Tunnel - Ground Penetrating Radar (GPR)
Target of Investigation
Ground penetrating radar (GPR) can be used for condition assessment of tunnel linings and other concrete members of concrete, tile-lined concrete, and shotcrete tunnels.(1) For condition assessment of concrete members, GPR can be used to detect the following:
- Rebar locations.
- Moisture intrusion and corrosion-related problems.(1)
- Subsurface defects, such as tile debonding, voids, and delaminations.(1)
Description
GPR testing is a widely used nondestructive evaluation technique for detecting subsurface structural elements and anomalies in structures. GPR operates by sending discrete electromagnetic wave pulses into a structure and then capturing the reflections from layer interfaces (e.g., objects and defects) within the structure. Collected data are processed and analyzed to create a condition map of the structure or an image of the subsurface structure, including rebars and defects.
Two types of GPR systems are available: air coupled and ground coupled. Air-coupled systems have antennas that are positioned above the surface of the structure and can be mounted on vehicles for relatively fast scanning (figure 1). Ground-coupled systems have antennas that must remain in contact with the surface while collecting data (figure 2). Air-coupled GPR systems evaluate tunnels faster than ground-coupled systems. Air-coupled systems can locate defects within 1 ft (0.3 m) of their actual locations and are generally used as a scanning tool to indicate where to perform more indepth testing with other systems, such as ground-coupled radar systems.(1) Although ground-coupled systems are slower, they provide deeper penetration and higher resolution data. Groundcoupled systems can determine depth of a defect within 10 percent of its actual depth, and when reference cores are used, results are within 5 percent of a defect’s actual depth. These systems are also able to detect defects up to 4 ft (1.2 m) deep when the defects contain significant air pockets or moisture.
![This photo shows a mechanical arm installed on a moving platform. The air-coupled ground penetrating radar antenna is attached to the end of the mechanical arm and is about 1 foot (0.3 meters) in front of the tiled tunnel wall. Three workers are on the moving platform to control the mechanical arm and the platform.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-36.png)
A. Moving platform with GPR.
![This photo shows a mechanical arm and air-coupled ground penetrating radar antenna from a side view.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-37.png)
B. Boom and GPR.
Figure 1. Photos. Air-coupled GPR system mounted on a truck boom.(1)
![This photo shows a ground-coupled ground penetrating radar antenna scanning a tunnel ceiling. The antenna is a box with cable exiting the side. The antenna is supported from below by a metal frame.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-38.png)
Figure 2. Photo. Ground-coupled GPR system.(1)
A typical GPR system has the following components:
- Data collection module.
- Computer controller.
- Air- or ground-coupled antenna.
- Distance indicator.
- Survey vehicle (air coupled).
- Scanning apparatus to which the system is mounted (ground coupled) (optional).
Physical Principle
GPR operates by sending discrete electromagnetic wave pulses (with a frequency range of
100–5,000 MHz) into a structure and then capturing the reflections from layer interfaces or other reflectors within the structure. Radar obeys the laws governing reflection and transmission of electromagnetic waves and is affected by the electrical properties of the material: conductivity and the dielectric constant.(1)
Air-Coupled GPR
As shown in figure 3, the largest peak in an air-coupled GPR reading is the reflection from the surface. The amplitudes before the direct couple are internally generated noise, and they should be removed from the trace prior to signal processing. Reflections that occur after the surface echo represent significant interfaces below the surface, and the measured travel time is related to the depth of the layer or defect. Air-coupled GPRs can measure surface dielectric constants by comparing reflected signals from the liner surface and a steel plate at the same distance. The equation for calculating the dielectric constant is presented in figure 4. Normal concrete usually has a dielectric value between 8 and 12. Values above this range indicate excessive moisture; values below this range indicate significant air voids.(1)
![This illustration is a schematic showing the reflection of an air-coupled ground penetrating radar (GPR) signal from a pavement structure. The box at the top of the figure represents a GPR antenna. Below this box is a section that represents the concrete liner, and below that is another section that represents rock and earth layers. Arrows represent the propagation and reflection of the GPR signal at the interface of multiple layers. At each layer, some of the signal is reflected back toward the antenna.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-39.png)
