A three dimensional approach for tracking cracks in bridges using GPR

Corrosion associated with reinforcing bars is the most significant contributor to bridge deficiencies. The corrosion is usually caused by moisture and chloride ion exposure. The reinforcing bars are attacked by corrosion and yield expansive corrosion products. These oxidation products occupy a larger volume than the original intact steel and internal expansive stresses lead to cracking and debonding. There are some conventional inspection methods for the detection of the reinforcing bar's corrosion but they can be invasive and destructive, often laborious, and lane closure is required and it is difficult or unreliable for any quantification of corrosion. For these reasons, bridge engineers always prefer more to use the ground penetrating radar (GPR) technique. In this work a novel numerical approach for three dimensional tracking and mapping of cracks in the bridge is proposed. The work starts from some interesting results based on the use of the 3D imaging technique in order to improve the potentiality of the GPR to detect voids, cracks or buried objects. The numerical approach has been tested on data acquired on a bridge by using a pulse GPR system specifically designed for bridge deck and pavement inspection. The equipment integrates two arrays of Ultra Wide Band ground coupled antennas, having a main working frequency of 2 GHz. The two arrays are using antennas arranged with a different polarization. The cracks, associated often to moisture increase and higher values of the dielectric constant, produce a not negligible increase of the signal amplitude. Following this, the algorithm, organized in preprocessing, processing and postprocessing stages, analyzes the signal by comparing the value of the amplitude all over the domain of the radar scan.     

Invariant properties and rotation transformations of the GPR scattering matrix

We analyze the properties of the scattering matrix associated with the incident and scattered electric fields used in GPR. The elements of the scattering matrix provide information produced by different polarizations of the incident wave field. Rotationally invariant quantities such as trace, determinant and Frobenius norm lead to images that combine the information contained in the four elements of the scattering matrix in a mathematically simple and sound manner. The invariant quantities remove the directional properties implicit in the dipolar field used in GPR allowing the application of standard processing techniques designed for scalar fields, such as those used in seismic data processing. We illustrate the non-directional properties of the invariants using a 3D simulation of the wavefield produced by a point scatterer. The estimation of the azimuth angle of elongated targets is also explored using rotation transformations that maximize alternatively the co-polarized or the cross-polarized responses. The angle estimation is essentially an unstable process, particularly if low amplitudes or noisy data are involved. We apply the Frobenius norm 6S6_F as a criterion for selection of the best amplitudes to use for a more stable and significant angle estimation. The performance of our formulation was tested with synthetic data produced by a 3D model of an air-filled metal pipe buried in a homogeneous halfspace. The images resulting from the invariants show a clear diffraction hyperbola suitable for a scalar wavefield migration, while the azimuth of the pipe is neatly resolved for amplitudes selected with 6S6_F ≥ 0.4. A field experiment conducted above an aqueduct pipe illustrates the proposed methods with real data. The images obtained from the invariants are better than those from the individual elements of the scattering matrix. The azimuth estimated using our formulation is in agreement with the probable orientation of the aqueduct. Finally, a field experiment above a buried air-filled barrel shows that combining the information in the way proposed in this work may lead to an improved image of the subsurface target, the cost to pay is the lost of directional information contained in the scattering matrix. In general, we claim that the methods proposed in this work can be useful to analyze the information acquired by multicomponent GPR surveys using standard scalar wavefield algorithms.     

Investigation of the age and migration of reversing dunes in Antarctica using GPR and OSL,with implications for GPR on Mars

GPR provides high resolution images of aeolian strata in frozen sand in the McMurdo Dry Valleys of Antarctica. The results have positive implications for potential GPR surveys of aeolian strata on Mars. Within the Lower Victoria Valley, seasonal changes in climate and a topographically-constrained wind regime result in significant wind reversals. As a consequence, dunes show reversing crest-lines and flattened dune crests. Ground-penetrating radar (GPR) surveys of the dunes reveal sets of cross-strata and low-angle bounding surfaces produced by reversing winds. Summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation. Solar radiation which heats the valley floor melts ice cements making sand available for transport. At the same time, solar heating of the valley floor generates easterly winds that transport the sand, contributing to the resultant westward dune migration. The location of the dune field along the northern edge of the Lower Victoria Valley provides some shelter from the powerful föehn and katabatic winds that sweep down the valley. Topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation, migration and preservation of the dune field. Optically-stimulated luminescence (OSL) ages from dune deposits range from 0 to 1.3 kyr showing that the dune field has been present for at least 1000 yr. The OSL ages are used to calculate end-point migration rates of 0.05 to 1.3 m/yr, which are lower than migration rates reported from recent surveys of the Packard dunes and lower than similar-sized dunes in low-latitude deserts. The relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment.     

