Ground Penetrating Radar Acquisition Research Articles

Comparative Study of GPR Acquisition Methods for Shallow Buried Object Detection

This paper investigates the use of ground-penetrating radar (GPR) technology for detecting shallow buried objects, utilizing an air-coupled stepped frequency continuous wave (SFCW) radar system that operates within a 2 GHz bandwidth starting at 500 MHz. Different GPR data acquisition methods for air-coupled systems are compared, specifically down-looking, side-looking, and circular acquisition strategies, employing the back projection algorithm to provide focusing of the acquired GPR data. Experimental results showed that the GPR can penetrate up to 0.6 m below the surface in a down-looking mode. The developed radar and the back projection focusing algorithm were used to acquire data in the side-looking and circular mode, providing focused images with a resolution of 0.1 m and detecting subsurface objects up to 0.3 m below the surface. The proposed approach transforms B-scans of the GPR-based data into 2D images. The provided approach has significant potential for advancing shallow object detection capabilities by transforming hyperbola-based features into point-like features.

Mapping snow depth on Canadian sub-arctic lakes using ground-penetrating radar

Abstract. Ice thickness across lake ice is mainly influenced by the presence of snow and its distribution, which affects the rate of lake ice growth. The distribution of snow depth over lake ice varies due to wind redistribution and snowpack metamorphism, affecting the variability of lake ice thickness. Accurate and consistent snow depth data on lake ice are sparse and challenging to obtain. However, high spatial resolution lake snow depth observations are necessary for the next generation of thermodynamic lake ice models to improve the understanding of how the varying distribution of snow depth influences lake ice formation and growth. This study was conducted using ground-penetrating radar (GPR) acquisitions with ∼9 cm sampling resolution along transects totalling ∼44 km to map snow depth over four Canadian sub-arctic freshwater lakes. The lake snow depth derived from GPR two-way travel time (TWT) resulted in an average relative error of under 10 % when compared to 2430 in situ snow depth observations for the early and late winter season. The snow depth derived from GPR TWTs for the early winter season was estimated with a root mean square error (RMSE) of 1.6 cm and a mean bias error of 0.01 cm, while the accuracy for the late winter season on a deeper snowpack was estimated with a RMSE of 2.9 cm and a mean bias error of 0.4 cm. The GPR-derived snow depths were interpolated to create 1 m spatial resolution snow depth maps. The findings showed improved lake snow depth retrieval accuracy and introduced a fast and efficient method to obtain high spatial resolution snow depth information. The results suggest that GPR acquisitions can be used to derive lake snow depth, providing a viable alternative to manual snow depth monitoring methods. The findings can lead to an improved understanding of snow and lake ice interactions, which is essential for northern communities' safety and wellbeing and the scientific modelling community.

A framework for efficient soil architecture mapping using ground-penetrating radar

Ground-penetrating radar (GPR) is more and more prevailing in soil survey as a geophysical method due to its non-invasive and high efficiency. For subsurface soil architecture mapping, its quality relies not upon only the accuracy of GPR evaluation itself but also on often overlooked survey strategy. This study focuses on the latter one and proposed a new survey strategy to promote the commonly-used survey strategy that subjectively employing multiple parallel two-dimensional GPR acquisition lines. A preliminary survey roughly capturing the spatial variability of soil architecture is suggested to provide two key parameters, the spacing and the direction of the parallel survey lines, for the subsequent commonly-used survey design. Thus, the final survey can not only take advantage of the dense space sampling rate of GPR itself along the survey direction with the highest variability of the subsurface soil architecture, but also well controls the mapping quality and efficiency. Field experiments and numerical tests demonstrate that the new survey strategy could effectively reduce mapping uncertainties originating from unmatching sampling rate and interpolation algorithm selection. Besides, a suitable mapping quality and efficiency can be guaranteed but preventing unnecessary too dense sampling rate, namely low mapping efficiency. This indicates the proposed strategy would be useful for relevant soil surveys using geophysical methods.

