Ground Penetrating Radar Antenna Research Articles

Boundary recognition of tunnel lining void from ground-penetrating radar data

Abstract Relatively low ground-penetrating radar antenna frequencies are usually selected for tunnel inspection due to the thickness of the lining, which causes the geometric features of voids in the image to be blurred and difficult to identify. Therefore, the boundary points of the voids are determined by combining reflection and attenuation coefficients, and the geometric features are constructed to identify the voids. Depending on the electromagnetic differences between the void and other mediums in the lining, when the electromagnetic wave propagates from the lining concrete to the void, the reflection coefficient is positive, and the phase of the reflected wave is the same as the incident wave. Conversely, the phase of the reflected wave is opposite to the incident wave. Therefore, the boundary point can be determined in the one dimension time-waveform diagram (A-Scan) based on the phase change. Moreover, the amplitude of the electromagnetic wave attenuates exponentially in the concrete, but it attenuates slowly in the void. Also, electromagnetic waves exhibit high-frequency characteristics in the lining but low-frequency characteristics in the voids. Boundary points that conform to the variation of amplitude and frequency characteristics in the void are screened. These boundary points are then constructed in two-dimensional scan data (B-Scan) to identify the voids by using the geometry of the voids. This method is applied to the Hu Sa Tunnel. The voids are successfully identified in the mileage section YK81+310–YK81+422 of the Hu Sa Tunnel, and the depth of cover and the area of voids are correctly estimated.

Coplanar integration of antipodal vivaldi antenna with metasurface antennas for ground penetrating radar and 5–GHz WIFI applications

In this paper, we design a novel antenna with dual-band and multi-radiation modes by integrating the two metasurface binary array antennas (MBAAs) into the antipodal Vivaldi antenna (AVA). Metasurface binary array antenna 1 (MBAA-1) and metasurface binary array antenna 2 (MBAA-2) are symmetrically embedded on two radiation patches of AVA. When AVA is excited, vertical down radiation can be achieved. When MBAA-1 or MBAA-2 is excited, horizontal forward or backward radiation can be achieved. The simulated results show that AVA and MBAAs have the −10 dB return loss bandwidths of 163.6% and 9.3% and the realized gain of 5.5–8.1 dBi and 8.0–10.0 dBi cover the Ground Penetrating Radar (GPR) band (0.6–6.0 GHz) and the 5–GHz WIFI band (4.9–5.9 GHz), respectively. The test results are in good agreement with the simulated results. The proposed antenna realizes the integrated design of 5–GHz WIFI and GPR antennas, which can be applied to the antenna system design of intelligent robots and UAVs, and contribute to the integration development of wireless communication and radar in the future.

Brief communication: Monitoring active layer dynamics using a lightweight nimble ground-penetrating radar system – a laboratory analogue test case

Abstract. Monitoring active layer dynamics is critical for improving the understanding of near-surface thermal and hydrological processes in the cryosphere. This study presents the laboratory test of a low-cost ground-penetrating radar (GPR) system within a laboratory experiment of active layer freezing and thawing monitoring. The system is an in-house-built low-power monostatic GPR antenna coupled with a reflectometer piloted by a single-board computer (SBC) and was tested prior to field deployment. The correspondence between the frozen front electromagnetic (EM) reflection and temperature allowed us to test the ability of the system to closely monitor the frozen front and bottom of the active layer reflection.

Diurnal Monitoring of Moisture Content of Scots Pine and Small-Leaved Lime Trunks Using Ground Penetrating Radar (GPR) and Increment Cores

Ground penetrating radar is non-invasive technology suitable for mapping moisture content variations since it shows high sensitivity to changes in water saturation. In this work we used a GPR tomography approach to estimate moisture content within two small-leaved lime (Tilia cordata) and two Scots pine (Pinus sylvestris) trunks. Additional information was derived using the method of GPR zero-offset. GPR data was collected in Moscow (diurnal monitoring in September 2021) using a shielded GPR antenna working at 1500 MHz. Moisture values derived from GPR data were compared with the values obtained directly by measuring sampled wood cores gravimetrically. A good agreement was observed between GPR-derived moisture content and core sample-derived values. Notwithstanding GPR-derived moisture content is about two times higher than core sample-derived values, a strong linear relation with a determination coefficient more than 0.8 is observed. Diurnal monitoring did not reveal any significant changes in moisture content inside the trunks. It can be concluded that the period of early autumn in the Moscow region is characterized by a constant moisture content of the small-leaved lime trunk during the day.

