Abstract
Subsurface drainage systems are commonly used to remove surplus water from the soil profile of a poorly drained farmland. Traditional methods for drainage mapping involve the use of tile probes and trenching equipment that are time-consuming, labor-intensive, and invasive, thereby entailing an inherent risk of damaging the drainpipes. Effective and efficient methods are needed in order to map the buried drain lines: (1) to comprehend the processes of leaching and offsite release of nutrients and pesticides and (2) for the installation of a new set of drain lines between the old ones to enhance the soil water removal. Non-invasive geophysical soil sensors provide a potential alternative solution. Previous research has mainly showcased the use of time-domain ground penetrating radar, with variable success, depending on local soil and hydrological conditions and the central frequency of the specific equipment used. The objectives of this study were: (1) to test the use of a stepped-frequency continuous wave three-dimensional ground penetrating radar (3D-GPR) with a wide antenna array for subsurface drainage mapping and (2) to evaluate its performance with the use of a single-frequency multi-receiver electromagnetic induction (EMI) sensor in-combination. This sensor combination was evaluated on twelve different study sites with various soil types with textures ranging from sand to clay till. While the 3D-GPR showed a high success rate in finding the drainpipes at five sites (sandy, sandy loam, loamy sand, and organic topsoils), the results at the other seven sites were less successful due to the limited penetration depth of the 3D-GPR signal. The results suggest that the electrical conductivity estimates produced by the inversion of apparent electrical conductivity data measured by the EMI sensor could be a useful proxy for explaining the success achieved by the 3D-GPR in finding the drain lines.
Highlights
The installation of subsurface drainage systems comprised of buried drainage pipe networks has been a common practice for decades in order to enhance the water removal capability of naturally poorly drained soils
The typical signature of a drainage pipe in the 3D-ground penetrating radar (GPR) data is a hyperbolic pattern in vertical profiles and a linear pattern in horizontal time/depth slices (Figure 5) when the instrument is moved perpendicular to the drain line direction
Due to the fact that the GPR signal is propagated into the subsurface as an elongated cone of energy and “sees” buried features both in front of it and behind it [50,61,65], the arrival times of reflections retrace a hyperbolic pattern with the apex of the hyperbola coinciding with the location of the drain line and the extent of linear pattern in the horizontal slice representing the length of the drain line
Summary
The installation of subsurface drainage systems comprised of buried drainage pipe networks has been a common practice for decades in order to enhance the water removal capability of naturally poorly drained soils. Due to limited information on subsurface drainage installations, it is difficult to understand the hydrology and solute dynamics and plan effective mitigation strategies, such as constructed wetlands, saturated buffer zones, bioreactors, and nitrate and phosphate filters [9,10,11,12,13]. Apart from these environmental aspects, there are practical reasons motivating investment in improved drain line mapping. To enhance the drainage efficiency in agricultural areas with established drainage systems, new drain lines can be installed in between the old ones, which requires knowledge of the precise location of the latter [14,15]
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