3D HBIM Model and Full Contactless GPR Tomography: An Experimental Application on the Historic Walls That Support Giotto’s Mural Paintings, Santa Croce Basilica, Florence—Italy

GPR (Ground Penetrating Radar) is a technology widely applied today in the field of non-destructive investigations. From its first applications, the field of use has involved environmental and archaeological contexts and, only recently, non-destructive investigations for the diagnostics of existing buildings, including historical ones. In the latter, the GPR is, in particular, addressed to the NDT diagnostics of walls, which often support paintings of considerable value. In this study, GPR technology was used to investigate the walls of the Bardi Chapel in the Santa Croce Basilica in Florence, which features Giotto’s frescoes. The GPR acquisition was performed with a three-antenna module that, through multiple scans, allowed to reconstruct the full 3D tomography of the three main walls of the chapel. The development of a customized system made it possible to extend the investigation area to almost the entire wall and to avoid contact between the instrument and the wall paint, thus safeguarding its integrity. The collected data, once inserted in a unified framework of the HBIM model, geo-referenced, and equipped with the information content, allowed us to evaluate the masonry structure and to generate masonry data for subsequent seismic vulnerability assessments.

Drone-Borne Ground-Penetrating Radar for Snow Cover Mapping

Ground-penetrating radar (GPR) is one of the most commonly used instruments to map the Snow Water Equivalent (SWE) in mountainous regions. However, some areas may be difficult or dangerous to access; besides, some surveys can be quite time-consuming. We test a new system to fulfill the need to speed up the acquisition process for the analysis of the SWE and to access remote or dangerous areas. A GPR antenna (900 MHz) is mounted on a drone prototype designed to carry heavy instruments, fly safely at high altitudes, and avoid interference of the GPR signal. A survey of two test sites of the Alpine region during winter 2020–2021 is presented, to check the prototype performance for mapping the snow thickness at the catchment scale. We process the data according to a standard flow-chart of radar processing and we pick both the travel times of the air–snow interface and the snow–ground interface to compute the travel time difference and to estimate the snow depth. The calibration of the radar snow depth is performed by comparing the radar travel times with snow depth measurements at preselected stations. The main results show fairly good reliability and performance in terms of data quality, accuracy, and spatial resolution in snow depth monitoring. We tested the device in the condition of low snow density (<200 kg/m3) and this limits the detectability of the air–snow interface. This is mainly caused by low values of the electrical permittivity of the dry soft snow, providing a weak reflectivity of the snow surface. To overcome this critical aspect, we use the data of the rangefinder to properly detect the travel time of the snow–air interface. This sensor is already installed in our prototype and in most commercial drones for flight purposes. Based on our experience with the prototype, various improvement strategies and limitations of drone-borne GPR acquisition are discussed. In conclusion, the drone technology is found to be ready to support GPR-based snow depth mapping applications at high altitudes, provided that the operators acquire adequate knowledge of the devices, in order to effectively build, tune, use and maintain a reliable acquisition system.

FDTD‐based sensitivity analysis of GPR acquisition parameters for accurate detection of buried cylindrical objects

Ground-penetrating radar (GPR) is a non-destructive technique used to assess the subsurface and locate any buried objects such as subterranean utilities. It emits electromagnetic radiations and detects the reflected signals from underground objects. Nevertheless, many GPR acquisition factors cannot easily be accounted for in evaluating the performance of the GPR system. To ensure more correct and accurate detection of the GPR signals reflected by a buried cylindrical object in a homogenous medium, sensitivity analysis of GPR data is proposed in this work. The numerical simulator gprMax, which is based on finite-difference time-domain, is used to generate GPR data. The data analysis results showed that the GPR measurement accuracy is more sensitive to the incident signal waveform than the other parameters. Thus, one of the major characteristics of GPR that can affect its accuracy is the incident waveform.

