Dead Dog Vent Field
The major scientific objective for Site 858 (Hole 858G) (Fig. 4) was to replace the existing CORK and thermistor string as part of an active hydrologic flow experiment designed to enable temporal monitoring of changes in subseafloor temperature and pore pressure. Observations by the Alvin submersible in 1993 established that the pore pressure in the drill hole had returned to seafloor hydrostatic conditions, indicating a failure of the borehole seal, and that the datalogger had eventually failed due to flow of hydrothermal fluid through the base of the reentry cone. Because the hole was observed to be flowing in 1993 and because it was cased to basaltic basement, it represented a unique opportunity to sample formation fluids from the basalt that could be compared to fluids venting through the overlying sediment cover only a few tens of meters away from the hole. Therefore, measuring the temperature profile and obtaining high temperature borehole fluids were important ancillary objectives for this site. A planned hole-to-hole hydrologic experiment involving monitoring the pressure in Hole 858G in response to a transient pressure pulse to be induced by drilling in Hole 857D, which is located 1.6 km to the south, dictated that the temperature structure in Hole 858G should be disturbed as little as possible during operations.
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