Showing papers in "Subsurface Sensing Technologies and Applications in 2005"

Radius estimation for cylindrical objects detected by ground penetrating radar

Abstract: In this paper, a model is presented for the hyperbolic signature of a buried cylindrical target that takes into account the radius of the cylinder, This allows for cylinders of arbitrary radii to be detected and characterized uniquely from a single radargram, resulting in a more accurate estimation of the relative permittivity of the surrounding medium and of the depth, in addition to the radius information. This is achieved by subjecting the radargrams to a series of image processing stages followed by a curve-fitting procedure specifically developed for hyperbolae. The fitting technique is applied on a variety of real hyperbolic signatures that are collected from a controlled test site, The results indicate this technique is fully capable of successfully estimating the depth and radius to within 10%, which validates the method and justify the assumptions used.

Microwave Non-Destructive Testing of Wood and Similar Orthotropic Materials

Abstract: A non-destructive method is described to estimate fiber (grain) direction, moisture density, and dry density of an orthotropic material such as wood, from measurements of the complex attenuation of microwaves transmitted through the material. The complex attenuation in an orthotropic material has a tensor character, similar to other tensor quantities such as stress and strain. The same transformation equations and Mohr’s Circle constructions apply to rotations of material axes. Bulk material properties can be identified from the principal attenuations, and the fiber (grain) direction from the principal directions. The orthotropic nature of the material increases the number of independent electrical properties that can be measured and gives useful opportunities for data combination and consistency checking. In a series of microwave attenuation measurements using wood samples, the standard error for identification of grain angle was 0.9° (total measured range =−90° to +90°, for moisture density 3.7 kg/m3 (total measured range = 40–50 kg/m3), and for dry wood density 16 kg/m3 (total measured range = 325–625 kg/m3).

Frequency dependance of the soil dielectric properties derived from ground penetrating radar signal inversion

Abstract: The accuracy at which the subsurface electromagnetic properties can be identified from full wave inversion of ground penetrating radar (GPR) signals relies on the appropriateness of the model describing their frequency dependence. In this paper, we focus on the characterization of the frequency dependence of the dielectric permittivity and electric conductivity of a sandy soil subject to different water contents from inversion of GPR measurements. Based on previous studies of Lambot et al. the methodology relies on an ultrawide band (UWB) stepped-frequency continuous-wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic (TEM) horn antenna. Forward modeling of the radar signal is based on linear system transfer functions for describing the antenna, and on the exact solution of Maxwellrsquos equations for wave propagation in a horizontally multilayered medium representing the subsurface. Model inversion, formulated by the classical least-squares problem, is carried out iteratively using advanced global optimization techniques. The frequency dependence of the electromagnetic properties of the sandy soil is characterized by performing inversions of the radar signal in different and subsequent limited frequency bands, in which the electromagnetic parameters are assumed to be constant. We observed that over the entire frequency band considered in this study (1–3 GHz), the dielectric permittivity of the sand remains constant with frequency, whatever the water content is. In contrast, the electric conductivity increases significantly from 1GHz to 3 GHz, and this effect increases with water content. The frequency dependence of the electric conductivity may be adequately described using a simple linear relationship. This approach is advantageous since it limits the number of parameters to be optimized in the inverse modeling procedure.

Near-surface Interface Detection for Coal Mining Applications using Bispectral Features and GPR

Abstract: The use of ground penetrating radar (GPR) for detecting the presence of near-surface interfaces is a scenario of special interest to the underground coal mining industry. The problem is difficult to solve in practice because the radar echo from the near-surface interface is often dominated by unwanted components such as antenna crosstalk and ringing, ground-bounce effects, clutter, and severe attenuation. These nuisance components are also highly sensitive to subtle variations in ground conditions, rendering the application of standard signal pre-processing techniques such as background subtraction largely ineffective in the unsupervised case. As a solution to this detection problem, we develop a novel pattern recognition-based algorithm which utilizes a neural network to classify features derived from the bispectrum of 1D early time radar data. The binary classifier is used to decide between two key cases, namely whether an interface is within, for example, 5 cm of the surface or not. This go/no-go detection capability is highly valuable for underground coal mining operations, such as longwall mining, where the need to leave a remnant coal section is essential for geological stability. The classifier was trained and tested using real GPR data with ground truth measurements. The real data was acquired from a testbed with coal–clay, coal–shale and shale–clay interfaces, which represents a test mine site. We show that, unlike traditional second order correlation based methods such as matched filtering which can fail even in known conditions, the new method reliably allows the detection of interfaces using GPR to be applied in the near-surface region. In this work, we are not addressing the problem of depth estimation, rather confining ourselves to detecting an interface within a particular depth range.

