Properties of surface waveguides derived from inversion of fundamental and higher mode dispersive GPR data

Thin surface layers of high-permittivity material (e.g., water-saturated soil) can cause a pronounced dispersion of ground-penetrating radar waves. The dispersion characteristics depend on the permittivity and thickness of the effective surface waveguide and the permittivity of the material below it. As for the analogous seismic case, the values of these parameters can be estimated by applying a scheme that includes calculating the phase-velocity spectra, picking dispersion curves from the spectra, and inverting the dispersion curves using a combined local and global minimization procedure. Here, this scheme is extended by incorporating higher order modes. Explicit expressions for the cutoff frequencies of the fundamental and higher order transverse electric (TE) and transverse magnetic (TM) modes are derived. These expressions demonstrate that the cutoff frequency decreases as the thickness and/or permittivity of the waveguide increase and/or the permittivity of the lower half-space decreases. In addition, the TE cutoff frequencies are shown to be generally lower than the TM cutoff frequencies. Numerical modeling of realistic models demonstrates the presence of the higher order modes with the correct cutoff frequencies. Application of the modified inversion scheme to synthetic and field data demonstrates its efficacy in providing the required physical property information. In particular, better-constrained models are obtained by including higher order modes in the inversion