Railway track inspection using GPR

Swiss Federal Railways (SBB) inspect their railway tracks at regular intervals. The first step of track renewal planning is a geotechnical study. Inspection is focused on the thickness of the ballast, on subsoil material penetrating upwards into the ballast and on geotechnical properties of subgrade and subsoil materials. Up to now, the inspection has been done mainly by digging trenches at evenly spaced intervals and in locations of special interest. In order to evaluate the benefits and limits of GPR railway track inspections, three GPR surveys were carried out on three different railway lines. Data were acquired using a mobile system travelling at 10 km/h. Subsequent to radar data acquisition, trenches were dug. The positioning of some of the trench locations was based on preliminary GPR results in order to support the interpretation of GPR data. Only those trenches were available during interpretation of radar data. In addition, SBB performed their usual investigation programme. This provided an opportunity for checking the radar results in great detail.     

The inspection of retaining walls using GPR

In hilly regions, retaining walls along roads, motorways and railway lines are numerous. In some cases the knowledge of the details of the construction is limited. If rehabilitation work becomes necessary, a detailed knowledge of the construction is desirable for the improved planning of maintenance and repair. This paper describes the application of Ground Penetrating Radar (GPR) for the inspection of retaining walls. The work was carried out in two steps. First, an investigation was carried out on large retaining walls at a Swiss motorway within the framework of a service contract. This included the development of an apparatus enabling high precision positioning of the antennas on the walls. Second, a pilot study was performed on a smaller wall with optimized acquisition and processing parameters. This included the use of antennas with different orientations and the fusion of the two corresponding datasets as well as true 3-D data processing. This paper describes the approaches to data acquisition and processing in the form of the two case studies. Results from different acquisition and processing strategies are compared and the benefits and limits are discussed.     

Multidimensional GPR array processing using Kirchhoff migration

We compare the ability of several practical ground-penetrating radar (GPR) array processing methods to improve signal-to-noise ratio (SNR), increase depth of signal penetration, and suppress out-of-plane arrivals for data with SNR of roughly 1. The methods include two-dimensional (2-D) monostatic, three-dimensional (3-D) monostatic, and 3-D bistatic Kirchhoff migration. The migration algorithm is modified to include the radiation pattern for interfacial dipoles. Results are discussed for synthetic and field data. The synthetic data model includes spatially coherent noise sources that yield nonstationary signal statistics like those observed in high noise GPR settings. Array results from the model data clearly indicate that resolution and noise suppression performance increases as array dimensionality increases. Using 50-MHz array data collected on a temperate glacier (Gulkana Glacier, AK), we compare 2-D and 3-D monostatic migration results. The data have low SNR and contain reflections from a complex, steeply dipping bed. We demonstrate that the glacier bed can only be accurately localized with the 3-D array. In addition, we show that the 3-D array increases SNR (relative to a 2-D array) by a factor of three.     

Delineating alluvial aquifer heterogeneity using resistivity and GPR data

Conceptual geological models based on geophysical data can elucidate aquifer architecture and heterogeneity at meter and smaller scales, which can lead to better predictions of preferential flow pathways. The macrodispersion experiment (MADE) site, with >2000 measurements of hydraulic conductivity obtained and three tracer tests conducted, serves as an ideal natural laboratory for examining relationships between subsurface flow characteristics and geophysical attributes in fluvial aquifers. The spatial variation of hydraulic conductivity measurements indicates a large degree of site heterogeneity. To evaluate the usefulness of geophysical methods for better delineating fluvial aquifer heterogeneities and distribution of preferential flow paths, a surface grid of two-dimensional ground penetrating radar (GPR) and direct current (DC) resistivity data were collected. A geological model was developed from these data that delineate four stratigraphic units with distinct electrical and radar properties including (from top to bottom) (1) a meandering fluvial system (MFS); (2) a braided fluvial system (BFS); (3) fine-grained sands; and (4) a clay-rich interval. A paleochannel, inferred by other authors to affect flow, was mapped in the MFS with both DC resistivity and GPR data. The channel is 2 to 4 m deep and, based on resistivity values, is predominantly filled with clay and silt. Comparing previously collected hydraulic conductivity measurements and tracer-plume migration patterns to the geological model indicates that flow primarily occurs in the BFS and that the channel mapped in the MFS has no influence on plume migration patterns.     

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.     