Random and Non-Random Uncertainties in Precision GPR Measurements: Identifying and Compensating for Instrument Drift

Abstract: The precision of ground penetrating radar (GPR) data is of increasing interest, as the technology finds ever increasing applications to near surface geophysical studies. Our group has undertaken a series of studies to identify the precision and accuracy with which GPR traveltimes, velocities and interval properties can be estimated under controlled conditions, recently reporting that random errors in two-way traveltime and velocity are on the order of ±0.7 ns and ±0.001 m/ns, respectively, at the 95% confidence level. The high degree of precision in that dataset makes it possible to observe non-random patterns in the independent estimates of the same parameter, suggesting possible systematic biases in the data. One source of systematic error that workers often encounter is the effect of back-scattering from above ground features. Another that we documented in our previous study is that the material properties of the subsurface may change from one survey to the next. A third possibility is non-random instrumentation error, one source of which, and the focus of the present report, is a variation in the time base. We address ways to identify its presence, to assess its influence on estimating GPR parameters, and how and when to compensate for its effects. Using a static transmitter (Tx) and receiver (Rx) offset to record a series of GPR traces over a nominal period of one hour, we find that time base drift can be significant for up to 25 min after turning on the instrument. Fortunately, the type of drift that we have found to be most apparent—time zero drift—can be readily identified and compensated for, if one employs the air phase at a number of Tx–Rx offsets. An invariant condition in a GPR survey is that the true velocity of the direct air phase should be the velocity of an electromagnetic pulse in free space. Thus, upon carefully determining the observed velocity of the air phase, and using the ratio between it and the true value of 0.29979 m/ns, one can correct the entire time base of a radargram. Guidelines for when to apply the air phase compensation technique are based on whether the observed velocity of the air phase falls outside the limits of an acceptable precision under the most ideal circumstances. We illustrate the compensation procedure using two CMPs collected on different days at the same survey position. Analyzing the uncompensated data unsurprisingly yields two different subsurface velocities, however the depth estimates of the same subsurface reflector differ by 0.5 m, which for this site is physically unlikely. The difference between the observed and the true air phase velocities for both data sets exceeded the minimum expected error according to our guidelines, thus after applying our time base correction, the difference between the depth estimates improves to 0.05~m.

Propagation of Radar Pulses from a Horizontal Dipole in Variable Dielectric Ground: A Numerical Approach

Abstract: We numerically investigate the propagation of radar type short pulses from a horizontal dipole in the presence of some simple models of inhomogeneous ground with a flat surface. We use 3-dimensional (3-D) pseudospectral time domain (PSTD) and 2-D finite difference time domain forward modeling to determine the range and azimuth dependence of electric field components and to simulate events in a moveout profile. The models are: (i) a uniform half space with either high or low conductivity; (ii) a vertical dielectric wedge; (iii) a surface thin layer with a monocline wedge overlaid on the dielectric half space. Our homogeneous results agree with the analytical solutions, and more clearly show the significant vertical electric field component, which occurs for all models. The incorporation of an anomalous dielectric quadrant does not affect the air wave and only complicates ground wave propagation near the boundary. Modeling of a monocline dielectric wedge shows predictable subsurface reflections and refractions, some of which are highly dispersive events, depending on the direction of propagation. We present two field examples that appear to demonstrate some of our findings. We conclude that air waves make suitable references for moveout profiles regardless of dielectric complications, and that our results provide some insight into the interpretation of unique events seen in moveout profiles.

Estimation of Temporal Changes of Volumetric Soil Water Content from Ground-Penetrating Radar Reflections

Abstract: Time series of ground-penetrating rader traces are used to monitor temporal changes of soil water content down to about 1.5 m. The results are compared with high-resolution time series of time-domain reflectometry measurements at a few locations in the soil profile. We find a reasonable agreement between the two approaches and discuss the differences in terms of their respective limitations.

