Crustal Imaging and Characterization Team
Subsurface Characterization of Hydrogeologic Properties of Intrabasinal Faults
- Investigate the use of geophysical (primarily aeromagnetic) techniques to
predict the hydrogeologic nature of faults in the subsurface
- Understand which lithologic or structural parameters related to faulting are
important for input to hydrologic modeling
Specific objectives are to
- Better understand the sources of aeromagnetic anomalies at intrabasin faults
in general, in order to establish which parameters--juxtaposed lithologies,
fault geometry, fault-related sedimentation, etc. --are responsible for the
variable aeromagnetic signatures across faults.
- Further investigate preliminary results of rock-magnetic studies that suggest
magnetization increases with grain size, implying that coarse-grained, potentially
permeable, material can be detected by aeromagnetic surveys, especially where
it is concentrated at faults.
- Examine faults in outcrop to better understand the hydrogeologic properties
of the faults in relation to their geophysical expression, to establish their
role in basin development and affect on paleoflow, and to determine the range
of variability of the hydrogeologic parameters.
- Develop geophysical/geologic models for specific faults using constraints
provided by additional geophysical methods and subsurface drilling.
- Examine the upscaling problem of extrapolating information from the very
local scale of outcrop examinations to a more regional scale of aeromagnetic
investigations to an even more regional scale of a ground-water flow model
and whether the relations between geologic, hydrogeologic, and geophysical
parameters change in importance or not at the different scales.
The strategy to accomplish these objectives is to
- Concentrate geophysical, geologic, and rock-property work on the San Ysidro
fault, chosen because it is buried at one end, gradually exposed to deeper
levels along strike, has a good aeromagnetic expression, is located in the
vicinity of several boreholes, and seems fairly typical of other significant
faults in the basin.
San Ysidro Fault
Key Findings:
Fault systems that offset and influence deposition of sediments in extensional
basins played key roles in the development of critical alluvial aquifers in
the U.S. desert southwest. Our multi-disciplinary investigations of the San
Ysidro and other faults in the northern Albuquerque basin have provided several
valuable insights into the influence of fault zone material on ground water
flow and how aeromagnetic data can be used to predict aquifer heterogeneity
at faults.
Field mapping and structural characterization studies in conjunction with permeametry
results indicate that faults we have investigated act as partial barriers to
lateral groundwater flow and thus compartmentalize the basin aquifer system.
In particular, permeametry tests indicate that a clay-rich "gouge",
which is consistently present in fault zones, has nearly six orders of magnitude
lower permeability than the surrounding faulted rocks. This low permeability
would present a significant barrier to lateral ground water flow. Major heterogeneities
due to juxtaposition of sedimentary units with different hydraulic properties
and variable cementation of fault-related bodies also commonly combine to impede
ground water flow.
Our observations, combined with recent evidence from InSAR data of fault control
on subsidence due to ground-water withdrawal, suggest that the degree of aquifer
compartmentalization may be more significant than currently realized. Our current
investigations are focusing on understanding the origin and development of the
fault zone characteristics, which help us understand their prevalence in the
subsurface, and on numerical modeling, which will indicate the severity of the
problem for various applications of ground-water modeling.
To understand how to better use the aeromagnetic expression of
the faults to predict the nature of aquifers compartmentalized by faults, we
investigated the San Ysidro fault using rock-property measurements, geophysical
analysis, and geologic information. We characterized the magnetic properties
of various sediment units through analysis of magnetic susceptibility measurements
from 254 sites. Using this characterization, geologic cross-sections, and analysis
of the aeromagnetic data (as shown above), we determined that the largest contrasts
in magnetic properties occur consistently at the lower levels of the fault,
where the basin aquifers are juxtaposed against the basin-floor bedrock, and
more sporadically at the upper levels of the fault, where coarse-grained material
is juxtaposed against fine-grained material within the basin aquifer system.
This understanding may eventually lead to development of analytical tools to
predict what units are juxtaposed at faults in other parts of the basin from
aeromagnetic data. Our current research is focused on refining our understanding
of the variability of the magnetic properties of sediments along strike and
development of an approach to analyze the aeromagnetic data.
Publications:
REPORTS
- Grauch, V. J. S., 2001, Aeromagnetic mapping of hydrologically important
faults, Albuquerque basin, New Mexico: Proceedings of Symposium for the Application
of Geophysics to Engineering and Environmental Problems (SAGEEP), March 4-5,
2001, Denver, CO, CD-ROM, 12 p.
- Grauch, V. J. S., 2001, High-resolution aeromagnetic data, a new tool for
mapping intrabasinal faults: An example from the Albuquerque basin, New Mexico:
Geology, v. 29, p. 367-370.
- Grauch, V. J. S., Hudson, M. R., and Minor, S. A., 2001, Aeromagnetic expression
of faults that offset basin fill, Albuquerque basin, New Mexico: Geophysics,
v. 66, 707-720.
- Williams J. M. and Rodriguez, B. D., 2003, Magnetotelluric data release
across the Hubbell Springs fault area, Middle Rio Grande Basin, New Mexico:
U.S. Geological Survey Open-File Report 03-0115, 101 p.
Available on-line at http://pubs.usgs.gov/of/2003/ofr-03-115/.
ABSTRACTS
- Caine, J.S., Minor, S.A., Grauch, V.J.S., and Hudson, M.R., 2002, Potential
for fault zone compartmentalization of groundwater aquifers in poorly lithified,
Rio Grande rift-related sediments, New Mexico, Geological Society of America
Rocky Mountain Section Meeting, Cedar City, Utah, Geological Society of America
Abstracts with Programs, v.34, p. A-59.
- Grauch, V.J.S., Hudson, M.R., Minor, S.A., and Caine, J.S., 2003, Using
rock property measurements to understand aeromagnetic anomalies related to
intrabasin faults: A case study from the central Rio Grande Rift, USA: Proceedings
of the XXIII General Assembly of the International Union of Geodesy and Geophysics
(IUGG), June 30-July 11, 2003, Sapporo, Japan, p. B263.
- Grauch, V.J.S and Hudson, M.R., 2002, Implications of significant distances
between major faults and large vertical displacements based on geophysical
evidence, central Rio Grande rift, New Mexico: Geological Society of America
Abstract with Programs, v. 34, n. 6, p. 452.
- Hudson, M.R., Grauch, V.J.S., Minor, S.A., Caine, J.S., and Hudson, A.M.,
2001, Rock magnetic properties, magnetic anomalies, and intrabasin faulting:
Santa Fe Group basin fill, Rio Grande rift, New Mexico: EOS, Transactions,
American Geophysical Union, v. 82. n. 47, p. 338-339.
RELATED PUBLICATIONS
- Grauch. V.J.S., 2002, High-resolution aeromagnetic survey to image shallow
faults, Dixie Valley geothermal field, Nevada: U.S. Geological Survey Open-File
Report 02-0384, available on-line at http://pubs.usgs.gov/of/2002/ofr-02-0384/.
- Smith, R.P., Grauch, V.J.S., and Blackwell, D.D., 2002, Preliminary Results
of a High-Resolution Aeromagnetic Survey to Identify Buried Faults at Dixie
Valley, Nevada: Geothermal Resources Council Transactions, v. 26, p. 543-546.
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For technical further information contact: Tien Grauch
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