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- Volume 11, Issue 2, 2013
Near Surface Geophysics - Volume 11, Issue 2, 2013
Volume 11, Issue 2, 2013
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Absorption and frequency shift of GPR signals in sandy and silty soils: empirical relations between quality factor Q, complex permittivity and clay and water contents
Authors Tina Wunderlich and Wolfgang RabbelABSTRACTThe arrival time, shape and amplitude of a reflected GPR wavelet depend mainly on the real and imaginary parts of dielectrical permittivity and electrical conductivity. All these parameters are strongly affected by the water and clay content of soils. From a comparison of the amplitude and shape of the direct and reflected wavelets the quality factor can be determined independently as an additional soil parameter. To investigate the relationships between the influencing factors, we conducted high‐frequency GPR reflection measurements on soil samples with varying clay and water contents. The factor, derived from the spectral ratio method, is found to be between 5–15 for our samples that range from about 5–90% in sand content, 5–65% in silt content, 3–63% in clay content and 0–37% in pore water content. A multivariate non‐linear empirical function linking water and clay content to agrees well with the observations. We found that the ratio of the real and imaginary parts of the dielectric permittivity depends basically on the clay content. Therefore, it may become possible to determine the clay content of soils in situ from a combination of GPR and conductivity measurements. The values determined from spectral ratios are confirmed by synthetic radargrams and agree with the observed decrease of the central frequency of GPR reflections caused by absorption.
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Time‐lapse monitoring of DNAPL in a controlled cell
Authors Luciana Orlando and Beatrice RenziABSTRACTA multi‐component GPR antenna is used to perform time‐lapse measurements in a controlled experiment simulating DNAPL release. The DNAPL monitoring is based on the delay time of a reflection from the back side of a cell and on spectral analysis of traces.
The study shows that co‐polar and cross‐polar antennas detect a similar pull‐up of the reflection from the back of the cell induced by the DNAPL. The amplitude of the reflection from the DNAPL is weaker in the cross‐polar data with respect to the co‐polar data and in actual cases it could not be detected by both antenna configurations. In addition we observe a change in the amplitude spectra of the traces collected at the DNAPL release with respect to those collected in an uncontaminated area. The amplitude spectrum variations occurred mainly in the co‐polar antennas. The spectra of cross‐polar antennas show variations over time that are not easily linked to the DNAPL position. We observed that the data collected 141 hours after the first DNAPL injection and two hours of water flow, show a pull‐up of the reflection from the back of the cell and variations in the amplitude spectra of the traces located at the same position as the DNAPL injection. This suggests that the DNAPL probably remained trapped by sediments and was not totally removed by the water flow. Forward models, simulating the experiment, confirmed that the DNAPL induces a pull‐up of the reflection from the back of cell and showed that in a controlled experiment the DNAPL produces a reflection whose amplitude depends on the DNAPL saturation. In a real case the presence of small amounts of DNAPL can be at best barely visible because of induced small amplitude reflections. This is truer if no GPR data are available from before the DNAPL spill, so there is little chance that GPR responses can be positively attributed to DNAPL presence.
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Comparison of GPR and unilateral NMR for water content measurements in a laboratory scale experiment
Authors C. Ferrara, V. Di Tullio, P.M. Barone, E. Mattei, S.E. Lauro, N. Proietti, D. Capitani and E. PettinelliABSTRACTSeveral 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 ( = 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.
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Numerical evaluation of a full‐wave antenna model for near‐field applications
Authors A.P. Tran, C. Warren, F. André, A. Giannopoulos and S. LambotABSTRACTIn this study, we numerically evaluated a full‐wave antenna model for near‐field conditions using a Finite‐Difference Time‐Domain (FDTD) antenna model. The antenna is effectively characterized by a series of source and field points and global reflection/transmission coefficients, which by using analytical solutions of Maxwell’s equations, enable us to significantly reduce computation times compared to numerical approaches. The full‐wave GPR model was calibrated by a series of radar data from numerical measurements performed above an infinite perfect electrical conductor (PEC). The calibration results provided a very good agreement with data from the FDTD antenna model giving a correlation coefficient of 0.9995. The model was subsequently verified by using it to invert responses from the FDTD antenna model to reconstruct the electrical properties of an artificial medium subject to 8 scenarios of layering, thickness and electrical properties. Full‐wave inverse modelling enabled us to very well reproduce the GPR data both in time and frequency domains, resulting in accurate estimations of the dielectric permittivity even with a two‐layered medium with highly contrasting electrical properties. The relative errors of the permittivity estimation were less than 5% for all medium scenarios and antenna heights. Inversion also provided very good estimations of the electrical conductivity when this parameter was relatively high but poor results were obtained for low conductivities. Surface response analysis showed that the model was more sensitive to permittivity than conductivity and more sensitive to high than low conductivities. Our modelling approach shows great potential to apply full‐wave inversion for retrieving the electrical properties of the subsurface from near‐ and far‐field radar measurements.
