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Evaluating Ground Penetrating Radar Use for Water Infiltration Monitoring

UMR 8148 CNRS-UPS, Laboratoire IDES, Université Paris Sud 11, Bâtiment 504, 91405 Orsay cedex, France

View larger version (21K): [in this window] [in a new window]   FIG. 1. Experiment setup: water was injected down the 81-cm-long tube inserted in a hole while a pair of transmitter and receiver antennae (S and R, respectively) were recording a trace from the surface at 1-s intervals. The piezometer recorded the depth of the water in the tube. A tap regulated the water flux to keep the water level at 66 cm below the surface.

View larger version (12K): [in this window] [in a new window]   FIG. 2. Cumulative volume of infiltrated water vs. time.

View larger version (17K): [in this window] [in a new window]   FIG. 3. Snapshots resulting from the HYDRUS-2D modeling. Isolines = 0.425 are plotted with dots. Isolines = 0.05 are drawn with plain lines during the water injection and dashed lines after infiltration was stopped. Arrows point toward the radar transmitter S and receiver R.

View larger version (18K): [in this window] [in a new window]   FIG. 4. Water content along the transect A–B–C through the infiltration bulb of Fig. 3 at three times.

View larger version (16K): [in this window] [in a new window]   FIG. 5. One-dimensional models created to study the transition zone effect on ground penetrating radar data. In Model 1 (dashed line), the transition zone is made of 25 layers of 5-mm thickness with a relative dielectric permittivity increase from 5 to 31 with a step of 1. In Model 2 (plain line), the transition zone consists of a single homogeneous layer of relative dielectric permittivity 17.2 with a thickness of 12.5 cm.

View larger version (17K): [in this window] [in a new window]   FIG. 6. Simulated traces for Model 1, in which the transition zone is made of 25 layers, and Model 2, in which the transition zone consists of a single homogeneous layer.

View larger version (12K): [in this window] [in a new window]   FIG. 7. Model geometry used for ground penetrating radar data simulations, where S and R are the transmitter and receiver antennae, r are the dielectric permittivities, R is the radius of the external cylinder, and A, B, C, and D are zones of reflection of the electromagnetic wave.

View larger version (34K): [in this window] [in a new window]   FIG. 8. Simulated traces corresponding to the model used for ground penetrating radar simulations (Fig. 7). Each trace corresponds to a cylinder radius R in the range 2 to 30 cm. No amplitude gain was applied. The main reflections are named A to E.

View larger version (98K): [in this window] [in a new window]   FIG. 9. Recorded traces with fixed antennae during the experiment. The water tap was opened at time 0, closed at 1280 s, and there was no more water at the bottom of the hole at 1300 s. The data has been band-passed and the whole median trace has been removed from each trace to show differences. No amplitude gain was applied. Reflections A and D are highlighted by gray lines.

View larger version (123K): [in this window] [in a new window]   FIG. 10. Closeup of the recorded traces shown in Fig. 9 with different data processing. The mean of a running 30-trace window was removed from each trace.

View larger version (12K): [in this window] [in a new window]   FIG. 11. Distance from the injection point to the wetting front in the antennae direction, with its uncertainty as a function of time, retrieved from picked two-way travel times on Reflection A measured in Fig. 9.

View larger version (87K): [in this window] [in a new window]   FIG. 12. Cube of surface ground penetrating radar data obtained by difference between the data sets acquired before and after the water injection. Gray dotted surfaces correspond to the two reflections underlined in gray in Fig. 9 after 1300 s.