Measuring Soil Water Content with Ground Penetrating Radar: A Decade of Progress (original) (raw)
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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.
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
Vadose Zone Journal, 2004
properties of the subsurface. In that respect, GPR constitutes a promising high resolution characterization We explore the possibility of measuring a continuously variable tool. However, despite considerable research devoted to soil moisture profile by inversion of a ground penetrating radar (GPR) signal. Synthetic experiments were conducted to demonstrate the well-GPR, its use for assessing quantitatively the subsurface posedness of the inverse problem for the specific case of identifying properties has been constrained by a lack of appropriate a soil moisture profile in hydrostatic equilibrium with a water table. GPR systems and signal analysis methods. Ground pen-In this case, the profile agrees with the water retention curve of the etrating radar has been used to identify soil stratigraphy soil. The analysis subsequently extends to an actual case study in con-(Davis and Annan, 1989; Kung and Lu, 1993; Boll et al., trolled outdoor conditions on a large tank filled with sand. Due to 1996), to locate the water table (Nakashima et al., 2001), the presence of a discontinuity in the actual dielectric profile, inversion to follow wetting front movement (Vellidis et al., 1990), of the continuous model (Model 1) led to poor results. Only the to measure soil water content (Greaves et al., 1996; surface soil moisture was well estimated. Including the observed dis
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.
We applied inverse modelling of zero-offset, air-raised ground-penetrating radar (GPR) data to measure soil surface water contents over a bare agricultural field. The GPR system consisted of a vector network analyser combined with a low-frequency 0.2–2.0 GHz off-ground monostatic horn antenna, thereby setting up an ultra-wideband stepped-frequency continuous-wave radar. A fully automated platform was created by mounting the radar system on a truck for real-time data acquisition. An antenna calibration experiment was performed by lifting the whole setup to different heights above a perfect electrical conductor. This calibration procedure allowed the flittering out of the antenna effects and antenna-soil interactions from the raw radar data in the frequency domain. To avoid surface roughness effects, only the lower frequency range of 0.2–0.8 GHz was used for signal processing. Inversions of the radar data using the Green’s functions were performed in the time domain, focusing on a time window containing the surface reflection. GPR measurements were conducted every 4 m along a transect of 100 m. In addition, five time-domain reflectometry measurements were randomly recorded within the footprint of the GPR antenna. A good agreement was observed between the GPR and time-domain reflectometry soil water content estimates, as compared to the previous study performed at the same test site using a higher frequency 0.8–1.6 GHz horn antenna. To monitor the dynamics of soil water content, a pair of time-domain reflectometry probes was installed at 8 cm depth near the footprint of the GPR antenna and both time-domain reflectometry and GPR measurements were carried out for a period of 20 days. A good agreement of the trend was observed between the time-domain reflectometry and GPR time-lapse data with respect to several precipitation events. The proposed method and truck-mounted setup appear to be promising for the real-time mapping and monitoring of surface soil moisture contents at the field scale.
