Abstract
The electromagnetic (EM) soil properties are dynamic variables that can change considerably over time, and they fundamentally affect the performance of Ground Penetrating Radar (GPR). However, long-term field studies are remarkably rare and records of the EM soil properties and their seasonal variation are largely absent from the literature. This research explores the extent of the seasonal variation of the apparent permittivity (Ka) and bulk electrical conductivity (BEC) measured by Time Domain Reflectometry (TDR) and their impact on GPR results, with a particularly important application to utility detection. A bespoke TDR field monitoring station was specifically developed and installed in an anthropogenic sandy soil in the UK for 22months. The relationship between the temporal variation of the EM soil properties and GPR performance has been qualitatively assessed, highlighting notably degradation of the GPR images during wet periods and a few days after significant rainfall events following dry periods. Significantly, it was shown that by assuming arbitrary average values (i.e. not extreme values) of Ka and BEC which do not often reflect the typical conditions of the soil, it can lead to significant inaccuracies in the estimation of the depth of buried targets, with errors potentially up to approximately 30% even over a depth of 0.50m (where GPR is expected to be most accurate). It is therefore recommended to measure or assess the soil conditions during GPR surveys, and if this is not possible to use typical wet and dry Ka values reported in the literature for the soil expected at the site, to improve confidence in estimations of target depths.
Highlights
The goal of remote sensing of the subsurface, in urban environments, is to create an accurate 3D representation of the ground and all other buried features
Ground Penetrating Radar (GPR) and other remote sensing technologies are often reliable creators of 2D maps, but accurate depth estimations remain a major challenge, which is recognised by the ASCE (2002) guidelines, which do not attribute depth information to geophysical survey data in contrast to the BSI (2014) specification
Due to the limited depth of the manual excavation the density and gravimetric water content (GWC) were not measured for the deeper horizon. pH and organic matter (OM) were measured from samples taken during the Time Domain Reflectometry (TDR) field installation
Summary
The goal of remote sensing of the subsurface, in urban environments, is to create an accurate 3D representation of the ground and all other buried features. The key benefit of TDR is that it operates on similar principles to GPR and is well suited for supplementing GPR data during monitoring applications as it can provide more detailed insights on the spatial and temporal variations of the soil properties Both TDR and GPR work at a similar frequency range (i.e. approximately 10 MHz to 1 GHz) and the two techniques can be directly compared and used effectively in combination. The seasonal variation of soil properties has been investigated in the past in order to understand their effect on the GPR ability to detect soil layers (Boll et al, 1996; Kowalsky et al, 2001; Lunt et al, 2005; Zhang et al, 2014) These studies discussed only the results of selected field campaigns not repeated over the years, possibly failing to account for more rare extreme conditions. An analysis of the potential errors in depth estimates induced by changing soil properties, vitally significant for accurate 3D models, is presented
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