In situ sediment thermal measurements were made during Leg 190 using the Adara APC temperature tool, the WSTP, and the Davis-Villinger temperature probe (DVTP) (Davis et al., 1997). Formation pore pressures were also measured using a prototype DVTP modified to include a pressure port and sensor. The instruments and procedures are summarized below.
The Adara temperature tool fits directly into the cutting shoe on the APC and can therefore be used to measure sediment temperatures during regular piston coring. The tool consists of electronic components, including battery packs and a data logger, and a platinum resistance-temperature device calibrated over a temperature range of 0°-30°C. A photograph of the components can be found in Fisher and Becker (1993). The thermal time constant of the cutting shoe assembly into which the Adara tool is inserted is ~2-3 min. The only modification to normal APC procedures required to obtain temperature measurements is to hold the corer in place for ~10 min after cutting the core. During this time, the Adara tool equilibrates toward the in situ temperature of the sediments.
The Adara tool logs data on a microprocessor contained within the instrument. Following deployment, the data are downloaded for processing on PC computers. The tool can be preprogrammed to record temperatures at a range of sampling rates. Sampling rates of 5 s were used during Leg 190.
A typical APC measurement consists of a mudline temperature record lasting 10 min for the first deployment at each borehole and 2 min on subsequent runs, a pulse of frictional heating when the piston is fired, a period of thermal decay that is monitored for 10 min, and a frictional pulse upon removal of the corer. Before reduction and drift corrections, nominal accuracy of Adara temperature data is estimated at 0.1°C.
The WSTP can be configured with or without a fluid-sampling capability. When the probe tip is configured for temperature measurement only, the tip has a shorter time constant such that (1) the frictional heat pulse associated with insertion of the probe can be assumed to approximate more closely a line source of heat and (2) insertion of the instrument is less likely to fracture semilithified sediments.
In operation, the WSTP is mounted inside a core barrel and lowered down the drill pipe by wireline while the bit is held above the bottom of the hole. The tool is held briefly above the mudline to measure the temperature of bottom water. The tool is then lowered and latched into place, with the probe tip extending 1.1 m ahead of the bit. The drill string is lowered, and the probe is forced into the bottom of the hole. A colleted delivery system allows the probe to retract inside the bit if the formation is too hard to penetrate. With an APC/XCB bottom hole assembly, the bit can be decoupled from the tool after penetration so that the probe will not be disturbed by drill-string heave. Insertion of the probe significantly disturbs formation temperatures. If the instrument can not be left in position to allow this disturbance to decay completely, extrapolation to thermal equilibrium is required.
The DVTP is described in detail by Davis et al. (1997). The probe is conical and has two thermistors, one 1 cm from the tip of the probe and the other 12 cm above the tip. A third thermistor, referred to as the internal thermistor, is in the electronics package. Thermistor sensitivity is 1 mK in an operating range of -5° to 20°C, and the total operating range is -5° to 100°C. The thermistors were calibrated at the factory and on the laboratory bench before installation in the probe. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s2. Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval.
In a DVTP deployment, mudline temperatures (within the drill pipe) are measured for 10 min during the first run within each hole and for 2 min during subsequent runs, before descent into the hole for a 10-min equilibration interval in the bottom. The time constants for the sensors are ~1 min for the probe tip thermistor and ~2 min for the thermistor at 12 cm from the tip. Only data from the probe tip thermistor were used for estimation of in situ temperatures.
Similar data reduction procedures were used for the three temperature tools. The transient thermal decay curves for sediment thermal probes are known to be a function of the geometry of the probes and the thermal properties of the probe and the sediments (Bullard, 1954; Horai and Von Herzen, 1985). Analysis of data requires fitting the measurements to analytical or synthetic decay curves calculated based on tool geometry, sampling interval, and tool and sediment thermal properties. For the DVTP tool, thermal decay data are analyzed by comparison to computed type curves using the software program CONEFIT, developed by Davis et al. (1997). However, it is generally not possible to obtain a perfect match between the synthetic curves and the data because (1) the probe does not reach thermal equilibrium during the penetration period; (2) contrary to ideal theory, the frictional pulse upon insertion is not instantaneous; and (3) temperature data are sampled at discrete intervals, so that the exact time of penetration is uncertain. Thus, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data. The data collected more than 20-50 s beyond penetration usually provide a reliable estimate of equilibrium temperature. The thermal decay curves for the measurements were initially fit on board assuming formation thermal conductivity of 1.0 W/(m·°C) for all of the data. Thermal conductivities measured in the shipboard laboratories (see "Physical Properties") were used for final estimation of in situ temperatures and for calculation of heat flow. Laboratory thermal conductivity measurements were not corrected for in situ conditions.
Prior to Leg 190, one of the DVTP tools was modified at the Geological Survey of Canada as a prototype to allow simultaneous measurement of formation temperature and pressure. A new tip was designed that incorporates both a single thermistor in an oil-filled needle and ports to allow hydraulic transmission of fluid pressures outside the probe to a precision Paroscientific pressure gauge within the probe. A standard data logger was modified to accept the pressure signal instead of the second thermistor signal in the normal DVTP described above.
In 1999, the prototype was successfully tested on a wire from an oceanographic ship in sediments in 2600 m of water on the eastern flank of the Juan de Fuca Ridge (E. Davis, R. Macdonald, and R. Meldrum, unpubl. data). These tests demonstrated that (1) the thermal response of the modified tip remains virtually identical to that of the unmodified DVTP and (2) smooth pressure decay curves are recorded after penetration, such that useful extrapolations to in situ pore pressures are possible with measurement times on the order of 30 min. This pressure response is qualitatively similar to but slower than the thermal response, and the model for the characteristic response of pressure to the displacement and sediment deformation associated with penetration is more complex than the model used to estimate in situ temperatures from the decay of the frictional heating pulse. Construction of a complete analytical or numerical model of the pressure response had not been completed by the time of Leg 190, so the stations were recorded for 30 min or as long as deemed operationally safe, and shipboard extrapolations to estimated formation pressures must be considered preliminary until thoroughly processed postcruise.