Masonry texture reconstruction for building seismic assessment: Practical evaluation and potentials of Ground Penetrating Radar methodology

Within the context of masonry texture identification and geometrical reconstruction, Ground Penetrating Radar (GPR) could emerge as an effective survey method, due to its high-resolution, scalable and non-destructive approach. This study has methodologically assessed operational advantages and weaknesses of the methodology when implemented for a seismic assessment purpose. In particular, the scope was to examine acquisition and post-processing strategies that could reliably highlight the presence or absence of specific construction features on which to develop a quantitative indication of the quality of the masonry. For this purpose, a high frequency 3D GPR acquisition on a plastered masonry wall incorporating different brick geometries, has been carried out. The acquisition was made along two different survey directions and analysing unfocussed and migrated radar slices. Thanks to a favourable antenna pattern alignment and achieved resolution enhancement, the focussed GPR data collected along the vertical mortar joints orientation have proven to be able to properly reconstruct the wall texture, providing therefore detailed and crucial information on its geometrical appearance.

Assessment of health status of tree trunks using ground penetrating radar tomography

Typical tree health assessment methods are destructive. Non-destructive Ground Penetrating Radar (GPR) technique can provide a diagnostic tool for assessing the health status of live tree trunks based on internal dielectric permittivity distribution. Typical GPR acquisition technique—common-offset—is not helpful in providing robust and high-resolution quantitative results. In the current work we evaluate the capabilities of GPR tomography on locating tree-decays in a number of different tree species, imaging the interval structure of a tree and quantitative estimation of moisture content (MC) based on distribution of dielectric permittivity, directly related to MC. The measurements described in this work were made on the trunks of live trees of different species in different conditions: a healthy English oak (Quercus robur), a dry Siberian fir (Picea obovata), a Horse chestnut (Castanea dentata) and a European aspen (Populus tremula) with rot inside. The results of the suggested approach were confirmed by resistography. Different parts of the trunk (bark, core, sapwood), as well as and affected areas differ in moisture content, so the method of GPR tomography allowed us to see both the structure of the trunk of a tree, and the presence and dimensions of defects.

A high school student's introduction to geophysics through acquisition, processing, and interpretation of GPR data from marked and unmarked grave sites

During the 2019–2020 school year, a high school student engaged in a science fair project that involved acquiring, processing, and interpreting geophysical data. The project used ground-penetrating radar (GPR) to identify response signatures of marked and unmarked grave sites in Fairlawn Cemetery in Stillwater, Oklahoma. During the investigation, the student was introduced to GPR acquisition equipment, seismic processing algorithms, and interpretation techniques. With the help of local university outreach and community mentors, the student was able to complete the science fair project with a positive outcome.

GPR research on damaged road pavements built in cut and fill sections

We present the GPR (Ground Penetrating Radar) results dealing with flexible road pavements located on cut and fill sections. The aim is to evaluate the road damage (particularly the ramified cracks) taking into consideration also the cut and fill sections height and the traffic load. GPR evaluation was carried out on 14 sites selected in the secondary non-urban road network of L’Aquila, a medium-size urban area of Central Italy. Stress induced by traffic load generally affects a road section thickness of about 1.5 m. A monostatic 1600 MHz GPR antenna was used because it is quite high-resolution when inspecting road damage. GPR acquisition was carried out in damaged and adjoining undamaged road sites, to compare GPR data of the two areas. GPR data analysis was based on the sweep-rectified power approach to evaluate the radar signal attenuation curve vs. depth, which permitted to single out different road types of damage and to discuss the factors which caused them.

The contribution of geophysical prospecting to the multidisciplinary study of the Sarno Baths, Pompeii

Abstract This paper focuses on the contribution of geophysical measurements to the study of the Sarno Baths in Pompeii in the context of the MACH project funded by the University of Padua. Between 2016 and 2017, a series of geophysical measurements were carried out inside and outside the Sarno Baths in order to support the multidisciplinary study of this archaeological site. High-frequency ground penetrating radar (GPR) antennas have been applied to the characterization of horizontal and vertical structural elements (vaults and walls) of the building, where GPR acquisitions and electrical resistivity tomography (ERT) were used in the open field to identifying possible archaeological features. In general, the results made by these geophysical measurements provide new information supporting both structural analysis and preliminary archaeological excavation conducted in this area.

