scispace - formally typeset

Journal ArticleDOI

A signal processing methodology for assessing the performance of ASTM standard test methods for GPR systems

01 Mar 2017-Signal Processing (Elsevier)-Vol. 132, pp 327-337

TL;DR: This work proposes a GPR signal processing methodology, calibrated and validated on the basis of a consistent amount of data collected by means of laboratory-scale tests, to assess the performance of the above standard test methods for GPR systems.

AbstractGround penetrating radar (GPR) is one of the most promising and effective non-destructive testing techniques (NDTs), particularly for the interpretation of the soil properties. Within the framework of international Agencies dealing with the standardization of NDTs, the American Society for Testing and Materials (ASTM) has published several standard test methods related to GPR, none of which is focused on a detailed analysis of the system performance, particularly in terms of precision and bias of the testing variable under consideration. This work proposes a GPR signal processing methodology, calibrated and validated on the basis of a consistent amount of data collected by means of laboratory-scale tests, to assess the performance of the above standard test methods for GPR systems. The (theoretical) expressions of the bias and variance of the estimation error are here investigated by a reduced Taylor's expansion up to the second order. Therefore, a closed form expression for theoretically tuning the optimal threshold according to a fixed target value of the GPR signal stability is proposed. Finally, the study is extended to GPR systems with different antenna frequencies to analyze the specific relationship between the frequency of investigation, the optimal thresholds, and the signal stability.

Topics: Ground-penetrating radar (51%)

Summary (3 min read)

1. INTRODUCTION

  • Ground penetrating radar (GPR) is an increasingly popular non-destructive testing (NDT) technique that emits a short pulse of electromagnetic energy into the subsurface [1, 2].
  • In Germany, instructions on the use of radar systems for non-destructive testing in civil engineering [10] and for gaining inventory data of road structures [11] are available.
  • Furthermore, a novel pre-processing method for GPR signals, based on the minimum gradient method, is discussed in [28].
  • In line with the above and according to the guidance provided by the mentioned ASTM standards, this paper is (to the best of the authors’ knowledge) the first study that focuses on the ASTM SNR test, thereby aiming at providing a detailed analysis of the bias and variance of the testing variable under consideration (i.e. the SNR).

2. BASIC FRAMEWORK ON GPR PRINCIPLES AND REFERENCE ASTM

  • STANDARDS 2.1 GPR working principles and main applications.
  • The hardware of a GPR system utilized for the measurement of the subsurface conditions usually consists of a transmitter and a receiver antenna, a radar control unit, and suitable data storage and display devices.
  • Measurements can be traditionally performed in two main survey configurations, namely, with ground-coupled or air-coupled antennas, as a function of the main purposes and type of survey.
  • To cite a few, the authors can mention the evaluation of layer thicknesses [44] and subsurface moisture [45, 46], the assessment of damage conditions in hot-mix asphalt (HMA) layers [47], load-bearing layers and subgrade soils [48-51], the inspection of concrete structures [52, 53].

2.2 ASTM standard test methods

  • The ASTM society is an international organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services.
  • For air-launched antennas, four main tests are mentioned and worthy to be verified, namely, i) the signal-to-noise ratio (SNR), ii) the signal stability, iii) the linearity in the time axis and time window accuracy, and iv) the long-term stability test.
  • To the best of the authors’ knowledge, no in-depth study is provided in the literature about the analysis of the compliance with the SNR test of a GPR signal, which conversely appears to be one of the main requirements by the manufacturers.
  • The Standard recommends to perform the SNR test on each of the above 100 waveforms and to take the average signal-to-noise value of the 100 waveforms as reference “signal-to-noise of the system”.

3. SIGNAL PROCESSING METHODOLOGY FOR ASTM STANDARD

  • The SNR test method, as defined by the D 6087 – 08 ASTM Standard [24], is here taken as the reference parameter for assessing the performance of the GPR signals, in terms of bias and variance.
  • Only three papers investigating on the precision of the GPR measurements have been found in the literature.
  • Nevertheless, none of these papers provides any discussion about the precision of the SNR test method, especially in terms of bias.
  • Hence, bias stands for the average difference to be expected between the estimator and the underlying parameter.
  • Afterwards, the authors provide such a discussion in Section 3.2, evaluating the bias and variance of the testing variable under consideration.

