Combining Surface and Downhole Geophysical Methods to Identify

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					                        Combining Surface and Downhole Geophysical Methods
                          to Identify Karst Conditions in North-central Iowa

                          J. E. Wedekind, M. A. Osten, E. Kitt, and B. Herridge

Abstract
          Karst conditions in the glaciated terrain of north-central Iowa are often difficult to ascertain during
typical site reconnaissance. Surface and downhole geophysical techniques are useful to better characterize
karst features where they are hidden by a generally featureless topography. An iterative investigative
strategy, involving high-resolution surface geophysical techniques, complemented with downhole
geophysical logging and rock coring, proved effective in providing sufficient information to adequately
characterize bedrock conditions in this difficult setting.
          An 800-acre site in north-central Iowa was one of several proposed for a coal-fired electric
generating facility. Concerns arose over the possibility of karst conditions at the site that could impact
facility design and construction. Although several shallow closed topographic lows, described as sinkholes
by soil scientists, were mapped on the property, no subsurface investigations had been conducted in the
vicinity to document bedrock quality or karst development. An investigation using high-resolution surface
geophysical techniques to focus an exploratory drilling program was proposed. The boring program was
coupled with downhole geophysics to document the bedrock quality and establish better stratigraphic
control and to determine the structural character of the shallow bedrock.
          Surface geophysical techniques included micro-topography using a differential global positioning
system (DGPS) to produce a digital terrain model (DTM) that would identify and locate topographical
expressions to a resolution of less than 0.5 foot. A high-resolution electromagnetic (EM) conductivity
survey was conducted simultaneously in an attempt to identify areas of higher moisture content that may
indicate areas of incipient karst development. The results of these surveys were used to identify anomalies
that were then surveyed using ground penetrating radar (GPR) to provide three-dimensional imaging and
identify specific features to drill. Finally, a shallow seismic survey that utilized Multichannel Analysis of
Surface Waves (MASW) was conducted after drilling to provide better definition of shallow solutional
features and to compare this emerging technology with the other methods.
          Two candidate geophysical anomalies were core-drilled to depths ranging from 50 to 150 ft with
particular attention applied to identification of subsurface voids. Boreholes were then investigated using
downhole video, caliper, natural gamma, resistivity, and spontaneous potential geophysical logging. The
caliper was used to identify the locations of bedrock cavities. The natural gamma was used to identify
shale aquitards that may affect the distribution of solutional activity and also serve as stratigraphic marker
horizons. Downhole video provided indications of groundwater pathways and verified fabric geometry of
the fractures and solutional features.
          Results of the investigation suggest that the DTM and EM were effective for rapid identification
of bedrock surface anomalies and soil compositional anomalies over a large area that served to focus the
more expensive invasive tools. As a result of signal loss from fine-grained and relatively higher moisture
content soils, these techniques could not identify the location of deeper bedrock voids. GPR penetration
was limited in areas of clayey till. However, the GPR successfully detected soil surface anomalies where
subsequent drilling discovered silt-filled grikes. MASW identified the of the bedrock surface, and was
successful in verifying a previously identified filled grike. In summary, this study illustrates that coupling
area-wide surface geophysics with a limited boring program can adequately define the bedrock character
across large areas where topographic clues are lacking.




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posted:9/4/2010
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