Estimation of Surface Soil Moisture by a Multi‐Elevation UAV‐Based Ground Penetrating Radar

AbstractThe measurement of soil moisture is important for a wide range of applications, including ecosystem conservation and agricultural management. However, most traditional measurement methods, for example, time‐domain reflectometry (TDR), are unsuitable for mapping field scale variability. In this study, we propose a method that uses an unmanned aerial vehicle (UAV) to support a ground penetrating radar (GPR) system for spatial scanning investigation at different elevations above ground level. This method measures the surface reflectivity to estimate the soil moisture, exploiting the linear relationship between the ratio of the reflected and the direct wave amplitudes along with the reciprocal of GPR antenna height. This relationship is deduced in this study based on the point source assumptions of a transmitter antenna and ground reflections, which is confirmed by numerical simulation results using the gprMax software. Unlike previous air‐launched GPR methods, the UAV‐GPR method presented here removes the limitations of a steady transmitter power and a fixed GPR survey height and the need for calibration of antenna transfer functions and geophysical inversion calculations, and thus is simpler and more convenient for field applications. We test the method at field sites within the riparian zone and a river‐island grassland adjacent to the Yangtze River. The results from the field study illustrate comparable measured soil moisture to those obtained invasively using TDR. The root mean square error of surface reflectivity and soil moisture values between UAV‐GPR with eight antenna height investigations and TDR in the grassland are 0.03 and 0.05 cm3/cm3, respectively.

Design and Optimization of a Bowtie Antenna for Ground Penetrating Radar (GPR) System

Design and Optimization of a Bowtie Antenna for Ground Penetrating Radar (GPR) System

Robotic Inspection of Underground Utilities for Construction Survey Using a Ground Penetrating Radar

Ground penetrating radar (GPR) is a very useful nondestructive evaluation (NDE) device for locating and mapping underground assets prior to digging and trenching efforts in construction. This paper presents a novel robotic system to automate the GPR data-collection process, localize underground utilities, and interpret and reconstruct the underground objects for better visualization, allowing regular nonprofessional users to understand the survey results. This system is composed of three modules: (1) an omnidirectional robotic data-collection platform that carries a RGB-D camera with inertial measurement unit (IMU) and a GPR antenna to perform automatic GPR data collection and tag each GPR measurement with visual positioning information at every sampling step, (2) a learning-based migration module to interpret the raw GPR B-scan image into a two-dimensional (2D) cross-section model of objects, and (3) a three-dimensional (3D) reconstruction module, i.e., 30.0% GPRNet, to generate underground utility model represented as fine 3D point cloud. Comparative studies were performed on synthetic data and field GPR raw data with various incompleteness and noise. Experimental results demonstrated that our proposed method achieves a higher GPR imaging accuracy in mean intersection over union (IoU) than the conventional back-projection (BP) migration approach, and 6.9%−7.2% less loss in Chamfer distance (CD) than point cloud model reconstruction baseline methods. The GPR-based robotic inspection provides an effective tool for civil engineers to detect and survey underground utilities before construction.

Sparsity enhanced MRF algorithm for automatic object detection in GPR imagery.

This study addressed the problem of automated object detection from ground penetrating radar imaging (GPR), using the concept of sparse representation. The detection task is first formulated as a Markov random field (MRF) process. Then, we propose a novel detection algorithm by introducing the sparsity constraint to the standard MRF model. Specifically, the traditional approach finds it difficult to determine the central target due to the influence of different neighbors from the imaging area. As such, we introduce a domain search algorithm to overcome this issue and increase the accuracy of target detection. Additionally, in the standard MRF model, the Gibbs parameters are empirically predetermined and fixed during the detection process, yet those hyperparameters may have a significant effect on the performance of the detection. Accordingly, in this paper, Gibbs parameters are self-adaptive and fine-tuned using an iterative updating strategy followed the concept of sparse representation. Furthermore, the proposed algorithm has then been proven to have a strong convergence property theoretically. Finally, we verify the proposed method using a real-world dataset, with a set of ground penetrating radar antennas in three different transmitted frequencies (50 MHz, 200 MHz and 300 MHz). Experimental evaluations demonstrate the advantages of utilizing the proposed algorithm to detect objects in ground penetrating radar imagery, in comparison with four traditional detection algorithms.

