A Ground-penetrating radar (GPR) is a non-destructive technique used for investigating the characteristics of the subsurface. It is a real-time NDT instrument that employs high-frequency radio waves to study the underground surface. This technique provides high-resolution data within a short period.
GPR employs electromagnetic waves to image the subsurface. GPR is a geophysical survey method that can survey concrete, masonry, asphalt, and ground. The article explains the working and application of GPR in the construction industry.
Working Principle of Ground Penetrating Radar (GPR)
The GPR system consists of a transmitter, antenna, and radargram (control unit). The transmitter emits pulses of electromagnetic radiation into the surface to be surveyed. The difference in permittivity is an indication of the change in sub-surface features.
Some of the electromagnetic energy reflects when any change is encountered. The antenna receives these reflected waves, and the corresponding variations are recorded. The information is interpreted and displayed on the radargram. The time taken by the reflected signals to travel back is measured, which is an indication of the depth and location of interruption.
The GPR waves penetrate through different soil debris, water, concrete, and different materials that have different dielectric and conductive properties. These differences are observed in the GPR waves based on which the GPR data is interpreted.
The data images are finally displayed on the screen, which requires an experienced GPR operator to accurately determine the results.
Data Collection in Ground Penetrating Radar (GPR)
The data collection methodologies in concrete and ground are explained below:
GPR Data Collection for Concrete
GPR is used in concrete to determine the location of reinforcement bars, conduits, or post-tensioned cables present in the concrete element. This is conducted by employing a high-frequency GPR system.
The GPR can be conducted in simple line scans, which determines the thickness of concrete. To determine a specific target within the concrete, the scan can be performed in grid formats. The results are obtained for different parts of the concrete in the form of images. These are combined to get a final 3D map of the picture.
GPR Data Collection for Ground
GPR can be used to analyze the location and depth of any object underground. Underground analysis can be conducted using various types of GPR equipment. The depth and the size of the target decide the kind of GPR instrument required.
In this process, the GPR emits and receives a signal at a rate of thousand times per second. The field operator analyses this immediately and stores it in the system. It is later taken for further data analysis.
Features of Ground Penetrating Radar (GPR)
- GPR can be used for detecting and distinguishing metallic and non-metallic materials.
- GPR works best when used in a medium that reflects high variation in electromagnetic properties. Metallic materials like steel reinforcement in concrete is one such right application.
- Large variation between the electromagnetic properties of the target material (for example, rebar) and the surrounding material (For example, concrete) makes GPR give the best results.
- The important target materials detected by GPR include concrete, air-pockets or voids, metal, plastics, geological features, variation in ground strata, excavated areas, ground disturbances, and backfilled areas.
- GPR is not a good choice in grounds with water clogged or clayey soils, as these mediums are weak in signal making. It cannot also penetrate through metallic surfaces or thick reinforcement in concrete.
Penetration Depth of GPR
The depth up to which GPR can penetrate is dependent on:
- Medium to Scan
- Transmitted Wave Frequency (frequency of the antenna)
- Radiated Power
The penetration depth decreases when the electrical conductivity of the medium is high. This is because of the high attenuation of the waves.
High frequency gives high-resolution results but lower penetration depth. Lower frequency gives great depth with low resolution. Hence, the choice of frequency is dependent on the objectives and the depth of the survey path. Always test more than one frequency and fix a final one.
For high-resolution applications like rebar or conduit locations, a high frequency of 1000 MHz can be used, which gives a penetration depth of 24 inches. For using GPR in soils, a lower frequency of 12.5 to 500MHz gives a deeper penetration.
Frequently Asked Question
The GPR system consists of a transmitter, antenna, and radargram. The transmitter emits pulses of electromagnetic radiation into the surface to be surveyed. The difference in permittivity is an indication of change in sub-surface features.
Some of the electromagnetic energy reflects when any change is encountered. The antenna receives these reflected waves, and the corresponding variations are recorded. The information is interpreted and displayed on the radargram. The time taken by the reflected signals to travel back is measured, which is an indication of the depth and location of interruption.
The important target materials detected by GPR include concrete, air-pockets or voids, metal, plastics, geological features, variation in ground strata, excavated areas, ground disturbances, and backfilled areas.
The depth up to which GPR can penetrate is dependent on:
Medium to scan, Transmitted Wave Frequency (frequency of the antenna) and Radiated Power
The penetration depth decreases when the electrical conductivity of the medium is high. This is because the high value attenuates the waves.
High frequency gives high-resolution results but lower penetration depth. Lower frequency gives great depth with low resolution. Hence, the choice of frequency is dependent on the objectives and the depth of the survey path. Always test more than one frequency and fix a final one.
For high-resolution applications like rebar or conduit locations, a high frequency of 1000 MHz can be used, which gives a penetration depth of 24 inches. For using GPR in soils, a lower frequency of 12.5 to 500MHz gives a deeper penetration.
Read More: Non-Destructing tests on RC Structures- Basic Methods and Purposes