Pressure, volume, temperature states within the VC-2B corehole, Valles Caldera, New Mexico, U.S.A.

Two different instruments were used over a 17.5 month period to make high-resolution temperature measurements in the VC-2B corehole. The first tool transmits data to the surface and has a 300°C limitation imposed by the logging cable; the second tool has a design capability of 400°C and stores data in an onboard computer. The pressure in the corehole was calculated from an assumed equation of state that accounts for the NaCl dissolved in the borehole fluids. The maximum thermodynamic state achieved in VC-2B (136 bars, 295°C) occurred at a bottom hole depth of 1.76 km. Temperature data and the equation of state were used to calculate the pressure within a closed vessel withdrawn from the corehole. These calculations support experimental observations that fluid sampling devices used in VC-2B leak and return anomalously large amounts of fluid to the surface. The extension of the methods used herein to the investigation of subseafloor systems that approach the critical point of sea water is discussed.