Spectral bandwidth enhancement of GPR profiling data using multiple-frequency compositing

The amplitude spectrum of ground penetrating radar (GPR) reflection data acquired with a particular antenna set is normally concentrated over a spectral bandwidth of a single octave, limiting the resolving power of the GPR wavelet. Where variously-sized GPR targets are located at numerous depths in the ground, it is often necessary to acquire several profiles of GPR data using antennas of different nominal frequencies. The most complete understanding of the subsurface is obtained when those frequency-limited radargrams are jointly interpreted, since each frequency yields a particular response to subsurface reflectivity. The application of deconvolution to GPR data could improve image quality, but is often hindered by limited spectral bandwidth.We present multiple-frequency compositing as a means of combining data from several frequency-limited datasets and improving the spectral bandwidth of the GPR profile. A multiple-frequency composite is built by summing together a number of spatially-coincident radargrams, each acquired with antennae of different centre frequency. The goal of the compositing process is therefore to produce a composite radargram with balanced contributions from frequency-limited radargrams and obtain a composite wavelet that has properties approximating a delta function (i.e. short in duration and having a broad, uniform spectral bandwidth).A synthetic investigation of the compositing process was performed using Berlage wavelets as proxies for GPR source pulses. This investigation suggests that a balanced, broad bandwidth, effective source pulse is obtained by a compositing process that equalises the spectral maxima of frequency-limited wavelets prior to summation into the composite. The compositing of real GPR data was examined using a set of 225, 450 and 900 MHz GPR common offset profiles acquired at a site on the Waterloo Moraine in Ontario, Canada. The most successful compositing strategy involved derivation of scaling factors from a time-variant least squares analysis of the amplitude spectra of each frequency-limited dataset. Contributions to the composite from each nominal acquisition frequency are clear, and the trace averaged amplitude spectrum of the corresponding composite is broadened uniformly over a bandwidth approaching two-octaves. Improvements to wavelet resolution are clear when a composite radargram is treated with a spiking deconvolution algorithm. Such improvement suggests that multiple-frequency compositing is a useful imaging tool, and a promising foundation for improving deconvolution of GPR data.     

Measurements of the direction of the solar wind using interplanetary scintillation

EISCAT observations of the interplanetary scintillation of a single source were made over an extended period of time, during which the orientation of the baselines between the two observing sites changed significantly. Assuming that maximum correlation between the scintillations observed at the two sites occurs when the projected baseline is parallel to the direction of plasma flow, this technique can be used to make a unique determination of the direction of the solar wind. In the past it has usually been assumed that the plasma flow is radial, but measurements of eleven sources using this technique have indicated conclusively that in at least six cases observed at mid or high heliocentric latitude there is a significant non-radial component directed in four cases towards the heliocentric equator and in two cases towards the pole.     

Fields of local stresses of different levels and second order fractures close to the off of a strike slip fault

In the work 2D and 3D fields of stresses of several scale levels close to the off of the main fault (vertical strike-slip fault) in conditions of compression are mathematically calculated and investigated. The solution is found for the elastic task for a 2D “horizontal” field; a 3D field of stresses is obtained by the imposition of a “vertical” unaxis compression. It is shown that the surroundings of the fault are subdivided into three (not two, as is usually considered) regions of types of predictable secondary fractures: “extension,” “strike-slip fault,” and “compression.” In regions close to the off of the main fault, three different microregions occur. The type of destruction in these microregions depends on the parameters of the outer load. Natural and model data of second order fractures that are compared with the calculated data are examined and generalized. The performed investigation is important for the determination of the genesis of secondary fractures, located close to the main fault. The calculated parageneses of secondary fractures may be used for the estimation of the stress tensor type of the regional field.     

Increasing waveform inversion efficiency of GPR data using compression during reconstruction

We propose a method to avoid large amounts of data storage and reading during the reconstruction of a forward–backward time-stepping (FBTS) method for ground-penetrating radar (GPR) data using compression and decompression. The reconstruction example shows that the reconstruction result is acceptable if the compression ratio is less than 90%.     

金属矿井下环境对GPR测试的干扰因素分析

探地雷达技术（GPR）在地下金属矿中应用日益广泛,然而复杂的井下环境时常影响雷达的测试效果.鉴别、分析各种干扰因素,有利于消除和抑制环境噪音在雷达中产生测试伪信号.本文结合作者在地下金属矿中的测试经验,探讨了GPR在金属矿井下测试过程中遇到的几种常见的干扰因素,分析其在雷达图象中呈现基本特征,并提出相应的处理措施与方法.     