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Spectral velocity analysis for the determination of ground‐wave velocities and their uncertainties in multi‐offset GPR data
Authors Göran Hamann, Jens Tronicke, Colby M. Steelman and Anthony L. EndresABSTRACTIn many hydrological applications, ground‐wave velocity measurements are increasingly used to map and monitor shallow soil water content. In this study, we propose an automated spectral velocity analysis method to determine the direct ground‐wave (DGW) velocity from common midpoint (CMP) or multi‐offset ground‐penetrating radar (GPR) data. The method introduced in this paper is a variation of the well‐known spectral velocity analysis for seismic and GPR reflection events where velocity spectra are computed using different coherency measures along hyperbolas following the normal moveout model. Here, the unnormalized cross‐correlation is computed between waveforms across data gathers that are corrected with a linear moveout equation using a predefined range of velocities. Peaks in the resulting velocity spectra identify linear events in the GPR data gathers like DGW events and allow for estimating the corresponding velocities. In addition to obtaining a DGW velocity measurement, we propose a robust method to estimate the associated velocity uncertainties based on the width of the peak in the calculated velocity spectrum. Our proposed method is tested on synthetic data examples to evaluate the influence of subsurface velocity, surveying geometry and signal frequency on the accuracy of estimated ground‐wave velocities. In addition, we investigate the influence of such velocity uncertainties on subsequent soil water content estimates using an established petrophysical relationship. Furthermore, we apply our approach to analyse field data, which were collected across a test site in Canada to monitor a wide range of seasonal soil moisture variations. A comparison between our spectral velocity estimates and results derived from manually picked ground‐wave arrivals shows good agreement, which illustrates that our spectral velocity analysis is a feasible tool to analyse DGW arrivals in multi‐offset GPR data gathers in an objective and more automated manner.
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Asymptotic solution for a scattered field by cylindrical objects buried beneath a slightly rough surface
Authors M.A. Fiaz, F. Frezza, L. Pajewski, C. Ponti and G. SchettiniABSTRACTThe problem of scattering of electromagnetic waves from a set of cylindrical objects buried beneath a slightly rough surface is undertaken with the cylindrical wave approach as a method of analysis. The small perturbation method is used to compute the scattered field from a rough surface. Numerical results for a rough surface with a sinusoidal profile are obtained with asymptotic evaluation.
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Estimating the 3D position and inclination of a planar interface with a directional borehole radar
Authors Satoshi Ebihara, Hideaki Kawai and Kazushige WadaABSTRACTIn this paper, we present a signal processing method suitable for estimating the position of a planar interface and its inclination using a directional borehole radar. The receiving directive antenna in a radar system is a coaxial‐fed circular dipole array antenna in a borehole (CFCAB), which we have proposed previously. The method combines least‐square fitting with a model of plane‐wave incidence and uses an inverse boundary scattering transform. This approach makes it possible to estimate parameters from measurements collected at only two narrowly separated depths of a radar sonde. We give a numerical example to verify the proposed method. In this simulation, the array data were generated with the Method of Moments modified for a borehole radar. We conducted field experiments in the Nakatatsu mine in Japan, where there is a fault in the skarn. We constructed a 3D image of the fault with a directional borehole radar and the signal processing method. The image agrees well with the information obtained from boring core samples and observations in the gallery.
