Air-coupled Ground Penetrating Radar Research Articles

Comparative Study of Ground Penetrating Radar Systems with Time Delay Methods for Accurate Pavement Thickness and Dielectric Constant Estimation

The applicability of using ground penetrating radar (GPR) as a non-destructive technology for estimating the thickness and dielectric constant of pavement layers has been extensively documented in the literature. Time delay methods (TDMs) have been the most commonly used by researchers. However, various studies have reported different results with respect to the suitability of different GPR systems and TDMs for dielectric constant and thickness estimation. Thus, this study aims to compare the use of ground-coupled GPR and air-coupled GPR (AC-GPR) for estimating the dielectric constant of pavement materials. In the context of TDMs, the study examines three methodologies for assessing the traveled distance in the investigated layer. The study involves laboratory experiments using asphalt and dry sand. Also, it comprises a sensitivity analysis to examine the effect of different experimentation setups. The study employed two GSSI antennas: 51600S (GC-51600S) and GSSI 42000S (AC-42000S). Using the manufacturer’s recommended operation setups, the results generally indicated the possibility of simplifying the TDM calculation by considering the 42000S and 51600S antennas as monostatic for pavement materials with different dielectric constants (≥3) with an error rate within 5%. Moreover, the study indicates that using AC-GPR may be inappropriate for common midpoint analysis using the default mounting setups. These findings provide valuable insights into the suitability and limitations of different GPR systems and calculation methods for accurately estimating pavement’s dielectric constant and thickness.

Development and application of air-coupled GPR antenna for coal-rock interface detection in mines

Rapid and high precision detection of coal-rock interface is one of the key problems in intelligent coal mining. In this paper, an air-coupled ground penetrating radar (GPR) antenna is developed for coal-rock interface detection in fully mechanized mining face, which can realize non-contacting, high precision, high-resolution detection during the mining process. The main features of the air-coupled GPR antenna are ultra-wideband, narrow beam, and high gain. For the vertical polarization pattern at 1200 MHz, the main lobe amplitude is 6.29dBi and the main lobe width (3 dB) is 108.3°, as well as the relative bandwidth of the antenna can reach 51%. For design validations, the novel air-coupled GPR antenna developed in this paper is tested in mine, the detection error and the average error percentage are ±1.2 cm and 8.6%, respectively, which shows the superior detection ability of the novel air-coupled GPR antenna and the applicability in complex geological environment, providing technical support for the identification of coal-rock interface in intelligent coal mining.

Backcalculation of Thickness and Dielectric Properties of Asphalt Concrete Using Numerical Simulation of Ground Penetrating Radar

Ground penetrating radar (GPR) is an essential tool for the non-destructive evaluation of pavement conditions. The accuracy of GPR analysis depends on the correct prediction of the layer dielectric property. In this study, a finite-difference time domain simulation-based two-step backcalculation algorithm was developed to determine the layer thickness and dielectric constant from air-coupled GPR data. The two-step backcalculation algorithm was developed by combining a surrogate-based global optimizer and artificial neural network for efficiency and accuracy. The GPR backcalculated results were compared with the field conditions of three pavement sections. The results show that the backcalculation algorithm could predict the layer thicknesses with almost 95% to 99.7% accuracy, whereas the prediction accuracy of the dielectric constants was between 90% and 98.5%. Furthermore, it was observed that the variations of dielectric constants in each lift of the asphalt concrete layer could be used to predict the presence of probable distress in the field.

Detectability of concealed cracks in the asphalt pavement layer using air-coupled ground-penetrating radar

The air-coupled ground-penetrating radar (GPR) could be used to detect the concealed cracks in the asphalt pavement layer at traffic-speed. However, the wide electromagnetic footprint of air-coupled antenna, strong pavement layer reflections, and external noise may cause detection inaccuracies. This study investigates the detectability of concealed cracks in asphalt pavement using the air-coupled GPR. An edge detection-based method is proposed to enhance the reflections from the concealed cracks. A simulation model was built to investigate the antenna footprint and reflection features of concealed cracks in the asphalt pavement layer. Laboratory tests were performed to validate the proposed enhancement method. The minimum detectable crack sizes are 10 mm and 15 mm using the 2 GHz antenna in the simulation and laboratory tests, respectively. The proposed method is automatic and robust at low spatial sampling rate, which may allow traffic-speed survey. It increases the signal-to-noise ratio from 8.38 to 13.62.