3.1 Optimal threshold tuning

  • Nevertheless, no further information is provided on how the threshold has been set, nor to which accuracy (or signal stability) of the GPR equipment this threshold corresponds.
  • The authors provide the signal processing methodology to evaluate the proper threshold for the specific GPR equipment, according to a fixed desired (i.e. target) level of accuracy.
  • First of all, the authors evaluate the SNR of the system as the average SNR over a number of L trials as follows: 𝑆𝑁𝑅 = 1 𝐿 ∙ ∑ 𝑆𝑁𝑅𝑗 𝐿 𝑗=1 , (4) In particular, SNRj is the SNR of the j-th experiment, and defined as the ratio between the mean signal and the noise peaks, respectively.
  • Since all the terms of the sum in (4) represent SNRs of different experiments, they also represent physically independent and hence statistically independent random variables.
  • Then, let us now define with PACC the desired (or target) level of accuracy (in percentage) requested to the GPR system.

3.2 Performance evaluation

  • It is evaluated the performance of the SNR test method, by theoretically assessing the bias and the variance of the estimation error of the testing variable in (4).
  • In the following analysis, the authors first evaluate the bias and the variance of SNR̂𝑗, and then they compute the bias and variance in (11).
  • The algebraic expressions (12) and (13) are trivially derived from a computation of the partial derivatives of the expressions in (4).
  • It has to be noted that both the estimations in (22) and (23) vary with 1/K, meaning that the SNR estimator is consistent (i.e. as the number of considered radar waveforms K becomes larger and larger, the estimate tends to the true value).

4.1 Laboratory set-up

  • Several laboratory-scale tests were performed according to the set-up shown in Fig.
  • This may be mostly due to the deflections induced to the surveying apparatus by the combination of remarkable traffic speeds and damaged conditions of the pavement surface when performing GPR measurements in real roads.
  • Considerations on the signal response in near-field and far-field conditions can also be drawn.
  • The floor under the antennas was covered by a 200 cm × 200 cm copper sheet acting as perfect electric conductor (PEC), and capable to completely reflect the propagation waves and generating a pulse with maximum amplitude.
  • Measurements at each of the aforementioned heights were performed using three air-coupled GPR systems with different central frequencies of 1 and 2 GHz.

4.2 Experimental outcomes

  • The assumptions made in the previous Section have been firstly empirically verified.
  • As in the previous case, the authors can notice a maximum variation of the signal and noise peak within the 6% for the 2 GHZ EU equipment, whereas this value is between the 6-7% in the case of the 2GHz NA system.
  • Variances of the noise peak for 100 consecutive radar traces.
  • Hence, the authors can now evaluate the performance of the SNR test method, by computing the bias and variance of the estimation errors, according to (21) and (22).
  • The threshold value modifies accordingly to h. Figs. 10 and 11 show how the accuracy of the GPR signal varies versus the threshold in the cases of the 2GHZ EU and the 2GHZ NA GPR systems, respectively.

5. CONCLUSION

  • This paper has devised a signal processing methodology for assessing the performance of the international standard test methods released by the American Society for Testing and Materials (ASTM) about the application of GPR techniques.
  • The theoretical expressions for the bias and variance of the estimation error have been evaluated by a reduced Taylor’s expansion up to the second order.
  • Therefore, a closed form expression for theoretically tuning the optimal threshold according to a fixed target value of the GPR signal stability has been proposed.
  • The overall study has been extended to three air-coupled GPR systems with different antennas to analyze the specific relationship between the frequency of investigation, the optimal thresholds, and the signal stability.
  • The results achieved from several trials at the laboratory scale confirm the consistency of such a methodology for assessing the performance of these international standard test methods for GPR systems.

