Mapping of agricultural subsurface drainage systems using a frequency-domain ground penetrating radar and evaluating its performance using a single-frequency multi-receiver electromagnetic induction instrument

Authors: KOGANTI, TRIVEN - Aarhus University; VAN DE VIJVER, ELLEN - Ghent University; Allred, Barry; GREVE, MOGENS - Aarhus University; RINGGAARD, JORGEN - Ramboll; IVERSEN, BO - Aarhus University

Submitted to: Sensors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary: A high success rate was achieved in finding the drain lines at five sites using a three-dimensional ground penetrating radar (3D-GPR), but the results at the other seven sites were less successful. The 3D-GPR system was particularly successful in mapping the drain lines oriented either perpendicular or at an angle to the 3D-GPR survey direction. The discrepancies and offset between the actual location of the drain lines mapped by the 3D-GPR and the pre-existing drainage system maps can be related to the inaccuracies associated with the processes involved from drainage design to documentation of the installations and later digitalization of maps. The inverse relationship between both the average global and localized signal penetration depth (PD) of the 3D-GPR versus mean soil electrical conductivity, EC, at a depth interval of 0–1.5 m, was generally confirmed, corroborating the expected influence of EC on signal attenuation. Hence, the EC measured by the electromagnetic induction (EMI) sensor can act as ancillary information to explain the success achieved by the 3D-GPR system in finding the drain lines, and in a complementary manner, providing information on the spatial variability of soil properties that are of importance to precision agriculture. Although, the correlations between localized PD and soil EC are not statistically evident across all the sites, this research was novel because it showcased the use of a 3D-GPR system for mapping subsurface drainage and an attempt was made to calculate the 3D-GPR global and localized PDs’.

Technical Abstract: Subsurface drainage systems remove excess water from the soil profile thereby improving crop yields in poorly drained farmland. Knowledge of the position of the buried drain lines is important: 1) to improve understanding 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 for enhanced soil water removal efficiency. Traditional methods of drainage mapping involve the use of tile probes and trenching equipment. While these can be effective, they are also time-consuming, labor-intensive and invasive, thereby entailing an inherent risk of damaging the drainpipes. Non-invasive geophysical soil sensors provide a potential alternative solution. Previous research has focused on the use of time-domain ground penetrating radar (GPR), with variable success depending on local soil and hydrological conditions and the central frequency of the specific equipment employed. The objectives of this study are to 1) test the use of a stepped-frequency continuous wave (SFCW) 3D-GPR (GeoScope Mk IV 3D-Radar with DXG1820 antenna array) for subsurface drainage mapping, and 2) to evaluate the performance of a 3D-GPR with the use of a single-frequency multi-receiver electromagnetic induction (EMI) sensor (DUALEM) in-combination. The 3D-GPR system offers more flexibility for application to different (sub)surface conditions due to the coverage of wide frequency bandwidth. The EMI sensor simultaneously provides information about the apparent electrical conductivity (ECa) for different soil volumes, corresponding to different depths. This sensor combination was evaluated on twelve different study sites with various soil types with textures ranging from sand to clay till. While the 3-D GPR showed a high success rate in finding the drain pipes at five sites (sandy, sandy loam, loamy sand and organic topsoils), the results at the other seven sites were less successful due to limited penetration depth (PD) of the 3D-GPR signal. The results suggest that the electrical conductivity estimates produced by the inversion of ECa data measured by the DUALEM sensor could be a useful proxy to evaluate the success achieved by the 3D-GPR in finding the drain lines. The high attenuation of electromagnetic waves in highly conductive media limiting the PD of the 3D-GPR can explain the findings obtained in this research.