A Compact Wideband Semi-Elliptical Dipole Antenna for Boat-Borne GPR

In this article, a compact wideband semi-elliptical dipole antenna for boat-borne ground penetrating radar (GPR) is presented. The antenna is a semi-elliptical dipole for obtaining wideband performance. In addition, to reduce the size of the antenna, a V-shaped slot structure was applied. Furthermore, resistors are loaded between the slots of the V-shaped structure to improve the time-domain performance of the antenna. The compact wideband semi-elliptical dipole antenna is fabricated and measured. The measurement results show that the reflection coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert \text{S}_{\mathbf {11}}\vert$ </tex-math></inline-formula> ) of the proposed antenna in the range of 170–247.6 MHz is less than−10 dB. The realized gain of the proposed antenna is 0.98 dBi in the center frequency. Finally, the field experiments are carried out and the boat-borne GPR with the proposed antenna can clearly map the bottom of the river.

Mapping inland water bathymetry with Ground Penetrating Radar (GPR) on board Unmanned Aerial Systems (UASs)

Bathymetry of inland water bodies is essential for river maintenance and flood risk management. Traditionally, in shallow water bodies, bathymetry is retrieved by operators wading through the water body with Real Time Kinematic (RTK) Global Navigation Satellite System (GNSS), whilst in deeper waters, it is retrieved with sonar instruments on manned or unmanned boats. In the past, researchers have documented the use of Ground Penetrating Radar (GPR) on boats (i.e. water-coupled GPR) for monitoring the bathymetry of frozen and non-frozen water bodies. Furthermore, GPR has been used on helicopters for monitoring ice and snow thickness. However, deployment of GPR on board Unmanned Aerial Systems (UASs) in non-frozen inland water bodies with electric conductivity higher than 100 μS/cm (as is common in most inland waterbodies in non-polar regions) is unexplored. In this paper, we document the possibility to use drone-borne and water-coupled GPR in several cross-sections located in three different waterbodies (1 lake and 2 rivers) in Denmark. These waterbodies had different bed sediment materials and vegetation conditions, an electric conductivity varying from 200 to 340 μS/cm and depths up to 2.5 m. Drone-borne GPR showed accuracy similar to water-coupled GPR when compared to RTK GNSS ground-truth measurements, with a Mean Absolute Error (MAE) of approx. 8 cm. The only limitations of drone-borne GPR were i) more restrictive minimum depth requirement (typically 0.8–1.1 m for drone-borne GPR, while 0.3–0.4 m for water-coupled GPR) ii) requirement to fly the GPR antenna at altitudes of approx. 0.5 m above the water surface to avoid high spreading losses and strong surface clutter events hiding the signal. Finally, GPR measurements were benchmarked against traditional sonar measurements, showing that GPR measurements significantly outperform sonar measurements in waterbodies with medium or high density of aquatic vegetation.

Developments in 3D Visualisation of the Rail Tunnel Subsurface for Inspection and Monitoring

Railway Tunnel SubSurface Inspection (RTSSI) is essential for targeted structural maintenance. ‘Effective’ detection, localisation and characterisation of fully concealed features (i.e., assets, defects) is the primary challenge faced by RTSSI engineers, particularly in historic masonry tunnels. Clear conveyance and communication of gathered information to end-users poses the less frequently considered secondary challenge. The purpose of this review is to establish the current state of the art in RTSSI data acquisition and information conveyance schemes, in turn formalising exactly what constitutes an ‘effective’ RTSSI visualisation framework. From this knowledge gaps, trends in leading RTSSI research and opportunities for future development are explored. Literary analysis of over 300 resources (identified using the 360-degree search method) informs data acquisition system operation principles, common strengths and limitations, alongside leading studies and commercial tools. Similar rigor is adopted to appraise leading information conveyance schemes. This provides a comprehensive whilst critical review of present research and future development opportunities within the field. This review highlights common shortcomings shared by multiple methods for RTSSI, which are used to formulate robust criteria for a contextually ‘effective’ visualisation framework. Although no current process is deemed fully effective; a feasible hybridised framework capable of meeting all stipulated criteria is proposed based on identified future research avenues. Scope for novel analysis of helical point cloud subsurface datasets obtained by a new rotating ground penetrating radar antenna is of notable interest.

Evaluation of Air-Cavities behind Concrete Tunnel Linings Using GPR Measurements

The primary objectives of this study are to investigate the interference of ground penetrating radar (GPR) signals with air-cavities behind concrete linings subjected to three experimental parameters (GPR antenna frequency, thickness of concrete lining, and thickness of air-cavities), and their effects on the evaluation of important engineering parameters (thickness of concrete, and length and thickness of air-cavities) for tunnel structure inspection using GPR. For these purposes, first, a concrete model specimen is fabricated in the laboratory, which simulates air-cavity defects with various thicknesses (0 mm, 30 mm, 100 mm, 200 mm, and 300 mm) behind concrete with four different thicknesses (100 mm, 200 mm, 300 mm, and 400 mm) in tunnel structures. Second, a series of GPR survey is performed on the fabricated concrete model specimen using three ground-coupled GPR antennas with nominal center frequencies of 400 MHz, 900 MHz, and 1600 MHz. Third, a practical signal interpretation method is proposed to evaluate air-cavities in concrete tunnel linings subjected to the three experimental parameters. It is demonstrated that the cross-correlation analysis is effective for identifying air-cavities behind concrete, and for characterizing the length of the air-cavities. In addition, the thicknesses of air-cavities are estimated by a new approach in this study, measuring the apparent time delay of partially overlapped electromagnetic wave reflections within air-cavities. The results in this study provide a fundamental basis to improve GPR signal interpretation techniques for the evaluation of air-cavities behind concrete tunnel linings.

Mutual Coupling Suppression of GPR Antennas by Depositing Wideband Meta-Absorber with Resistive Film.

The direct wave between the transceiver antenna negatively affects the dynamic range and imaging quality of ground penetrating radar (GPR). Suppressing this direct wave is a vital problem in enhancing the performance of the whole GPR system. In this paper, a Minkowski-fractal metamaterial absorber (MMA) with the resistive film is proposed in the GPR transceiver antenna to reduce the mutual coupling. The simulated and measured results indicate that this MMA has an effective wideband absorption in 1.0-8.0 GHz. And the thickness of MMA is only 0.007 (with respect to 2.0 GHz). This wideband MMA can reduce the mutual coupling of the proposed GPR transceiver antenna by an average of 10 dB. And it also mitigates the time-domain ringing problem of the transmit antenna. Real-world experiments demonstrate that the direct wave from the transmitting antenna can be reduced and the target echo arriving at the receiving antenna can be increased if this MMA is placed in the proposed transceiver antenna. This resistive film-based MMA offers great promise in realizing low-cost, compact, and lightweight GPR antennas, which can also be extended to high-frequency microwave imaging.

Impact of stress on GPR signal in concrete

Prestressed concrete is present in many civil engineering structures such as dams or containment buildings. Due to aging of the structures (creep, shrinkage, corrosion…) prestressing losses can occur. Thus, it is important to be able to know the stress state in order to ensure structural safety. The current techniques, such as the crossbow-test used for assessing the tension in the prestressing cables, are semi-destructive. However, some structures such as containment buildings cannot withstand intrusive testing. It is thus necessary to use non-destructive techniques to assess the concrete stress in order to monitor the prestressing losses. The aim of this study is to assess the concrete stress variations by electromagnetic techniques. More practically, the study focuses on the impact of compressive stress on the propagation of Ground Penetrating Radar (GPR) waves in concrete. The measurements are done with GPR antennas with a center frequency of 1,5 GHz on concrete specimens under compressive stress. Different GPR measurement configurations (reflection or transmission) are used. Tests are performed on saturated and dry samples and with different orientations of the antennas compared to the loading direction. The signals are analyzed in both time and frequency domain. When the antenna is parallel to the direction of loading, results show that an increase of stress induce a decrease of the magnitude and a time offset of the signal. These outcomes are observed during the loading as well as during the unloading. The results do not depend on the hydric state of the slab. However, they depend on the direction of the wave propagation compared to the loading direction.