Did you find this useful? Give us your feedback

...read more

Content maybe subject to copyright    Report

1
A SIGNAL PROCESSING METHODOLOGY FOR ASSESSING THE
PERFORMANCE OF ASTM STANDARD TEST METHODS FOR GPR SYSTEMS
Francesco Benedetto
1,*
, Fabio Tosti
2
1
Signal processing for Telecommunications and Economics Lab., Roma Tre University, Via Vito Volterra 62, 00146,
Rome, Italy; e-mail: francesco.benedetto@uniroma3. (*Corresponding author)
2
School of Computing and Engineering, University of West London (UWL), St Mary's Road, Ealing, W5 5RF, London,
United Kingdom; Fabio.Tosti@uwl.ac.uk
ABSTRACT - Ground penetrating radar (GPR) is one of the most promising and effective non-
destructive testing techniques (NDTs), particularly for the interpretation of the soil properties.
Within the framework of international Agencies dealing with the standardization of NDTs, the
American Society for Testing and Materials (ASTM) has published several standard test methods
related to GPR, none of which is focused on a detailed analysis of the system performance,
particularly in terms of precision and bias of the testing variable under consideration. This work
proposes a GPR signal processing methodology, calibrated and validated on the basis of a
consistent amount of data collected by means of laboratory-scale tests, to assess the performance
of the above standard test methods for GPR systems. The (theoretical) expressions of the bias and
variance of the estimation error are here investigated by a reduced Taylor’s expansion up to the
second order. Therefore, a closed form expression for theoretically tuning the optimal threshold
according to a fixed target value of the GPR signal stability is proposed. Finally, the study is
extended to GPR systems with different antenna frequencies to analyze the specific relationship
between the frequency of investigation, the optimal thresholds, and the signal stability.
Keywords - Ground Penetrating Radar, GPR calibration, performance evaluation, civil
engineering application.

2
1. INTRODUCTION
Ground penetrating radar (GPR) is an increasingly popular non-destructive testing (NDT)
technique that emits a short pulse of electromagnetic energy into the subsurface [1, 2]. When such
a pulse strikes an interface between layered materials with different electrical properties, part of
the wave reflects back, and the remaining energy continues to the next interface, thereby
penetrating in depth before being absorbed. GPR is capable to collect the reflections of the
electromagnetic waves at the interface between two different dielectric materials. It is relatively
easy to recognize a GPR signal, since the return signal is shaped very similar to the emitted one.
The depth, shape and electromagnetic properties of the scattering of the reflecting object affect
the time delay, as well as the differences in phase, frequency and amplitude. GPR is a technology
with a wide flexibility of usage. It is mainly application-oriented , with structure and electronics
relatively variable according to the target characteristics, such as type and constituent materials,
the environmental context, and the spatial scale of applications. A variety of areas, e.g., civil and
environmental engineering [3], geology, archaeology, forensic and public safety [4], planetary
sciences [5] are therefore increasingly interested by the application of this tool.
Nevertheless, few recognised international standards exist in the area of GPR, and a certain
amount of inhomogeneous recommendations can be encountered in different countries.
Moreover, the levels of knowledge, awareness and experience on the use of GPR may vary very
considerably across countries. This results into a general need for the GPR users to know the most
appropriate good practices to be followed in terms of GPR measurements and the expected quality
level of the results. A small number of National and International standards includes general
recommendations for performing geophysical surveys of the subsurface. Many of these focus on
civil engineering applications, with the area of transport infrastructures being the most regulated.
Within the European framework, few GPR-related National and International guidelines have
been issued if compared to the usage of this tool. It is worth to mention the Italian contribution
within the field of the underground utility detection [6], along with the guidelines released by IDS

