Conventional Ground Penetrating Radar Research Articles

Application of Ground-Penetrating Radar with the Logging Data Constraint in the Detection of Fractured Rock Mass in Dazu Rock Carvings, Chongqing, China

Geologic interpretation results from conventional ground-penetrating radar (GPR) detection methods tend to have a certain degree of uncertainty. In order to improve the reliability of ground-penetrating radars in the detection of rock mass fissures in grottoes, this study proposes a ground-penetrating radar detection method with the logging data constraint, which is applied to detect the fractured rock mass in the Baodingshan Scenic Area of Dazu Rock Carvings, Chongqing, China. First, conventional logging and borehole televiewer data were compared and verified, yielding detailed lithological and wellbore fissure information. Next, electromagnetic wave velocity was calibrated using GPR profile and the depth of the stratigraphic interface determined by borehole data. Utilizing this calibrated velocity, we are able to accurately calculate the depth values of anomalies in GPR interpretation profiles. Subsequently, we compared the preliminary GPR interpretation profile with the borehole televiewer images. After eliminating false anomalies caused by interference, we obtained more reliable location information for detection targets such as fissures, fracture zones, and weak interlayers. The results of fissure detection in the Dazu Rock Carvings indicate that the detection results of ground-penetrating radar are verified and supplemented under the constraints of stratigraphic and well-wall fissure information obtained by logging. This effectively mitigates the influence of multiplicity and false anomalies of GPR detection on interpretation results. GPR with the logging data constraint enhances the accuracy of the fissure detection results, providing novel technical means for the protection and restoration of grotto relics.

Improved Detection of Buried Elongated Targets by Dual-Polarization GPR

Ground-penetrating radar (GPR) has been widely applied to the detection and delineation of buried targets in the subsurface. Compared with conventional single-channel GPR, polarimetric GPR has been proven to possess an improved ability to detect and characterize an elongated object in the subsurface. This letter proves that the scattering signals from a cylinder in two orthogonal polarization channels have a phase difference of about 90° when its diameter-to-wavelength ratio is about 0.05–0.33. Consequently, a polarization-difference imaging method, which shifts the VV component by −90° and subtracts it from the HH component, is proposed for the improved detection and imaging of a subsurface elongated object. Its effectiveness is verified by numerical, laboratory, and field tests on buried rebars and pipes. The signal-to-clutter ratios of the reconstructed GPR images can be improved by up to 4.5 dB by considering the phase difference between the dual-polarization components.

A modified common midpoint approach for GPR radars

Ground Penetrating Radar (GPR) plays an important role among the non-destructive methods used to analyze, measure and collect data from pavement layers, building structures and archaeological sites. A GPR device consists of a radar able to get an image, technically called radargram, of the subsurface. Due to the decreasing costs of calculation power, it is now possible to analyze and interpret radargrams more efficiently than in past. While technological advancements and improvements in the post-processing of data have increased the power of this process, it is also important to consider survey technique as a key factor in nondestructive testing. The aim of this paper is to propose a modification of a well-known survey technique called Common Mid-Point (CMP). The conventional CMP survey technique requires that the transmitter and receiver of the GPR device be physically separated to survey a site. This provides the benefit of determining the speed of light in the host medium from the slant of the hyperbola reflection of a buried target. The CMP process unifies the inspection technology into a single device, making the use of GPR more accessible to robots on construction sites. The method documented in this paper is based on a mathematical model for adapting the conventional CMP for conventional GPR radar devices, in which transmitter and receiver cannot be separated physically.

Nondestructive Inspection of Tree Trunks Using a Dual-Polarized Ground-Penetrating Radar System