Research and Application of Compact Air-Coupled Ground Penetrating Radar in Thin-Layer Asphalt Thickness Testing and Vibration Resistant Calibration Method

For high-precision continuous thickness measurement of thin-layer asphalt loose paving, the paper proposes a compact ground penetrating radar (GPR) solution with smaller volume than the conventional air-coupled antenna. It takes into account the minimum identifiable thickness of the GPR tailing characteristics of radar signals, the common thickness range of thin-layer asphalt, and the vibration of construction machinery. In general, the overlap between bottom reflection and surface reflection of thin-layer asphalt can be solved by subtracting the reflection of the asphalt surface from the calibration waveform file of a metal plate. However, because the operating environment of asphalt paving is not suitable to realize the frequent static calibration of metal plates, the paper designs a bow-tie antenna loaded with high loss absorbent material substrate, which significantly improves the signal tailing phenomenon and greatly reduces the size of the air-coupled antenna. At the same time, in view of dynamic change of paver vibration and the requirements of the construction site, less calibration is more advantageous. Based on the full-amplitude reflection method, a fitting formula for the dielectric constant of asphalt suitable for mechanical vibration is proposed in this paper, to further improve the engineering applicability of the detection system. Finally, static calibration asphalt thickness, dynamic measured dielectric constant asphalt thickness, and manual sampling thickness under three construction conditions are compared, and the capability of the compact air-coupled GPR detection system proposed in this paper is comprehensively verified.

Ground surface detection method using ground penetrating radar signal for sugarcane harvester base-cutter control

Sugarcane harvesters, which can automatically adjust the base-cutter height according to the topography, have attracted increasing interest. Existing ground probing approaches mainly rely on contact machineries, which exhibit slow responses and damage sugarcane plants. This paper presents a rapid and non-contact terrain sensing method using ground penetrating radar (GPR) to realise the automatic height control of the base-cutter. In this study, a 1.6-GHz air-coupled GPR antenna was first mounted on a knockdown roller to measure the vertical distance between the ground and the antenna. The geographic relationship between the detection height and cutter cylinder length was established, based on which the base-cutter height was adjusted by changing the cylinder length. The obtained GPR signal was analysed to locate the ground surface. Post-process techniques were used to enhance the GPR signal contrast between the air and ground layer. To evaluate the penetrating ability and accuracy of the GPR method, sugarcane stalks were densely placed on the soil surface to mimic the extreme conditions of sugarcane fields. The influences of the height/depth of respective highlands/potholes, slope of the ground, and the moisture content of the soil on the GPR imaging were systematically investigated. The numerical and laboratory results confirmed the high accuracy of the proposed GPR method. Hence, a field experiment was conducted, and the soil surface was clearly visualised by GPR imaging. This study demonstrates the significant potential of the GPR-based method for the detection of a sugarcane field surface, which verifies its feasibility for use in manufactured automatic sugarcane harvesters. • A ground penetrating radar (GPR) was proposed to sense sugarcane terrain. • Simulation and lab experiments were used to investigate the GPR penetration ability. • The GPR can clearly visualise the ground layer, even when covered with stalks. • A 1.6-GHz GPR antenna achieved high ground detection accuracy in a field survey.

Material Permittivity Estimation Using Analytic Peak Ratio of Air-Coupled GPR Signatures

Ground penetrating radar (GPR) is widely applied for civil engineering, such as for building health assessments or pavement inspections. In these applications, information on the material permittivity is important for internal structure inspection and characterization. However, in the case of a large building area, a lightweight, fast and effective permittivity measurement technique is required. In addition, the estimation method for different GPR types and range configurations must be generalized for adaptability and wide applicability. This paper proposes a material permittivity estimation technique for air-coupled GPR that effectively works for mobile and large observation missions. The method employs the peak ratio of analytic representation signals between the antenna direct coupling and the material surface reflection. An interpolation algorithm is applied to the reference characteristic curves generated by the finite-difference time-domain technique to adapt the change in radar range configuration. The proposed method has been tested to estimate the permittivity in three different construction materials and measure two metal rebar depths inside reinforced concrete. The experimental results show that the method works with acceptable accuracy. Therefore, the method is sufficiently promising to be utilized in real applications, especially those that require adaptability and mobility, such as ground and aerial vehicle-based radar systems.