A. GPR propagation and reflection.
![This illustration depicts waveforms of the ground penetrating radar signal. Each curve in the graph is an A-scan. The first obvious peak on each A-scan represents the direct couple. The second peak, which is the largest, represents the air–liner interface. The third peak represents the back wall reflection.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-40.png)
B. Multiple GPR A-scans.
Figure 3. Illustrations. Signal from an air-coupled GPR.(2)
![epsilon a equals the square of open bracket one plus open parenthesis A subscript 1 over A subscript m close parenthesis all over one minus open parenthesis A subscript 1 over A subscript m close parenthesis close bracket.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-41.png)
Where:
εa = dielectric of the liner surface.
A1 = amplitude of reflection from the surface.
Am = amplitude of reflection from a large metal plate on the surface.
Ground-Coupled GPR
The physical principles of ground-coupled GPR systems are similar to those of air-coupled GPR systems. Although slow compared to air-coupled systems, ground-coupled systems provide better depth penetration and a higher number of data readings. Thus, ground-coupled systems are much better suited for indepth data collection and defining subsurface interfaces or defects.
A GPR signal attenuates as it travels in a structure. Signal attenuation depends on the geometric attenuation, signal scattering, reflections, and thermal losses. Two-way travel time and reflection amplitudes are recorded with a receiver antenna. When the measurements are made over sequential survey points, they can be viewed as a GPR B-scan or profile (figure 5). The thickness of a layer within the structure can be calculated from two-way travel time. Rebars are seen as bright hyperbolas. Amplitudes of reflections from top-layer rebars are normalized with respect to concrete in good condition to plot the attenuation as a condition map. A deterioration threshold value of the attenuation can be established using ground truth (cores or other nondestructive test methods) following interpretation of the data.(3) Heterogeneities, such as voids and water, have different dielectric values and densities from the concrete material; GPR can show irregular reflections caused by these heterogeneities.
![This image has two components. The top component is a two-dimensional cross section of a concrete element. Two horizontal bars labeled “Rebar Mat” are embedded in the concrete. Above the upper rebar and below the lower rebar are circles representing rebars that run vertically. The leftmost upper circle is labeled “Corroded Rebar.” Resting on the top surface of the concrete are two rectangles representing a moving ground penetrating radar antenna. The rectangle on the left is labeled “Antenna.” An arrow points to the other rectangle, which has two components of the antenna, labeled “Transmitter” and “Receiver.” Two shaded triangular areas in the surface cross section are labeled “Conical Wave Front.” The lower component of the figure is the two-dimensional image from a B-scan. There are distinct hyperbolas in the scan showing reflections from each of the five upper rebars. The four right peaks are labeled “Top Rebar Reflection Hyperbola (Sound).” The peak on the left, corresponding to the corroded rebar, is less distinct and labeled “Top Rebar Reflection Hyperbola (Corroded).” There is also a secondary hyperbola, which corresponds to the secondary row of rebar.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-45.png)
Figure 5. Image. Attenuation of a GPR signal.(4)
Data Acquisition
Air-Coupled GPR
Air-coupled GPR manufacturers recommend following their system-specific testing procedures when collecting data. These procedures are available in the user manuals supplied by the manufacturers. For tunnel operations, the following are also recommended:(1)
- Before collecting data on a tunnel lining, personnel should collect at least 50 waveform traces on a metal plate measuring at least 16 ft2 (4.87 m2). The operating height of the antenna should be between 12 and 18 inches (0.3 and 0.45 m). The resulting data will be used to calculate the surface dielectric.
- During data collection on the tunnel lining, data should be collected with 1 ft (0.3 m) or less between survey lines.
- Within a tunnel, global positioning systems cannot be used, so other methods (e.g., survey wheels) should be used.
Ground-Coupled GPR
As with air-coupled GPR systems, manufacturers of ground-coupled GPR systems recommend following their system-specific testing procedures when collecting data. These procedures are available in the user manuals supplied by the manufacturers.
Data Processing
Data preprocessing, processing, and interpretation can be done using analysis software. Preprocessing operations, which do not change the signal content of the original data, include data channel splitting, data scaling, data reversing, and zero-level correction. Processing operations consist mainly of filtering operations and amplitude and dielectric value calculations. These operations are fully reversible and changeable. The primary objective of processing is to make GPR data more informative and easy to interpret. Software can be used to “pick” individual rebars for condition assessment analysis. Figure 6 shows raw, preprocessed, and processed data.
![This screenshot is of a B-scan of raw data. There are a few straight horizontal lines indicating direct couplings. The x-axis represents distance, and the y-axis represents depth.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-42.png)
A. Raw GPR data.
![This screenshot is of a B-scan of preprocessed data. The straight lines have been removed by deleting data with two-way travel times less than certain values. The x-axis represents distance, and the y-axis represents depth.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-44.png)
B. Preprocessed GPR data.
![This screenshot is of a B-scan of processed data. A constant signal has been removed, leaving variations in the data. The x-axis represents distance, and the y-axis represents depth.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-43.png)
C. Processed GPR data.
Figure 6. Screenshots. Raw, preprocessed, and processed GPR data.(1)
Software can calculate the depths of suspected defects using the two-way travel time between the anomaly and the antenna, the x-coordinate of the pick (selected rebar) within the profile, the surface dielectric values, and the wave velocity within the concrete. The signal attenuation at the top-layer rebar can also be calculated.
Data Interpretation
Data interpretation should be performed by personnel with extensive training and experience. The surface dielectric profiles can be stitched together to create color-coded dielectric surface maps for visualization of the tunnel lining’s condition. Figure 7 shows color-coded maps for dielectric values of a tunnel wall in summer, winter, and autumn. The attenuation of the signal at the top-layer rebar can represent the condition of the concrete because a corrosive environment (characterized by moisture, chlorides, rust, and cracks) will highly attenuate the signals.(5) A contour map of concrete condition (attenuation) can be created to present concrete deterioration.
![This figure contains three color-coded graphs depicting the dielectric values measured on a tunnel liner during different seasons. From top to bottom, the graphs represent data measured in summer, winter, and autumn. The minimum dielectric value in the map is 4.5, and the maximum value is 9. For each graph, the x-axis is distance ranging from 150 to 300 meters, and the y-axis is the height ranging from 1.5 to 3 meters. The average dielectric values are highest in summer and lowest in winter. High dielectric values are indicated by arrows at the bottom of each graph. In the summer and winter graphs, the high dielectric values are located at the distances of 173, 213, 224, 232, 253, 273, and 293 meters. In the autumn graph, the high dielectric values are located at the distances of 173, 193, 213, 224, 253, 273, and 293 meters.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-48.png)
1 m = 3.3 ft.
Figure 7. Contour map. Dielectric values of tunnel wall in summer, winter, and autumn.(1)
Suspected defects, such as voids, can also be detected as anomalies in GPR B-, D-, and C-scans. Voids tend to appear as pronounced, irregular bright-spot anomalies with higher amplitudes than the surrounding material. Internal thicknesses of voids are difficult to estimate because the bottoms of voids tend not to be imaged by GPR data.(6) Figure 8 shows one B-scan of a tunnel liner, from which the liner surface and layered interfaces can be recognized. The surface dielectric values along the B-scan are also presented in figure 8. Figure 9 shows an example of a void from a GPR C-scan.
![This graph is a color-coded ground penetrating radar B-scan of a tunnel liner. The x-axis is distance in feet, and the y-axis is depth in inches. The different colors indicate different signal amplitudes of the scan’s layers. A layer near the top of the graph is labeled “Tunnel Lining Surface Reflection.” A layer near the bottom of the graph is labeled “Possible Lining Interface.” A plot at the bottom of the graph is labeled “Surface Dielectric.” A peak at the center of the surface dielectric plot is labeled “One Area Selected For Testing.”](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-47.png)
1 inch = 25.4 mm; 1 ft = 0.3 m.
Figure 8. Image. Air-coupled GPR data showing subsurface anomalies and the surface dielectric value.(1)
![This graph presents a ground penetrating radar C-scan. The x-axis is distance and ranges from 0 to about 1.2 inches. The y-axis is distance and ranges from negative 0.6 to 0 inches. The depth of the C-scan is 12 inches. A circled area indicates a potential void zone. A mix of bright and dark areas in the center of the figure indicates potential voids.](https://infotechnology.fhwa.dot.gov/wp-content/uploads/2021/05/image-46.png)
z = depth.
1 inch = 25.4 mm.
Note: The circled area is approximately 24 by 20 inches (0.6 by 0.5 m).
Figure 9. Image. GPR C-Scan showing a potential void at a depth of 12 inches (0.3 m).(1)
Advantages
Advantages of GPR testing include the following:
- Well-established field data collection processes.
- Rapid test methods.
- Reliable and repeatable results.
Limitations
Limitations of GPR testing include the following:
- Defects (delamination and voids) are detectable only if they contain significant air pockets or are filled with water.(7)
- The presence of a potential delamination is inferred from the properties associated with advanced corrosion and the formation of corrosion products.
- Air-coupled GPR systems should not be relied on to determine the depths of defects.
- Extensive training and experience are required for operation, data processing, and data interpretation.
- Steel fibers in shotcrete and steel slag in some concrete prevent signal penetration.
- Salts in concrete (from deicing operations or seawater) may cause signal-penetration problems.
- External electromagnetic radiation (from cell phone, radio, and television antennas) can cause signal degradation.
- Global positioning systems cannot be used within the confines of the tunnel.
References
- Wimsatt, A., White, J., Leung, C., Scullion, T., Hurlebaus, S., Zollinger, D., Grasley, Z., et al. (2014). Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings, Report No. S2-R06G-RR-1, Transportation Research Board, Washington, DC.
- Arnold, J.A., Gibson, D.R.P., Mills, M.K., Scott, M., and Youtcheff, J. (2011). “Using GPR to Unearth Sensor Malfunctions.” Public Roads, 74(4), Federal Highway Administration, Washington, DC.
- Gucunski, N., Feldmann, R., Romero, F., Kruschwitz, S., Abu-Hawash, A., and Dunn, M. (2009). “Multimodal Condition Assessment of Bridge Decks by NDE and its Validation.” Proceedings of the 2009 Mid-Continent Transportation Research Symposium, Iowa State University, Ames, IA.
- Gucunski, N., Imani, A., Romero, F., Nazarian, S., Yuan, D., Wiggenhauser, H., Shokouhi, P., Taffee, A., and Kutrubes, D. (2013). Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report No. S2-R06A-RR-1, Transportation Research Board, Washington, DC.
- Dinh, K., Gucunski, N., Kim, J., and Duong, T.H. (2016). “Understanding depth-amplitude effects in assessment of GPR data from concrete bridge decks.” NDT&E International, 83, pp. 48–58, Elsevier, Amsterdam, Netherlands.
- Parkinson, G. and Ekes, C. (2008). Ground Penetrating Radar Evaluation of Concrete Tunnel Linings, 12th International Conference on Ground Penetrating Radar, Birmingham, United Kingdom.
- Heitzman, M., Maser, K., Tran, N.H., Brown, T., Bell, H., Holland, S., Ceylan, H., Belli, K., and Hiltunen, D. (2013). Nondestructive Testing to Identify Delaminations Between HMA Layers, Report No. S2-R06D-RR-1, Transportation Research Board, Washington, DC.