Location of buried plastic pipes using multi-agent support based on GPR images

This work focuses on the generation of tools to aid inspection and identify buried plastic pipes in water supply systems (WSS). In our study we use ground penetrating system (GPR) images as a non-destructive method of obtaining information without altering the system conditions and the environmental characteristics. A viability study for extracting features, and an approach to the above-mentioned application based on multi-agent systems are addressed in this paper. Firstly, we use intensive matrix manipulation of the GPR output for preprocessing the images. As a result, two matrices are produced that classify initial data based on the original radargram of the wave amplitude parameter. Then the plastic pipe characteristics that offer an enhanced likelihood of location are defined. This procedure is evaluated through two case-studies. One study corresponds to a simple case (one pipe) and the other corresponds to various pipes (made of different materials). Both cases were developed under controlled laboratory conditions. The obtained results are promising, and we show that automatic plastic pipe location has been achieved. The main contributions of the procedures proposed in this work are: firstly, highly skilled GPR prospection operators become unnecessary for plastic pipe location using GPR images; and secondly, we have opened a route to further classification that makes use of other methodologies.     

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.     

Temporal observations of a seasonal snowpack using upward‐looking GPR

An increase of the spatial and temporal resolution of snowpack measurements in Alpine or Arctic regions will improve the predictability of flood and avalanche hazards and increase the spatial validity of snowpack simulation models. In the winter season 2009, we installed a ground‐penetrating radar (GPR) system beneath the snowpack to measure snowpack conditions above the antennas. In comparison with modulated frequency systems, GPR systems consist of a much simpler technology, are commercially available and therefore are cheaper. The radar observed the temporal alternation of the snow height over more than 2·5 months. The presented data showed that with moved antennas, it is possible to record the snow height with an uncertainty of less than 8% in comparison with the probed snow depth. Three persistent melt crusts, which formed at the snow surface and were buried by further new snow events, were used as reflecting tracers to follow the snow cover evolution and to determine the strain rates of underlaying layers between adjacent measurements. The height in two‐way travel time of each layer changed over time, which is a cumulative effect of settlement and variation of wave speed in response to densification and liquid water content. The infiltration of liquid water with depth during melt processes was clearly observed during one event. All recorded reflections appeared in concordance with the physical principles (e.g. in phase structure), and one can assume that distinct density steps above a certain threshold result in reflections in the radargram. The accuracy of the used impulse radar system in determining the snow water equivalent is in good agreement with previous studies, which used continuous wave radar systems. The results of this pilot study encourage further investigations with radar measurements using the described test arrangement on a daily basis for continuous destruction‐free monitoring of the snow cover. Copyright © 2010 John Wiley & Sons, Ltd.     

基于辛算法模拟探地雷达在复杂地电模型中的传播

近年来,探地雷达(GPR)凭借其快速、高效、无破损等特点,已经广泛应用于浅地层目标探测中.数值模拟是研究探地雷达电磁波在地下结构中传播规律的有效手段.辛算法是一种保持Hamilton系统总能量不变的时域数值计算方法.本文提出了基于一阶显式辛分块龙格库塔方法的探地雷达数值模拟方法.通过对比本文算法与时域有限差分方法计算结果可知,在同等计算精度下,本文算法可以节省25%的计算时间.并基于本文算法对两个复杂GPR模型进行正演模拟,得到模拟GPR探测wiggle图,这有助于更好的理解和分析实测雷达数据.     

Finding of low-contrast formations in the solar corona using a low contrast method

Coronal holes, bright coronal points, filaments, and prominences are among the initial factors responsible for variability of the space weather conditions. Radioheliographic data on low-contrast formations contain valuable information necessary for studying conditions of origination, peculiarities of evolution, and prediction of solar-terrestrial relations. It is important to identify these formations on the solar disk when physical properties of coronal holes are revealed. The algorithm based on the Wiener-Tikhonov filter modification with controlled parameters and a high-frequency contrast filter was developed in order to isolate low-contrast formations in the solar corona brightness distributions obtained at a wavelength of 5.2 cm from the Siberian solar radio telescope observations. In this case low-contrast sources are isolated in two main stages: (1) HF noise smoothing based on an evolutionary filter with controlled parameters and (2) contrasting of sources using an HF filter. The evolutionary filter regularization parameters and the dimensions of an HF contrast filter mask are selected depending on the signal-to-noise ratio and dimensions of the studied region based on the results of preliminary data processing. The corresponding software has been developed in order to identify low-contrast objects on the Sun’s radio images using this method. The algorithm is used to isolate filaments and coronal holes and the results of this usage are presented in this work.     