SFCW Radar with an Integrated Static Target Echo Cancellation System.

Continuous Wave (CW) radars systems, especially air-coupled Ground-Penetrating Radar (GPR) or Through-Wall Imaging Radar (TWIR) systems, echo signals reflected from a stationary target with high energy, which may cause receiver saturation. Another effect caused by reflection of stationary targets is noticeable as background within a radargram. Nowadays, radar systems use automatic gain control to prevent receiver saturation. This paper proposes a method to remove stationary targets automatically from the received signal. The method was designed for a radar system with a moving platform, with an assumption that the distance between the surface and target is constant. The design is proposed of an SFCW radar with an integrated system for real-time multiple static target Echo Cancellation (EC). The proposed EC system removes the static target using active Integrated Circuit (IC) components, which generate the corresponding EC signal for each frequency step of the SFCW radar and sum it with the received echo signal. This has the main advantage of removing even multiple echoes at any distance, and excludes the need for a high-dynamic-range receiver. Additionally, the proposed system has minimal impact on the radar size and power consumption. Besides static target removal, the antenna coupling can be removed if the signal appears to be constant. The operating frequency was selected between 500 MHz and 2.5 GHz, due to the limitation of the used electronic components. The experimental results show that the simulated target’s echo using a cable with a known length could be suppressed to up to 38 dB. Experimental results using a moving radar platform and the real environment scenario with static and dynamic targets, show that the proposed EC system could achieve up to 20 dB attenuation of the static target. The system does not affect any other target of interest, which can even move at any distance during the measurement. Therefore, this could be a promising method for further compact implementation into SFCW radars, or any other radar type that generates CW single frequencies.

GPR Monitoring of Artificial Debonded Pavement Structures throughout Its Life Cycle during Accelerated Pavement Testing

The paper gives an overview of a ground penetrating radar (GPR) experiment to survey debonding areas within pavement structure during accelerated pavement tests (APT) conducted on the university Gustave Eiffel’s fatigue carrousel. Thirteen artificial defect sections composed of three types of defects (Tack-free, Geotextile, and Sand-based) were embedded during the construction phase between the top and the base layers. The data were collected in two stages covering the entire life cycle of the pavement structure using four GPR systems: An air-coupled ultra-wideband GPR (SF-GPR), two wideband 2D ground coupled GPRs (a SIR-4000 with a 1.5 GHz antenna and a 2.6 GHz-StructureScan from GSSI manufacturer), and a wideband 3D GPR (from 3D-radar manufacturer). The first stage of the experiments took place in 2012–2013 and lasted up to 300 K loadings. During this stage, the pavement structure presented no clear degradation. The second stage of experiments was conducted in 2019 and continued until the pavement surface demonstrated a strong degradation, which was observed at 800 K loadings. At the end of the GPR experiments, several trenches were cut at various sections to get the ground truth of the pavement structure. Finally, the GPR data are processed using the conventional amplitude ratio test to study the evolution of the echoes coming from the debonded areas.

A Practical Approach to Condition Assessment of Asphalt-Covered Concrete Bridge Decks on Korean Expressways by Dielectric Constant Measurements Using Air-Coupled GPR.

The main objectives of this study are to investigate the variations of the dielectric constant of concrete on Korean expressways by using a 1 GHz air-coupled Ground Penetrating Radar (GPR) system and to develop a practical approach to the condition assessment of concrete bridge decks with asphalt overlay on Korean expressways by dielectric constant measurements. A total of 684 GPR investigations of 601 actual concrete bridge decks, which are in service between 2 and 43 years, were carried out during the period between 1999 and 2013. Statistical analysis revealed that the dielectric constant of asphalt-covered concrete bridge decks reduced with service age and this trend continued until service age of over 40 years. As a result, this study provides a practical dielectric constant curve that could be used for condition evaluation of top concrete in asphalt-covered bridge decks with consideration of concrete age. Based on regression analyses of the GPR field survey data and experiences through the field survey, a double cut-off dielectric constant criterion was proposed for condition assessment of asphalt-covered concrete bridge decks on Korean expressways. In addition, a GPR field survey was performed at an actual bridge on the Yeongdong expressway in Korea to test the proposed GPR signal interpretation method. The field survey results provide fundamental data to better understand the variation of the dielectric constant of concrete in actual bridges with asphalt overlay and to develop a practical approach to condition assessment of asphalt-covered concrete bridge decks on Korean expressways by dielectric constant measurements using air-coupled GPR.

