HR: 0800h
AN: V41A-0395    [Abstracts]
TI: Integration of 3D MT Resistivity Imaging With Borehole Petrology, Temperature and Resistivity Log Data to Characterize the Geothermal Resource at Glass Mountain, California
AU: * Cumming, W B
EM: wcumming@wcumming.com
AF: Cumming Geoscience, 4728 Shade Tree Lane, Santa Rosa, CA 95405, United States
AU: Mackie, R L
EM: randy@geosystem.us
AF: GSY-USA, Inc., PMB# 643 2261 Market St., San Francisco, CA 94114, United States
AB: The integration of resistivity images from magnetotelluric (MT) surveys with supporting geophysics, geology and borehole data shows that the resistivity pattern to depths of at least 3000 m in the Glass Mountain geothermal resource area is primarily controlled by interface conduction in temperature-dependent hydrothermal clays. Over 200 MT stations, 350 TDEM stations and 500 gravity stations are integrated into the analysis along with temperature and petrology data from 26 boreholes from 500 to 3000 m and a resistivity well log from 340 to 2800 m depth. These data cover the Glass Mountain geothermal area within the 7 by 12 km ring fracture at the summit of Medicine Lake Volcano and extend down its flanks. The integration of the geophysical data illustrates the relative effectiveness of 1D, 2D and 3D MT inversions and highlights limitations of some conventional joint analyses such as the use of TDEM to correct MT static distortion. The most effective conceptual integration is a comparative analysis of the 3D MT resistivity imaging with respect to detailed borehole petrology, resistivity and temperature logs. The 3D MT inversion images and well data illustrate the petrophysical origin of two prominent resistivity transitions in the Glass Mountain geothermal resource area. Near surface, unaltered volcanics have relatively high resistivity, over 200 ohm-m. At depths from 100 m to 700 m, there is a transition from the resistive unaltered volcanics to a 2 to 10 ohm-m zone correlated with low resistivity zeolite and smectite hydrothermal clay alteration found at temperature lower than 200°C. Where temperature exceeds 200°C, smectite clay becomes unstable and alters to more resistive illite, accounting for a transition to 10 to 100 ohm-m resistivity at 500 to 1400 m depth. The correlation of resistivity with temperature-sensitive clay alteration provides a basis for interpreting subsurface temperature throughout the volume imaged by the 3D MT inversion to 3000 m depth in the Glass Mountain area, as confirmed by data from the 26 boreholes.
DE: 0699 General or miscellaneous
DE: 0925 Magnetic and electrical methods (5109)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting