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Ground
Penetrating Radar Response to Laminations in Sediments
Ground Penetrating Radar
Imaging of Tephra Deposits
Geophysical Signatures of
Submarine Groundwater Discharge
Resistivity Imaging in Karst
Geophysical Signatures of
Active Faults in Central Costa Rica
Parallel Processing
and Inversion
Ground Penetrating Radar Response to Laminations in Sediments
In ground penetrating radar (GPR) investigations of sedimentary stratigraphy,
radar wavelengths (typically tens of cm to meters) are commonly greater than
the thickness of strata being imaged (mm to tens of cm scale). When this is
the case, the radar record represents an interference pattern in which there
is not a one-to-one correlation between reflection events and contacts within
sediments. Such studies are still useful as they shed light on the attitudes
of packages of strata. We are investigating techniques for extracting
additional information about internal layering within laminated sedimentary
packages. In particular we are looking at the spectral signatures of GPR
profiles over sequences of thin beds. Swagata Guha is carrying out this work with a mix of field
studies in barrier island settings (Waites Island,
SC, Jekyll Island, Georgia, Santa Rosa Island, Florida) and simulations of
radar wave propagation using finite difference time model (FDTD) models. The
Waites
Island studies are part of a larger investigation
of of SC barrier island history and geomorphology
by Eric Wright
and others at Coastal
Carolina University.
The current studies follow on investigations with Harry Jol and Laura Moore of packages of
strata in Utah and Washington, respectively.
References:
Kruse et al (2005)
Swagata Guha
(M.S.Thesis)
Guha et al. (2004)
Moore et al. (2004)
Kruse and Jol (2003)
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Geophysical Signatures of
Submarine Groundwater Discharge
Groundwater flows upward or outward into oceans via springs and more diffuse
seeps. Assessing flow paths and flow volumes for submarine groundwater
discharge is useful for establishing water budgets for estuaries (such as Tampa
Bay) and for tracking contaminant flow
at coasts. Traditional methods of estimating seepage rates (seepage meters or
geochemical tracer tests) rely on point measurements that may be
prohibitively expensive for large reconnaissance studies. As part of
studies on the Gulf of Mexico (funded by EPA GOM program) and in
Tampa Bay (in
collaboration with the USGS), we are testing several rapid-reconnaissance or
remote sensing methods to map regional seepage patterns. These methods
utilize conductivity and temperature differences between seeping groundwater
and surrounding seawater. Arnell Harrison and Matt
Weiss are comparing results of marine resistivity survey with Rn tracer estimates of seepage in Sarasota Bay and the
lower Suwannee estuary, respectively, For exploring very close to the
shoreline, in water too shallow for commercial marine resistivity systems,
Jason Greenwood (M.S. 2004) and Arnell Harrison
have also tested the conditions under which land-based EM systems (EM-31 and
EM-34 of Geonics, Inc. are useful for measuring
seabed conductivity. We find that the EM-31 is generally more useful, with
more stable results in this setting, although penetration depths are very
limited (<~few meters). These studies are being conducted in collaboration
with Jeff Chanton and Bill Burnett of FSU and Peter
Swarzenski of the USGS.
References:
Jason Greenwood (M.S. Thesis)
AGU abstracts
Greenwood,
W.J., and S.E. Kruse, Under Review
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Resistivity Imaging in Karst
To identify optimal survey geometries for imaging sinkholes in covered
karst terrain, we are beginning by simulating surveys with 3D models using
the groundwater modeling package Modflow. (This is possible because the
equations for current flow and Darcy's law are equivalent.) We can then
exploit the graphics capabilities of Modflow driver packages and their
utility for simulating complex geometries. Beverly Griffin is developing the
Modflow processing scheme and will be testing the methods against resistivity
surveys conducted over sinkholes at USF's Geopark.
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Geophysical signatures of active faults in Costa Rica
While on sabbatical in Costa Rica, I began work with Walter Montero of the
University of Costa Rica on a suite of geophysical studies across active
faults in central Costa Rica. We ran resistivity and magnetic surveys across
parts of the Agua Caliente
fault system and bounding and cross-basin faults on what Walter has
identified as the Irazu-Turrialba pull-apart basin.
Fatin Tutak is currently
analyzing magnetic surveys across the intrabasin
and bounding faults on the southeast portion of the pull-apart.
References:
S. Kruse and W. Montero (2004)
Kruse, S., J. Schneider, and J. Greenwood (2004)
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Ground Penetrating
Radar Imaging of Tephra Deposits
Recently we have run GPR profiles on a variety of tephra
deposits in Cerro Negro volcano, Nicaragua, and Poás
and Irazú volcanoes, Costa Rica. This method has
remarkable imaging capabilities in recent tephra
fallout ans surge deposits. In the dry, highly
resistive tephra of the Cerro Negro basaltic cinder
cone, distinct deposits are clearly imaged between 2 and 20 m depth. The
lowermost coherent reflection is presumed to be the contact with underlying
pre-Cerro Negro lavas and weathered tephra
deposits. Within the 2-20 m package, individual reflecting horizons are
clearly resolved. On the wetter deposits in Costa Rica, radar wave velocities
are slower and finer scale resolution is possible. Ash, pumice and paleosol units on Irazú and Poás volcanoes show velocities as shallow as 40-70 cm to
be imaged with 200 MHz antennas, with depth penetration typically 5 to 8
m. Comparison of trench observations and radar profiles indicates that
strong radar reflections are produced by iron-rich zones at the water table
and soil-ash contacts. Other features visible in the profiles are small (tens
of cm) sub-vertical offsets of nearly horizontal units, and diffractions or
disruptions in horizontal units presumed to reflect >~30 cm blocks. These
studies are being conducted in collaboration with Chuck Connor and Kristin
Martin of USF, and Raul Mora, Carlos Ramirez, and Guilllermo
Alvarado of UCR.
References:
S.E. Kruse et al (2006)
S. E. Kruse et al. (2004)
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Parallel
Processing and Inversion
Application of parallel processing techniques decreases
computational time and increases computational efficiency, allowing for a
more complete exploration of geophysical models. As a result, solutions based
on these models become more robust. At USF, we apply parallel processing to
computationally expensive modeling problems involving stochastic parameter sampling,
code optimization, and nonlinear data inversion. Example codes include: a
numerical model to determine tephra deposition and
accumulation for assessing possible hazards of volcanic eruptions, 3D
inversion of magnetic data to image subsurface geologic structures, and
inversion of tephra isopach
maps to determine eruption parameters.
Inversion
of magnetic data
Inversion
of tephra isomass data
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