Evaluation of the Depth of Deteriorations in Concrete Bridge Decks with Asphalt Overlays Using Air-Coupled GPR: A Case Study from a Pilot Bridge on Korean Expressway

In this recent work, a ground penetrating radar (GPR) technique was proposed to evaluate the deteriorated depth of concrete bridge decks with asphalt overlays in the Korea expressway network. Air-coupled GPR was utilized in the measurement of the relative permittivity of concrete on bridge decks with asphalt overlays and the electromagnetic (EM) wave attenuation of the concrete cover of top reinforcing bars (rebars) in the pilot bridge in public service. In addition, 13 core samples were obtained from the bridge deck to carry out a detailed survey that includes visual inspection of the deterioration and measurement of chloride content with depth. The validity of the GPR technique was examined by comparing it with the results of the field investigation. Moreover, the correlation of the deteriorated depth with either the relative permittivity or EM wave attenuation was established. Results show that a GPR signal analysis method based on a dual-criteria (relative permittivity and EM wave attenuation) is more effective in analyzing the deterioration characteristics and evaluating the deteriorated depth of concrete bridge decks with asphalt overlay compared to the analysis method based on one of the two GPR properties. Results of the field test are considered to be significant wherein it establishes a relationship between the GPR property and deterioration characteristics of the bridge decks. Moreover, results show the practical applicability of the GPR technique in evaluating the deteriorated depth of the bridge decks with asphalt overlay.

Evaluation of the feasibility of Common Mid-Point approach for air-coupled GPR applied to road pavement assessment

Ground Penetrating Radar (GPR) is commonly used in pavement assessment, manly for measuring pavement layer thickness. Asphalt layer thickness is the most relevant input for flexible pavements assessment. The present study evaluates the feasibility of the Common Mid-Point (CMP) method, for a coreless GPR approach, using air-coupled antennas. Three test sections, with different asphalt layer thicknesses, were evaluated using a pair of 1.8 GHz air-coupled antennas. Results highlight the satisfactory performance of the CMP method with air-coupled antennas. The estimation of the asphalt layer thickness leads to a linear coefficient of determination greater than 0.95 when compared with the measured thickness from cores. The CMP method presented lower average absolute errors for the thinner asphalt layers (about 6 cm) in comparison with the SRM method (6% vs 18%). For thicker layers (from 11 to 13 cm), the average absolute errors for both methods were similar (approximately 7%).

Seasonal Variation and Age-related Changes in the Relative Permittivity of Concrete Bridge Decks on Korea Expressways

The purposes of this study were to investigate the seasonal variations in the relative permittivity of actual concrete bridge decks of various ages on Korea expressways, and to propose a practical way to calibrate two critical effects, to assess the condition of the concrete bridge decks with better accuracy. For these purposes, fifty-three actual concrete bridge decks on Korea expressways were scanned using an air-coupled ground penetrating radar (GPR) system, and the relative permittivity of the concrete was evaluated from the GPR data in the field. A statistical analysis showed that the relative permittivity of the concrete bridge decks in Korea are closely correlated with seasonal changes and concrete ages. For concrete bridge decks aged 10 years or older, the effect of concrete age was not as significant as that of relative humidity of the air on relative permittivity of concrete. Based on the results, a simple calibration process was proposed to suppress the effects of seasonal variation and age of concrete on the measured relative permittivity of the concrete decks on actual bridges. As will be discussed, the resulting relative permittivity method produced better agreement with the current condition of the concrete bridge decks as determined by visual inspection and other complementary methods (energy attenuation of GPR signals and corrosion potential measurements) than without the calibration process.