An automatic recognition algorithm for GPR images of RC structure voids

Ground penetrating radar (GPR) is a powerful tool for detecting defects in and behind reinforced concrete (RC) structures. However, the traditional way of interpreting GPR data involves considerable manpower and is time-consuming. The aim of this study is to illustrate a new approach to recognize GPR images of RC structure voids automatically. Firstly, synthetic GPR images are created by FDTD method. As multiple waves caused by steel bars seriously interfere with the target echo signals, it is difficult to identify targets from the forward modeling images. According to the periodicity of multiple waves from steel bars, the predictive deconvolution method is used to suppress those waves and the outcome is preferable. Then, the support vector machine (SVM) algorithm is proposed to automatically recognize voids in GPR images. The automatic identification procedure includes four steps: 1) collecting training data, 2) extracting features from GPR images, 3) building the SVM model and 4) identifying the voids automatically. The results show that the proposed method provides a suitable tool to locate the cover depths and lateral ranges of the voids, and the trained SVM model gives a favorable outcome when noise (no more than 5%) is added to a synthetic GPR image.     

Resolution of GPR bowtie antennas: An experimental approach

Since target reflections directly depend on the emitted pulse characteristics, a key factor for carrying out a successful GPR survey is to know as much as possible about the transmission features of the antennas used. This information is very important in order to choose the right antennas and set the appropriate configuration parameters for a specific survey. With this in mind this paper deals with the development of a set of laboratory experiments on the resolution capabilities of three bowtie antennas at frequencies of 500, 800 and 1000 MHz. Results from these measurements give a first estimation of the resolution of the antennas under test, showing the advantage of performing experiments rather than relying only on theoretical assumptions. The results are also expressed in terms of the central wavelength for each antenna and compared with some theoretical estimations proposed in the specialized bibliography.     

基于纵横波解耦的三维弹性波逆时偏移

纵横波波场分离是弹性波偏移方法的必要条件,通过纵横波成像的差异可以获取更多地下介质的信息.目前所用的纵横波波场分离方法多采用Helmholtz分解,这样得到的波场不仅物理意义发生了变化,振幅和相位也会发生改变.本文采用纵横波解耦的弹性波方程,将其应用于三维介质,对比分析了纵横波解耦方法相对传统Helmholtz分解方法在相位、振幅上的优势.将该解耦的波场分离方法应用于弹性波逆时偏移,能得到相位、振幅和物理意义不受改变的偏移结果.但是该解耦方法分离得到的纵横波波场均为矢量场,将该波场分离方法用于弹性波逆时偏移,还需要解决矢量场如何得到标量成像结果的问题.本文引入了Poynting矢量,通过Poynting矢量对矢量波场进行标量化,这样就能得到保振幅、相位,且无极性反转的标量PP和PS成像结果.同时针对S波Poynting矢量求取不准确的问题,采用拟S波应力场和S波速度场得到了更加准确的S波Poynting矢量.理论计算证明了本文采用的3D波场解耦的矢量波场分离方法的正确性和引入Poynting矢量对矢量波场进行标量成像的有效性.     

MULTICOMPONENT COMMON-RECEIVER GATHER MIGRATION OF SINGLE-LEVEL WALK-AWAY SEISMIC PROFILES1

Seismic data are usually separated into P-waves and S-waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common-receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity. Here the propagation of the elastic wavefield for a given mode (e.g. P-S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far-field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common-receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component. This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split-step Fourier-excitation-time imaging condition depth migration. The raytracing to get the excitation-time imaging condition also provides the expected polarization for the post-migration projection. The same downward extrapolated wavefield can be used for both the P-P and P-S migrations, providing a flexible and efficient route to the migration of multicomponent data. The technique is illustrated on a synthetic example and a single-level Walk-away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P-P reflector below the geophone and localized P-P and P-S scatterers at the level of the geo-phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface.     

