THE EFFECTIVENESS OF USING GPR TO MONITOR CHANGES IN SOIL WATER CONTENT IN CLAYEY FLOODPLAIN SOILS DUE TO RAIN EVENTS BEFORE STREAM MODIFICATIONS
ALGEO, Jonathan T., Geology, Bucknell, 701 Moore Avenue, Lewisburg, PA 17837, Sodamoeba@gmail.com
Greater infiltration of natural rain water into the subsurface decreases the amount of rapid discharge to streams. The Miller Run watershed in Lewisburg, PA experiences significant rapid discharge during rainfall events due to impervious surfaces, naturally clayey soil and urbanization of the Miller Run channel. In this study, the effectiveness of using GPR common mid-point soundings (CMP) to monitor soil water content is evaluated in the clay-rich environment of the watershed in order to monitor planned engineering modifications to the floodplain. Using non invasive geophysical methods insures that the research won't modify the environment being studied. We used several geophysical methods in order to determine the general distribution of clay and determine the subsurface structure. EM-38 was used to locate an area where the conductivity was spatially uniform. Subsurface layer thicknesses were assessed using both DC resistivity based upon electric properties and seismic refraction based upon the acoustic properties.
After significant storm events totaling at least a half an inch of rainfall, 200 MHz GPR was used to collect a series of CMP data. Measurements were taken 10 hours, 29 hours, and six days after the end of a two inch rain event in July 2012. The arrival time for a shallow reflection was used to determine the velocity of the GPR signal. The velocity changed from .0507 m/ns at the first measurement to .0521 m/ns to .0719 m/ns by the final measurement. The depth estimate for the reflection based on the reflection analysis is 0.8 m ± 0.1 m for each CMP dataset. Analysis of the ground wave arrival times also showed a significant increase in velocity, from .055 m/ns at the first measurement to .057 m/ns to .064 m/ns by the final measurement. This implies that, even without a reflection, GPR can be used to monitor changes in soil water content in the clay soils within the Miller Run watershed.