Integrated Electrical Resistivity Tomography and Ground Penetrating Radar Measurements Applied to Tomb Detection

The integrated use of electrical resistivity tomography (ERT) and ground penetrating radar (GPR) measurements, and in particular the joint analysis of 2D and 3D data, can represent a valid solution for target identification at complex archaeological sites. A good example, in this respect, is given by the case study of a Phoenician–Punic necropolis in the archaeological site of Nora, in southern Sardinia (Italy), where GPR and ERT measurements were collected before site excavation. In this specific case, the mix of soil and air in the buried chambers, as well as the orientation and the complex spatial distribution of these structures into the sandstone bedrock, generated a number of anomalies difficult to interpret only using 2D results. Only the integration of all GPR and ERT data in a 3D view, and the comparison with archaeological evidence after the excavation, allowed a solid interpretation of geophysical anomalies visible in the 2D sections. Overall, this case study demonstrates the efficiency of the combined use of GPR and ERT acquisitions and shows how, in general, only the joint analysis of 2D data and in a 3D view can help the interpretation of the real distribution of the buried archaeological remains at similar archaeological complex sites.

Measuring Soil Water Content with Ground Penetrating Radar: A Decade of Progress

Core Ideas There has been tremendous progress in GPR as a tool for soil water content determination. Numerous studies have shown the potential of GPR to detect and map SWC. We highlight new possibilities and achievements for GPR acquisition and processing strategies. Quantitative SWC detection and hydrological parameter estimation are possible using GPR. We encourages other communities to embrace GPR as a tool for SWC determination. Tremendous progress has been made with respect to ground penetrating radar (GPR) equipment, data acquisition, and processing since the establishment of GPR as a tool for soil water content determination in vadose zone hydrology about 25 yr ago. In this update, we aim to provide a critical overview of recent advances in vadose zone applications of GPR with a particular focus on new possibilities for multi‐offset and borehole GPR measurements, the development of quantitative off‐ground GPR methods, full‐waveform inversion of GPR measurements, the potential of time‐lapse GPR measurements for process investigations and hydrological parameter estimation, and recent improvements in GPR instrumentation. We hope that this update encourages the soil hydrology, groundwater, and critical zone community to embrace GPR as a viable tool for soil water content determination and the elucidation of subsurface hydrological processes.

Non-invasive estimation of moisture content in tuff bricks by GPR

Measuring water content in buildings of historical value requires non-invasive techniques to avoid the damage that sample taking or probe insertion may cause to the investigated walls. With this aim, a stepped frequency ground penetrating radar (GPR) system was tested to assess its applicability in moisture measurements of porous masonry elements. The technique was tested on a real scale wall made with yellow Neapolitan tuff bricks, a material commonly found in historical buildings of Campania (Southern Italy). First, the antenna was calibrated to find its characteristic transfer functions. Then 64 GPR acquisitions, coupled with gravimetric measurements of the volumetric water content, were performed on the tuff wall in laboratory controlled conditions. A full inverse modelling of the GPR signal on tuff was used to retrieve dielectric permittivity and electrical conductivity of tuff at various water contents. By linking these characteristic electromagnetic parameters to the water content, the calibration relationships specific for yellow Neapolitan tuff are defined, which can be used for moisture measurements by GPR in real case studies. The experimental results lead to a robust identification of clearly defined monotonic relationships for dielectric permittivity and electrical conductivity. These are characterized by high values of the correlation coefficient, indicating that both parameters are potentially good proxies for water content of tuff. The results indicate that GPR represents a promising indirect technique for reliable measurements of water content in tuff walls and, potentially, in other porous building materials.