3
(Ingegneria dei Sistemi) enterprise [7] and the ASG (Associazione Società di Geofisica)
geophysical association [8]. These provide useful theoretical and practical insights on GPR, along
with a number of key and application-oriented data processing algorithms. In France, the detection
of buried utilities has been thoroughly tackled [9]. In Germany, instructions on the use of radar
systems for non-destructive testing in civil engineering [10] and for gaining inventory data of
road structures [11] are available. In Scandinavia, recommendations were developed within the
Mara Nord Project on the use of GPR in several applications, such as the measurement of air
voids in asphalt concrete [12], road construction quality controls [13], bridge deck surveys [14],
road rehabilitation projects [15] and in-site investigations [16].
Still at the European level, a number of standards and codes introduced by the European
Telecommunications Standards Institute (ETSI) regulate the use of GPR and its emissions of
electromagnetic radiation. Such documents focus on the common technical requirements [17], the
specific conditions for ground and wall probing radar (WPR) applications [18], the main technical
characteristics and test methods [19], the levels of compliance [20] with the Radio and
Telecommunications Terminal Equipment (RTTE) Directive [21], as well as with one code of
practice in respect of the control, use and application of GPR and WPR systems and equipment
[22]. On the other hand, three main standards released by the American Society for Testing and
Materials (ASTM) guide the use of GPR toward the investigation of the subsurface [23], the
evaluation of asphalt-covered concrete bridge decks [24], and the determination of pavement-
layer thickness [25]. In more details, according to the ASTM classification on the standard
categories, the above documents can be classified into i) Standard Guides (i.e., [23]), namely, an
organized collection of information or series of options that does not recommend a specific course
of action, and ii) Standard Test Methods (i.e., [24] and [25]), wherein specific test procedures for
assessing the stability of the GPR signal are discussed, such as the signal-to-noise-ratio (SNR)
test.

4
Notwithstanding the estimate of the SNR is not new to the GPR community, to the best of our
knowledge there are no papers related to the SNR test as defined by the ASTM standards. Many
works on signal processing procedures for the assessment and improvement of the SNR in GPR
investigations can be found in the literature. In [26], the authors propose to enhance the GPR
signal with the KarhunenLoève transform (KLT), whereas the work in [27] aims at improving
the SNR of a GPR signal by introducing an enhanced-signal-based method, with the noise
variance being estimated by a clustering technique. Furthermore, a novel pre-processing method
for GPR signals, based on the minimum gradient method, is discussed in [28]. Within the most
established signal processing techniques performed in the GPR area we can cite time and
frequency analyses [29], time varying band-pass filtering [30], deconvolution [31], velocity
analysis [32], migration [33] and compressive sensing [34], as well as the attribute analysis and
classification [35]. The main purpose of all such techniques is to enhance the SNR of the GPR
signal. They commonly focus on the SNR of the received GPR signal, wherein the noise is
assumed as the back-scattered noise from the subsurface after carrying out a GPR survey.
Conversely, in this paper we are assuming the noise as the amount of clutter that is in the GPR
equipment, also known as systematic error. Thereby, we are focusing on the signal stability of a
GPR system, during the calibration phase and before an investigation is carried out. The
evaluation of this type of internal noise is extremely important to perform automated signal
processing by GPR, thereby ensuring that the quality of the GPR waveforms is suited for
purposes. According to this, the ASTM standards [23-25] define some tests to verify the stability
of the GPR signal, such as the SNR test, and the (short- and long-term) signal stability tests.
Notwithstanding their higher scientific level with respect to similar National and International
standards, three main failings in the ASTM standards can be singled out, namely, i) how to select
the optimal threshold and to which level of signal accuracy (or stability) this threshold
corresponds; ii) the lacking of a detailed analysis of the system performance, particularly in terms
of precision and bias of the testing variable under consideration; and iii) the use of a few central
frequencies of investigation, which may not allow to supply a comprehensive overview of the

