Measuring the Soil Water Content Profile of a Sandy Soil with an Off-Ground Monostatic Ground Penetrating Radar (original) (raw)
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Measuring Soil Water Content with Ground Penetrating Radar: A Decade of Progress
Vadoze Zone Journal, 2018
Tremendous progress has been made with respect to ground penetrating radar (GPR) equipment, data acquisition, and processing since the establishment of GPR as a tool for soil water content determination in vadose zone hydrology about 25 yr ago. In this update, we aim to provide a critical overview of recent advances in vadose zone applications of GPR with a particular focus on new possibilities for multi-offset and borehole GPR measurements, the development of quantitative off-ground GPR methods, full-waveform inversion of GPR measurements, the potential of time-lapse GPR measurements for process investigations and hydrological parameter estimation, and recent improvements in GPR instrumentation. We hope that this update encourages the soil hydrology, groundwater, and critical zone community to embrace GPR as a viable tool for soil water content determination and the elucidation of subsurface hydrological processes.
Water Resources Research, 2006
We report on a laboratory experiment that investigates the effect of soil surface roughness on the identification of the soil electromagnetic properties from full-wave inversion of ground-penetrating radar (GPR) data in the frequency domain. The GPR system consists of an ultrawide band stepped-frequency continuous-wave radar combined with an off-ground monostatic horn antenna. Radar measurements were performed above a rectangular container filled with a loose sandy soil subject to seven water contents and four random surface roughnesses, including a smooth surface as reference. Compared to previous studies, we have reduced the modeling error of the GPR signal for the smooth surface case thanks to improved antenna transfer functions by solving an overdetermined system of equations based on six model configurations instead of only three. Then, the continuously increasing effect of surface roughness on the radar signal with respect to frequency is clearly observed. In close accordance with Rayleigh's criterion, both the radar signal and the inversely estimated parameters are not significantly affected if the surface protuberances are smaller than one eighth of a wavelength. In addition, when this criterion is not respected, errors are made in the estimated parameters, but the inverse solution remains stable. This demonstrates the promising perspectives for application of GPR for noninvasive water content estimation in agricultural and environmental field applications.
Journal of Hydrology, 2007
Two ground-penetrating radar (GPR) techniques were used to estimate the shallow soil water 3 content at the field scale. The first technique is based on the ground wave velocity measured 4 with a bistatic impulse radar connected to 450 MHz ground-coupled antennas. The second 5 technique is based on inverse modeling of an off-ground monostatic TEM horn antenna in the 6 0.8 to 1.6 GHz frequency range. Data were collected on a 8 by 9 m partially irrigated 7 intensive research plot and along four 148.5 m transects. Time domain reflectrometry, 8 capacitance sensors, and volumetric soil samples were used as reference measurements. The 9 aim of the study was to test the applicability of the ground wave method and the off-ground 10 inverse modeling approach at the field scale for a soil with a silt loam texture. The results for 11 the ground wave technique were difficult to interpret due to the strong attenuation of the GPR 12 signal, which was related to the silt loam texture at the test site. The root mean square error of 13 the ground wave technique was 0.076 m³m -³ when compared to the TDR measurements and 14 0.102 m³m -³ when compared with the volumetric soil samples. The off-ground monostatic 15 GPR measured less within field soil water content variability than the reference 16 measurements, resulting in a root mean square error of 0.053 m³m -³ when compared with the 17 TDR measurements and an error of 0.051 m³m -³ when compared with the volumetric soil 18
Comparison of soil water content estimation equations using ground penetrating radar
Journal of Hydrology
Soil water content has an important impact on many fundamental biophysical processes. The quantification of soil water content is necessary for different applications, ranging from large-scale calibration of global-scale climate models to field and catchment scale monitoring in hydrology and agriculture. Many techniques are available today for measuring soil water content, ranging from point scale soil water content sensors to global scale, active and passive, microwave satellites. Geophysical methods are important methods, used for several decades, to measure soil water content at different scales. Among these methods, ground penetrating radar has been shown to be one of the most reliable and promising ones. Soil water content measurement using ground penetrating radar requires the application of parametric equations that will convert the measured dielectric permittivity to water content. While several studies have been performed to test equations for soil water content sensors such as time domain reflectometry, a few studies have been performed to test different formulae for application to ground penetrating radar. In this study, we compare available formulae for converting dielectric permittivity obtained from detailed laboratory scale measurement of reflected waves using ground penetrating radar. Four soils covering a wide