A combined strategy for landmine detection and identification using synthetic GPR responses

Synthetic Ground Penetrating Radar (GPR) target responses may be successfully used for buried landmine classification purposes. This paper demonstrates that accurately simulated one-dimensional temporal signatures can be employed as reference waveforms for efficient clutter suppression and improved target detection/recognition. The proposed methodology is a combined approach consisting of a cross-correlation based identification algorithm and an energy based detection algorithm. The former can be implemented before conducting the detection as an additional filtering step in the form of a similarity constraint between measured and synthetic scattered signals. The application of the combined method to experimental data yields a clear gain in the detection sensitivity, particularly for those mines which are most difficult to detect through scattered energy considerations alone. Moreover, an adapted Inverse Distance Weighted (IDW) averaging has been incorporated to enhance the quality of the imaging and to rise the Signal-to-Clutter ratio (SCR) of the resulting maps. This strategy can help to substantially reduce the number of false alarms and speed up the clearance labors.     

区间B样条小波有限元GPR模拟双相随机混凝土介质

基于可分离小波理论,由一维区间B样条小波尺度函数的张量积构造二维B样条小波基,并将它作为GPR波动方程求解的插值函数,通过引入转换矩阵,实现小波系数空间与雷达电磁场之间的转换.应用Galerkin算法,推导了二维区间B样条小波有限元GPR波动方程离散格式,求出了2阶1尺度与2阶2尺度BSWI尺度函数的积分值及联系系数,给出了该算法的详细求解过程.编制了BSWI的Matlab模拟程序,应用该程序对两个典型实例进行了正演,结果表明：BSWI能采用较少的单元达到与FEM相似的精度,而BSWI算法尺度提升能提高解的精度,但耗时会急剧增加.最后,将BSWI算法应用于双相随机混凝土模型,说明随机介质模型理论能灵活、有效地描述实际混凝土介质的分布,正演剖面与实测剖面特征更相符,能更真实地模拟雷达波的传播过程,可为提高GPR的探测效果和解释准确性提供理论基础.     

利用水平井分段压裂微地震裂缝方位监测确定最大水平主应力方位

<正>水平井是石油钻井技术进步的产物,因其水平段在深度方向起伏很小,可以简化为地下几百米至几千米深度处的一个完全水平的空心管线,管线壁称为套管;在套管壁上射出若干簇孔,称为射孔;射孔使管线内外的流体可以进行交换,深度通常大于1m;射孔指向方向垂直于套管壁及水平段走向,人工压裂裂缝沿着最大水平主应力方向扩展,这是由人工裂缝方向判断最大水平主应力方向     

Characterizing the energy distribution around GPR antennas

Antenna height, orientation, shielding and subsurface properties all impact GPR responses. Although the basic concepts are generally understood, clarifying the key relationships can aid interpretation. Our long term goal is to develop easily parameterized models of transmitting and receiving components of GPR systems that will provide for quantitative interpretation of data acquired with real systems. Our first step was to develop modeling capacity and response presentation tools to help with development; the initial results have been both informative and forced a better understanding of near and far field.We are using three-dimensional (3D) finite-difference time-domain (FDTD) modeling. Model results can be presented in a variety of forms. In this paper, we focus on presenting the emitted energy characteristics and use radiation pattern display format for infinitesimal dipoles, resistively loaded dipoles and shielded dipoles. Patterns are computed for a range of environments such as free-space and over loss-less half-spaces with various properties. The energy distribution patterns are presented to investigate the behavior with distance away from the transmitter feed point, and as a function of height above the ground surface.The numerical simulations provide expected insights plus demonstrate the benefit of ground-coupling and the impact of shielding on GPR responses. Further, using the total radiated energy parameter is a novel method for displaying directivity, for GPR transient emissions.     

基于GprMax正演模拟的探地雷达根系探测敏感因素分析

应用探地雷达对植物根系进行探测的有效性已得到证实.但由于根系结构复杂,根围环境异质性强,针对基于探地雷达得到的根系探测数据的解读尚处于经验积累阶段.本研究首先通过对比根系探地雷达实测信号图像和模拟信号图像,证实了利用GprMax模拟探地雷达探测植物根系的有效性.其次通过定义不同根系空间结构和电性参数场景,模拟了不同条件下根目标反射信号的差异,并对影响探地雷达探测植物根系有效性的敏感因素进行了初步分析.模拟结果有助于探地雷达野外根系探测图谱的解译,为探地雷达在植物根系探测中的应用积累经验.