David Redman

Borehole GPR measurement of soil water content during an infiltration experiment

Borehole ground penetrating radar (GPR) measurements of water content within the unsaturated zone were performed during infiltration experiments using both a uniform and point source of water at the soil surface. Horizontal GPR antenna access tubes were installed at a depth of 1 m to measure the water content within the horizontal plane underlying the surface water sources. The zero offset profile and multiple offset gather modes of data collection were used to produce velocity profiles and tomograms that were converted to water content. Time domain reflectometry (TDR) probes provided near-surface measurements of water content to complement the deeper borehole GPR data. For the point source, the water content tomograms and profiles showed that the zone of increased water content was restricted to a 1 m diameter zone, centered below the point source. For the uniform source, significant variability in water content within the studied area was observed possible due to preferential flow of water through the soil profile.

A study of GPR vertical crack responses in pavement using field data and numerical modelling

The application of ground penetrating radar (GPR) as a non-destructive technique for characterization of pavement structure on road networks has gained considerable attention during recent years. High resolution ground coupled GPR has the potential to provide important additional information on pavement deterioration, defects and cracks, the last being the focus of this study. Crack geometry and the electrical properties of the pavement surrounding the crack can be quite variable, resulting in often complex and hard to interpret data. Therefore, FDTD numerical modelling has been employed to help understand a range of GPR vertical crack responses observed in a variety of pavements.

Quantifying GPR transient waveforms in the intermediate zone

In this paper we examine the transient electromagnetic field variation around dipole antennas placed on the surface of a half-space. To achieve this we employ three-dimensional (3D) finite-difference time-domain (FDTD) numerical modelling. We have previously shown how antenna height, shielding and ground properties impact the directionality and energy flow. Here, we report how the transient fields around a dipole change their amplitude, shape and frequency content. Further, we demonstrate how the aforementioned attributes change as we move away from the source.

Quantifying GPR responses

Antenna height, shielding and subsurface properties all impact the observed GPR responses. While the basic concepts are generally understood, our long term goal is to develop parameterized models that will provide for quantitative interpretation of data acquired with real systems. Our first step was to develop modelling capacity and response presentation tools to help with development. We are using three-dimensional (3D) finite-difference time-domain (FDTD) modelling. Model results are then presented in a variety of forms. In this paper we compute emitted energy characteristics and display in radiation pattern format for infinitesimal dipoles, resistively loaded dipoles and shielded dipoles. Patterns are computed for free-space and over loss-less half-spaces with various properties as a function of height above the surface. The numerical simulations show the advantage of ground-coupling and the impact of shielding on GPR responses. Further, we demonstrate that total radiated energy is a very effective means of characterizing radiated signal directivity for GPR transient emissions.

Measuring Soil Water Content with the Ground Penetrating Radar Surface Reflectivity Method: Effects of Spatial Variability

A ground penetrating radar (GPR) system elevated ~1 m above the surface can be used to determine the near surface (<0.5 m) water content by measuring the surface reflectivity at the air/ground interface. A vehicle mounted GPR surface reflectivity system could efficiently map water content over large areas compared to the limited coverage possible with time domain reflectometry (TDR), currently the most widely used and accepted technique in agricultural and hydrological applications. Field studies performed at local test sites have shown that the GPR surface reflectivity method was able to map the water content distribution but there were substantial differences observed between these measurements and those acquired with TDR. These discrepancies are attributed to the different sampling volumes for the two methods and to inaccuracies in the surface reflectivity method, resulting from surface scattering and spatial variability in water content. Numerical modeling studies have been performed to investigate the effects of a horizontally stratified water content distribution on water content estimates obtained using the surface reflectivity method. Modeling of a two-layer case has demonstrated that the measured water content could, depending on the thickness of the upper layer, be overestimated for a wet layer over dry layer and underestimated for a dry layer over a wet layer. The field and modeling results have provided insight into the limitations and potential problems that will need to be addressed when using the GPR surface reflectivity method for estimating soil water content.

Forensic Geophysics: how GPR could help police investigations

Police regularly use GPR to uncover buried caches of drugs, money, weapons as well as locate unmarked graves. GPRs versatility and sensitivity to buried objects has lead to an ever widening use in forensics. While GPR does not deliver the fantasy results portrayed on some TV shows, GPR can provide powerful insight to forensics specialists needing to conduct non-destructively detailed subsurface site investigations.