range of textures were used and the measured soil water contents were compared with values obtained from gravimetric measurements. Results showed that the dielectric mixing model of Roth et al. (1990) provided the best fit for individual soil textural classes, except for sandy soils. However, for all data combined the dielectric mixing model performed much better with significant difference in coefficient and determination and root mean square error. Empirical equations developed from calibration of time domain reflectometry performed poorly when applied to estimation of soil water content obtained from ground penetrating radar. Differences in sample volume, frequency of operation and data analysis between ground penetrating radar and time domain reflectometry, suggest to use more flexible and robust electromagnetic mixing formulae, allowing for incorporating the dielectric properties of constituents materials and geometrical features of the media. Sensitivity analysis was then performed to provide detailed information for the most accurate application of the selected dielectric model. Sensitivity analysis showed that the geometric parameter α and the dielectric permittivity of the solid phase ∊ s are the two most sensitive parameters, determining important variations in the estimation of soil water content. Based on these results, these two parameters are suggested as fitting parameters, to be selected if the model is fitted to data. Otherwise, the model can be successfully used without calibration, as presented in this study, by using α = 0.5 (as also suggested by the authors) and ∊ s = 4, which is an average value for soil minerals. methods available for measuring SWC. Among geophysical techniques, Ground Penetrating Radar (GPR) is a powerful and promising one. GPR has the advantage of covering larger areas with respect to point-based measurements typical of soil moisture sensors such as Time Domain Reflectometry (TDR), filling the gap between point scale and large scale satellite-based measurements. SWC can be obtained by performing different types of analysis and
Water Resources Research, 2006
1] We analyze the common surface reflection and full-wave inversion methods to retrieve the soil surface dielectric permittivity and correlated water content from airlaunched ground-penetrating radar (GPR) measurements. In the full-wave approach, antenna effects are filtered out from the raw radar data in the frequency domain, and fullwave inversion is performed in the time domain, on a time window focused on the surface reflection. Synthetic experiments are performed to investigate the most critical hypotheses on which both techniques rely, namely, the negligible effects of the soil electric conductivity (s) and layering. In the frequency range 1-2 GHz we show that for s > 0.1 Sm À1 , significant errors are made on the estimated parameters, e.g., an absolute error of 0.10 in water content may be observed for s = 1 Sm À1 . This threshold is more stringent with decreasing frequency. Contrasting surface layering may proportionally lead to significant errors when the thickness of the surface layer is close to one fourth the wavelength in the medium, which corresponds to the depth resolution. Absolute errors may be >0.10 in water content for large contrasts. Yet we show that full-wave inversion presents valuable advantages compared to the common surface reflection method. First, filtering antenna effects may prevent absolute errors >0.04 in water content, depending of the antenna height. Second, the critical reference measurements above a perfect electric conductor (PEC) are not required, and the height of the antenna does not need to be known a priori. This averts absolute errors of 0.02-0.09 in water content when antenna height differences of 1-5 cm occur between the soil and the PEC. A laboratory experiment is finally presented to analyze the stability of the estimates with respect to actual measurement and modeling errors. While the conditions were particularly well suited for applying the common reflection method, better results were obtained using fullwave inversion.
Verification of Ground Penetrating Radar for Soil Water Content Measuring
EGU General Assembly Conference Abstracts, 2009
Spatially distributed water at the land surface is a vital natural resource for human being and ecosystems. Soil water content at vadose zone at regional scale controls exchange of moisture and energy between Earth surface and atmosphere, at the catchment scale-the separation of precipitation into infiltration, runoff and evapotranspiration, at the field scale-plant growing, at the small plot scale-pathway of water flow through soil profile. Hydrologist, agronomists, soil scientists and others looking for technology providing soil water content measurements across a range of spatial range. Ground penetrating radar is not destructive method of measurement for diverse application was tested in the field for mapping a spatial distribution of soil water content during infiltration event at chestnut soil of Saratov Region, Russia. A Common-MidPoint method was used to calibrate GPR OKO with a 400 MHz antenna. At experimental plot of 50x50 m a range of 36 boreholes equipped by vertical access tubes (10 distance between) for TDR PR2 with 4 predefined depths of soil moisture measurements was prepared. TDR PR2 equipment used for measurements was calibrated on special experimental setup with soil from plot. Data sets of parallel measurements of soil water content by TDR at 4 depths of borehole locations and GPR at trace lines along ranges of boreholes were used to produce soil water content maps with geo-statistical methods.