Non Destructive Geophysical Monitoring of Water Content and Fluid Conductivity Anomalies in the Near Surface at the Border of an Agricultural

Abstract: We use in this paper advanced geophysical techniques for the characterization and monitoring of subsurface properties such as porosity, water content and electrical conductivity of water. Ground Penetrating Radar (GPR) and electrical conductivity measurements were recorded monthly during one year at the border of a corn field. Velocity analyses of multioffset GPR data were conducted to determine total porosity and to monitor vertical transport of water from the soil surface to the water table. The use of novel and original techniques for GPR processing (GPR velocity estimation by the Common Reflection Surface (CRS) method, kriging applied to GPR velocity) improved the estimate and the resolution of GPR velocity maps compared with the classical Normal MoveOut (NMO) and the bi-linear interpolation. Electrical resistivities were used to determine the effective porosity. The combination of GPR and electrical data permitted to estimate the electrical conductivity of water and to highlight high conductivity zones, possibly due to contamination by agricultural fertilizers. Independent determinations (grain size fractions, electrical conductivity, major ion content of water samples and porosity) were obtained, that validate our geophysical investigation. This study demonstrates the efficiency of non destructive geophysical approaches for providing accurate models of water content, porosity and electrical conductivity of water down to a depth of several meters in a poorly conductive soil.

Application of GPR in the study of a modern alluvial megafan: The case of the Taquari river in Pantanal wetland, West-Central Brazil

Abstract: A novel and timely ground penetrating radar (GPR) survey has been carried out in Pantanal from Mato Grosso State, west-central Brazil. Fieldwork was carried out on February/2001 and August/2002 in an attempt to understand avulsion processes that are occurring within Taquari alluvial megafan. The main objectives were to map channel, crevasse and floodplain morphology, as well as active sedimentary bedforms. Many GPR profiles were acquired in the medium and lower Taquari River course. Subaqueous megaripples and exposed sand bars were identified in the medium fan area. Similar features were observed in the lower fan channels, where there were many crevasses in the marginal levees. During the flooding seasons, the flow splays out on to the floodplain, where new distributary channels are being formed. GPR data show that the lower fan, Taquari channel is topographically higher than the adjacent floodplain, thus favoring avulsion as a natural process of river course shifting. GPR data obtained during the wet and dry seasons, together with sedimentology information have been very important in characterizing the fluvial dynamics, and avulsion phenomena.

Joint Interpretation of Geophysical Data for Archaeology: A Case Study

Abstract: Interpret-joints within geophysical data recorded in a complex area where ruins do not outcrop and only earthenware remains within the surficial layer are present. The study area, located in central Italy, consists of Roman, medieval and modern ruins that are included in reworked sediments. The geology is formed by inhomogeneous alluvial sediments (sand and gravel) several meters thick with diamagnetic character. To reduce the ambiguity in the subsurface reconstruction, a joint interpretation of georadar, magnetic and electrical tomography data was performed. The georadar was chosen to reconstruct detailed subsurface features, the electrical tomography to distinguish resistive bodies (stones, voids, etc.) from conductive (cavities filled by clay) and, because of the diamagnetic character of in situ sediments, the magnetic method was chosen to detect the earthenware ruins. The geophysical data were controlled by excavation, which detected silos of 1 m in diameter and a concrete layer at a few centimeters from the topographic surface. Time slices in the georadar data allowed us to detect the silos and to define the lateral edge of the concrete layer. Silos were also indirectly detected by the magnetic data because of the earthenware present in the filling sediments. Electrical tomography detected the concrete layer and an ancient anthropogenic surface of few centimeter depths. The study demonstrates that, because the geophysical methods are based on different physical characteristics, they can have different resolution and therefore detect different bodies.