In recent years, trees in European countries have been increasingly endangered by emerging infectious diseases (EIDs). In the United Kingdom, this has been observed to affect whole woodlands and forests, threatening the existence of some types of trees. Although quarantine measures have been taken to limit the spreading of such diseases, this has not yet been effectively controlled leading to millions of trees affected by EIDs. Ground-penetrating radar (GPR) has proven effective in identifying critical features on diseased trees for detection of EIDs spread. However, the irregular shape of tree trunks and their complex internal structure represent real challenges for conventional GPR measurements and data processing methodologies. In this research, a dual-polarised GPR system is used to detect internal decay in tree trunks using novel signal processing methodologies. A polarimetric correlation filter based on Bragg Scattering on a 3D Pauli feature vector and an arc-shaped Kirchhoff migration are discussed in detail. The proposed polarimetric correlation filter is utilised to enhance the signal-to-noise ratio (SNR) of B-scans due to bark and tree trunk high-loss properties of tree trunks. Meanwhile, an arc-shaped Kirchhoff migration algorithm is performed to counteract the influence of the bark irregularity. The proposed data processing framework is successfully validated with measurements on a real tree trunk, where cross-sections were subsequently cut for comparison purposes. Outcomes from the proposed methodology demonstrate a high consistency with the features observed on the tree trunk cross-sections, indicating the reliability of the proposed detection scheme for assessing tree-decay associated with EIDs.

Detection of road cavities in urban cities by 3D ground-penetrating radar

Cavities under urban roads have increasingly become a great threat to traffic safety in many cities. As a quick, effective, and high-resolution geophysical method, ground-penetrating radar (GPR) has been widely used to detect and image near-surface objects. However, the interpretation of field GPR data remains challenging. For example, it is hard to distinguish reflections caused by road cavities or other urban utilities with a conventional 2D GPR survey. The superiority of 3D GPR in data interpretation is demonstrated by a laboratory experiment. Two pipes and a glass-made cavity buried in a sandpit show similar hyperbolic reflections in the 2D GPR profiles; thus, they are difficult to discriminate. In contrast, their geometric shapes and dimensions are readily identified in the 3D image reconstructed from the synthetic 3D GPR data set. Based on these ideas, we have developed a car-mounted 3D GPR system with two antenna arrays oriented in different polarization directions and have detected more than 100 cavities in three Chinese cities over the past year. The field data of two such cavities are presented. As a result, the cavity depth, horizontal size, and height can be accurately estimated from the 3D GPR data set. Laboratory and field experimental results indicate that 3D GPR possesses potential in the detection and recognition of road cavities and utilities in the complex urban environment.

The Effectiveness of Large-Scale, High-Resolution Ground-Penetrating Radar Surveys and Trial Trenching for Archaeological Site Evaluations—A Comparative Study from Two Sites in Norway

The use of large-scale, high-resolution ground-penetrating radar surveys has increasingly become a part of Norwegian cultural heritage management as a complementary method to trial trenching surveys to detect and delineate archaeological sites. The aim of this article is to collect, interpret and compare large-scale, high-resolution ground-penetrating radar (GPR) survey data with results from trial trenching and subsequent large-scale excavations, and to extract descriptive and spatial statistics on detection rates and precision for both evaluation methods. This, in turn, is used to assess the advantages and disadvantages of both conventional, intrusive methods and large-scale GPR surveys. Neither method proved to be flawless, and while the trial trenching had a better overall detection rate, organic and charcoal rich features were nearly just as easily detected by both methods. Similarly, the spatial representability was similar, even though the total detection rates were lower with the GPR. This can be used as an argument in advance of integrating full-coverage GPR results into a site evaluation scheme, preferably in combination with other methods. Overall, these analyses have highlighted drawbacks and possibilities in both methods that are important contributions in understanding how to use them and integrate them in future site evaluations.

Radio Frequency Tomography for Nondestructive Testing of Pillars

Pillars represent some of the commonest supporting elements of modern and historical buildings. Nondestructive testing methods can be applied to gain information about the status of these structural elements. Among them, ground penetrating radar (GPR) is a popular diagnostic tool for the assessment of concrete structures. Despite several theoretical and experimental studies on concrete structural evaluation by GPR have been reported, little work has been done so far with respect to pillars. Owing to their circular geometry, pillars are complex multiscattering environments, which render the interpretation of the radar images very challenging. This article deals with the application of radio frequency tomography as a nondestructive technique for imaging the inner structure of pillars. The main goal of the study is the assessment of the imaging performance that can be obtained in comparison to conventional GPR exploiting a multimonostatic configuration. Accordingly, potentialities and performance of multimonostatic and multiview/multistatic measurement configurations are herein investigated in the inverse scattering framework. For each measurement configuration, the regularized reconstruction of a point-like target and the spectral content are evaluated. The data inversion is carried out by means of the truncated singular value decomposition scheme. Tomographic reconstructions based on full-wave synthetic data are shown to support the comparative analysis.