3D imaging of a reservoir analogue in point bar deposits in the Ferron Sandstone, Utah, using ground-penetrating radar

Most existing reservoir models are based on 2D outcrop studies; 3D aspects are inferred from correlation between wells, and so are inadequately constrained for reservoir simulations. To overcome these deficiencies, we have initiated a multidimensional characterization of reservoir analogues in the Cretaceous Ferron Sandstone in Utah. Detailed sedimentary facies maps of cliff faces define the geometry and distribution of reservoir flow units, barriers and baffles at the outcrop. High‐resolution 2D and 3D ground‐penetrating radar (GPR) images extend these reservoir characteristics into 3D to allow the development of realistic 3D reservoir models. Models use geometric information from mapping and the GPR data, combined with petrophysical data from surface and cliff‐face outcrops, and laboratory analyses of outcrop and core samples. The site of the field work is Corbula Gulch, on the western flank of the San Rafael Swell, in east‐central Utah. The outcrop consists of an 8–17 m thick sandstone body which contains various sedimentary structures, such as cross‐bedding, inclined stratification and erosional surfaces, which range in scale from less than a metre to hundreds of metres. 3D depth migration of the common‐offset GPR data produces data volumes within which the inclined surfaces and erosional surfaces are visible. Correlation between fluid permeability, clay content, instantaneous frequency and instantaneous amplitude of the GPR data provides estimates of the 3D distribution of fluid permeability and clay content.     

Remigration of ground-penetrating radar data

Optimal electromagnetic wave propagation velocities and subsurface images for ground-penetrating radar (GPR) data can be specified by using an imaging scanning method. In addition to time-migrating the unmigrated GPR section, we remigrate the already time-migrated section by a one-step remigration operator using different velocities. This creates many time-migrated images for different constant migration velocities. In this way, the computation time for time-migration is very much reduced. Time-migrated reflector images `propagate' when the constant migration velocity is continuously changed. For this `propagation' there exists a wave-equation-type partial differential equation. Each time-migrated section can thus be viewed as a snapshot for a certain migration velocity. The time-migrated reflector images behave like `waves', called image waves. This is applied to real GPR data acquired over a concrete body within which a steel cable frame is buried. The method produces a quick velocity scan to find a reliable migration velocity leading to the best time-migrated image.     

GPR reflection characteristics and depositional models of mud volcanic sediments—Wushanting mud volcano field, southwestern Taiwan

Ground penetrating radar (GPR) survey was conducted in the Wushanting mud volcano field (Yanchao, Kaohsiung) using a 500 MHz antennae, which allowed high-resolution imaging of subsurface structures. Seven GPR reflection characteristics are recognized. Sigmoid GPR reflection patterns resulted from a recent mud lobe deposited above an underlying older mud lobe front. Contorted GPR facies resulted from recent mud flow which encountered obstacles. Subparallel reflections resulted from mud volcano deposits of limited flowability, low velocity and gentle gradient. Hummocky reflection patterns are formed by interfingering of recent mud lobes building onto low land. Disrupted GPR facies were due to lateral breaks of continuity from mud cracks, which, according to field observation, can provide channels for erosion and form deeper erosion gullies. GPR time slices of different depths are rendered as a three-dimensional model. Approximately orbicular GPR reflection characteristics can indicate arcuate stacked mud lobe fronts of different periods. Some depositional models to explain GPR reflection characteristics can be founded upon observations of recent sedimentary phenomena. The models of this study may be applied to paleoenvironments and the depositional evolution of mud volcanoes in similar geological settings.     

基于速度估计的改进型线性变换有限差分偏移在探地雷达中的应用

为了解决传统线性变换有限差分偏移算法对变速介质的高敏感性问题,对加入了速度变量的标量波动方程进行线性变换,在横向非均匀介质条件下,详细推导了包含混合因子项的线性变换差分偏移方程,实现了横向非均匀介质中陡倾角反射界面的正确归位,同时改进型算法可以灵活地选择混合因子项,这样就增强了原算法对复杂地质问题的适应能力,在此基础上...     

Application of the Brewster angle to quantify the dielectric properties of ground ice formations

Previous studies of ground ice using moveout type ground-penetrating radar (GPR) surveys indicate that the dielectric permittivity can constrain the type of ground ice present in the subsurface. Due to the high-loss nature of the active layer over permafrost targets, however, the signal strength of GPR signals is often insufficient to resolve the basal boundary required for determining the dielectric permittivity of an underlying unit. We apply a non-conventional antenna orientation and post-processing method to determine the dielectric permittivity of the unit underlying the lowest resolvable boundary. We conduct moveout surveys using a 450 MHz GPR with collinear parallel oriented antennas on two adjacent ground ice formations in the region of Thomas Lee Inlet, Devon Island, Nunavut. We exploit the Brewster angle to calculate the approximate dielectric permittivity of ground ice formations below the active layer. The results agree within 1 dielectric unit with on-ice permittivity measurements made during a complementary study of the site.     

