logger electronics. This board contains a microprocessor, a 16-bit analog to digital (A/D) converter, a real-time clock, and nonvolatile memory for as many as 65,000 readings.

Electrical power is supplied by two standard lithium batteries mounted on a second circuit board, which is easily serviceable and replaceable. The battery capacity allows at least 600,000 readings equaling continuous operation for ~1 week at a sampling rate of 1 s. Even if there is a total loss of battery voltage, the recorded data are preserved in nonvolatile memory. The APCT-3 electronics can safely be operated at ambient temperatures from –10 to 60C. Nominal temperature resolution of the complete instrument is 2.5 mK over a range from –6 to +55C, and 1.0 mK at temperatures 25C (Fig. F2). This range should be adequate for the vast majority of shallow environments in which APC coring is likely to be attempted during IODP.

Communication with the APCT-3 temperature logger is accomplished using custom software (WinTemp) and an interface box that connects to the contact pins of the logger on one side (Fig. F1B) and to the RS-232 port (or universal serial bus [USB] port by adapter) of a personal computer (PC) running Microsoft Windows 2000/XP. WinTemp was originally developed for use with MTLs and provides a graphic interface for programming and data recovery. Using a logger setup dialog, it is possible to set the logger's real-time clock or synchronize it with the clock of the PC. The operator sets logging start date and time, logging duration, and a constant sample interval during deployment ranging from 1 s to 255 min. The software also provides functionality to retrieve temperature data from the logger, clear the tool's memory, check battery voltage, and display real-time readings of digital counts, resistance, and apparent temperature.

After recovering data from a tool, WinTemp displays calculated temperatures in text format, and data can be saved in a binary file (*.wtf), a WinTemp-specific format, or exported to an ASCII-format file (*.dat). The latter file consists of a multiline header, which includes the logger identification number and columns of measurement dates, times, A/D converter readings, computed thermistor resistances, and computed temperatures. Computations of resistance and temperature are based on the information contained in a calibration file (*.wtc), which is assigned to an individual logger. Tool-specific calibration information is stored in the binary data files along with the logger readings.

Antares supplies calibration files with every delivered logger. The contents of these files are based on the specifications of the electronic components comprising the APCT-3 data logger and the resistance-temperature curves supplied by the manufacturer of the thermistors. These calibration files offer an absolute accuracy on the order of 0.1C, but considerably greater accuracy on the order of 0.001–0.002C can be achieved through careful calibration and processing, as discussed below.

Once data are converted to temperatures using the calibration coefficients, additional processing is required to estimate undisturbed formation temperature. As with all penetrating subseafloor temperature measurement tools, a tool response function must be used to extrapolate observations (e.g., Bullard, 1954; Davis et al., 1997; Horai, 1985). This function depends on tool geometry, distribution of frictional heat generated during tool penetration, and thermal properties of the sediments and the tool. For conventional oceanic heat flux measurements the analytical F(a,t) function is used as a reference for comparison with observations (Bullard, 1954; Carslaw and Jaeger, 1959). Use of the F(a,t) function presumes, however, that the geometry of the probe is well described by a semi-infinite solid cylinder, which is not the case for the more complex geometry of the APCT-3 tool.

An analytical solution for a one-dimensional radial geometry was developed for the first-generation APCT tool (Horai, 1985), with a central cylinder of sediment, a ring of metal, and an infinite surrounding region of sediment. This approach, like that for a thin cylindrical probe, is based on the assumption that the tool behaves as if it extends vertically well away from the sensor and that heat transport following tool insertion is purely radial within a homogeneous medium. This approach was adapted for use in software having a graphical interface (TFit) that was developed as part of the second-generation APCT tool.

Decay curves are calculated for different sediment thermal conductivity values, and a curve is usually selected on the basis of independent observations of thermal conductivity from needle probe measurements on recovered core. It is common practice during processing to allow an arbitrary shift in time between modeled and measured decay curves so as to minimize the misfit between measured and modeled decay curves. This time shift is intended to correct for a series of nonideal behaviors, including incomplete knowledge of tool insertion time, finite time for insertion (possibly involving multiple frictional pulses), finite tool response time, incomplete thermal coupling between the sensor and the shoe, modification to sediment properties both inside and outside the tool, short-term advection immediately adjacent to the coring shoe, and nonhorizontal heat conduction.

As a practical matter, there is often considerable uncertainty in sediment thermal properties, including heterogeneity, immediately adjacent to the APC coring shoe, so the user must process data using a range of assumed sediment thermal conductivities in order to evaluate uncertainties. Even in the case of ideal tool response (i.e., no motion of shoe during temperature decay following the initial friction spike), it is not possible to use statistical-fit criteria to determine the effective thermal conductivity; essentially all decay curves can be made to fit the data, albeit on the basis of different time shifts. In cases where the decay curve is not ideal, it may be necessary to use only part of the record and different record segments may indicate different equilibrium temperatures. As a result of these uncertainties, a range of extrapolated temperatures will be generated, and it is often not possible to determine the equilibrium temperature with uncertainties <0.1–0.2C.

In addition, the geometry of the APC coring shoe is not one-dimensional and radial. The shoe is tapered at its front end, and the probe tip is located close to the front of the shoe (Fig. F1A). As a result, the measured temperature in the shoe may decay according to a function that differs from that derived for a one-dimensional, radial analytical model. Additional deviations from the one-dimensional model may result from heterogeneities in sediment properties (natural or induced by tool penetration) and associated frictional heating or conduction of heat vertically along the coring shoe.

The complete APCT-3 project includes considerable modeling and analysis, much of which is still underway, to evaluate uncertainties in extrapolated temperatures and improve standard procedures. For the purposes of prototype testing during Expedition 311 and the results presented in this volume, we have used TFit software developed for the second-generation APCT tool. This allowed rapid analysis of available data and comparison with data collected using earlier tools. We will reanalyze Expedition 311 data later when the rest of APCT-3 modeling and software development is complete.