Estimation Diameter of Buried Pipe Using Principle of Ground Penetrating Radar and Electromagnetic Locator

A comprehensive utility map presents a complete information for users to interpret and extract the information. Its importance for underground utility database to support future infrastructure planning. However, there is not all information is available such as material, voltage, colour and diameter of pipe. The surveyor needs to observe the surrounding area of survey such as utility marker and utility box. There are lots of utility marker was missing and damage due to the road maintenance and construction work. Hence, to ensure a complete information could be presented on the map, the surveyor needs to find a method to gather the information. In this study the geophysics method was used to identify the diameter of buried pipe. The integration of geophysics principles is tested between Electromagnetic Locator (EML) and Ground penetrating Radar (GPR). The test was conducted at Jalan B2-B9, Taman Melawati, Kuala Lumpur where five points were marked on the ground as point 1, 2, 3, 4 and 5. The EML with 33kHz &amp; 65kHz frequency have been used to locate the position and depth of buried pipe using Direct Connection technique. The GPR antenna with 700MHz frequency were tested to penetrate the subsurface up to 2.5 meter, respectively. The post processing is carried out to enhance the quality of GPR radargram profile. Thus, the velocity of hyperbola was calibrated trough curve fitting process to get the actual depth of buried pipe. The result of the study shown that the diameter of pipe is 100mm in which corresponded to the information at the pipe marker. EML and GPR is a useful method to detect the diameter of pipe, although it is required field verification and the proper selection of antenna frequency.

Drone applications in hydrogeophysics: Recent examples and a vision for the future

The recent boom of commercially available drone technology capable of supporting geophysical imaging systems has led to several practitioners adopting the platforms to collect geophysical data. This has led to significant opportunities for studying hydrologic systems, as these platforms enable more efficient data collection and data collection in difficult-to-access locations. Recent examples from snow and stream studies show the viability of these platforms for studying complex systems in difficult locations and the added value of data provided by the technology. Small unoccupied aerial system (sUAS) ground-penetrating radar (GPR) surveys conducted over water bodies and snow have proven beneficial for determining bathymetry in dangerous swift water and depth of snowpack in mountainous regions. Challenges arise due to decoupling of the GPR antenna and the ground surface, which limits signal penetration. Additionally, multifrequency or multichannel GPR imaging remains a challenge due to the payload limitations of an sUAS. Drone-mounted thermal infrared data collection enables the generation of thermal orthomosaic images with unprecedented detail and spatial range for quantifying groundwater fluxes to surface water bodies and reduces issues with infrared reflection common in ground-based studies. Several drone-mounted electromagnetic studies also show utility in efficient, high-resolution mapping of near-surface soil conductivity. Overall, the use of drone platforms for studying hydrologic systems remains a frontier in science applications that needs further refinement before wide-scale adoption. The preliminary studies presented here hint that the development of robust geophysical data acquisition tools based on UAS will lead to an explosion of applications to near-surface hydrologic problems.

Railway ballast layer inspection with different GPR antennas and frequencies

Ground penetrating radar (GPR) is a popular technology for inspecting railway ballast layer, mainly on the ballast fouling level. However, different GPR antennas with different frequencies are suitable for different inspection emphasis and diverse railway lines (weather and sub-structure). In addition, the full-scale track model (with subgrade) for experimental tests was not seen in earlier studies. For further application of GPR in China, the GPR inspections (with 400 MHz, 900 MHz and 2 GHz antennas) were performed on a 30 m long full-scale track and three railway lines (different weather and sub-structure). Results show that ballast layer inspection should be performed mainly with the 2 GHz antenna and supplemented by the 400 MHz and 900 MHz antennas. The weather has great influence on the results of GPR inspection. This study is helpful for supplementing the guidance of ballast layer inspection with GPR.

