Integrated GPR and unilateral NMR approach to estimate water content in a porous material (original) (raw)
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Comparison of GPR and unilateral NMR for water content measurements in a laboratory scale experiment
Near Surface Geophysics, 2013
Several factors affect antenna-soil coupling in a Ground Penetrating Radar (GPR) survey, like surface roughness, lithology, lateral heterogeneities, vegetation, antenna height from the surface and water content. Among them, lithology and water content have a direct effect on the bulk electromagnetic properties of the material under investigation. It has been recently pointed out that the wavelet of the early-time portion of a radar signal is correlated to the shallow subsurface dielectric properties of a material. This result indicates that some information on such properties can be directly extracted from the analysis of GPR early-time traces. In the present paper, we use the early-time GPR signal, in terms of average envelope amplitude computed on the first half-cycle, to map the near-surface (few centimetres) lateral distribution of dielectric parameters, induced by changing the shallow water content on a concrete slab. This controlled experiment was specifically designed to study the effect of water content variations on antenna-material coupling, minimizing the influence of both surface roughness and heterogeneity. The quantitative control of the water in the shallow portion of the slab is performed by using a portable unilateral Nuclear Magnetic Resonance (NMR) sensor, which is able to determine the water content in the material on the basis of the measured proton density. The results show a matching pattern of the physical parameters measured with the two different techniques and a very high degree of linear correlation (r = 0.97) between the radar early-time signal average amplitude and the intensity of the NMR signal, which is proportional to the proton density, i.e., to the water content. This experiment suggests that the early-time approach could be used as a fast and high-spatial resolution tool for qualitatively mapping water content lateral variations in a porous material at shallow depth, using a ground-coupled single-offset antenna configuration and that a quantitative evaluation of the moisture content would require a calibration procedure. In a survey design several factors should be taken into consideration, like the investigation depth, the spatial resolution, the characteristics of the medium (e.g., granular or solid), the physical properties of the material, the site conditions and the reliability of the retrieved physical parameter in terms of water content estimator. If the required investigation depth is limited to a few metres, GPR represents one of the best options in terms of spatial resolution, fast acquisition time, extension of the investigated area and repeatability of the measurements (Annan 2004; Jol 2009; Barone et al. 2010). Such a technique is based on radio waves propagating through the medium and the water content value can be indirectly retrieved from the measurement of a signal velocity. In particular, several radar methodologies like the
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
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.
STUDY OF HYDROGEOLOGY BY USING GROUND PENETRATING RADAR TECHNIQUES
Ground penetrating radar (GPR) is a near surface geophysical method that can give high determination image of the dielectric properties of the features from few tens of meters on the surface. GPR is a geophysical method to capture subsurface feature using electromagnetic waves with the frequency band of 10-1000MHz. This paper examine about the different uses of GPR in hydrogeological studies. The interpreted GPR image is helps to evaluate hydrogeologic properties, such as water content, porosity, and permeability. Its non-invasive abilities make GPR an attractive contrasting option to the traditional methods utilized for subsurface characterization and to study the behavior of groundwater head near pumping well. It has been widely used to map subsurface formations and to recognize fate and transport of contaminants in groundwater.
Ground Penetrating Radar as tool for nondestructive evaluation of soil
2011
A major risk during the construction of highways, rail w ys, civil and industrial buildings is represented by the existence in the soil of ammunition and unexploded mines. Supplementary, in localities, during the excavation, pipes, electrical cables, archaeological sites, the voi ds might be found. The modern method for their detection is Ground Penetrating Radar (GPR) which gives necessary in formation at the scanning of the surface to be examined. Due to the high frequency, the maximum depth at which the objects can be detected is limited to 4 4.5 m. In the same time, the radar-grams interpretation is quite complicate, requiring rich experience and a corresponding database. The obtained experimental data and s ome signal and image processing procedures are presented in this paper in order to make a correct evaluation of type of buried ammunitions, including antitank mines, as caliber, dimensions and their location in the horizontally plan as well in depth.
This paper focused on the effect of moisture and chloride content on the Ground Penetrating Radar (GPR) amplitude of concrete slab. The GPR is used in detecting both corrosion agents on 13 number of concrete slab samples with water-to-cement-ratio of 0.7. Radar measurements were employed on a fixed point of the samples to measure two signals referred to as the direct and reflected radar amplitudes. Simple signal processing on the collected data was implemented by MATLAB software to compute the attenuation of peak-to-peak amplitude of direct waves and reflected waves which is normalized with respect to the peak-to-peak amplitude of direct wave recorded in air. From the analysis, a strong linear relationships (R2 = 0.82 and 0.96) for water content variation of direct and reflected wave were found. However, a very weak linear relationship with R2 of 0.31 for chloride content variation of direct wave was found but showed strong linear relationship (R2 = 0.95) for the reflected wave. This findings showed that both moisture and chloride content have a measurable influence on both GPR signals, which enable the GPR usage on detecting the amount of both corrosion agents in concrete.
We exploit a combination of transmission and reflection Ground Penetrating Radar (GPR) techniques to characterize limestone at different scales (from sub-metric/metric to decametric). The integrated method is based on Multi-Fold (MF) GPR techniques to obtain radar response at different offsets (source-receiver separation) corresponding to different incidence angles. MF GPR measurements are combined with transmission tomography where possible (isolated blocks). The MF datasets allow imaging of rock conditions, characterization of rock, fractures and filling materials. Imaging of joints, fractures, voids and heterogeneous volumes is performed by means of pre-stack and post-stack time and depth imaging methods. In this work experiments are carried out from the surface or on isolated blocks using a pulse GPR system equipped with 250, 500 and 800 MHz central frequencies antennas. The results show that the integrated multi-fold/tomographic method allows enhanced characterization of limestone, compared with conventional sonic methods and proves to be highly sensitive to subtle variations in the lithological and physical properties of the rock mass.
Monitoring the evolution of water and chloride in concrete using GPR full-waveform inversion
2011 6th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), 2011
The assessment of ageing concrete structures is a major technical challenge faced by the Civil Engineers of today. Concrete bridges are exposed to corrosion from the spreading of de-icing salts on roads during winter. Corrosion is problematic because it reduces the reinforcement's load carrying capacity and a long-term consequence can be structural failure. This paper describes an experiment that consisted in exposing a concrete specimen to a controlled wetting-drying cycle using a saline solution. This setup enabled the concentration of chlorides to be distributed in gradients inside the specimen. A novel methodology to estimate material properties, based on ground penetrating radar (GPR) full-waveform inversion, was used to process GPR traces. In this paper, each reflection present in the trace was regarded independently. This procedure was applied to a dataset acquired with bistatic off-ground GPR antennas. Measurements were carried out above the test specimen at regular time lapses. This allowed monitoring of the electromagnetic parameters of the top and bottom concrete specimen surfaces during the wettingdrying cycle.