The TPC tool is designed to continuously record temperature, pressure, and conductivity at the face of a modified ODP APC piston assembly (Figs. F2, F3, F4) during deployment, core collection, and recovery. The APC piston was modified to contain the TPC tool components (Fig. F3), and the tool has a maximum design pressure of 10,000 psi, which corresponds to ~6.9 km total hydrostatic depth.
Making measurements at the piston face allows for standardization of the effects of cooling during core ascent. This location was chosen for practical reasons: (1) the APC piston could be easily modified to incorporate TPC sensors; (2) the data logging electronics and battery could be housed in the existing piston corer subassembly, making this an autonomous downhole tool; (3) no special downhole tool runs would be necessary to collect TPC data, causing little disruption to the tempo of coring operations; and (4) gas can collect in a recess on the piston face, making electrical conductivity measurements possible.
Because there is a large contrast in electrical properties between a gas phase (lower conductivity) and wet sediment and/or seawater (higher conductivity), the conductivity sensor was designed to detect the formation of a headspace gas phase at the top of a sediment core (Fig. F2). This information aids in analyzing the temperature and internal pressure data obtained during core ascent by providing an independent confirmation of headspace formation.
The temperature and conductivity sensors are recessed in shallow wells on the piston face for mechanical protection and for the accumulation of a headspace gas phase (Figs. F2, F3, F4). The pressure port opens directly onto the face of the APC piston. The sensors are threaded into the sensor head for ease of replacement.
Temperature is measured using a YSI 55036 thermistor encapsulated within a thin-walled stainless steel probe (3/16 in diameter x 1/4 in long) with a pressure rating of 10,000 psi (Logan Enterprises, West Liberty, Ohio, USA). The time constant for this thermistor probe is 1.5 s, and the accuracy is ±0.05°C. Pressure is measured using a transducer designed for use in corrosive downhole environments and for temperature stability (model 211-37-520; Paine Corp., Seattle, Washington, USA). Measurement error is ±0.15% of the full-scale reading of 10,000 psi. The conductivity probe is a miniature bulkhead connector with an inconel body and three gold-plated 0.040-in-diameter Kovar pins (PMS-series; Kemlon Products and Development, Pearland, Texas, USA). Pin spacing is 3.2 mm.
Design objectives for the electronics included minimal power consumption, low component count, 12-bit or better analog-to-digital (A/D) resolution, low long-term sensor drift, vibration tolerance, large amounts of nonvolatile data storage, and ease of programming and component replacement. Three elements compose the electronics package: (1) the sensor conditioning electronics, (2) an off-the-shelf microprocessor unit, and (3) the batteries. Low-power 3.3-V CMOS semiconductor components were selected to minimize current drain. The discrete components are surface-mounted on a narrow multilayer printed circuit board that supports the detachable microprocessor unit. To prevent polarization of the conductivity electrodes, the conductivity sensor is excited using an alternating-current signal generated by a simple oscillator circuit. After signal conditioning, the analog signals from the sensors are fed into either a Maxim MAX147 12-bit A/D converter (thermistor and conductivity) or a Cirrus CS5509 16-bit A/D converter (pressure). These A/D converters are connected to a Persistor CF1 microprocessor unit via the Queued Serial Peripheral Interface implemented by the Motorola 68338 central processing unit. Data are stored in a 48-MB CompactFlash card hardwired to the CF1 microprocessor unit. Communications with the CF1 occurs through an RS-232 interface. The CF1 is programmed in C, using the Metrowerks CodeWarrior programming environment. TPC data were collected at 1-s intervals throughout tool deployment.
Power is supplied by two double-C lithium thionyl chloride batteries within a 1-in-diameter x 9-in-long battery pack that provides 7.3 V with a 100-mA rating. The electronics/battery assembly was designed for no less than 100 hr of continuous operation.
Data transfer can be accomplished while the TPC tool is installed in the APC drilling string through a RS-232 communications port on the face of the piston. This data port is a three-pin keyed bulkhead connector (PMJ-series; Kemlon Products and Development, Pearland, Texas, USA). During coring operations, this port is O-ring sealed with a faceplate that is easily removed for access (Fig. F4).