5
results in line with the wider range of central frequencies used in GPR applications. To the best
of our knowledge, only one paper [36] can be found in the literature wherein the issues in the
ASTM standards are tackled by checking the GPR signal stability versus the systematic error of
the GPR system. In particular, the paper by Rial et al. [36] represents an effort to set-up a strategy
for verifying the stability of performances in GPR systems in terms of the electromagnetic
radiated fields. In addition, the paper in [36] focuses on the (short- and long-term) signal stability
tests, whereas no discussion has been included about the SNR test. Nevertheless, this activity is
dramatically relevant as the starting point to develop a methodology for calibrating GPR devices
and verifying proper operation.
In line with the above and according to the guidance provided by the mentioned ASTM
standards, this paper is (to the best of the authors knowledge) the first study that focuses on the
ASTM SNR test, thereby aiming at providing a detailed analysis of the bias and variance of the
testing variable under consideration (i.e. the SNR). In particular, this work proposes a simple GPR
signal processing procedure (calibrated and validated on the basis of a consistent amount of data
collected from laboratory-scale tests), to evaluate the precision and bias of the GPR signal under
investigation, by a reduced Taylor’s expansion up to the second order. Therefore, we propose a
closed form expression for theoretically tuning the optimal threshold, according to a fixed target
value of the GPR signal stability. Finally, the study is performed with several GPR systems (i.e.
exploiting antennas tuned to frequencies different from 1 GHz), analyzing the specific
relationship between the frequency of investigation and the optimal thresholds.
The remainder of this paper is organized as follows. In the first half of Section 2, the GPR
working principles as well as the main applications of this NDT are discussed. The second half
of Section 2 illustrates the conventional ASTM standard test methods, highlighting the weakness
points of such methodologies. In Section 3, a signal processing procedure for threshold tuning is
provided, as well as a closed form expression for theoretically evaluating the optimal threshold
according to a fixed target level of GPR signal accuracy (or stability). Then, a theoretical analysis

Citations
More filters

Journal ArticleDOI
R.M. Gray1
01 Dec 1976

348 citations


01 Feb 2015

191 citations


Journal ArticleDOI
19 Feb 2019
TL;DR: Results have proven the viability of the proposed signal processing method for data acquired on flexible pavements using GPR, provided recommendations on use of specific processing stages depending on survey requirements and quality of the raw dataset.
Abstract: Effective quality assurance and quality control inspections of new roads as well as assessment of remaining service-life of existing assets is taking priority nowadays. Within this context, use of ground penetrating radar (GPR) is well-established in the field, although standards for a correct management of datasets collected on roads are still missing. This paper reports a signal processing method for data acquired on flexible pavements using GPR. To demonstrate the viability of the method, a dataset collected on a real-life flexible pavement was used for processing purposes. An overview of the use of non-destructive testing (NDT) methods in the field, including GPR, is first given. A multi-stage method is then presented including: (i) raw signal correction; (ii) removal of lower frequency harmonics; (iii) removal of antenna ringing; (iv) signal gain; and (v) band-pass filtering. Use of special processing steps such as vertical resolution enhancement, migration and time-to-depth conversion are finally discussed. Key considerations about the effects of each step are given by way of comparison between processed and unprocessed radargrams. Results have proven the viability of the proposed method and provided recommendations on use of specific processing stages depending on survey requirements and quality of the raw dataset.

36 citations


Journal ArticleDOI
Abstract: This paper reports on the ground-penetrating radar (GPR)-based assessment of railway ballast which was progressively “polluted” with a fine-grained silty soil material. It is known how the proper operation of a ballast track bed may be undermined by the presence of fine-grained material which can fill progressively the voids between the ballast aggregates and affect the original strength mechanisms. This occurrence is typically defined as “fouling”. To this effect, a square-based methacrylate tank was filled with ballast aggregates in the laboratory environment and then silty soil (pollutant) was added in different quantities. In order to simulate a real-life scenario within the context of railway structures, a total of four different ballast/pollutant mixes were introduced from 100% ballast (clean) to highly-fouled (24%). GPR systems equipped with different air-coupled antennas and central frequencies of 1000 MHz and 2000 MHz were used for testing purposes. Several processing methods were applied in order to obtain the dielectric permittivity of the ballast system under investigation. The results were validated using the “volumetric mixing approach” (available within the literature) as well as by performing a numerical simulation on the physical models used in the laboratory. It is important to emphasize the significance of the random-sequential absorption (RSA) paradigm coupled with the finite-difference time-domain (FDTD) technique used during the data processing. This was proved to be crucial and effective for the simulation of the GPR signal as well as in generating synthetic GPR responses close to the experimental data.

31 citations


Cites background from "A signal processing methodology for..."

  • ...The tank was laid above a 2m × 2m copper-made perfect electric conductor (PEC) sheet, which allowed to reflect completely the waves propagating through the investigated material [36]....

    [...]