Hydrogeophysical characterization of transport processes in fractured rock by combining push‐pull and single‐hole ground penetrating radar experiments

AbstractThe in situ characterization of transport processes in fractured media is particularly challenging due to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, such as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We attempted to reduce this uncertainty by coupling push‐pull tracer experiments with single‐hole ground penetrating radar (GPR) time‐lapse imaging. The experiments involved different injection fractures, chaser volumes and resting times, and were performed at the fractured rock research site of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions, we used both fixed and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixed‐antenna experiment, time‐varying GPR reflections allowed us to track the spatial and temporal dynamics of the tracer during the push‐pull experiment. During the moving antenna experiments, we clearly imaged the dominant fractures in which tracer transport took place, fractures in which the tracer was trapped for longer time periods, and the spatial extent of the tracer distribution (up to 8 m) at different times. This demonstrated the existence of strongly channelized flow in the first few meters and radial flow at greater distances. By varying the resting time of a given experiment, we identified regions affected by density‐driven and ambient flow. These experiments open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport phenomena in fractured rock formations.

Data acquisition and processing parameters for concrete bridge deck condition assessment using ground-coupled ground penetrating radar: Some considerations

Ground penetrating radar (GPR) is a non-destructive geophysical technique that is widely used to determine the relative condition of reinforced concrete. This paper presents case studies from Missouri, USA, where a ground-coupled GPR system was used to assess the condition of eleven concrete bridge decks. The main goal of this paper is to develop appropriate acquisition and processing parameters in order to conduct rapid, efficient, and cost-effective assessment of bridge decks. To accomplish this goal, the GPR data sets were collected with slightly different acquisition parameters and processed using different parameters. The quality of the results and the time required for each bridge deck survey are analyzed. Additionally, several experimental data sets were collected across a 12th concrete bridge deck to examine the influence of weather conditions on reflection amplitude values, since amplitude analysis is used in this study. Based on the authors' experience and findings, appropriate GPR acquisition and processing parameters are suggested and described for use of the ground-coupled GPR method for bridge deck assessment.

Characterization of the porosity distribution in the upper part of the karst Biscayne aquifer using common offset ground penetrating radar, Everglades National Park, Florida

• We use common offset GPR to rapidly characterize porosity in karst limestone. • We demonstrate the ability of GPR to characterize subsurface heterogeneities. • The CRIM petrophysical model is used to build field scale GPR-based porosity models. The karst Biscayne aquifer is characterized by a heterogeneous spatial arrangement of porosity and hydraulic conductivity, making conceptualization difficult. The Biscayne aquifer is the primary source of drinking water for millions of people in south Florida; thus, information concerning the distribution of karst features that concentrate the groundwater flow and affect contaminant transport is critical. The principal purpose of the study was to investigate the ability of two-dimensional ground penetrating radar (GPR) to rapidly characterize porosity variability in the karst Biscayne aquifer in south Florida. An 800-m-long GPR transect of a previously investigated area at the Long Pine Key Nature Trail in Everglades National Park, collected in fast acquisition common offset mode, shows hundreds of diffraction hyperbolae. The distribution of diffraction hyperbolae was used to estimate electromagnetic (EM) wave velocity at each diffraction location and to assess both horizontal and vertical changes in velocity within the transect. A petrophysical model (complex refractive index model or CRIM) was used to estimate total bulk porosity. A set of common midpoint surveys at selected locations distributed along the common-offset transect also were collected for comparison with the common offsets and were used to constrain one-dimensional (1-D) distributions of porosity with depth. Porosity values for the saturated Miami Limestone ranged between 25% and 41% for common offset GPR surveys, and between 23% and 39% for common midpoint GPR surveys. Laboratory measurements of porosity in five whole-core samples from the saturated part of the aquifer in the study area ranged between 7.1% and 41.8%. GPR estimates of porosity were found to be valid only under saturated conditions; other limitations are related to the vertical resolution of the GPR signal and the volume of the material considered by the measurement methodology. Overall, good correspondence between GPR estimates and the direct porosity values from the whole-core samples confirms the ability of GPR common offset surveys to provide rapid characterization of porosity variability in the Biscayne aquifer. The common offset survey method has several advantages: (1) improved time efficiency in comparison to other GPR acquisition modes such as common midpoints; and (2) enhanced lateral continuity of porosity estimates, particularly when compared to porosity measurements on 1-D samples such as rock cores. The results also support the presence of areas of low EM wave velocity or high porosity under saturated conditions, causing velocity pull-down areas and apparent sag features in the reflection record. This study shows that GPR can be a useful tool for improving understanding of the petrophysical properties of highly heterogeneous systems such as karst aquifers, and thus may assist with the development of more accurate groundwater flow models, such as those used for restoration efforts in the Everglades.

