HR: 1340h
AN: H23C-1511    [Abstracts]
TI: The Effects of Gravity-Induced Stresses on Hydrogeologic Properties Near Ridges: Examples From two Rift Basins
AU: Savage, W Z
EM: savage@usgs.gov
AF: U. S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: * Morin, R H
EM: rhmorin@usgs.gov
AF: U. S. Geological Survey, Denver Federal Center, Denver, CO 80225, United States
AU: Matter, J M
EM: jmatter@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Rivard, C
EM: crivard@nrcan.gc.ca
AF: Geological Survey of Canada, 490, rue de la Couronne, Quebec, QC G1K 9A9, Canada
AU: Goldberg, D S
EM: goldberg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AB: Twenty Mesozoic rift basins have been identified along the northeast coast of North America. At two of these, the Minas Basin in Nova Scotia and the Newark Basin in New York, steep topographic ridges formed initially by extensional tectonic processes have been evaluated for groundwater potential by obtaining geophysical logs in nearby water wells. The two locations have common lithologic and topographic characteristics, whereby a basalt cap overlying sedimentary rocks is exposed along a ridge flank. Despite these similarities, the basalts in Nova Scotia serve as a local water supply whereas the wells in New York are non-producing. Analyses of acoustic televiewer logs and measurements from flowmeter/pumping tests locate and distinguish the permeable fractures from the general fracture population. In addition, a complementary finite-element stress model is used to simulate ridge geometry from topographic maps and incorporates rock elastic properties as determined from processing full-waveform sonic logs. Results derived from field data and modeling indicate that gravity-induced stresses near ridges result from a lack of buttressing along the free faces and are amplified by spreading effects due to a contrast in Poisson's ratio between the basalts and the sedimentary rocks. These conditions act to enhance the permeability of fractures striking subparallel to the ridge axis in Nova Scotia. Conversely, the predominant fracture population in New York strikes perpendicular to the ridge axis and to the maximum horizontal principal stress. Consequently, there are few fractures preferentially aligned with the stress field and no hydraulically conductive features are recognized in these rocks. The hydrogeologic properties of the basalts near the ridge flanks at these two sites are markedly different because of the orientation of pre-existing fractures and their response to imposed stresses as derived from topography and lithostratigraphy.
DE: 1822 Geomechanics
DE: 1829 Groundwater hydrology
DE: 1835 Hydrogeophysics
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting