Inference of vertical soil moisture distribution using high-frequency CMP and reflection traveltime analysis

High-frequency ground-penetrating radar (GPR) surveys were used to investigate temporal water content variations in a vertical soil column characterized by stratified clean sand deposits over multiple annual cycles. Reflection profiling and common-midpoint (CMP) soundings were coincidently performed using 900 MHz antennas across a 2 m intensive monitoring profile. Our ability to identify fixed reflection events along a vertical soil profile permits inference of soil water flux across defined soil intervals in a non-invasive manner. Soil moisture contents were estimated from two-way traveltime measurements between seasonally coherent stratigraphic interfaces in the upper 2-3 m of soil. Interval thicknesses between stratigraphic interfaces were estimated from normal-moveout velocity analysis of coincidently collected CMP soundings. Interval traveltimes from reflection profiles were then converted to wave velocity using the interval thickness estimates and a volumetric water content estimate using an appropriate petrophysical relationship. The GPR effectively characterized long (e.g., seasonal trends) and short-period (e.g., distinct wetting events) variations in vertical soil moisture distribution.