HR: 0800h
AN: H41F-0469    [Abstracts]
TI: Modeling GPR Reflections from a Water Table. Why High GPR Frequencies don't Image the Water Table?
AU: * BANO, M
EM: maksim.bano@eost.u-strasbg.fr
AF: EOST Universit‚ Louis Pasteur (UMR-7516), 5 rue Rene Descartes, Strasbourg, 67084 France
AU: LOEFFLER, O
EM: olivier.loeffler@eost.u-strasbg.fr
AF: EOST Universit‚ Louis Pasteur (UMR-7516), 5 rue Rene Descartes, Strasbourg, 67084 France
AB: Ground Penetrating Radar (GPR) is a geophysical method that uses high frequency (from 10 to 1500 MHz) electromagnetic (EM) waves to image the shallow subsurface. Numerous studies of GPR imaging the water table are shown in the literature over the last ten years. In all these studies the central frequency of the antennae is about 100 MHz. We simulate a water table (water level at 72 and 48 cm depth) by injecting water in a sand box that contains also some buried objects. The box was filled with fine calibrated sand having diameters between 0.3 and 0.5 mm. GPR monostatic profiles (with 900 and 1200 MHz monostatic antennae) are performed in order to estimate the depth of the water level. The GPR data, for the water tables at the 72 and 48 cm depth, respectively, do not show any clear reflections from the top of the saturated zone. This might be because of the existence of a capillary fringe above the water table. The capillary fringe is the zone in which water rises by capillarity from the water table toward the surface. The degree of saturation of the capillary fringe decreases gradually upwards implying an increase of the velocity of EM waves. The thickness (h) of the capillary rise depends on the porosity and the mean grain diameter of the sand. For a given sandy soil the value of this thickness is not affected by the depth of the water table. In the following analyses we fixed this value to 30 cm, which agrees with the one given in the literature. To model the reflections from a water table we consider that our medium is composed of a dry sand layer (upper layer), an unsaturated layer (capillary fringe) and a fully-saturated layer (water table). The velocities (V1>V3) of the first and third layers are considered to be constant, while the velocity of the second layer (capillary fringe) decreases linearly from V1 to V3. We used a 1D wavelet modeling method in the frequency domain and considered different frequencies from 100 to 1000 MHz. Therefore the wavelength (lambda) varies from 1 m to 10 cm for a velocity of 0.1 m/ns. For each frequency the simulation is performed by using two models: with and without the capillary fringe (three layers and two layers model). We remark that for lambda/2 > h (thickness of the capillary fringe) the amplitude of the reflections for the two models is nearly the same. On the other hand, for lambda/2 < h the amplitude of the reflections for the three layers model (considering the capillary fringe) is much weaker than in the case of two layers model (without considering the capillary fringe). By using a Finite Difference Time Domain (FDTD) modeling method, we try also to model the reflections coming from the water table and compare them with the previous results. Examples of real and synthetic radar reflection data will be shown in order to illustrate the methodology presented here.
UR: http://Phineas.u-strasbg.fr
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: Fall Meeting 2005