Photographing layer thicknesses and discontinuities in a marble quarry with 3D GPR visualisation

An important problem of marble-quarry management is assessing the quality and the homogeneity of quarry blocks before excavation. In this study, we decided to image the limestone, which we studied in a marble quarry, in terms of layer thickness, discontinuities and cavities using the ground-penetrating-radar (GPR) method. The method was successfully applied to detect and map the fractures with the cavities in a marble layer according to depth in the Ankara City Polatli Town (Turkey) region, which represents upper Miocene–Pliocene lacustrine carbonate rocks.This paper is based on interactive transparent 3D visualisation of the 2D GPR profiles to determine changes in layer thickness and discontinuities. In addition, this paper indicates the importance of the appropriate opacity-function construction to obtain transparent 3D visualisation. Firstly we acquired and processed parallel 2D GPR profile data, then we assigned two different amplitude–colour ranges using a limited number of colours to determine the layer thickness and its discontinuities separately. We obtained a 3D volume using parallel 2D GPR data and displayed a limited amplitude range by arranging an opacity function. Therefore, we obtained transparent 3D blocks for thickness and discontinuities, and we formulated an interactive 3D display to image the horizontal, vertical and inclined discontinuities and their directions in the x–y plane versus depth. The GPR results were compared with the petrographical investigation on the basis of textural and mineralogical compositions. The vesicular textures within carbonate platform were supported by the GPR results.     

探地雷达小波域三维波动方程偏移

阐述了矩阵多分辨分析理论中的标准形式与非标准形式,并以Hilbert算子为例,说明了算子多分辨表示的压缩效果,为小波域偏移算法奠定了理论基础.从三维雷达波动方程出发,利用爆炸反射原理和浮动坐标变换,推导出三维探地雷达波动方程差分格式,并通过方程分裂算法及小波多分辨算法,在小波域求解波场外推矩阵,进而得到探地雷达小波域三维波动方程偏移算法,在此基础上,开发了探地雷达小波域偏移处理程序,并把该程序应用于三个球体空洞的3-D正演结果及实际的雷达数据中,通过对比偏移处理前后的雷达资料,得知该三维偏移算法能使3-D正演剖面中的反射波归位、绕射波收敛,极大地提高了雷达剖面的分辨率,有利于探地雷达资料的地质解释.     

机载探地雷达的进展以及数值模拟

机载探地雷达可能解决危险环境或广域条件下的近地表探测问题,用于解决环境、生态或军事方面的问题.然而由于种种原因,该技术的发展却显得比较慢.为了推进该技术的发展,本文介绍了目前世界范围内机载探地雷达的进展,并利用时间域有限差分法对一些典型模型进行数值模拟,并用特定的偏移成像方法对模拟结果进行成像.目前存在的机载探地雷达主要有三种类型:第一种为将常规探地雷达天线悬挂在直升飞机上,第二种为针对机载探地雷达开发的雷达系统,第三种为具有探地能力的合成孔径雷达.数值模拟结果表明,不管是水平地面的情况下,还是起伏界面的情况下,机载探地雷达都能清楚探测一定深度范围内的地下目标.可见,机载探地雷达是存满希望的一种方法.     

Monitoring subsurface changes over time with cross-well electromagnetic tomographyt1

A fast imaging technique is developed to deduce the spatial conductivity distribution in the earth from low-frequency (> 1 MHz) cross-well electromagnetic measurements. A sinusoidally oscillating, vertically orientated, magnetic dipole employed as a source, and it is assumed that the scattering bodies are azimuthally symmetric about the source dipole axis. The use of this model geometry reduces the 3D vector problem to a more manageable 2D scalar form. Additional efficiency is obtained by using the Born series approximation which is derived from nonlinear integral equations that account for the scattered magnetic fields generated by inhomogeneities embedded in a layered earth. Stabilization of the inversion problem is accomplished through the use of bounding constraints and a regularization method which results in a smooth model that fits the data to the desired noise level. The applicability of cross-well electromagnetics for imaging and monitoring changes caused by subsurface processes has been tested by simulating plumes of conductive fluid with 2D models. The images that result from inverting these synthetic data indicate that the vertical resolution of the method is better than the horizontal, increasing the noise decreases the image resolution, and incorporating a priori knowledge in the form of positivity constraints improves the results. Although higher operating frequencies are usually associated with better resolution, frequencies as low as 100 Hz can produce acceptable images in simulated oilfield environments. The imaging scheme has been applied to data collected during a salt-water injection experiment at the Richmond Field Station test site in Richmond, California. Both the data and the resulting images clearly reveal the presence of the plume and indicate that it is migrating towards the north-northwest rather than spreading symmetrically about the injection well. Applying the imaging code to synthetic data generated by a 3D sheet model verifies the interpretation of these results.     

