Monitoring Vertical Soil Moisture Dynamics using GPR Reflection Traveltimes
Colby Steelman and Anthony Endres
Event name: 24rd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Geophysics in Rivers and Streams
Publication date: 10 April 2011
Info: Abstract, PDF ( 64.81Kb )
High-frequency ground-penetrating radar (GPR) surveys were used to investigate temporal soil moisture variations within a shallow vadose zone environment over multiple annual cycles between 28 August 2006 and 22 October 2008. Reflection profiling and common-midpoint (CMP) soundings were concurrently performed using 900 MHz antennas along a single 2.0 m transect characterized by stratified clean sand deposits. Surveys were conducted at intervals ranging from one day to four weeks across a wide range of wet, dry and frozen soil periods. Soil conditions were evaluated using measured traveltimes to four seasonally coherent stratigraphic interfaces. the presence of fixed stratigraphic events allowed us to monitor soil moisture changes within well-defined intervals, which significantly improves our ability to characterize hydraulic processes (e.g., infiltration, redistribution, drainage). Measured traveltimes were converted to velocity using the average interval thicknesses calculated from normal-moveout velocity analysis of CMP data which was then converted to an equivalent volumetric water content using a complex refractive index model. the GPR measurements effectively characterized temporal changes in vertical soil moisture distribution and migration along a 3.0 m vertical profile; the magnitude and duration of long-term seasonal moisture trends and short term fluctuations due to major precipitation events were quantified in terms of integrated water equivalents contained within each soil interval. in addition, direct ground wave velocity measurements from CMP soundings were used to examine the effects of soil wetting/drying and freezing/thawing cycles on the migration of moisture within the active root zone which significantly improved our ability to characterize dynamic processes at the air-ground interface and its coupling with deeper moisture profile variations.