HR: 14:10h
AN: H43J-03    [Abstracts]
TI: GPR guided waves for the estimation of water content in shallow soil layers
AU: * Cassiani, G
EM: giorgio.cassiani@unimib.it
AF: Dipartimento di Scienze Geologiche e Geotecnologie, Universita' di Milano Bicocca, Milan, MI 20126 Italy
AU: Strobbia, C
EM: claudio.strobbia@eucentre.it
AF: EUCentre, Via Ferrata, 1, Pavia, 27100 Italy
AB: The estimation of moisture content changes in shallow soil layers is only apparently a simple problem. Invasive techniques, based on sampling and weighing, are fraught with practical difficulties. GPR used in surface-to-surface configuration has increasingly been proposed as a means to obtain fast and inexpensive images of soil moisture content over large areas at shallow depth. The technique is based on the measurement of direct GPR wave velocity in the ground. However, the success of the method relies heavily on the assumption that identifying such direct arrival is straightforward. We show that this is not always the case. The example we present is that of a stratified medium composed of a sequence of alternated high and low velocity layers (or semi-spaces, like air), where the thickness of the slow layers is comparable with the dominant wavelength of the propagating GPR signal. We demonstrate that, under such conditions, dispersive guided waves are propagated through the ground, and the velocity dependence on frequency is controlled by the dielectric properties of the layers as well as by their thicknesses and depth sequence. Note that, in general, first arrivals do not correspond to the bulk GPR velocity in the uppermost layer. We analyze the possibility of inverting the dispersion curve (phase velocity versus frequency) for such governing parameters, and we analyze the sensitivity of the method. This approach is applied to a mountain slope site, where soil cover is only 1-2 m thick, and the underlying bedrock undergoes variations in water saturation, thus giving potentially rise to a multi-layer GPR waveguide.
DE: 1835 Hydrogeophysics
SC: Hydrology [H]
MN: Fall Meeting 2005