Bistatic Landmine and IED Detection Combining Vehicle and Drone Mounted GPR Sensors

This work proposes a novel Ground Penetrating Radar (GPR) system to detect landmines and Improvised Explosive Devices (IEDs). The system, which was numerically evaluated, is composed of a transmitter placed on a vehicle and looking forward and a receiver mounted on a drone and looking downwards. This combination offers both a good penetration and a high resolution, enabling the detection of non-metallic targets and mitigating the clutter at the air–soil interface. First, a fast ray tracing simulator was developed to find proper configurations of the system. Then, these configurations were validated using a full wave simulator, considering a flat and a rough surface. All simulations were post-processed using a fast and accurate Synthetic Aperture Radar (SAR) algorithm that takes into account the constitutive parameters of the soil. The SAR images for all configurations were compared, concluding that the proposed contribution greatly improves the target detection and the surface clutter reduction over conventional forward-looking GPR systems.

Velocity analysis of CMP gathers acquired by an array GPR system ‘Yakumo’: results from field application to tsunami deposits

This paper examines the capability of a multi-static ground penetrating radar (GPR) system ‘Yakumo’ to estimate a vertical profile of dielectric constants of stratified geological layers. Yakumo is equipped with eight transmitting and eight receiving antennas. By using Yakumo, we propose to obtain an eight-channel common mid-point (CMP) gather from the 64-channel multi-static radar dataset acquired at each point on the survey line. From each CMP gather, a vertical profile of interval velocities can be estimated by velocity spectrum analysis. By a laboratory sandbox experiment, we validate that the thickness of an 83 cm sand layer is estimated with an error of less than 4%. A field survey carried out on tsunami deposits demonstrates that our GPR method can provide the spatial distribution of layer velocities of the sedimentary structure and that the results are comparable with the geological logs at three points and a conventional common-offset GPR profile.

Method for attenuation assessment of GPR data from concrete bridge decks

Ground penetrating radar (GPR) is a well-accepted technology for condition assessment of concrete bridge decks. Such assessment has been mainly based on the measurement of attenuation of a signal reflected from the top rebar mat. Recently, a new approach for GPR data interpretation has been proposed, which is based on the correlation analysis of time-series GPR data. Motivated by that same research, the current paper presents an alternative method for performing the GPR attenuation analysis. In this method, instead of requiring time-series or baseline data, semi-simulated waveforms are developed and employed in a correlation analysis. With only one reflection retained from direct coupling, these waveforms mimic A-scans collected on a completely deteriorated bridge deck. The obtained results are then plotted in a form of contour maps of correlation coefficients, in which a higher value coefficient indicates more deteriorated concrete. The method was validated through its implementation on three bare concrete bridge decks. The condition maps from the proposed GPR correlation analysis correlate well to those obtained using the conventional GPR amplitude approach, and condition maps from several other nondestructive evaluation (NDE) techniques. However, in comparison to the conventional amplitude analysis, the proposed method provides a better description of the overall deterioration of bridge decks.

Application of NDT methods to improve the detection of underground linear objects

Identification of underground objects and rectilinear discontinuities such as tunnels, tubes etc. is quite complicated. This is due to the fact that their width is quite small compared to the cross-section of ground penetrating radar (GPR) beams. This implies low reflectivity and hence very low signal-to-noise ratio (SNR) per scan. In this paper, we propose a new method for improving the SNR of remote sensing devices such as GPR, that use electromagnetic (EM) waves for detection of underground objects. The proposed method for improving the detectability of small-width objects is based on measuring the anisotropy of soils, in a similar fashion as in non-destructive testing methods. The proposed method performs ‘orthogonal’ scans of a parcel of the tested soil, with subsequent processing of the received signals using a special algorithm. No ‘progressive scan’ is required. Mathematical expressions for calculation of SNR are derived in a general form. The performance of the proposed method is investigated by simulations of real cases, when the reflected signal from the object is ‘drowned in the noise’. Numerical simulations of the investigated cases show that SNR can be increased from 10 dB up to several 10's dB, compared to the traditional method of progressive scanning of the soil under test. It is shown that the presented version of the orthogonal scanning method can yield simultaneous improvement of both SNR and selectivity. The horizontal resolution is improved to about 1 m, in contrast to 5–6 m in conventional GPR.

