Kansas Geological Survey, Open File Report 96-49
GPR Study of The Drum Limestone, part 9 of 9
Conclusions
High-resolution ground-penetrating radar was only partially
successful at imaging the Drum Limestone, being limited to very
shallow penetration depths (< 3 m) due to rapid signal
attenuation and instrument noise. A lack of dielectric constant
contrasts at deeper levels may also have contributed to the
paucity of reflections. The lack of signal penetration in
the Drum Limestone, particularly in areas of high near surface
reflectivity, may be due to high conductivity layers at shallow
depths combined with the high-frequency nature of the signal
being used. The quality of the GPR data was best from near
the surface to depths of 1 m, with some bounding surfaces and
cross-beds appearing as high-amplitude reflections. Although
lower in reflection strength, dipping clinoforms and other
bounding surfaces were also imaged 1-3 m below the surface.
The geometry of these reflections correlated fairly well with
beds exposed along the highwall of the quarry at depths and
station locations where the usable outcrop and GPR data
overlapped. GPR data along line D2, which is perpendicular
to the outcrop face was also useful because it allowed 3-D
correlation of the bed forms imaged on line D1 into the
outcrop face. The combined GPR interpretation confirmed
the strike and dip direction of cross-beds measured on the
limestone surface behind the outcrop, indicated the strike
and dip direction of cross-beds not exposed at the surface
behind the outcrop, and indicated the trend and dip of a
channel-like feature which was covered along the outcrop face.
Results from this site suggest that high-resolution GPR is
an excellent tool for imaging the internal stratigraphy of
limestones near the surface, provided that high-conductivity
soils or rock units above the limestone do not significantly
attenuate the signal. Full-wavelength resolution with this
technique is on the order of a few decimeters (depending on
the velocity of the unit), and penetration depths can reach
at least 3 m depending on lithology. Signal penetration in
other areas and other limestone formations may be slightly
deeper with a 500 MHz antenna as indicated by results at other
limestone sites
(Kruger et al., 1995;
Martinez et al., 1995a, b).
High-resolution GPR is also useful for imaging the near-surface
structure of limestones, as indicated by the transition from
dipping to relatively horizontal beds along the profile. These
aspects suggest that GPR can be used to extend stratigraphic
and structural information from a limestone unit away from the
outcrop exposure, and if needed, create a grid of data to achieve
a 3-D image of the unit in question. The correlations between
geology and GPR in this report also suggest that GPR can be
used as a remote sensing tool, even when outcrop information
is sparse or unavailable.
Acknowledgments
We would like to thank Dr. Neil Anderson from the University of
Missouri-Rolla, for the use of UMR's GPR equipment. We would
also like to thank Mike Shoemaker for assisting in GPR data
collection, Tim Carr for making Kansas Geological Survey funds
available for the survey, and the Heartland Cement Co. for
allowing access to the quarry site.
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Kansas Geological Survey, Open-File Report 96-49
Placed online Jan. 1997
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