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Numerical Modeling of GPR to Determine the Direct Ground Wave Sampling Depth

^a Department of Agricultural Engineering, University of Peradeniya, Peradeniya, KY 20400, Sri Lanka
^b Sensors and Software Inc., Mississauga, ON, L4W 3R7, Canada
^c Dep. of Land Resource Science, University of Guelph, ON, N1G 2W1, Canada
^d Dep. of Earth Science, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

View larger version (8K): [in a new window]   Fig. 1. Common GPR wave fronts from a line source (s) kept at the air–soil interface: A and B are spherical waves, C is lateral (head) wave, and b is evanescent wave in air. Direct ground wave (GW) is a combination of B near the interface and b (after Annan, 1973).

View larger version (14K): [in a new window]   Fig. 2. Schematic diagram of conceptual two-layer model (2 by 4 m domain) for GPR direct ground wave modeling for the dry soil over wet soil model. r is relative dielectric permittivity. The air layer at the top was always 0.2 m thick.

View larger version (44K): [in a new window]   Fig. 3. Simulated wave fronts from a 1000-MHz line source on the soil for a two-layer soil model. The vertical positions of the soil surface and dry or wet layer are 250 and 275 cm, respectively, in the model: (A) dry over wet soil layer case, (B) wet over dry soil layer case.

View larger version (33K): [in a new window]   Fig. 4. Some raw data obtained from CMP surveys using GPRMAX2D with 450-MHz frequency: (A) dry (r = 5) soil over wet (r = 15) soil layer model, (B) wet (r = 15) soil over dry (r = 5) soil layer model. The stars give locations of ground wave peaks used to determine the velocity.

View larger version (23K): [in a new window]   Fig. 5. (A, C) Variation of the ground wave velocity and (B, D) the estimated soil moisture for four different frequencies obtained from dry soil over wet soil model.

View larger version (23K): [in a new window]   Fig. 6. (A, C) Variation of the ground wave velocity and (B, D) the estimated soil moisture for four different frequencies obtained from wet soil over dry soil model.

View larger version (15K): [in a new window]   Fig. 7. Variation of the GPR ground wave influence depth with respect to the wavelength in both the dry over wet and wet over dry layer cases.

View larger version (25K): [in a new window]   Fig. 8. Variation of GPR ground wave sampling depth (predicted using Eq. [4]) with respect to soil moisture content for four different frequencies. Predicted sampling depths by Du (1996) and Sperl (1999) are also included for comparison.

View larger version (15K): [in a new window]   Fig. 9. Comparison of apparent moisture content estimated using the calculated ground wave velocity and true average moisture content. Overall RMSE = 0.017 m3 m–3.