High resolution aquifer characterization using crosshole GPR full‐waveform tomography: Comparison with direct‐push and tracer test data

AbstractLimited knowledge about the spatial distribution of aquifer properties typically constrains our ability to predict subsurface flow and transport. Here we investigate the value of using high resolution full‐waveform inversion of cross‐borehole ground penetrating radar (GPR) data for aquifer characterization. By stitching together GPR tomograms from multiple adjacent crosshole planes, we are able to image, with a decimeter scale resolution, the dielectric permittivity and electrical conductivity of an alluvial aquifer along cross sections of 50 m length and 10 m depth. A logistic regression model is employed to predict the spatial distribution of lithological facies on the basis of the GPR results. Vertical profiles of porosity and hydraulic conductivity from direct‐push, flowmeter and grain size data suggest that the GPR predicted facies classification is meaningful with regard to porosity and hydraulic conductivity, even though the distributions of individual facies show some overlap and the absolute hydraulic conductivities from the different methods (direct‐push, flowmeter, grain size) differ up to approximately one order of magnitude. Comparison of the GPR predicted facies architecture with tracer test data suggests that the plume splitting observed in a tracer experiment was caused by a hydraulically low‐conductive sand layer with a thickness of only a few decimeters. Because this sand layer is identified by GPR full‐waveform inversion but not by conventional GPR ray‐based inversion we conclude that the improvement in spatial resolution due to full‐waveform inversion is crucial to detect small‐scale aquifer structures that are highly relevant for solute transport.

A Fast Iterative Interpolation Method in f-k Domain for 3-D Irregularly Sampled GPR Data

We propose a new iterative interpolation method for ground penetrating radar (GPR) data based on projection onto convex sets (POCS) and frequency-wavenumber (f-k) zone-pass filtering. The main purpose of this method is to simplify the three-dimensional (3-D) GPR data acquisition; it can also be used for conventional GPR data that include some missing traces. This approach allows the reconstruction of the image from sparsely sampled data that violate the Nyquist criterion. The method can fully use 3-D information and works well with irregularly sampled data. Compared to other iterative interpolation methods, the proposed method needs many fewer iterations and can avoid aliasing. The algorithm is demonstrated with both synthetic data and real GPR measurements from a sand pit experiment. In the latter, the target could still be well reconstructed after 80% of the traces were randomly removed. The average spatial interval of the resampled data was 12 cm, which is much larger than the necessary interval as calculated using the Nyquist criterion. Detailed discussions on the large gap recovery issue and aliasing issue are also provided. The results indicate that it should be possible to reduce the data acquisition density in many GPR applications.

Cross-polarized GPR imaging of fracture flow channeling

Ground penetrating radar (GPR) can be used to image fractures and monitor fluid flow in the subsurface. Conventional GPR imaging uses single-polarization, co-polarized acquisition. We examine the use of cross-polarized GPR signals for imaging flow channeling in a discrete horizontal fracture. Numerical modeling (FDTD) demonstrates that when the fracture channel is oriented at an oblique angle to the survey line, depolarization of the GPR signal results in scattered energy along the cross-polarized components. When the channel is oriented parallel or orthogonal to the survey line, all scattered energy is captured by the co-polarized components and no signal is present in the cross-polarized orientation. Multipolarization, time-lapse 3D GPR field data were acquired at the Altona Flat Rock test site in New York State. The GPR surveys were conducted during background fresh fracture water conditions and during a natural gradient saline tracer test which was used to highlight flow channels along a sub-horizontal fracture. Amplitude analysis of the cross-polarized data reveals flow channeling that is in agreement with the co-polarized GPR images and with independent hydraulic tests. This investigation demonstrates that cross-polarized components of GPR signals can be used to enhance imaging of flow channels in fractured media.

Influence of Heterogeneous Soils and Clutter on the Performance of Ground-Penetrating Radar for Landmine Detection

Ground-penetrating radar (GPR) has been studied for landmine detection and identification. Since this application employs higher frequencies as compared to conventional largescale GPR measurements, the GPR performance is greatly influenced by soil properties and their spatial heterogeneity. In order to study the influence of soil heterogeneity on GPR performance, three types of soil were investigated. From the soil heterogeneity, GPR clutter was modeled with the aim of assessing the difficulty encountered in successful GPR performance. A handheld dual-sensor system that combines a metal detector and GPR was tested in these three test soils, and its performance for identifying buried objects was evaluated. The GPR performance obtained from the test showed a clear correlation with the modeled GPR clutter. Hence, the present study illustrates that clutter plays a major role in the detection of small objects in heterogeneous soil by GPR.

Improvements in crosshole GPR full-waveform inversion and application on data measured at the Boise Hydrogeophysics Research Site

Abstract Crosshole ground penetrating radar (GPR) tomography has been widely used and has the potential to improve the obtained subsurface models due to its high spatial resolution compared to other methods. Recent advances in full-waveform inversion of crosshole GPR data show that higher resolution images can be obtained compared to conventional ray-based GPR inversion because it can exploit all information present in the observed data. Since the first application of full-waveform inversion on synthetic and experimental GPR data, the algorithm has been significantly improved by extending the scalar to a vectorial approach, and changing the stepped permittivity and conductivity update into a simultaneous update. Here, we introduce new normalized gradients that do not depend on the number of sources and receivers which enable a comparison of the gradients and step lengths for different crosshole survey layouts. An experimental data set acquired at the Boise Hydrogeophysics Research Site is inverted using different source–receiver setups and the obtained permittivity and conductivity images, remaining gradients and final misfits are compared for the different versions of the full-waveform inversion. Moreover, different versions of the full-waveform inversion are applied to obtain an overview of all improvements. Most improvements result in a reducing final misfit between the measured and synthetic data and a reducing remaining gradient at the final iteration. Regions with relatively high remaining gradient amplitudes indicate less reliable inversion results. Comparison of the final full-waveform inversion results with Neutron–Neutron porosity log data and capacitive resistivity log data show considerably higher spatial frequencies for the logging data compared to the full-waveform inversion results. To enable a better comparison, we estimated a simple wavenumber filter and the full-waveform inversion results show an improved fit with the logging data. This work shows the potential of full-waveform inversion as an advanced method that can provide high resolution images to improve hydrological models.

Void detection beneath reinforced concrete sections: The practical application of ground-penetrating radar and ultrasonic techniques

Ground-penetrating radar (GPR) and ultrasonic ‘pulse echo’ techniques are well-established methods for the imaging, investigation and analysis of steel reinforced concrete structures and are important civil engineering survey tools. GPR is, arguably, the more widely-used technique as it is suitable for a greater range of problem scenarios (i.e., from rebar mapping to moisture content determination). Ultrasonic techniques are traditionally associated with the engineering-based, non-destructive testing of concrete structures and their integrity analyses (e.g., flaw detection, shear/longitudinal velocity determination, etc). However, when used in an appropriate manner, both techniques can be considered complementary and provide a unique way of imaging the sub-surface that is suited to a range of geotechnical problems. In this paper, we present a comparative study between mid-to-high frequency GPR (450 MHz and 900 MHz) and array-based, shear wave, pulse-echo ultrasonic surveys using proprietary instruments and conventional GPR data processing and visualisation techniques. Our focus is the practical detection of sub-metre scale voids located under steel reinforced concrete sections in realistic survey conditions (e.g., a capped, relict mine shaft or vent). Representative two-dimensional (2D) sections are presented for both methods illustrating the similarities/differences in signal response and the temporal–spatial target resolutions achieved with each technique. The use of three-dimensional data volumes and time slices (or ‘C-scans’) for advanced interpretation is also demonstrated, which although common in GPR applications is under-utilised as a technique in general ultrasonic surveys. The results show that ultrasonic methods can perform as well as GPR for this specific investigation scenario and that they have the potential of overcoming some of the inherent limitations of GPR investigations (i.e., the need for careful antenna frequency selection and survey design in order to image through the rebar meshes). More importantly, we show that standard GPR data collection, processing and visualisation techniques can be used with both types of data without users needing to change existing operational protocols or survey criteria. ► First published comparison of 3D GPR and ultrasonics for voids under concrete. ► Ultrasonics and GPR show comparable performances. ► Ultrasonics capable of overcoming imaging limitations of GPR for rebars. ► 3D timeslicing of ultrasonics shown to perform well with standard processing.