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Optimization of acquisition setup for cross‐hole: GPR full‐waveform inversion using checkerboard analysis
Authors Max Oberröhrmann, Anja Klotzsche, Harry Vereecken and Jan van der KrakABSTRACTTomographic inversions of cross‐hole ground‐penetrating radar provide images of electromagnetic properties of the shallow subsurface and are used in a wide range of applications. Whereas the resolutions of ray‐based methods like first‐arrival traveltime and first‐cycle amplitude tomography are limited to the scale of the first Fresnel zone, full‐waveform inversions incorporate precise forward modelling using the full recorded signal for a solution of Maxwell’s equation, which results in sub‐wavelength resolutions. In practice, the method can be time‐consuming in data acquisition and expensive in computational costs. To overcome these expenses, a semi‐reciprocal acquisition setup with a reduced number of transmitters and an interchange of transmitter and receiver boreholes instead of a one‐sided equidistant setup in either borehole yielded promising results. Here, this optimized, semi‐reciprocal acquisition setup is compared to a dense, equidistant, one‐sided acquisition setup measured at the field site Krauthausen, Germany. The full‐waveform inversion results are evaluated using the checkerboard test as a capable resolution analysis tool to determine resolvabili‐ties. We introduced also a new method of time‐zero correction by a cross‐correlation of a zero‐offset profile with corresponding horizontal traces of each multi‐offset gather. The obtained experimental results from Krauthausen combined with the checkerboard analysis indicate the main three‐permittivity layers that correspond with different porosities. Also fine‐layered structures within these main layers were reliably imaged. We conclude that the use of the semi‐reciprocal setup is optimum for acquisition speed, inversion speed and obtained permittivity inversion results. Our results indicate that conductivity results are better for denser transmitter‐receiver setups.
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Maxwell curl equation datuming for GPR based on the Kirchhoff integral solution and application in a tunnel grouting test
Authors Yonghui Zhao, Xiongyao Xie, Jiansheng Wu, Jun Chen and Shuangchen GeABSTRACTIn two‐dimensional (2D) ground‐penetrating radar (GPR) data, the reflections from detection targets in depth can be severely obscured by strong scattering generated from near‐surface non‐target structures. By using GPR as a geotechnical non‐destructive testing device, it is still a challenge to eliminate the strong scattering caused by near‐surface rebars in a tunnel lining and to image and asses the grouting condition behind the tunnel lining. This study proposes a method for the reconstruction of GPR images, termed Maxwell curl equation datuming. To eliminate the deleterious effect caused by near‐surface diffractive scattering, we have redefined the reference surface to an actual geologic/engineered interface by using Maxwell curl equation datuming methodology based on the Kirchhoff integral solution. The datuming procedure can redefine the reference surface to a deeper horizon on which GPR transmitters and receivers appear to be located. We conducted a comparison between the datuming and migration to synthetic examples and case studies are presented for a physical model and real GPR data for assessments of shield tunnel grouting in the Shanghai Metro line No.9. The results show that the datuming technique is able to eliminate the strong scattering related to near‐surface rebars in a tunnel lining and improves the quality of deeper images beneath the tunnel lining. The datuming images make it easy to identify the distribution of tunnel grouting.
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Tomographic GPR imaging using a linear inversion algorithm informed by FDTD modelling: a numerical case study of buried utility pipes monitoring
Authors T.M. Millington, N.J. Cassidy, L. Crocco and F. SoldovieriABSTRACTIn this paper, we present the results of an on‐going study into the development of a practical strategy for the interpretation and analysis of near‐surface GPR data in complex scenarios. In particular, we consider the problem of how the knowledge of the investigated scenario can be exploited to improve the diagnostic results using an approach based on the joint use of FDTD numerical modelling and linear tomographic inversion methods. The performance of the approach is evaluated using a simulated test‐case in which GPR data are collected in a complex utility‐pipe model. Prior knowledge of the investigation scenario is captured for the inversion using a three‐dimensional, full‐field FDTD modelling scheme to calculate the incident field and the Green’s functions, allowing the antenna geometry, the air‐ground interface and known subsurface a priori information to be accounted for. As input data to the inversion algorithm, we assume the raw GPR data preprocessed only by simple time‐gating, after Truncated Singular Value Decomposition (TSVD) resulting from the ‘informed linear’ model, are exploited to achieve a regularized solution of the problem. The results show that with just this basic assumption, the joint use of these two (forward and inverse) modelling techniques enhances tomographic imaging in very complex scenarios.
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Non‐destructive prospecting and virtual reconstruction of the chapel of the Holy Spirit in Lecce, Italy
ABSTRACTIn this contribution, we show the results of a GPR measurement campaign, combined with temperature and humidity measurements, gathered in a Renaissance monument of the 16th century in Lecce, Southern Italy. The data are processed by means of standard processing and the results interpreted with the aid of an archive research. Moreover, the reconstruction is inserted into a 3D virtual reconstruction of the monument, achieved by means of a laser scanner.
