During Leg 205, a combined temperature and pressure tool was employed to determine in situ temperature and pore pressure. The Davis-Villinger Temperature-Pressure Probe (DVTPP) is a modification of the original Davis-Villinger Temperature Probe (DVTP), which is described in detail by Davis et al. (1997) and summarized by Pribnow et al. (2000) and Graber et al. (2002). The tip is of conical shape, is 1.1 m long, and has a diameter of 8 mm at the lowermost end. In the original version, two thermistors were mounted in the probe, the first located 1 cm above the tip, the second 10 cm above the lower thermistor. A third thermistor, referred to as the internal thermistor, was located in the electronics package. The temperature measurement range is -5° to 160°C with a temperature resolution of 1 mK at ambient deep-sea temperatures, increasing to 10 mK at 120°C. The thermistors are calibrated using the temperature vs. resistance tables provided by the manufacturer and, in addition, adjusted for offset by using bottom water temperatures from oceanographic data and mudline records. Additionally, the probe contains an accelerometer with a total range of ±2 g and a resolution of 0.01 g. Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track vertical disturbances to the instrument package during the equilibration interval.
The DVTPP allows simultaneous measurement of both formation temperature and pressure. The upper thermistor was removed, and ports on the sidewalls of the tip allow hydraulic transmission of formation fluid pressures to a precision pressure gauge inside. The dimensions of the probe tip have not been changed in order to not affect the thermal response. The Paroscientific Digiquartz transducer pressure gauge provides a resolution of 4 Pa over a range of 70 MPa. The tool tip is shown in Figure F18.
The tool is typically deployed by lowering it by wireline to the mudline and pausing for 10 min to collect data. Subsequently, the tool is lowered to the base of the hole and latched in at the bottom of the drill string with the end of the tool extending 1.1 m below the drill bit. The extended probe is pushed into the sediment below the bottom of the hole, and pressure is recorded for ~30 min. If smooth pressure decay curves are recorded after penetration, extrapolations to in situ pore pressures are possible.
The transient thermal decay curves recorded by the thermistor of the DVTPP are a function of the geometry of the tool and the thermal properties of probe and sediments (Bullard, 1954; Horai and Von Herzen, 1985). Data analysis requires fitting the measurements to temperature decay curves based on probe geometry and thermal properties of the sediment. Pribnow et al. (2000) discuss data analysis procedures and uncertainties. The software used for temperature data processing is CONEFIT, developed by Davis et al. (1997). Resulting uncertainties in the in situ temperature estimates are caused by several factors: (1) the fact the probe does not reach thermal equilibrium during the penetration period and derived temperatures must be extrapolated; (2) contrary to ideal theory, the frictional pulse upon insertion is not instantaneous; (3) temperature data are sampled at discrete intervals so that the exact time of penetration is uncertain; and (4) the in situ thermal conductivity of the sediments is imperfectly known.
Figure F19 shows a typical temperature history recorded by the DVTPP. Various stages in the tool deployment history are marked. Mudline temperature is determined from the time the tool is held near the seafloor prior to penetration of the DVTPP. In this example, the mudline temperature is not yet stable because of the pumping, which is stopped exactly when reaching the mudline. Initial penetration is marked by a temperature pulse due to frictional heating. A second pulse is observed when the tool is extracted from the sediment. In the example shown in Figure F19, we also took bottom-hole and mudline temperatures after the penetration. The best-fitting time of penetration and in situ temperature are calculated by the shipboard analyst. The estimated uncertainty of the derived in situ temperature for good-quality measurements is 0.1°C (Pribnow et al., 2000), although the uncertainty may be considerably larger for poor-quality measurements. Temperature gradients may be better resolved than absolute values of temperature, provided the same tool is used to make all measurements at a given site. In order to calculate heat flow, the thermal conductivity of the sediment must also be specified. Thermal conductivities measured from the core interval closest to the DVTPP measurement were used (see "Physical Properties").
The pressure response of the DVTPP is qualitatively similar to, but slower than, the thermal response. The decay time is a function of the sediment diffusivity and the magnitude of the initial pulse, which is a function of the taper angle and diameter of the tool (Whittle et al., 2001; Heesemann, 2002). Shipboard extrapolations to estimate formation pressures are preliminary estimates. A more complete analysis will be done postcruise.
Miniaturized temperature data loggers (MTLs) have been deployed inside the OsmoSamplers for long-term monitoring of fluid temperature fluctuations inside boreholes (see Jannasch et al., this volume). Moreover, they were used to measure high-resolution temperature profiles in the boreholes simultaneous to the wireline logging and to determine the bottom water temperature, which is used for calibration of the DVTPP.
The MTL (Pfender and Villinger, 2002) consists of a 140-mm-long cylindrical body with an outside diameter of 15 mm, housing the electronics, and a thin-walled tip (20 mm long with an outer diameter of 4 mm) containing the temperature sensor. The pressure housing consists of high-strength corrosion-resistant steel and withstands a pressure equivalent of 6000 m water depth. The logger is shown in Figure F20. Programming of the logger and downloading the data are performed without opening the pressure case. A readout unit contacts the logger's tip and end cap with a voltage delivered by an RS232 interface from a PC, and a high-strength plastic isolator between the tip and main body maintains the voltage so that the communication unit works as a two-point connection for data transfer.
The electronics of the logger consist of a microprocessor, a 16-bit analog-to-digital (A/D) converter, a real-time clock, and nonvolatile memory for up to 64,800 measurements. The sample interval can be varied from 1 s to 255 min, which allows up to 18 hr of logging at a 1-s interval or a longest theoretical recording time of ~33 yr. The complete system is powered by a standard small-sized 3-V lithium battery. We use a thermistor with interchangeability of 0.1 K as a sensing element in order to achieve a resolution of 1 mK at typical deep-sea temperatures of 2°C. The temperature measurement range extends from -5° to 60°C. The absolute accuracy of the logger after calibration with a high-precision thermometer in a well-stirred water bath is <5 mK, which is adequate to resolve expected temperature fluctuations in the boreholes. The design of this temperature sensor in a thin-walled tip satisfies the need for a fast-response sensor. The thermal time constant of the system is ~2 s.