Time-frequency analysis of air-coupled GPR data for identification of delamination between pavement layers

Identification of delamination in pavement layers using ground penetrating radar (GPR) is still challenging due to its limited range resolution. This paper investigates the characteristics of the composite reflection from the air-filled delamination between pavement layers by time-frequency analysis. We firstly simulated GPR data from an asphalt pavement model with an embedded air gap of different thicknesses by the Green’s function method. Then we conducted a laboratory experiment on a delaminated asphalt pavement model. Both the numerical and laboratory experiment results demonstrate that both the peak instantaneous frequency and its amplitude are sensitive to the variation of the air gap thickness. When the thickness of the air gap is smaller than a quarter of the dominant wavelength, the peak instantaneous frequency of the composite reflection is higher than that of the normal interface reflection. We conclude that we can identify the delamination from the overall rise in the peak instantaneous frequency.

Accounting for Surface Refraction in Velocity Semblance Analysis With Air-Coupled GPR

The aim of this study is to evaluate the velocity semblance analysis technique for air-coupled common midpoint (CMP) measurements with a small antenna offset. The technique was originally developed for seismic surveys, assuming the small spread approximation. Owing to the strong refraction at the surface and shallow investigation depth, this assumption is not valid in the case of air-coupled ground-penetrating radar (GPR). To overcome this assumption, a modification to the method is proposed accounting for the refraction at the surface. Synthetic experiments were executed to demonstrate that the traditional method resulted in a persistent overestimation, whereas the modified method improved the results significantly. Two field experiments have been conducted to test the method under different field conditions. In a first experiment on a road test site, the modified method improved the estimation of depth and propagation velocity significantly. However, the technique failed to estimate the propagation velocity and depth or objects in a second field test, due to rough terrain conditions and noise in the data. Therefore, an additional modification was proposed, by incorporating in-line data as well. This improved the depth and velocity estimations significantly. Overall, this study demonstrates that the traditional velocity semblance analysis (TRAD) is not valid for air-coupled GPR. By accounting for the refraction at the surface and incorporating inline data, it is possible to successfully estimate depth and propagation velocity with small offset air-coupled GPR configurations.

Construction Management Method for Asphalt Paving Using Ground Penetrating Radar and an Infrared Camera

PURPOSES : The objective of this study is to propose a quality control and quality assurance method for use during asphalt pavement construction using non-destructive methods, such as ground penetrating radar (GPR) and an infrared (IR) camera. METHODS : A 1.0 GHz air-coupled GPR system was used to measure the thickness and in situ density of asphalt concrete overlay during the placement and compaction of the asphalt layer in two test construction sections. The in situ density of the asphalt layer was estimated based on the dielectric constant of the asphalt concrete, which was measured as the ratio of the amplitude of the surface reflection of the asphalt mat to that of a metal plate. In addition, an IR camera was used to monitor the surface temperature of the asphalt mat to ensure its uniformity, for both conventional asphalt concrete and fiber-reinforced asphalt (FRA) concrete. RESULTS : From the GPR test, the measured in situ air void of the asphalt concrete overlay gradually decreased from 12.6% at placement to 8.1% after five roller passes for conventional asphalt concrete, and from 10.7% to 5.9% for the FRA concrete. The thickness of the asphalt concrete overlay was reduced from 7.0 cm to 6.0 cm for the conventional material, and from 9.2 cm to 6.4 cm for the FRA concrete. From the IR camera measurements, the temperature differences in the asphalt mat ranged from to in the two test sections. CONCLUSIONS : During asphalt concrete construction, GPR and IR tests can be applicable for monitoring the changes in in situ density, thickness, and temperature differences of the overlay, which are the most important factors for quality control. For easier and more reliable quality control of asphalt overlay construction, it is better to use the thickness measurement from the GPR.