Journal ArticleDOI
Abstract: This paper presents an investigation into the relative dielectric permittivity of railway ballast using ground-penetrating radar (GPR). To this effect, the experimental tests are carried out using a container (methacrylate material) of dimensions 1.5 × 1.5 × 0.5 m. GPR systems equipped with different ground-coupled and air-coupled antennas and central frequencies of 600 MH, 1000 MHz, 1600 MHz and 2000 MHz (standard and low-powered antenna systems) are used for testing purposes. Several processing methods are applied to assess and compare the dielectric permittivity of the ballast system under investigation. A comparison of the results identifies critical factors as well as antennas and central frequencies most suitable for the purpose.

23 citations


References
More filters

Book ChapterDOI
15 Apr 2005
Abstract: Ground penetrating radar (GPR) is a nondestructive measurement technique, which uses electromagnetic waves to locate targets or interfaces buried within a visually opaque substance or Earth material. GPR is also termed ground probing, surface penetrating (SPR), or subsurface radar. A GPR transmits a regular sequence of low-power packets of electromagnetic energy into the material or ground, and receives and detects the weak reflected signal from the buried target. The buried target can be a conductor, a dielectric, or combinations of both. There are now a number of commercially available equipments, and the technique is gradually developing in scope and capability. GPR has also been used successfully to provide forensic information in the course of criminal investigations, detect buried mines, survey roads, detect utilities, measure geophysical strata, and in other applications. Keywords: ground penetrating radar; ground probing radar; surface penetrating radar; subsurface radar; electromagnetic waves

1,078 citations


"A signal processing methodology for..." refers background in this paper

  • ...Ground penetrating radar (GPR) is an increasingly popular nondestructive testing (NDT) technique that emits a short pulse of electromagnetic energy into the subsurface [1,2]....

    [...]


DatasetDOI
Abstract: A nondestructive technique using electromagnetic waves to locate objects or interfaces buried benea…

624 citations


Journal ArticleDOI
Abstract: This paper provides a status report of the Ground Penetrating Radar (GPR) highway applications based on studies conducted in both Scandinavia and the USA. After several years of research local transportation agencies are now beginning to implement GPR technology for both network and project level surveys. This paper summarizes the principles of operation of both ground-coupled and air-launched GPR systems together with a discussion of both signal processing and data interpretation techniques. In the area of subgrade soil evaluation GPR techniques have been used to nondestructively identify soil type, to estimate the thickness of overburden and to evaluate the compressibility and frost susceptibility of subgrade soil. In road structure surveys, GPR has been used to measure layer thickness, to detect subsurface defects and to evaluate base course quality. In quality control surveys, GPR techniques have been used for thickness measurements, to estimate air void content of asphalt surfaces and to detect mix segregation. Future developments are described where the technique has great potential in assisting pavement engineers with their new pavement designs and in determining the optimal repair strategies for deteriorated roadways.

468 citations


Book
01 Jan 1975
TL;DR: Information and Detection Theory Appendix: Circuit and System Noise.
Abstract: 1 Introduction 2 Signals and Spectra 3 Signal Transmission and Filtering 4 Linear CW Modulation 5 Exponential CW Modulation 6 Sampling and Pulse Modulation 7 Analog Communication Systems 8 Probability and Random Variables 9 Random Signals and Noise 10 Noise in Analog Modulation Systems 11 Baseband Digital Transmission 12 Digitization Techniques for Analog Messages and Computer Networks 13 Channel Coding and Encryption 14 Bandpass Digital Transmission 15 Spread Spectrum Systems 16 Information and Detection Theory Appendix: Circuit and System Noise

451 citations


Journal ArticleDOI
R.M. Gray1
01 Dec 1976

348 citations


"A signal processing methodology for..." refers background in this paper

  • ...Accordingly, the testing variable in (4) is asymptotically (L - 1) Gaussian as a direct consequence of the central limit theorem [60]....

    [...]


Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "A signal processing methodology for assessing the performance of astm standard test methods for gpr systems" ?

This work proposes a GPR signal processing methodology, calibrated and validated on the basis of a consistent amount of data collected by means of laboratory-scale tests, to assess the performance of the above standard test methods for GPR systems. Finally, the study is extended to GPR systems with different antenna frequencies to analyze the specific relationship between the frequency of investigation, the optimal thresholds, and the signal stability.