Structural Damage Detection Using an Embedded ETDR Distributed Strain Sensor

Abstract: Feasible application of the Electrical Time Domain Reflectometry (ETDR) distributed strain sensing technique for structural damage detection was investigated in this study. A small-scale concrete beam specimen with an embedded ETDR distributed strain sensor was tested in three-point bending. The resulting ETDR waveforms of the sensor were studied in conjunction with the load–displacement curve of the concrete beam and the video images of the specimen recorded during the test. It was shown that the embedded ETDR sensor was capable of detecting weak points in the structure resulted from structural defects. The embedded sensor also had the capability to detect a load-induced damage at its early stage when no surface crack line was visible. On the occurrence of an apparent crack damage that passed through the sensing line, the embedded sensor could locate its position with a precision within 0.25′′. Moreover, the ETDR distributed sensor had the capability to detect multiple cracks simultaneously along a single sensing line.

Statistical Algorithms for Designing Geophysical Surveys to Detect UXO Target Areas

Abstract: The U.S. Department of Defense is in the process of assessing and remediating closed, transferred, and transferring military training ranges across the United States. Many of these sites have areas that are known to contain unexploded ordnance (UXO). Other sites or portions of sites are not expected to contain UXO, but some verification of this expectation using geophysical surveys is needed. Many sites are so large that it is often impractical and/or cost prohibitive to perform surveys over 100% of the site. In such cases, it is particularly important to be explicit about the performance required of the surveys. This article presents the statistical algorithms developed to support the design of geophysical surveys along transects (swaths) to find target areas (TAs) of anomalous geophysical readings that may indicate the presence of UXO. The algorithms described here determine (1) the spacing between transects that should be used for the surveys to achieve a specified probability of traversing the TA, (2) the probability of both traversing and detecting a TA of anomalous geophysical readings when the spatial density of anomalies within the TA is either uniform (unchanging over space) or has a bivariate normal distribution, and (3) the probability that a TA exists when it was not found by surveying along transects. These algorithms have been implemented in the Visual Sample Plan (VSP) software to develop cost-effective transect survey designs that meet performance objectives.

Time-Frequency Signatures of Penetrable or Impenetrable Targets Buried in Lossy Half-Spaces

Abstract: The capability of ultra-wideband (UWB) radar or sonar systems for extracting signature information useful for target-recognition purposes is well known The signal processing techniques to be used in the microwave or ultrasonic cases are quite similar The frequency content of the interrogating signals is usually designed to match the size and type of target and environment present In the radar case, the complex permittivity of a soil can vary substantially with its moisture content Both moisture content and target-depth will alter the returned signature We investigate here the backscattered echoes of impenetrable or penetrable (ie, dielectric, in this case) bodies of revolution (BOR) of the same size and shape, as they are illuminated by an above-ground projector pointing downwards The targets are buried at a few representative depths, in soils of various moisture contents The echoes are simulated by the Method-of-Moments (MoM) method, and then used to determine the target signatures in the time, the frequency, and most importantly, in the joint time-frequency domains These signal processing methods also apply for the case of ultrasonic signals In the time-frequency domain the signatures are generated here by a pseudo-Wigner distribution (PWD) Time-frequency distributions can be used for actual target-classification purposes using measured data

Improvement of Electromagnetic Pulse Radiation Efficiency

Abstract: Practical aspects of effective generation of short electromagnetic pulses (EMP) and improvement of radiation efficiency are discussed in this paper. Descriptions of the EMP radiation based on the Poynting vector definition and energy transformation are presented below. Antennas classification based on energy behavior in the radiating system is proposed and discussed. Properties of kinetic energy accumulated antennas are analyzed. To generate electromagnetic pulse the kinetic energy accumulated antenna is combined with such opening switch as drift step-recovery diode (DSRD). Operation principles of the DSRD are discussed. Criteria of diodes selection for its application in drift step-recovery (DSR) mode are proposed and described. Methods of provision the DSRD operation condition with a balanced current driver are developed and analyzed. A prototype of antenna–generator that utilizes proposed operation principles was designed, developed and examined. Waveform and power spectrum are presented. Peak power more than 1 kW was achieved on the antenna terminal with 3 ns pulse duration and up to 200 kHz pulse repetition frequency (PRF). The prototype's power consumption didn't exceed 4 W under 100 kHz PRF condition. The proposed approach allows producing of stable nanosecond electromagnetic impulses with hundreds kilowatts peak power and PRF up to hundreds kHz.