Novel Miniaturized Antenna Designs for In-traffic Air-coupled Ground Penetrating Radar Systems

Ground penetrating radar (GPR) antennas are key elements for the operation of air-coupled, compact, low-cost systems that can be operated at road speeds to map subsurface defects such as corroded rebar, trapped moisture, voids, and pavement layers. This paper presents requirements and tradeoffs for such antennas, as well as a brief methodology for the design process to frame the context and boundary conditions of the antenna problem. Furthermore, the paper discusses a number of planar antennas that have been designed in printed circuit board (PCB) technology using low-cost, glass-reinforced epoxy laminate sheets such as FR4.The ultra-wide-band operation of antennas, along with other key parameters such as gain and beam width, is very important in the design of a useful GPR antenna for air-coupled operation. The interconnected relationship between antenna parameters limits the designs and constrains the upper bounds of the performance that can be achieved. Regulatory specifications such as FCC compliance create additional burden on the specifications and requirements.A rounded bowtie antenna, a slotted bowtie antenna and two types of Vivaldi antenna are designed, simulated, fabricated, and characterized. All are intended to operate within the 1.1–3.5 GHz frequency band and benefit from compact size while providing high gain to allow for the detection of pavement layers and rebar in bridge decks to a depth of up to 2 feet. In-field measurements of the antennas, together with the GPR system, are presented for static testing scenarios such as buried rebar in a sand box and concrete slab. The antenna testing over the sandbox and concrete slab demonstrates the great potential of utilizing the proposed antennas in air-coupled GPR systems, especially the compact rounded bowtie and slotted bowtie antennas.

Validation of Benefits of Automated Depth Correction Method

Current ASTM specifications provide an approach for directly correlating the top rebar reflection amplitude from ground-penetrating radar (GPR) to deck condition when corrosion is the primary mechanism for concrete deterioration. However, current specifications do not offer an approach to compensate for geometric spreading losses caused by the inevitable variability in as-built concrete rebar cover. Even when cover thickness meets specified tolerances, significant errors may occur in mapping the location and quantity of deteriorated concrete when these amplitude variations are unaccounted for in air-coupled or ground-coupled GPR investigations. The significance of this correction is demonstrated by comparing mapped deterioration quantities that have been corrected and not corrected for depth variation, versus those quantities from independent methods such as half-cell corrosion potential, chain drag, or impact echo. One manual and two automated processes for depth correction are presented. These three processes compare favorably with one another on several mapped decks. One of the automated methods, which sets a deterioration threshold calibrated with half-cell and chain drag results, has been shown to be as accurate as manual methods on numerous decks. This approach is recommended for further evaluation and incorporation within ASTM D6087-08: Standard Method for Evaluating Asphalt-Covered Bridge Decks Using Ground-Penetrating Radar.

Non-destructive pavement layer thickness measurement using empirical mode decomposition with GPR

Ground Penetrating Radar (GPR) is an effective Non-Destructive Testing (NDT) technique for highway pavement surveys, which is able to acquire continuous pavement data compared with traditional core drilling method. In this study, we proposed an accurate and efficient method to estimate the thickness of each pavement layer using an air-coupled GPR system. For this work, the main difficulties are estimating each pavement layer’s time delay and dielectric constant. We first give the basic signal model for pavement evaluation, and then present an Intrinsic Mode Functions (IMFs) product detector to determine each pavement layer’s time delay. This method is based on Empirical Mode Decomposition (EMD), which is an adaptive signal decomposition procedure and proved to be suitable for suppressing noises in GPR signal. The dielectric constant was determined by metal reflection measurement. The laboratory and highway experiments illustrate that the proposed thickness estimation method yields reasonable result, thus meets the requirements of practical highway pavement survey with massive GPR data.

Noncontact techniques for monitoring of tunnel linings

An investigation of tunnel linings is performed at two tunnels in the US using complimentary noncontact techniques: air-coupled ground penetrating radar (GPR), and a vehicle-mounted scanning system (SPACETEC) that combines laser, visual, and infrared thermography scanning methods. This paper shows that a combination of such techniques can maximize inspection coverage in a comprehensive and efficient manner. Since ground-truth is typically not available in public tunnel field evaluations, the noncontact techniques used are compared with two reliable in-depth contact nondestructive testing methods: ground-coupled GPR and ultrasonic tomography. The noncontact techniques are used to identify and locate the reinforcement mesh, structural steel ribs, internal layer interfaces, shallow delamination, and tile debonding. It is shown that this combination of methods can be used synergistically to provide tunnel owners with a comprehensive and efficient approach for monitoring tunnel lining conditions.