Velocity analysis from common offset GPR data inversion: theory and application to synthetic and real data

We implemented a procedure to estimate the electromagnetic (EM) velocity using common offset ground penetrating radar (GPR) data. The technique is based on the inversion of reflection amplitudes to compute the series of reflection coefficients used to estimate the velocity in each interpreted layer. The proposed method recursively calculates the incident angles at any interface, taking into account the offset between antennas, and needs as input, in addition to the picked amplitudes values, a reference amplitude for each analysed GPR trace and a velocity value for the first (shallowest) layer. The latter two parameters can be estimated directly from the available data or can be better constrained by further dedicated GPR acquisitions or by additional direct measurements. We critically evaluated the performances for both synthetic and real data acquired with different antenna frequencies and we demonstrated that the new method can be applied in several real situations. Despite the necessary approximations and simplifying hypotheses, the velocity values calculated for each layer are consistent with direct information and with cross-validations obtained considering profiles acquired using different antennas and various path directions. Tests of the method on synthetic and real data sets show that the errors in the calculated velocity fields are quite low and comparable with more demanding velocity analysis techniques. The obtained EM velocity field is crucial in many processing steps, such as, for example, true amplitude recovery, depth conversion and imaging, and provide essential information to characterize the subsurface materials.

Ground penetrating radar: 2-D and 3-D subsurface imaging of a coastal barrier spit, Long Beach, WA, USA

The ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar (GPR). The GPR method, which is based on the propagation and reflection of pulsed high frequency electromagnetic energy, provides high resolution (cm to m scale) and shallow subsurface (0–60 m), near continuous profiles of many coarser-grained deposits (sediments of low electrical conductivity). This paper presents 2-D and 3-D GPR results from an experiment on a regressive modern barrier spit at Willapa Bay, WA, USA. The medium-grained sand spit is 38 km long, up to 2–3.5 km wide, and is influenced by a 3.7-m tidal range (spring) as well as high energy longshore transport and high wave energy depositional processes. The spit has a freshwater aquifer recharged by rainfall.The GPR acquisition system used for the test was a portable, digital pulseEKKO™ system with antennae frequency ranging from 25 to 200 MHz and transmitter voltages ranging from 400 to 1000 V. Step sizes and antennae separation varied depending on the test requirements. In addition, 100-MHz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data (50×50 m sampled every meter). The 2-D digital profiles were processed and plotted using pulseEKKO™ software. The 3-D datasets, after initial processing, were entered into a LANDMARK™ workstation that allowed for unique 3-D perspectives of the subsurface. To provide depth, near-surface velocity measurements were calculated from common midpoint (CMP) surveys.Results from the present study demonstrate higher resolution from the 200-MHz antennae for the top 5–6 m, whereas the 25- and 50-MHz antennae show deeper penetration to >10 m. For the study site, 100-MHz antennae provided acceptable resolution, continuity of reflections, and penetration. The dip profiles show a shingle-like accretionary depositional pattern, whereas strike profiles show a horizontal and subhorizontal, nearly continuous reflection pattern. Results from the GPR experiment reveal upper shoreface reflections with dip towards the ocean at about 1–2°. The loss of signal from below a depth of 6–8 m indicates a lithofacies change because of the storm wave base.The parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy, continuity, and depth of penetration. The cross-polarization test exhibits reduced continuity of reflections and less depth of penetration, but dipping reflections are apparent.The grid pattern data provided a detailed view of 3-D geometry of individual reflections. High quality data were obtained, processed, and directly exported into a LANDMARK™ workstation for interpretation. The resulting interpretations of the upper shoreface beds from the test cube (50×50 m; total 2600 traces) are shown as vertical sections (slices), horizontal sections (time slices), contour maps, 3-D representations of individual beds, and an isopach map. The 3-D depositional framework allows a more detailed interpretation than widely spaced 2-D profiles.