Ground-Coupled GPR Height Dependence

The objective of this study is to understand the effect of height on ground penetrating radar (GPR) data for the near surface ground-coupled regime. The height range investigated was from 0.017 to ~0.083 of the GPR's center frequency wavelength in free space (λc). A shielded 250 MHz center frequency GPR placed above a homogeneous medium, with a perfect electrical conductor (PEC) layer located at its base, was numerically modeled using a three-dimensional (3D) ?nite-difference time-domain (FDTD) algorithm to understand the GPR's response dependence on height for relative dielectric permittivity (Kr) values from 4.5 to 18. The numerical modeling results show that the optimal height to minimize direct wavelet and banding interference is ~0.033- 0.050 λc (i.e., 40-60 mm at 250 MHz). Elevating the GPR above the surface also increases airwave interference from above surface objects. The numerical modeling results show the dependence of this interference on GPR height and the orientation of the above surface planar objects relative to the GPR. Field site profiles were collected over a subsurface utility with a commercial 250 MHz center frequency GPR at a range of heights. These profiles show some similar height dependence characteristics and some differences with the numerical modeling results. Since shields, carts and antenna designs vary with GPR manufacturers, data quality height dependence should be field tested to determine an optimal height. The numerical model and field data results provide an understanding of the tradeoffs in choosing an optimal height for GPR profiling, based on survey objectives.