Ground Penetrating Radar in Hydrogeophysics
T o meet the needs of a growing population and to provide us with a higher quality of life, increasing pressures are being placed on our environment through the development of agriculture, industry, and infrastructures. Soil erosion, groundwater depletion, salinization, and pollution have been recognized for decades as major threats to ecosystems and human health. More recently, the progressive substitution of fossil fuels by biofuels for energy production and climate change have been recognized as potential threats to our water resources and sustained agricultural productivity. Th e vadose zone mediates many of the processes that govern water resources and quality, such as the partition of precipitation into infi ltration and runoff , groundwater recharge, contaminant transport, plant growth, evaporation, and energy exchanges between the Earth's surface and its atmosphere. It also determines soil organic carbon sequestra-tion and carbon-cycle feedbacks, which could substantially impact climate change. Th e vadose zone's inherent spatial variability and inaccessibility precludes direct observation of the important subsurface processes. In a societal context where the development of sustainable and optimal environmental management strategies has become a priority, there is a strong prerequisite for the development of noninvasive characterization and monitoring techniques of the vadose zone. In particular, hydrogeophysical approaches applied at relevant scales are required to appraise dynamic subsurface phenomena and to develop optimal sustainability, exploitation, and remediation strategies. Among existing geophysical techniques, ground penetrating radar (GPR) technology is of particular interest for providing high-resolution subsurface images and specifi cally addressing water-related questions. Ground penetrating radar is based on the transmission and reception of VHF-UHF (30–3000 MHz) electromagnetic waves into the ground, whose propagation is determined by the soil electromagnetic properties and their spatial distribution. As the dielectric permittivity of water overwhelms the permittivity of other soil components, the presence of water in the soil principally governs GPR wave propagation. Th erefore, GPR-derived dielectric permittivity is usually used as surrogate measure for soil water content. In the areas of unsaturated zone hydrology and water resources, GPR has been used to identify soil stratigraphy, to locate water tables, to follow wetting front movement, to estimate soil water content, to assist in subsurface hydraulic parameter identifi cation, to assess soil salinity, and to support the monitoring of contaminants. Th e purpose of this special section of the Vadose Zone Journal is to present recent research advances and applications of GPR in hydrogeophysics, with a particular emphasis on vadose zone investigations. Th is special section includes contributions presented at the European Geosciences Union General Assembly 2006 (EGU 2006, Vienna, Austria) and the 11th International Conference on Ground Penetrating Radar (GPR 2006, Columbus, OH). Th e studies presented here deal with a wide range of surface and borehole GPR applications, including GPR sensitivity to contaminant plumes, new methods for soil water content determination, three-dimensional imaging of the subsurface, time-lapse monitoring of hydrodynamic events and inversion techniques for soil hydraulic properties estimation, and joint interpretation of GPR and electric resistivity tomography (ERT) data. Th e fi rst part of this special section deals with surface-based GPR applications. Because surface-based datasets can typically be acquired quite rapidly, they are attractive for providing information about subsurface variability Abbreviations: ERT, electric resistivity tomography; FDTD, fi nite-difference time-domain; GPR, ground penetrating radar.
Use of Ground Penetrating Radar to Study Spatial Variability and Soil Stratigraphy
Engenharia Agrícola
Ground Penetrating Radar (GPR) is a geophysical method that uses electromagnetic waves to study subsurface structure in different fields such as geology, agriculture and civil engineering. The wave penetration in the soil is strongly controlled by the electrical conductivity of soil components such as clay, organic matter, and water. In this study, tests were conducted in a floodplain in the Elizabeth Creek watershed (New Jersey-USA). We established one transect where measurements were completed using two techniques, common mid point (CMP) and constant offset profile (COP), both with 100-MHz frequency antennas. Measurements were also completed using 250 and 500 MHz shielded antennas. GPR showed good accuracy to study soil spatial variability and stratigraphy. Antennas of a higher frequency had less vertical investigation capacity and greater accuracy. In this study, it was not possible to clearly differentiate signals from organic matter and clay; this was the main limitation of the GPR system.
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...
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
Ground Penetrating Radar Measurements in a Controlled Vadose Zone: Influence of the Water Content
Vadose Zone Journal, 2004
. Stoffregen et al. (2002) used a lysimeter to measure changes in water content for 1 yr and correlated Ground penetrating radar (GPR) is a nondestructive method, results with GPR measurements. Dannowski and Yarawhich, as with other geophysical methods, has been successfully used to estimate the water content or hydraulic properties of soils. We manci (1999) used reflected radar waves and geoelectriperformed GPR measurements to calibrate and compare water con-cal measurements to determine the water content betent estimates with actual water contents in a sand box. A vadose tween the water table and the soil surface. zone was simulated by injecting water in a sand box. We obtained Ground penetrating radar reflection techniques are four GPR data sets: for dry sand, for sand with water tables at 72able to detect liquid contaminants (Brewster and Anand 48-cm depths, and for sand after drainage. Using the reflections nan, 1994; Sneddon et al., 2002) and, if combined with (or diffractions) from the bottom of the sand box (or objects buried