NUMERICAL MODELING & A FIELD TEST OF GROUND PENETRATING RADAR FOR SOIL MOISTURE CONTENT ESTIMATION
Numerical simulation of electromagnetic radiation was performed using GPRMAX2D to estimate the penetration depth of the ground penetrating radar (GPR) direct ground wave. Two modeling approaches were used with two different soil layers: i) dry soil over wet soil layer, ii) wet soil over dry soil layer. The depth to the wet layer was estimated by analyzing the two way travel time of the reflected event from the wet layer. In both models, the top layer thickness was gradually reduced and the simulation was performed using four different frequencies from 100 to 900 MHz. Simulation was carried out using the common mid point (CMP) survey type with antenna separations starting at 0 m. A very high correlation was found between the GPR direct ground wave influence depth and the radar wavelength, λ (λ = V/f), where V is the electromagnetic wave velocity and f is the frequency. No difference was found between the estimated depth to the wet layer using the reflected wave and the true depth of the model. Field experiments showed that the GPR ground wave could be used to estimate the spatial soil moisture variability at shallow depths when antenna offsets close to 1 m were used.
Ground-penetrating radar for correlation analysis of temporal soil moisture stability and land-slope
2012 14th International Conference on Ground Penetrating Radar (GPR), 2012
Knowledge of temporal surface soil moisture variability is an useful key in agriculture, surface hydrology and meteorology. In that respect, ground-penetrating radar (GPR) is a non-invasive and promising tool for high-resolution and large scale characterization. In the case of quantitative analysis, offground GPR signal modeling and full-waveform inversion has shown a great potential during the last decade. In this research, we applied GPR for time-laps measurements in an agricultural field along a 320 m single transect with a significant landslope for about 3 months. A 200-2000 MHz TEM-horn antenna situated 1.1 m above the ground, connected to a vector network analyzer (VNA) was used as an off-ground frequency-domain GPR. The accurate positioning was done using a differential GPS. All systems were mounted on a 4-wheels vehicle for realtime and automated mapping. The calibration of the antenna and using the GPR signal inversion permitted to the ground surface relative dielectric permittivity. Topp's model was used for transformation of the relative dielectric permittivity to soil moisture. The temporal stability of the field-average soil moisture was computed by indicators based on the relative difference of the soil moisture to the field-average. The results showed an excellent correlation amount of -0.9905 for temporal stability of soil moisture and slope variability.
Vadose Zone Journal, 2012
An integrated hydrogeophysical inversion approach was used to remotely infer the unsaturated soil hydraulic parameters from time‐lapse ground‐penetrating radar (GPR) data collected at a fixed location over a bare agricultural field. The GPR model combines a full‐waveform solution of Maxwell's equations for three‐dimensional wave propagation in planar layered media together with global reflection and transmission functions to account for the antenna and its interactions with the medium. The hydrological simulator HYDRUS‐1D was used with a two layer single‐ and dual‐porosity model. The radar model was coupled to the hydrodynamic model, such that the soil electrical properties (permittivity and conductivity) that serve as input to the GPR model become a function of the hydrodynamic model output (water content), thereby permitting estimation of the soil hydraulic parameters from the GPR data in an inversion loop. To monitor the soil water content dynamics, time‐lapse GPR and time do...
Ground-penetrating radar for temporal soil moisture variability analysis along a land slope
Knowledge of temporal surface soil moisture variability is an useful key in agriculture, surface hydrology and meteorology. In that respect, ground-penetrating radar (GPR) is a non-invasive and promising tool for high-resolution and large scale characterization. In the case of quantitative analysis, offground GPR signal modeling and full-waveform inversion has shown a great potential during the last decade. In this research, we applied GPR for time-laps measurements in an agricultural field along a 320 m single transect with a significant landslope for about 3 months. A 200-2000 MHz TEM-horn antenna situated 1.1 m above the ground, connected to a vector network analyzer (VNA) was used as an off-ground frequency-domain GPR. The accurate positioning was done using a differential GPS. All systems were mounted on a 4-wheels vehicle for realtime and automated mapping. The calibration of the antenna and using the GPR signal inversion permitted to the ground surface relative dielectric permittivity. Topp's model was used for transformation of the relative dielectric permittivity to soil moisture. The temporal stability of the field-average soil moisture was computed by indicators based on the relative difference of the soil moisture to the field-average. The results showed an excellent correlation amount of -0.9905 for temporal stability of soil moisture and slope variability.