Stakeholder needs for ground penetrating radar utility location

In the UK alone there are millions of miles of underground utilities with often inaccurate, incomplete, or non-existent location records that cause significant health and safety problems for maintenance personnel, together with the potential for large, unnecessary, social and financial costs for their upkeep and repair. This has led to increasing use of Ground Penetrating Radar (GPR) for utility location, but without detailed consideration of the degree of location accuracy required by stakeholders — i.e. all those directly involved in streetworks ranging from utility owners to contractors and surveyors and government departments. In order to ensure that stakeholder requirements are incorporated into a major new UK study, entitled Mapping the Underworld, a questionnaire has been used to determine the current and future utility location accuracy requirements. The resulting data indicate that stakeholders generally require location tolerances better than 100 mm at depths usually extending down to 3 m, and more occasionally to 5 m, below surface level, providing significant challenges to GPR if their needs are to be met in all ground conditions. As well as providing much useful data on stakeholder needs, these data are also providing a methodology for assessment of GPR utility location in terms of the factor most important to them — the degree to which the equipment provides location within their own accuracy requirements.     

Improvements in crosshole GPR full-waveform inversion and application on data measured at the Boise Hydrogeophysics Research Site

Crosshole ground penetrating radar (GPR) tomography has been widely used and has the potential to improve the obtained subsurface models due to its high spatial resolution compared to other methods. Recent advances in full-waveform inversion of crosshole GPR data show that higher resolution images can be obtained compared to conventional ray-based GPR inversion because it can exploit all information present in the observed data. Since the first application of full-waveform inversion on synthetic and experimental GPR data, the algorithm has been significantly improved by extending the scalar to a vectorial approach, and changing the stepped permittivity and conductivity update into a simultaneous update. Here, we introduce new normalized gradients that do not depend on the number of sources and receivers which enable a comparison of the gradients and step lengths for different crosshole survey layouts. An experimental data set acquired at the Boise Hydrogeophysics Research Site is inverted using different source–receiver setups and the obtained permittivity and conductivity images, remaining gradients and final misfits are compared for the different versions of the full-waveform inversion. Moreover, different versions of the full-waveform inversion are applied to obtain an overview of all improvements. Most improvements result in a reducing final misfit between the measured and synthetic data and a reducing remaining gradient at the final iteration. Regions with relatively high remaining gradient amplitudes indicate less reliable inversion results. Comparison of the final full-waveform inversion results with Neutron–Neutron porosity log data and capacitive resistivity log data show considerably higher spatial frequencies for the logging data compared to the full-waveform inversion results. To enable a better comparison, we estimated a simple wavenumber filter and the full-waveform inversion results show an improved fit with the logging data. This work shows the potential of full-waveform inversion as an advanced method that can provide high resolution images to improve hydrological models.     

探地雷达图像的正演合成与偏移处理

本文讨论了射线追踪法实现二维地电断面的探地雷达图像的正演合成问题,其中包括二维地电断面的数字拟合、多层连续界面介质中高频电磁波的反射与透射统一的运动学边界方程;根据Huygen's-Fresnel原理导出了均匀介质中二维管状体模型的衍射雷达波的计算公式;给出了用射线追踪法合成的二维地电模型的探地雷达图像;用衍射波公式合成了二维管状模型的雷达图像,并与物理模拟作了比较.在此基础上,用Kirchhoff积分法对数字模拟与物理模拟记录的雷达图像作了偏移处理,并给出了实例.     

A full non-metallic trailer for GPR road surveys

In this work, the design and construction of a special trailer designed to evaluate traffic infrastructure with GPR is presented. This trailer is adapted to the requirements of a multipurpose GPR system and different ground-coupled antennas to allow for multi-frequency surveys of roads without obstructing traffic. The prototype presented in this work is completely non-metallic, allowing massive data acquisition at a cruising speed up to 80 km/h. It provides the possibility of carrying two bowtie antennas simultaneously with a simple mechanism that allows for changing the elevation from the roadbed to the antenna/s for adequate and safe operation. The work includes the results of tests conducted to evaluate the performance of the system under different field work conditions.