What is GPR?
Ground-penetrating radar is a non-destructive technique for mapping buried structures and features below the surface. It works by transmitting high-frequency electromagnetic waves into the ground through an antenna.
GPR generally uses reflected signals in the microwave band between 10 megahertz (MHz) and 2.6 gigahertz (GHz). Its main advantage is that it does not require digging into the ground like traditional subsurface survey methods. Under the right conditions, GPR operators can use GPR to identify objects, and even subsurface voids near the surface.
A transmitter on the GPR devices emits the GPR signal into the medium. An antenna on the GPR device receives the signals that are reflected from the medium and the various objects in it. The GPR device then receives these return signals and displays them on a GPR data screen.
The effective depth of the GPR is determined by several factors such as the electrical conductivity of the medium, the frequency of the transmitted signal and its power. Higher frequencies are more easily blocked by the material, making GPR penetration more difficult.
A medium with greater electrical conductivity allows the equipment signal to travel farther, increasing the usable depth. For example, ice is highly conductive, allowing the technology to penetrate a great distance.
In contrast, solid materials such as granite do not readily absorb water, limiting their penetration by GPR to much shallower depths.
The GPR is:
- A precise, fast and high-resolution geophysical technique for the investigation of the underground.
- Non-invasive, non-destructive and totally safe.
- The only non-intrusive method capable of accurately locating non-metallic subsurface features and utilities (e.g., clay, concrete, fibreglass, PVC conduit, or fibre optic cables).
- A geophysical probing technique based on the transmission of pulsed electromagnetic energy into the subsurface and the measurement of the strength of the reflected energy.
- Successful when there is sufficient contrast in material properties (dielectric permittivity) between a buried target and its environment.
- Detect and map pipes, cables, structural fittings, voids, disturbed soils, material degradation, subsurface layers and buried objects.
- Acquired using transmits and receives antennas that can be mounted on a cart, tray or vehicle, or held in the hand.
- A technique that requires skilled and experienced personnel to acquire high-quality survey data and geophysical expertise to process and interpret the results.
How Does GPR Works?
A Ground-penetrating radar transmitter emits pulses of electromagnetic energy into the subsurface. Changes in the subsurface are detected based on differences in permittivity.
When a change in the subsurface is encountered, some of the electromagnetic energy is reflected back to the surface.
This phenomenon is detected by a receiving antenna and the variations of the return signal are recorded. The information is displayed on a radar-gram.
Although ground-penetrating radar can detect changes in the subsurface, it cannot determine the exact nature of the change. Some features have specific characteristics in the reflected wave pattern.
For example, reflections from metal surfaces have large amplitude, while reflections from a vacuum have opposite polarity. These characteristics help in the identification of detected features.
However, in some cases, it may be necessary to supplement a ground-penetrating radar survey with absolute data from boreholes, sample cores, test pits, etc.
GPR technologies can also be used for concrete scanning but also to examine the interior of structures, either to check the condition of the construction or to locate hidden objects. If in doubt, ask your operator for advice.
The Range of GPR
The accuracy of the time range of GPR equipment is extremely important when quantitative information about target depth and wave propagation velocity in the subsurface is required.
It is common for GPR operators to accept the time range indicated by the GPR system as accurate.
This can lead to inaccurate estimates of target penetration depth when repeating surveys with different time range settings or different configurations of GPR equipment.
This paper formalizes a simple but effective technique for measuring the true time range of a physically-based system.
GPR operators can use this technique to calibrate their system and evaluate the accuracy of the time range of their equipment.
The results of a comparison between the reported time range and measured values of the time range of some commercial GPR equipment are also presented.
The Limits of GPR Detection
As with any other geophysical technique, the performance of GPR detection is site-specific and it is not appropriate to use it in some locations. Consideration must be given to the expected subsurface conditions and the composition, location, and size of the target.
Anomalies in ground-penetrating radar detection depend on a detectable contrast in the subsurface electrical properties between the target of interest and the surrounding material.
In the absence of a detectable contrast, no anomaly will be evident.
The signal detected by GPR cannot penetrate through highly conductive material, such as under metal sheets or very wet soil, or in material saturated with saltwater or highly conductive fluid.
A speed-depth calibration should always be performed to obtain satisfactory depth estimates.
Processing and interpretation of GPR detection data can be complicated – specialized geophysical analysis and interpretation are often required.
GPR is not suitable for absolute measurement, for example, it can find wet areas, but cannot determine the actual moisture content.
GPR detection is an interpretive method, based on the identification of reflectors, which may not uniquely identify an object. It is important to have the additional information from soil testing or other geophysical methods to help resolve any confusion.
Like all surface geophysical methods, GPR is inherently limited by the decrease in resolution with depth.
Applications of GPR
Ground-penetrating radar (GPR) can be applied to a wide range of applications in many fields. It is absolutely essential in the construction process to ensure safe cutting and coring, saving time and money.
Most often it can be used in construction and engineering as a form of non-destructive testing on structures and pavements.
It can be used for concrete inspection and scanning, corrosion mapping, and utility locating and oil tank locating, to name just a few.
Other applications include earth sciences, where it can be used to study bedrock, as well as soils and groundwater, and water table profiling.
In addition, it can be used in archaeology to detect and map archaeological features, as well as by the military and law enforcement to detect dangerous objects such as landmines.
GPR is a non-destructive reconnaissance tool for general subsurface mapping (soil and rock horizons, depth to water table, etc.) and locating buried objects prior to an invasive investigation. GPR can assist in the planning and location of follow-up intrusive sampling programs (test pits, boreholes, coring).
Geology and Mining
- General subsurface object mapping (rock quality, fault and fracture detection, soil stratigraphy, bedrock depth).
- Subsidence and swallow holes investigations.
- Soil and aggregate mapping (soil composition and depth of compaction aggregates, quality and quantity).
- Water resources (groundwater delineation, water distribution in soil).
- Quarrying and mining mapping (tunneling, rock mass stability, mineral and mineralized zone delineation).
Non-Destructive Testing of Concrete
- Identification and location of embedded structures (metallic and non-metallic conduits, reinforcing bars and post tension cables) in concrete structures (e.g. bridges, dams, reservoirs, foundations, tunnels, runways, etc.)
- Identification and location of voids and areas of construction degradation (concrete layer thickness and water content).
- Quality assurance control of new structures or existing constructions for rehabilitation purposes.
- Delineation of landfills
- Brownfield site investigation: mapping of existing utilities, mock-up soil thickness, unknown cellars, voids and buried objects prior to excavation.
- Contaminant plume profiling (tank leaks, surface spills, pipe leaks, dumps).
- Soil stratigraphy mapping (soil conditions, compaction and water distribution).
- Location of underground storage tanks (steel, concrete or fiberglass).
- Location of underground storage drums
Detection and Mapping of Utilities
- Detection and mapping of buried structures such as plastic water and gas pipes, fiber optic ducts, concrete sewers, clay and asbestos cement drainage pipes as well as standard traceable steel, cast iron and power cables.
- Detection of leaks in underground pipes and assessment of the impact of leaks (presence of leaks or voids).
Investigation of Roads and Railroads
- Structural mapping of asphalt and concrete pavements (layer thickness, layer integrity, moisture content, voids) for effective maintenance and rehabilitation decisions.
- Railroad ballast thickness mapping.
- Identification of embedded utilities.
Finance and Insurance
- Property evaluation (building integrity, construction quality control).
- Risk assessment
- General survey of the subsurface, soil conditions, and target location around archaeological and forensic sites.
- Location of buried objects and remains or disturbed soils associated with a burial.
- Location and delineation of buried objects, walls and foundations, soil disturbances and hidden cavities.
Question and answer on GPR:
GPR systems are responsible for detecting rapidly changing radio signals. Detecting these signals for analysis and interpretation requires a considerable degree of electronic complexity in order to obtain high-quality data.
In this case, the GPR can interfere with the equipment present such as television, telephones, radios, etc.
The detection with GPR does not take much time to have a result.
The GPR is capable of measuring the thickness of concrete.
Clients have many GPR needs, from locating utilities and conduits, to locating rebar and other obstructions to determine an appropriate location for cutting or coring. The bottom line is that the GPR method is non-disruptive, fast, safe and effective.
GPR can be used in many areas such as:
- Medico-legal investigations
- Location of buried utilities
- Archaeological excavations
- Avalanche prediction
- Measurement of snow and ice thickness and quality
- Evaluation of mine sites
- Search for buried landmines and unexploded ordnance
The ground-penetrating radar is a safe, non-invasive tool that does not emit dangerous radiation.