Nondestructive Sensor Using Microwaves From Laser Plasma by Subnanosecond Laser Pulses

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Conventional ground-penetrating radar (GPR) requires large-aperture antennas or long-span measurements to survey a remote location precisely. We propose a laser-driven GPR (LGPR) as a new detection method. LGPR uses microwaves from laser-produced plasmas as remote transmitters and can survey a remote location using a compact instrument. We performed numerical simulations to investigate the radiation mechanism of microwaves from laser plasmas and confirmed the pulsewidth of the laser suitable for LGPR. Experiments with subnanosecond pulse lasers clarified the feasibility and detection performance of LGPR. </para>

The GPR refraction method

Refracted phases have been a recognized feature in wide‐angle common midpoint (CMP) and common shotpoint (CSP) ground‐penetrating radar (GPR) surveys for many years, but historically their use has been limited and overshadowed by conventional reflection profiling techniques. In truth, the GPR refraction method holds great promise, as we show using four field examples from three sites in the northeastern United States. Typical refractors are shallow bedrock or coarse‐grained unconsolidated sediments (where the dielectric constant K ≈ 6–7) beneath partially saturated fine‐grained sand or silt (K ≈ 15–16). Under appropriate conditions, refracted phases can be detected readily at transmitter–receiver (Tx–Rx) offsets of &gt;15 m and from depths of &gt;2 m. Adapting procedures from refraction seismology, we interpret GPR data from reversed CSP soundings using dipping planar interface models and delay time analyses. Results from the two approaches are quite consistent, further corroborated at one of the sites by a GPR reflection profile and directly verified by a series of hammer‐driven probes. In addition to refractions from higher velocity layers in the subsurface, the air refracted phase (the ubiquitous phase refracted from the air–earth interface) can provide important constraints on interpreting the subsurface—particularly useful when attempting to resolve a shallow interface (depth ≤ 0.5 m) whose GPR signature might be masked by interacting phases at early traveltime and small transmitter–receiver offsets. Finally, a cautionary note for interpreting conventional reflection profile data when delineating a higher velocity interface at a depth that might be on the order of the antenna separation or less. Since the refracted phase arrives earlier than the reflected phase for Tx–Rx offsets beyond the critical distance, in certain cases the first break on a reflection radargram may actually be a refraction. CMP or CSP soundings in support of conventional GPR profiles can resolve such concerns.

Gated stepped-frequency ground penetrating radar

This paper describes a newly developed stepped-frequency ground penetrating radar (SFGPR) prototype operating across the 10–620 MHz frequency band. The SFGPR system has been developed to improve the maximum penetration depth capability of ground penetrating radar (GPR) applications without degradation of resolving power. The radar has the unique capability to employ a short gate at each frequency step that can suppress unwanted strong signals relative to weak reflections from deep targets. The system performance is approximately 175 dB in ungated mode and 170 dB in gated mode, with reference to a 10 ms integration time and a 10 dB signal-to-noise ratio. The current system is suitable for operation in a vehicle and incorporates a real time display similar to conventional impulse GPR. It is capable of recording 50 depth profiles per second. Subsurface images obtained with the gated SFGPR of a perched water table on a sand island, a culvert in high-loss ground, and a 500 lb bomb buried at 1.7 m deep are presented. A frequency compensation procedure is used to improve image clarity. The weakest confirmed underground target has a signal power 100 dB down on the transmitted power. Such a signal would be near the noise floor of a commercial impulse GPR.