HR: 1340h
AN: T13A-1350    [Abstracts]
TI: Determination of Thermal Properties at the SAFOD Site Through Cross-Hole Temperature Monitoring
AU: * Williams, C F
EM: colin@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Grubb, F V
EM: fgrubb@usgs.gov
AF: US Geological Survey, Placer Hall, Sacramento, CA 95819
AU: Galanis, S P
EM: pgalanis@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: The USGS is investigating the thermal state of the San Andreas fault (SAF) at the site of the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, California, through a series of detailed heat-flow measurements in both the pilot hole, drilled in 2002, and the main fault-crossing hole, spudded in the summer of 2004. The proximity of the existing SAFOD pilot hole to the new, fault-crossing main hole (ranging from 5 to 10 m in the upper 1100 m) provides the opportunity to utilize pilot hole temperature measurements to monitor the thermal effects of the drilling disturbance in the fault-crossing hole. The time evolution and magnitude of the resulting temperature changes in the pilot hole reflect the bulk thermal properties of the rock mass between the two holes. Analysis of these temperature changes addresses a significant limitation on the accuracy of heat-flow measurements in both holes. This is the absence of whole-rock thermal conductivity determinations in the sediments from the upper 770 m of the drilled section, for which there are no cores or open-hole wireline logs. Although grain density and thermal conductivity measurements on drill cuttings from the pilot hole provide values for the rock matrix, significant uncertainty remains regarding porosity and whole-rock conductivity. The time series of pilot-hole temperature measurements have been compared with a two-dimensional numerical model of the radial propagation of the temperature disturbance from drilling the fault-crossing hole. Measurements of the temperature disturbance are ongoing as drilling continues, but preliminary results indicates that the thermal diffusivity of the sedimentary section averaged over 150 m depth intervals ranges from 5.5 to 8.5x10$^{-7}$ m$^{2}$ s$^{-1}$. Analysis of these estimates with the sediment mineral composition determined from drill cuttings yields a range of thermal conductivity from 2.2 to 2.6 W m$^{-1}$ K$^{-1}$ and indicates that heat flow in the sediments is approximately equal to the value of 91 mW m$^{-2}$ measured in the basement below 770 m.
DE: 8100 TECTONOPHYSICS
DE: 8107 Continental neotectonics
DE: 8130 Heat generation and transport
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5134 Thermal properties
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting