Ground Penetrating Radar (GPR)
Technique
Ground Penetrating Radar (GPR)
Goal(s)
Main objective
GPR is a geophysical method that allows for the analysis of the propagation capacity of electromagnetic waves (very short electromagnetic pulse (1–20 ns) in the frequency band of 10 MHz–2.5 GHz) through media with different dielectric constants. A transmitting antenna emits an electromagnetic signal into the ground, which is partly reflected at the interface between two different media with sufficient dielectric contrast and partly transmitted into deeper layers. Then, the reflections produced are recorded from the receiving antenna, which is either in a separate antenna box or in the same antenna box as the transmitter. The strength (amplitude) of the reflected fields is proportional to the change in the magnitude of the dielectric constant. If the time required to propagate to a reflector and back is measured, and the velocity of the signal propagation in the medium is known, the depth of the reflector can therefore be determined.
A detailed description of the methodology along with a deep theoretical background can be found in [1], [2], [3].
Description
Functioning mode
GPR applications in structures and infrastructures depend highly on the characteristics of the studied elements and on the problems that must be studied. Radar data can be acquired with a single antenna or with an array of antennas moved on the surface of the medium. The first case is usual in narrow and small zones or in inaccessible areas for large equipment (walls, columns, etc.), while the second one is used in wide areas without obstacles (during road/pavement inspections). GPR data is also used in boreholes with antennas designed for this purpose. The study of walls or small size constructive members is sometimes done with transillumination and radar tomography, but in the majority of the cases the results are provided as depth slides and pseudo 3D models.
Common-Offset Mode
The most prevalent data acquisition methodology is Common-Offset (CO), also called single-fold or single-offset. This is the common reflection mode, the simplest and fastest way of acquiring data with a GPR system. Using the CO mode, one or two antennas are moved by keeping constant the distance between transmitter and receiver. Generally, survey lines should be designed to run perpendicular to the preferred strike direction of the features under investigation. Regarding the polarization of the antennas, they should be Bow-tie and Vivaldi antennas with linear polarization (most commonly perpendicular to the direction of data collection).
3D Common-Offset Mode
In some GPR surveys, particularly in wide areas, the CO procedure is repeated at regular intervals and for a number of survey lines, which are usually located parallel to each other. Using 3D CO, more realistic images of the underground space are provided, which allows not only for the location, but also for the reconstruction of buried structures. These 3D images (or time-slices) provide accurate and intuitive display of the underground distribution, and appropriate spatial correlations between reflectors in depth.
Common-Mid-Point Mode
In the CMP configuration, also known Multi-Offset, multifold or wide-aperture, the separation between transmitter and receiver increases at each survey location. Both the transmitting and receiving antennas are moved apart about a common fixed center point along the survey line at a constant step. The antenna spacing is varied at a fixed location and the change of the two-way travel time of the electromagnetic wave from the reflectors is measured. The CMP mode can be used to calculate the velocity of propagation.
Transillumination Mode
Another possible survey mode is the transillumination (more commonly known as borehole radar) used for borehole and tomography studies. Employing this mode, the transmitting antenna is kept stable at a fixed location while the receiving antenna is moved along a survey line in constant step intervals. While in the CO mode the reflected signal is received, the CDP mode receives the direct wave produced.
Types
There are several GPR manufacturers and commercial equipment available, and some experimental prototypes also exist. Different GPR systems will have different capabilities according to the type of antennas and their frequency, which affect the operating speed, the resolution, the penetration and the sampling rate [4], [5]. The frequency and depth of penetration are related, with higher frequency pulses achieving lower penetration, but better resolution. Impulse GPR systems are the most widely used, with two main groups of GPR antennas, dipole and horn antennas, and with frequencies nowadays ranging from 10 MHz to 6 GHz. Currently, the most commonly used technology is the time-domain impulse radar. Additionally, several investigations point to the step-frequency radar as a potential technology for broad resolution range, although these systems do not allow real-time visualization of data during acquisition. UWB
GPR with horn antennas
GPR horn antennas were specifically designed for use in transport infrastructures evaluation, since they can operate at traffic speed. In the last ten years, this type of equipment has evolved from prototype status to routine use in pavement evaluation studies.
Horn antennas have frequencies ranging from 1 to 2.5 GHz, corresponding to penetration depths in the order of 1 m to 0.4 m, respectively. The minimum layer thickness that can be detected is about 50 mm, for 1 GHz antennas and 25 mm, for the higher frequencies.
The antennas are “air-coupled”, and normally they work mounted on a mobile vehicle and are suspended at a certain distance from the surface, typically ranging from 0.4 to 0.6 m. They perform measurements at traffic speeds (up to 80–120 km/h) without any interference with traffic, and therefore they are suitable for the evaluation of in-service pavements and bridge decks without major disturbance to road users.
GPR with dipole antennas
Dipole antennas were primarily developed for use in geological survey, normally ground-coupled. They have frequencies ranging between 10 MHz and 6 GHz. For transport infrastructure applications, the best results are obtained with antennas from 400 MHz to 2.5 GHz central frequency. In general, the higher the frequency, the lower the penetration depth and the higher the resolution is. For example, 1.5 GHz dipole antennas will give a penetration depth of 0.50 m, while the 400 MHz will give a penetration of 2.00 m.
Dipole antennas were mainly developed for use in contact with the surface, or suspended just above it (2–5 cm), and they are suitable for testing at maximum speeds of 20–30 km/h. In this condition, the radar signal is “ground-coupled”. Ground coupling introduces a stronger signal into the pavement, and therefore these antennas are normally employed for detailed studies over limited areas, as they allow one to obtain higher resolution.
GPR with antennas array multi-channel
GPR array multi-channel systems consist of a large number of closely-spaced antennas recording at the same time. Different multi-channel prototypes have recently been provided with different configurations, and they can include both air- and ground-coupled antennas. Commonly, such multi-static systems are composed of 4–16 couples of transmitting and receiving channels mounted in a parallel broadside configuration with a cross-line trace spacing of 4–12 cm, depending on the manufacturer. The main advantage is that they enable faster data collection by increasing the extension of the investigated area per time unit, and they make it easier for the operator to produce 3D images.
Generally, in transport infrastructure inspection, the antennas are mounted on a mobile vehicle to minimize traffic disruption. Mobile GPR is positioned connected to an external real-time kinematic (RTK) global navigation satellite system (GNSS) for trace tagging (georeferenced data) or to a distance measurement indicator (DMI) to control the distance trace-interval and to measure the travelled distance. The system also uses a computer navigation guided system to correctly follow profile direction and keep a constant overlap among parallel profiles without any physical marker on the ground surface.
Bore-hole antennas
Bore-hole antenna systems are often applied to GPR investigations of deeper formations. The applications of such systems include fracture characterization, foundation investigations, cavity and crack detection, as well as tunneling inspection. Measurement methods include single-hole reflection mode, crosshole mode (tomography) and surface-to-borehole mode. Available bore-hole antenna frequencies are 100 MHz, 150 MHz, 250MHz and 300 MHz, and the system can perform surveys deeper than 30 m, with special BH equipment and accessories capable of measuring as deep as 2500 m. The area of investigation has a radius of about 10–100m depending on the dielectric properties of the propagation media.
Process/event to be detected or monitored
The detection of discontinuities (changes in the dielectric properties of media) in subsoil/subsurface. Particularly for the diagnosis of bridges and tunnels, the GPR method has been demonstrated as effective for the detection of the following characteristics:
Masonry arch bridges
- Unknown geometries remaining in the interior of the bridge such as hidden arches and ancient profiles (shape) of the structure.
- Evidences of restorations and/or reconstructions in stonework.
- Existence of cavities and fractures/cracking in masonry.
- Moisture in masonry.
- Bridge foundations.
- Filling distribution in masonry.
- Thickness of ashlars (pavement, ring arch, spandrel walls, etc.).
Concrete bridges
- Estimation of concrete cover depth.
- Mapping reinforcing bars (deck and beams).
- Location of cable ducts and other utilities such as deck joints or drain grate.
- Damage detection on concrete (corrosion, cracking, spall, delamination, etc.).
- Moisture detection and water content estimation.
Tunnels
- Thickness of concrete segment/lining.
- Thickness of the backfill grouting layer.
- Damages in concrete lining and grouting layer.
- Damages (e.g., cracks/fissures, fractures and voids) behind tunnel linings.
- Moisture/water content.
- Depth and location of reinforcement (rebar).
- Inspection of other reinforced concrete structures (e.g., steel arch and shotcrete layer).
- Location of immersion joints.
- Identification of depth and presence of insulation material.
More information about the application of the GPR method on transport infrastructures inspection can be found in [6], [7], [8].
Physical quantity to be measured
(e.g. actions, displacements, deformations, dynamic structural properties, material properties including mechanical, electrical and chemical properties, relative displacements of the two sides of a crack, etc.).
The method is based on measuring the time arrival and amplitude of the reflected waves. The magnitude of the measured amplitudes and the propagation time depend on the dielectric properties of the medium materials that can be inferred analysing the arrival time, the amplitude and the frequency of the received signals. The amplitude value is the strength or intensity of the reflected fields that is proportional to the magnitude of the dielectric contrast between adjacent media. But the amplitude of the received signals depends on different factors apart from the strength of the field reflected on the discontinuity: (i) distance between the target and the antenna and (ii) dielectric properties of the medium; hence, these factors determines the geometrical spreading and the attenuation of the radar waves.
The vertical axis of a radargram (or raw data) indicates the travel-time in which the propagated wave reflects the target or discontinuity. Knowing the exact medium, dielectric properties and wave velocity of propagation, these travel-time scales can be converted into depths, which allows to estimate thicknesses in a quantitative manner.
The horizontal axis indicates the length or distance in the profile line, which gives an estimation of the spatial target dimensions. A GPR system typically operates with an odometer wheel attached to control the trace interval distance and the length of the profile line.
The interpretation of the GPR imaging is basically based on qualitative aspects and reflection patter recognition (geometry, signal polarity, ...).
Induced damage to the structure during the measurement
Nondestructive Testing
General characteristics
Measurement type
(static or dynamic, local or global, short-term or continuous, etc.)
The GPR data acquisition with air-launched antennas allows for a dynamic measurement at traffic speeds (up to 80–120 km/h). The measurement is continuous (each a certain distance or scan/m interval) through the GPR profile line. This configuration is most typically used for pavement and bridge deck monitoring (e.g. thicknesses, cracking, rebar, etc.).
The GPR data acquisition with ground-coupled antennas allows for both a dynamic or a static measurement (up to 60–80 Km/h). The measurement is continuous (each a certain distance or time interval) through the GPR profile line. This configuration can be used for pavement and bridge deck monitoring (e.g. thicknesses, cracking, rebar, etc.), as well as for more local or punctual surveying such as piers. The ground-coupled antennas provide deeper range of penetration and better resolution (better signal to noise ratio) when compared with air-coupled antennas.
Finally, the GPR data acquisition with ground-coupled array systems allows for both a dynamic measurement (up to 15 Km/h). This configuration is most commonly focused on pavement and bridge deck monitoring (e.g. 3D mapping of rebar, delamination/corrosion maps, etc.).
Measurement range
The higher the frequency, the lower the penetration depth and the higher the resolution is. Resolution is commonly referred to by its horizontal (or lateral) and vertical (longitudinal or range) resolution. The vertical resolution allows the differentiation of two adjacent signals in the time axis (or in depth) like different events, while the horizontal resolution indicates the minimum distance between two adjacent targets at the same depth to be detected as separate events. Nevertheless, the measurement range provided by the GPR is somewhat like uncertainty because these parameters are highly dependent on the bandwidth used and, thus, on the dominant frequency of the antenna with determines pulse duration, as well as on the wave velocity of propagation (dielectric permittivity of media) and signal attenuation.
The 500 MHz antennas have limited ground penetration, but they give a very high resolution map of the subsurface in the first 2–3 meters. Below this depth, lower frequency antennas work better (with signal penetration up to 10-15 m), but those with a central frequency below 100 MHz may have insufficient vertical resolution. Frequencies in the order of GHz are suited for very shallow studies (< 1m), and they are an especially effective tool for structure inspection: detection of cracks, estimation of wall thickness, moisture content inside structures, rebar detection, etc. Regarding to horizontal resolution, the HF band provides data with a range resolution from 0.5 to 0.1 m, while the LF band is limited to a resolution ranging from 0.5 m up to 2.5 m. On the other hand, the vertical resolution ranges between 4 cm and 1 cm for HF band, and between 5 cm and 0.5 m for LF band.
Measurement accuracy
Not applicable.
