Properties of precipitation-induced multilayer surface waveguides derived from inversion of dispersive TE and TM GPR data

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Abstract
Precipitation events result in a high water content of the shallow subsurface. When the water remains in the shallow subsurface, a shallow low-velocity layer can be generated which acts as a waveguide when the contrasts in permittivity are large enough. The electromagnetic waves emitted by ground-penetrating radar (GPR) systems are trapped within these thin surface layers and show pronounced dispersion, which depends on the velocities and thicknesses of the surface waveguide layers and the velocity of the material below it. Conventional traveltime techniques cannot be reliably applied because the different phases cannot be clearly identified. Recently developed techniques for inverting dispersed waveforms in transverse electric (TE) and transversemagnetic (TM) GPR data are used to provide information on the thickness and permittivity of a single-layer low-velocity waveguide induced by precipitation events. Repeated measurements at one location after two precipitation events show that, for increasing soil water content and increasing thickness of the low-velocity waveguide, an increased number of higher-order modes can be identified in the phase-velocity spectrum. The TE and TM inversion techniques are extended for multilayered waveguides, and inversion of synthetic and experimental data events show that the properties of a two-layer waveguide can be reliably reconstructed when at least two modes can be used in the inversion. The misfits between the picked and the synthetic dispersion curves are significantly reduced compared to a single-layer wave-guide inversion.

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