Monitoring of hydrological hillslope processes via time-lapse ground-penetrating radar guided waves

Hydrological and hydrogeological dynamics along mountain slopes control many important phenomena, such as shallow landslide triggering and flood generation. The governing factors include: soil thickness, slope and bedrock morphology, rainfall pattern and subsurface groundwater conditions, both in the vadose zone and under the water table. We present the results of a monitoring project undertaken on a large slope parcel in the Alpine region of northern Italy. Both direct (piezometers, tensiometers, etc.) and indirect (geophysical) methods have been used to characterize slope and bedrock morphology as well as changes in soil moisture content over time. In this note, we focus on the use of the ground-penetrating radar (GPR) in surface-to-surface configuration. Recently, the use of multi- and single-offset GPR has been advocated for intermediate scale monitoring of moisture content changes in agricultural soils, e.g. in vineyards. We investigate the applicability of similar techniques to hillslopes. The monitoring has been performed using a PulseEkko 100 radar system. The estimation of the soil moisture content is based on the differential arrival time of direct waves through the air and the soil itself. GPR data can also provide information about the bedrock morphology, with special regard to the degree of fracturing. Care must be taken in data interpretation, because the GPR signal propagates in the soil layer as a guided wave, having a dispersive character (phase velocity is function of frequency) and possibly different modes. Consequently, the wave¿s first arrival at a different offset cannot be simply interpreted as a direct wave through the shallow soil layer. Inversion of the dispersion curve ¿ phase velocity versus frequency ¿ must be performed to yield velocity and thickness of the soil layer as well as velocity of the bedrock. We show that GPR data, properly processed and inverted, carry significant information about the site structure and hydrological dynamics of mountain slopes