GPR antennas geometry and its impact over detection of near-surface water contamination through AVO Data analysis at laboratory site

Polarization is an important technique for ground penetrating radar (GPR) surveys, particularly when determining the shape, orientation, size, and precise location of any buried material. A linearly-polarized incident GPR wave experiences polarization change upon scattering from the host medium, which highly depends upon the technique and antenna orientation used for the acquisition of the GPR data. This paper focuses on the polarization of GPR data acquired at a laboratory test site. We kept the GPR antenna orientation and geometry in mind to understand how they impact the results while detecting subsurface contamination. Because of its direct indication ability, amplitude variation with offset (AVO) has been widely used to detect subsurface contamination; however, a technique to evaluate the distribution of subsurface contamination is not yet available; thus, in this study, we will also discuss the distribution of contamination using polarization of GPR data. Two major phenomena have been studied: the polarity of the data acquired and slice view with and without contamination in the testing tank, as well as trace, frequency, and amplitude plot during data analysis to determine the difference of plot with and without contamination. Our findings suggest that using the polarization geometry technique to obtain an accurate view of the subsurface contamination will not only improve the GPR AVO analysis but will also provide a clear distribution of contamination in the subsurface.

High Gain Compact UWB Antenna for Ground Penetrating Radar Detection and Soil Inspection.

An ultrawide bandwidth (UWB) antenna for ground-penetrating radar (GPR) applications is designed to check soil moisture and provide good-quality images of metallic targets hidden in the soil. GPR is a promising technology for detecting and identifying buried objects, such as landmines, and investigating soil in terms of moisture content and contamination. A paddle-shaped microstrip antenna is created by cutting a rectangular patch at one of its diametrical edges fed by the coplanar waveguide technique. The antenna is loaded by stubs, shorting pins, and a split-ring resonator (SRR) metamaterial structure to increase the antenna’s gain and enhance the bandwidth (BW) towards both the lower and higher end of the working BW. The antenna’s performance in soil inspection is studied in terms of the operating frequency range, different types of soil, different distances (e.g., 50 cm) between the antenna arrays and soil, S-parameters, and gain. Following this, the antenna’s ability to find a metallic target in the soil is tested, considering different array numbers, multi-targets, and locations. The antenna is designed on a thin layer of economic polytetrafluoroethylene (PTFE) substrate with dimensions 50 × 39 × 0.508 mm3 and works in the frequency range 1.9–9.2 GHz. In addition, two more resonances at 0.9 and 1.8 GHz are also achieved; hence, the antenna works for more than two application bands, such as the ISM- and L-bands. The measurement results validated excellent agreement with the simulated results. Furthermore, the recommended antenna offering a high gain of about 10.8 dBi and maximum efficiency above 97% proved able to discriminate between hidden objects and even recognize their shapes. Moreover, the reconstructed images show that the antenna can detect an object in the soil at any location.

Characterization of a ground penetrating radar shielded antenna using laboratory measurements, FDTD modeling and swarm global optimization

Full Waveform Inversion (FWI) is an optimization method that retrieves high-quality images of the ground's internal electromagnetic properties, such as permittivity, permeability, or conductivity. FWI requirements include an initial subsurface image of the parameters (starting point models), a wave propagation model, a cost function, and the source wavelet used during data acquisition. Usually, the source wavelet is estimated from the acquired data, or modelled from the antenna characteristics. In this study, the materials of the shielded antenna of a commercial Ground Penetrating Radar (GPR), developed by GSSI, are estimated using a global optimization method, from the observation measurements of the source signal. The estimated source is then used to model the wave propagation of the electromagnetic signal, and to estimate the electromagnetic parameters of the SEAM model via FWI. Experimental results show that the soil characteristics with the estimated source and pattern radiations retrieve better quality images than the inversion when the radiation pattern is neglected. In fact, the impact of using the correct source during the inversion is more evident when the initial model is distant from the correct solution.