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2D and 3D ground‐penetrating radar surveys with a modular system: data processing strategies and results from archaeological field tests
Authors Lieven Verdonck, Frank Vermeulen, Roald Docter, Cornelius Meyer and Rudolf KniessABSTRACTRecently, the use of ground‐penetrating radar (GPR) arrays with a large number of antenna elements in a fixed configuration has become more common. The investment needed for these systems is significant. In order to reduce the recording time in the field, an alternative is the use of several single GPR antennas in parallel (a ‘modular system’). Although this does not match the fast acquisition of detailed data sets by means of multi‐channel arrays, a system consisting in single antennas can gradually be expanded and investment can be spread over time. This paper presents a 2D and a full‐resolution 3D survey, conducted with a modular GPR instrument. A characteristic of these systems is that the cross‐line separation between transmitter‐receiver pairs is larger than the sampling distance prescribed by the Nyquist theorem. As a consequence, for 3D data collection, profiles have to be acquired between previously recorded ones, which requires high positioning accuracy. A completely identical response for different single GPR antennas is difficult to achieve. For the system tested, on less favourable soils this resulted in striping in the horizontal slices. Several methods (3D frequency‐wavenumber filtering, eigenimage filtering, mean profile filtering and filtering based on discrete wavelet transform, discrete ridgelet transform and linear Radon transform) were applied to two data sets exhibiting different kinds of linear noise and their capability to suppress artefacts was assessed. Although overall a reduction of the stripe patterns was achieved, mostly it was impossible to fully eliminate the noise in the time‐slices without low‐pass filtering in the cross‐line direction. For the 2D data, low‐pass filtering caused loss of some of the archaeological response and therefore was not applied. Mean profile filtering allowed the most reliable characterization of the archaeological structures.
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Deriving ice thickness, glacier volume and bedrock morphology of Austre Lovénbreen (Svalbard) using GPR
Authors A. Saintenoy, J.‐M. Friedt, A. D. Booth, F. Tolle, E. Bernard, D. Laffly, C. Marlin and M. GriselinABSTRACTAustre Lovénbreen is a 4.6 km2 glacier on the Archipelago of Svalbard (79° N) that has been surveyed over the last 47 years in order to monitor in particular the glacier evolution and associated hydrological phenomena in the context of nowadays global warming. A three‐week field survey during April 2010 allowed for the acquisition of a dense mesh of ground‐penetrating radar (GPR) data with an average of 14 683 points per km2 (67 542 points total) on the glacier surface. The profiles were acquired using Mala equipment with 100 MHz antennas, towed slowly enough to record on average every 0.3 m, a trace long enough to sound down to 189 m of ice. One profile was repeated with a 50 MHz antenna set to improve electromagnetic wave propagation depth in scattering media observed in the cirques closest to the slopes. The GPR was coupled to a GPS system to position traces. Each profile was manually edited using standard GPR data processing including migration, to pick the reflection arrival time from the ice‐bedrock interface. Snow cover was evaluated through 42 snow drilling measurements regularly spaced to cover the entire glacier. These data were acquired at the time of the GPR survey and subsequently spatially interpolated using ordinary kriging. Using a snow velocity of 0.22 m/ns, the snow thickness was converted to electromagnetic wave traveltimes and subtracted from the picked traveltimes to the ice‐bedrock interface. The resulting traveltimes were converted to ice thickness using a velocity of 0.17 m/ns. The velocity uncertainty is discussed from a common midpoint profile analysis. A total of 67 542 geo‐referenced data points with GPR‐derived ice thicknesses, in addition to a glacier boundary line derived from satellite images taken during summer, were interpolated over the entire glacier surface using kriging with a 10 m grid size. Some uncertainty analyses were carried out and we calculated an averaged ice thickness of 76 m and a maximum depth of 164 m with a relative error of 11.9%. The volume of the glacier is derived as 0.3487 ± 0.041 km3. Finally a 10 m grid map of the bedrock topography was derived by subtracting the ice thicknesses from a dual‐frequency GPS‐derived digital elevation model of the surface. These two data sets are the first step for modelling thermal evolution of a glacier and its bedrock, as well as the main hydrological network.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)