Investigation of the frequency dependent antenna transfer functions and phase center position for modeling off-ground GPR

Abstract

maximizing information retrieval from a single GPR measurement. Both phase (travel time) and amplitude information are inherently exploited. The technique relies on an accurate and computationally effective radar forward model. This includes internal antenna and antenna-soil interaction propagation effects through frequency dependent, complex scalar transfer functions and solves exactly the three-dimensional (3-D) Maxwell's equations for wave propagation in multilayered media. The proposed method was validated in laboratory conditions for identifying both the dielectric permittivity and electrical conductivity of a sandy soil subject to a range of water contents [7], to monitor the dynamics of water in a laboratory sand column and subsequently derive the soil hydraulic properties using hydrodynamic inverse modeling [8], to map the soil surface water content in field conditions [9], [10], and to estimate soil hydraulic properties from integrated hydrogeophysical inversion of time-lapse off-ground GPR data [11], [12].The exact location of the phase center of double ridged horns depends on the dimensions of the horn, specially on its flare angle and tapper, and on the frequencies of operation. Significant variation can be observed in the phase center with frequency, when the pattern angles extend across the main beam [3]. For large flare angles and/or high frequencies the phase center is closer to the apex and for small flare angles and/or low frequencies, the phase center moves toward the aperture of the horn [2]. Historically, the work of Walton et al.[13] on the double-ridged horn was important, and it is still used today for the design of these horns. Such horn antennas have recently been analyzed numerically [14]- [17], but the effect of frequency on the phase center position has not been quantified in these studies. Antenna analysis is nevertheless necessary for precise measurements of the source point above the ground and correctly parameterize the forward electromagnetic radar model.In this paper, we compared different methods for estimating the double ridged horn antenna phase center, namely, by extrapolation of peak-to-peak reflection values in the time domain (ψ c ) and by frequency-domain full-waveform inversion assuming frequency dependent phase centers (ψ c ( f )). For Abstract-We compared different methods to estimate the phase center of an ultra wideband ground penetrating radar (GPR) horn antenna operating off-ground, namely, (1) extrapolation of peak-to-peak reflection values in the time domain and assuming a fixed phase center, and (2), frequency-domain full-waveform inversion assuming a frequency dependent phase center. For that purpose, we performed radar measurements at different heights above a perfect electrical conductor (PEC). A double ridged horn antenna operating in the frequency range 0.8 -5.2 GHz was used. In the limits of the antenna geometry, we observed that antenna modeling results were not significantly affected by the position of the phase center, even when considering frequency d...

Validation of ground penetrating radar full-waveform inversion for field scale soil moisture mapping

“…According to the operating bandwidth of the antenna, the VNA emulated a stepped-frequency electromagnetic wave from 200 to 2000 MHz, with a frequency step of 6 MHz. For this off-ground configuration, antenna and soil-antenna interactions effects are modeled using frequency-dependent transfer functions for a farfield antenna configuration (Lambot et al, 2006a;Jadoon et al, 2010b).…”

GPR Modeling for Rapid Characterization of Layered Media

“…However, the antenna phase center can be placed at a fixed location like at the center of the antenna aperture. A study by Jadoon et al [26] proved that the linear transfer function model used to represent the antenna behavior inherently accounts for the gain and delay due to frequency dependent antenna phase center location through the calibration process.…”