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doi:10.2204/iodp.proc.346.109.2015

Downhole measurements

In situ temperature and heat flow

Five formation temperature measurements, including one at the mudline, were performed in Holes U1428A and U1429A using the APCT-3.

In Hole U1428A, in situ temperatures range from 10.30°C at 34.9 m CSF-A to 20.56°C at 120.4 m CSF-A (Table T22), with a linear downhole increase indicating that the gradient is uniform with depth (Fig. F57) despite a small offset of the measurement taken at 91.9 m CSF-A. A linear fit of temperature versus depth yields a geothermal gradient of 116°C/km. The trend line of the in situ temperature measurements intersects the seafloor at 6.26°C, giving a lower value than the average mudline temperature in the four APCT-3 measurements (7.61°C) (Fig. F57A).

Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndmann et al. (1974) (see “Physical properties” in the “Methods” chapter [Tada et al., 2015b]). Thermal resistance was calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole (Fig. F57). A heat flow of 126 mW/m² was obtained from the slope of the linear fit between in situ temperature and calculated in situ thermal resistance (Pribnow et al., 2000).

In Hole U1429A, in situ temperatures range from 8.93°C at 34.9 m CSF-A to 14.78°C at 118.1 m CSF-A (Table T22). There was an important uncertainty in fitting to the APCT-3 temperature equilibration curve to obtain the formation temperature at 118.1 m CSF-A, probably because of poor contact of the temperature probe with the formation during in situ temperature measurement. The temperature measured from Core 346-U1429-13H depth thus deviates from the linear downhole increasing gradient (Fig. F58) and was not taken into account in the successive calculations. A linear fit of temperature versus depth gives a geothermal gradient of 94°C/km. This is lower than the geothermal gradient measured at Site U1428 (116°C/km), located only 15 km away. The trend line of the in situ temperature measurements intersects the seafloor at 6.94°C (Fig. F58). This is lower than the average mudline temperature in the four APCT-3 measurements (7.77°C) and slightly warmer than the seafloor temperature calculated at Site U1428 (6.26°C, Fig. F57A). A heat flow of 88 mW/m² was obtained from the slope of the linear fit between in situ temperature and calculated in situ thermal resistance (Fig. F58) (Pribnow et al., 2000).

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