The optimal depth/thickness estimation from GPR data requires a correct calibration process by determining accurate dielectric permittivity and wave velocity of propagation in media. In some cases, for both air-launched and ground-coupled systems, calibration consists of comparing GPR data with cores. In other cases, and more particularly for air-launched antennas, calibration consists of comparing the amplitude of the reflected wave with the amplitude of the wave reflected on a metallic plate. Moreover, the dielectric permittivity of the media can be also determined with the common-mid-point (CMP) operating mode.
GPR manufactures mainly focuses on system accuracy, but measurements of unknown quantities should be accompanied by the uncertainty assessment [9], [10], [11].
Background (evolution through the years)
The first application of radio waves in detecting buried targets was proposed in two German patents by Leimbach and Lowy in 1910 [12], [13], as a methodology for shallow geological surveys. One of the patents was based on using the analysis of signals amplitude transmitted and received by dipole antennas placed in boreholes. The other patent described the use of surface antennas to detect underground ore and water deposits. This application was based on the previous German patent of the ‘telemobiloskop’ by Huelsmeyer in 1904, which allowed the detection of distant metallic objects by using electromagnetic waves. The method was improved in 1926 with the development of pulsed systems [14], based on the detection of reflections occurring in buried targets, allowing best depth resolution. The technique was developed during the following decades; the first applications being used to determine the thickness of a glacier in 1930. However, the method did not seem to generate enough interest during this first period, and commercially available devices appeared during the 1970s. This pioneer equipment was firstly used to analyse the radio waves’ penetration in ice and in different rocks and soil materials. Some of the first GPR studies in civil engineering appeared in mid 1970s, and were devoted to the detection of hidden utilities. Simultaneously, some first applications in pavements were also described. From these first works, the number of applications and developments of the methodology increased remarkably. GPR became a useful tool in the analysis of many structures and infrastructures, and nowadays it is successfully applied in a large number of cases: road and pavement analysis, detection of voids and cavities, study of bridges and tunnels, assessment of actual buildings and cultural heritage, archaeological surveys, forensics and mining detection, geotechnical evaluations and water management analysis.
In the last 10 years, thanks to different multi-channel prototypes and complex data analysis, manufacturers have provided with robust full-resolution GPR imaging solutions where the antenna responses of individual elements are much closer resulting in streamlined methodologies and consistent results [15], [16]. GPR array multi-channel systems consisting of a large number of closely-spaced antennas represented a new evolution for 3D mapping. These multi-channel systems speed up data acquisition and allow full wave-field recording at the same time, thus increasing the extension of the investigated area.
More recent research is focused on the design of innovative devices that facilitates surveying in complex or inaccessible structures such as columns, walls and roofs (e.g. a device consisting of a support for the antenna in rails moved with an electric motor [17]). In this way, in some cases, the use of drones has been also proposed and tested [18], [19], although further progress could be focused on the development of antennas and UAV (unmanned aerial vehicle) systems for surveying structures or parts of the structures with poor accessibility and large structures. With respect to tunnelling inspection, new GPR devices are been developed for higher speed data collection and complete cross section scanning towards fully automated tunnel inspection [20], [21], [22].
Performance
General points of attention and requirements
- Design criteria and requirements for the design of the survey
- Procedures for defining layout of the survey
- Design constraints (e.g. related to the measurement principles of the monitoring technologies)
It should be noted that there are technical standards regulating the electromagnetic emissions of GPR equipment. The following are the main standards in Europe, USA and Canada: European Telecommunications Standard Institute (ETSI) EN 203 066-1, ETSI EN 203 066-2,
ETSI EN 203 489-32 and ETSI EG 202 730, USA regulations on UWB-GPR: Part 15 of Federal Communications Commission (FCC) Regulations, and Industry Canada Radio Standards Specification RSS-220 (Issue 1) “Devices Using Ultra-Wideband (UWB) Technology.
Preparation
Procedures for calibration, initialisation, and post-installation verification
In Europe, there are no specific common guidelines about the application of GPR in pavement surveys, even though some proposals are developed in different countries [23]. Examples of those guidelines are in the Mara Nord Project [24] and in the British [25], [26] and Belgian [27] regulations. Many authors declare that an optimal surveying requires a GPR calibration process. The studies demonstrate that the correct calibration causes an extreme decrease in the error on the estimation of thickness measurements.
The guide ASTM D4748-98 [28] presents the procedures for the inspection of the upper layers of both bituminous and concrete pavements, using a short-pulse GPR. The methods included in this international standard are focused on the thickness evaluation of pavement layers. The guide includes technical topics such as calibration and standardization, procedures, calculation and reliability of the results.
The COST Action TU1208 has published some tests and recommendations for suitable GPR system performance compliance, obtained by scientists from Belgium (Belgium Road Research Center), Czech Republic (University of Pardubice), Portugal (National Laboratory for Civil Engineering), and Serbia (Faculty of Technical Sciences, Novi Sad) [29]. The D6087-08 standard, emitted by the ASTM [30], describes four procedures for the calibration of GPR systems equipped with air-coupled antennas. After a critical analysis of those procedures, four improved tests were proposed in [29], which can be carried out to evaluate the signal-to-noise ratio, short-term stability, linearity in the time axis, and long-term stability of the GPR signal.
Procedures for estimating the component of measurement uncertainty resulting from calibration of the data acquisition system (calibration uncertainty)
There is no standardization or guidelines regarding measurement (calibration) uncertainty. Only very few scientific papers in the literature have approached the uncertainty issue in GPR data [9], [10], [11], [31].
Requirements for data acquisition depending on measured physical quantity (e.g. based on the variation rate)
Performance
Requirements and recommendations for maintenance during operation (in case of continuous maintenance)
Not applicable.
Criteria for the successive surveying campaigns for updating the sensors
(The campaigns include: (i) Georeferenced frame, i.e. the global location on the bridge; (ii) Alignment of sensor data, relative alignment of the data collected in a surveying; (iii) Multi-temporal registration to previous campaigns; and (iv) Diagnostics.) Georeferenced grid or profile lines (at least the starting corner or initial point, respectively) in case of multi-temporal registration.
Reporting
Lifespan of the technology and required maintenance (if applied for continuous monitoring)
Not applicable. The GPR method does not offer continuous (long-term) monitoring.
Interpretation and validation of results
Expected output (Format, e.g. numbers in a .txt file)
Interpretation (e.g. each number of the file symbolizes the acceleration of a degree of freedom in the bridge)
Each number in the GPR binary data file represents an amplitude value. It consists of a matrix, composed by many sequentially stacked traces in the horizontal dimensions and samples in the vertical dimensions (trace data recorded sample by sample), which display individual amplitudes for each position in the matrix. Thus, the x axis represents the distance (number of trace) on the profile line, and the z axis represents the distance-time (number of sample) in which the reflection occurs.
Validation
- Specific methods used for validation of results depending on the technique
- Quantification of the error
- Quantitative or qualitative evaluation
The combination of GPR with complementary geophysical techniques is highly recommended to validate interpretation. Each particular method provides different information owing to the physical properties of the construction material, which allows for a more detailed investigation in the diagnosis of structures. In the case of reinforced concrete structures, GPR is widely complemented with seismic and ultrasonic pulse-echo technology, but also with profoscope (rebar detector), half-cell potential (corrosion analysis) and hammer sounding resistivity (concrete strength properties). Regarding the validation of quantitative measurements (thicknesses), the calibration and error estimation consists of comparing GPR data with cores.
Detection accuracy
Not applicable.
Detecting boundary between layers or discontinuities requires enough dielectric permittivity contrast between adjacent media. When the dielectric contrast is enough between two adjacent media to ensure detection, the other parameter controlling the detection accuracy is the spatial resolution and zone of influence around the propagation path of the GPR signal. In general, the higher the frequency, the lower the penetration depth and the higher the resolution is.
Resolution is commonly referred to by its horizontal (or lateral) and vertical (longitudinal or range) resolution. The vertical resolution, which allows the differentiation of two adjacent signals in the time axis (or in depth) like different events, mainly depends on the bandwidth used and, therefore, on the dominant frequency of the antenna which determines pulse duration (quarter of the wavelength criteria). On the other hand, horizontal resolution indicates the minimum distance between two adjacent targets at the same depth to be detected as separate events. This parameter mainly depends on the number of traces recorded that is directly adjusted before data acquisition, the bandwidth, and the depth of the reflector.
In general, low frequencies (50 – 200 MHz) are capable to reach a penetrating depth ranging from 10 to 3 m. The high frequency band (500 – 2000 GHz) provides a penetrating depth ranging from 3 to 0.5 m. Regarding to horizontal resolution, the HF band provides data with a range resolution from 0.5 to 0.1 m, while the LF band is limited to a resolution ranging from 0.5 m up to 2.5 m. The vertical resolution ranges between 4 cm and 1 cm for HF band, and between 5 cm and 0.5 m for LF band.
Data Transfer Rate
Constraints on Data Transmission Capabilities in terms of bandwidth and security depending on the amount of data obtained in the surveying campaigns, and the need for providing a secure transmission to the storage (Cloud)
Advantages
The main advantages of the GPR method are:
- Non-destructive and non-invasive method.
- Easy transportation.
- Fast data acquisition compared with other geophysical methods.
- Imaging of the subsoil with high resolution.
- Precise vertical and horizontal positioning.
- Results are displayed in real time.
Disadvantages
- Detecting boundary between layers or discontinuities requires enough dielectric permittivity contrast between adjacent media.
- The interpretation of radargrams is generally non-intuitive and requires considerable expertise to properly process and understand the measurements.
- Uncertainty/inaccuracy in depth/thickness estimation (pre-calibrated media characterization and accurate wave velocity of propagation are needed).
Barriers
The main limitations occur in the presence of high-conductivity materials (such as wet clay) and in heterogeneous conditions causing signal attenuation and complex scattering phenomena, respectively. The following limitations and drawbacks can occur during concrete bridge inspections:
- The steel/metal is a quasi-perfect reflector of the radar-waves, which facilitates the rebar detection, but deeper targets can be masked if it is a tight mesh. In this regard, collecting data in both polarizations (with the dipoles perpendicular and parallel to data collection direction) has benefits because the reflections produced by metallic targets perpendicular to the data collection direction are weakly seen in data collected with dipoles parallel to the scanning direction, so that other potential targets below them can be more easily detected.
- Overlapping reflections are usually observed between consecutive bars that is dependent on the spatial resolution of the antenna (higher frequencies provide a higher resolution), which may lead to the misinterpretation of closest or smaller-diameter bars. Resolution problems can also occur in sections with a too small concrete cover when measuring pavement thickness. The selection of the most appropriate central frequency of the antenna is therefore crucial herein.
- Another important parameter affecting the location accuracy is the horizontal sampling. Dense horizontal sampling (scan spacing), gives more accuracy on positioning and good quality data for further amplitude analysis, although its main consequence is a decrease in the survey speed. Conversely, using a low sampling rate may limit data visibility in the field and cause inaccurate adjustments of hyperbolas peaks when processing.
- The 3D data acquisition (especially with single GPR antennas) may incur an incorrect distance encoder calibration, position of the antenna on starting/ending grid lines, improperly configured survey grid, georeferencing, etc. The use of antenna arrays or automatic scanner systems makes the acquisition of 3D data easier and thus encourages a wider use of 3D techniques although, in practice, these systems usually have an excessive cost. For example, a higher sampling rate (higher number of traces recorded) decreases a survey speed, which could result in higher survey costs and, more importantly, could interrupt the traffic flow.
- The amplitude value is highly dependent on various factors such as different depth to the top mat of the reinforcing steel, weather conditions (e.g., humidity) during data acquisition, and concrete properties (e.g., density, porosity, etc.), which makes the detection of corrosion and delamination only difficult from the analysis of amplitude maps. Confirmation with the additional analysis of other signal attributes (e.g., signal attenuation, signal-to-noise ratio and velocity of propagation) or complementary NDT should be part of the process whenever possible.
With respect to tunnelling inspection, the following limitations and drawbacks can occur:
- Generally, tunnels are inspected manually, by maneuvering the GPR antenna over the surface of the tunnel, with single ground-coupled antennas using the mode of continuous acquisition, which is very slow and inefficient. In the process of tunnel inspection, longitudinal survey lines are most commonly arranged, with a range of three to six serial lines located at the vault, the left hance, the right hance, the left sidewall, the right sidewall and the inflected arch.
- The connection with external GPS devices for data referencing is obviously limited when surveying the interior of a tunnel structure. Therefore, wheel encoders are used during data acquisition to ensure the accuracy of ranging (trace interval) and location. So far, the following practical problems that typically occur are: (i) loss of contact between the antenna and the surface (the operator should ensure both the antenna and the survey wheel are in contact with the surface, while keeping a constant (uniform speed) and continuous rotation); (ii) deviation of the antenna with respect to the radar line (the operator should ensure the antenna position is consistent with the location of the acquisition line); and (iii) the presence of cables and conduits on the walls of the tunnel that makes it impossible to collect data in those areas.
- There are so many noisy signals from utilities (e.g., power cables) or metal (reinforcement) in the tunnel. Shotcrete-containing steel fibers cannot be inspected because fibers generate random electromagnetic scattering.
Existing standards
The American standard ASTM D6087-08 “Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Deck Using Ground Penetrating Radar” [30], the American SHRP 2- report S2-R06A-RR-1 “Nondestructive Testing to Identify Concrete Bridge Deck Deterioration” [32], the Mara Nord Project “Recommendations for guidelines for the use of GPR in bridge deck surveys” [33], the British technical specifications DMRB 3.1.7 “Design Manual for Roads and Bridges, Advice notes on the non-destructive testing of highway structures—advice note 3.5 BA 86/2006: Ground Penetrating Radar (GPR)” [34], the German BASt-report B55 “Examination of GPR in combination with magnetic techniques for the determination of moisture and salinity of concrete bridge decks with asphalt cover” [35], and the German document B10 “Recommendation for nondestructive testing of civil engineering structures by GPR” [36]. Additional standards including the assessment of bridge decks are: The American AASHTO R 37-04 “Standard Practice for Application of Ground Penetrating Radar (GPR) to Highways” [37], the American ACI 228.2R-98 “Nondestructive Test Methods for Evaluation of Concrete in Structures” [38], and the American NCHRP RR 848 “Inspection Guidelines for Bridge Post-Tensioning and Stay Cable Systems Using NDE Methods” [39].
Applicability
Relevant knowledge fields
- Civil Engineering.
- Rebar, linear metallic targets.
- Utilities.
- Roads and pavements.
- Railways, ballast and tunnels
- Concrete structures and buildings.
- Bridges.
- Foundations.
- Environmental investigations.
- Waste and pollution.
- Hydrology and water detection.
- Soil texture characterization.
- Root biomass.
- Ice sounding.
- Mining.
- Geology investigations.
- Fractures, faults.
- Stratigraphic studies.
- Bathymetry (freshwater).
- Dunes/sand environments.
- Volcanics.
- Archaeology and Cultural Heritage.
- Archaeology remains.
- Paleontology.
- Painting.
- Monuments.
- Statues.
- Ancient structures/buildings.
- Humanitarian assistance and forensics.
- Landmines/UXO/IED detection.
- Locating trapped people (avalanche, earthquake, etc.).
- Buried remains.
- Other emerging fields.
- Planetary exploration.
- Precision farming.
Performance Indicators
Relate the surveying technology being studied with PIs from WP3, provided in the document of the TU1406 COST ACTION.
- Cracks
- Delamination
- Spalling
- Holes
- Debonding
- Reinforcement bar / corrosion
- Loss of section
Type of structure
- Masonry bridges.
- Concrete bridges.
- Tunnels.
Spatial scales addressed (whole structure vs specific asset elements)
Air-coupled antennas were specifically developed for pavement (bridge deck) monitoring, allowing for measurements at traffic speed. Conversely, the ground-coupled antennas are most commonly used in specific parts of the bridge structure (e.g. beams, piers, etc.). Moreover, the ground-coupled array systems, are focused on 3D pavement/deck surveying.
Materials
Stone, brick, concrete/reinforced concrete, rock, wood.
Available knowledge
Reference projects
Assets4Rail “Measuring, Monitoring and Data Handling for Railway Assets; Bridges, Tunnels, Tracks and Safety Systems” EU H2020 Project (Grant Agreement ID 826250). COST Action TU1208 “Civil engineering applications of Ground Penetrating Radar”. LASTING-RTI2018-095893-B-C1. Life-Cycle Assessment of Existing Bridge Structures using Multiscale and Multisource Data. Spanish Ministry of Science and Innovation. OPAF-SIG: Observatory of masonry arch bridges. Integrated Management System (BIA2009-08012). Spanish Ministry of Science and Innovation. Geomatic techniques for the structural and dimensional analysis of historical bridges and their conservation (BIA2006-10259). Spanish Ministry of Education and Science. ESA-approved project “MOBI: Monitoring Bridges and Infrastructure Networks” (EOhops proposal 2045 (id 52479)).
Other
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