Geophysical Characterization of an Alluvial Aquifer
Bevin Bailey || 2010-2011
Subsurface characterization of the near surface is essential in many geologic, hydrologic and engineering investigations. Drilling boreholes for such purposes is generally not cost effective and cannot always define the existence or lateral continuity of subsurface features. Electric and electromagnetic geophysical survey methods offer a quick, non-invasive, and cost effective means to characterize the shallow subsurface. In this study, Bevin imaged the subsurface of a mountain stream floodplain in order to locate subsurface geologic features including depths to bedrock and the water table, and to determine the aquifer properties and degree of hydrologic connectivity. On site borehole data provided detailed information about the subsurface including direct measurements of depth to the water table and split spoon sediment samples; thus providing a means for direct comparison to near surface geophysical data. Bevin's results indicate that the relative degree of stratification of unconsolidated deposits and the sediment-bedrock interface can be imaged with GPR; however, due to the large amount of pseudo-stratigraphic reflectors, GPR data was unable to independently image the water table. Resistivity proved most useful for characterizing grain size of the subsurface, thus giving a reasonable method for characterization of subsurface aquifer properties. The combination of GPR and resistivity data therefore provides a powerful tool for characterizing a complex alluvial aquifer system.
Study Site: The Lee and Vivian Reynolds Greenway Park, Boone, NC
Above: The Greenway park along the south fork of the New River located in Boone, NC. Geophysical transects (GPR and Resistivity) are shown with yellow lines. Boreholes are shown with blue and white circles. Note that Transects 1-2 are nearby and straddle the boreholes. For regional location refer to the North Carolina county map inset.
Comparison of Resistivity Data to Borehole Split-Spoon Samples
Above: A) Stratigraphic columns of non-continuous sediment samples from each borehole colored in blue on the map above.
Colors were assigned based on the expected resistivity values with high resistivity being red and low being colored blue.
This allows direct comparison to our resistivity inverted sections.
B) Stratigraphic columns laid over Transect 1 resistivity data at the correct locations. In general, the resistivity
data shows excellent agreement with the borehole sediment samples expected resistivities. Thus, resistivity is a reliable tool
for resolving spatial variations in grain size at the ASU Greenway.
Determination of Radar Velocity: Common Midpoint Surveying
Above: Results from a common midpoint survey used to determine the subsurface radar velocity. The survey was conducted above a known sub-horizontal reflector. A hyperbola was fitted to several reflectors suggesting a relatively constant velocity of 0.051 m/ns. This is the subsurface radar velocity applied to subsequent GPR profiles to convert two-way time to depth.
Comparison of GPR Data to Water Table Levels in Borholes
Above: Location of the measured water table (yellow segments)from the four boreholes on Transects 1 (top) and 2 (bottom). The locations of the water table do typically correspond to reflectors, but these reflectors are not always continuous, are not always strong reflectors, and are not independently identifiable without direct measurements. Thus choosing which reflector represents the water table is not practical at the ASU Greenway site.