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Downhole measurements

Direct measurements of in situ pressure and temperature are critical to defining transport, diagenesis, and microbial activity in marine sediments. These data can be used to evaluate heat flux, fluid flow, and chemical transport through the system. Additionally, the data are valuable for validating shipboard and shore-based estimates of pressure and stress from consolidation experiments or interpreting porosity-depth data. The pressure and temperature fields also impact the strength behavior of sediments and therefore can be used to evaluate conditions that promote or impede fault slip.

In situ pressure and temperature measurements

During Expedition 322, a test deployment of the sediment temperature-pressure (SET-P) tool was used to evaluate deployment procedures and sensor calibrations. No in situ temperature and pressure measurements were made in the formation because of difficulties locating suitable target intervals of unlithified sand at Site C0011 (see "Downhole measurements" in the "Site C0011" chapter).

The SET-P tool incorporates a single thermistor in an oil-filled tip and ports that allow hydraulic transmission of formation pressure to an internal pressure gauge. A standard data logger records the pressure and temperature data. The temperature sensor in the SET-P tool operates over a range of 0°–85°C with a resolution of 0.001°C. The pressure sensor has a sensitivity of 70 Pa over a range of 0–70 MPa. All pressures and temperatures are recorded at 1 Hz. The SET-P tool also includes a three-component accelerometer. All accelerometer data are recorded at 10 Hz.

A typical SET-P deployment consists of connecting the SET-P tool to the colleted delivery system (CDS), lowering the tool string by wireline ~1000 m, and then taking a 30 s calibration measurement. The tool is lowered another 1000 m and a second 30 s calibration point is measured. The tool is then lowered to the seafloor for a 5 min calibration point. The drill bit is then raised ~2 m off the bottom of the hole. Subsequently, the tool string is lowered until the CDS engages in the bottom-hole assembly (BHA), with the tip of the tool extending 1.1 m below the drill bit. The SET-P tool is pushed into the sediment by lowering the drill bit to the bottom of the hole. Pressure and temperature are recorded for >30 min. No fluids are circulated during data collection at the calibration points or when the tool is in the sediment. The tool string is then recovered via wireline. During the Expedition 322 test run, only the 30 s calibration tests in the water column were completed; the SET-P tool was not advanced into the formation.