Ground penetrating radar works by emitting pulses of electromagnetic energy into the subsurface to detect differences in permittivity. Changes in the subsurface cause this electromagnetic energy to be reflected back to a receiving antenna on the surface, showing differences in electromagnetic property that indicate different features. The changes in the returning signal are displayed on a radargram.
While GPR can detect subsurface changes it cannot determine their exact nature. However, some features offer hints, such as with metallic surfaces that have a high amplitude or voids which offer a reverse polarity. Although ground penetrating radar surveys can help determine subsurface characteristics, they are often backed up with supplementary data from boreholes, cores or pits.
GPR works in a similar fashion to seismology, except it uses electromagnetic energy rather than acoustic energy. The high frequency radio waves used in GPR tend to be in the range of 10MHz to 2.6 GHz, with reflected energy occurring at boundaries where the subsurface electrical properties change.
Because radar works through movement, GPR requires the equipment to be moved to examine a survey area and detect differences in material composition. Although it can locate items or voids it cannot determine specific materials such as precious gems or gold.
The effective depth of a GPR investigation depends on a variety of factors, including the transmitted frequency, the radiated power and the ground’s electrical conductivity, with higher rates of conductivity attenuating the electromagnetic wave and reducing the penetration depth. In addition, higher frequencies do not penetrate as far as lower ones, although there is resolution with higher frequencies, which creates a trade-off between penetration and resolution.
Levels of subsurface penetration are also dependent on the material through which the GPR passes. Ice can offer several thousand metres of optimal penetration at low frequencies, while dry materials like concrete or granite as well as dry sandy soils may offer effective penetration depths of around 15 metres and moist or clay-heavy soils – as well as materials with high rates of electrical conductivity – may only provide penetration of a few centimetres.
The ability of a material to store and transmit charge energy from an electromagnetic field is measured by the dielectric constant (also known as relative dielectric permittivity or RDP). This impacts the velocity of a GPR signal and thereby the measured depth and signal footprint according to which geological material the signal is passing through. Dielectric constants use both the electrical and the magnetic properties of the material and are directly related to the velocity at which electromagnetic GPR waves pass through the ground.
GPR antennas are typically deployed in contact with the ground to provide the strongest possible signal strengths for the best results, but air-launched antennas are also used for some applications.
Ground penetrating radar is a fast and cost-effective method of non-disruptive subsurface surveying. The many advantages of GPR include:
1. Safe:
Being non-disruptive and unobtrusive, GPR is safe for use in public places
2. Materials:
GPR is capable of detecting both metallic and non-metallic objects as well for locating voids
3. Layers:
GPR can detect interfaces between layers of construction
4. Speed:
Fast data collection makes it suitable for scanning large areas
5. Access:
Only requiring single-sided access means that GPR is suitable for surveying floors, walls, tunnels, decks and more
6. Adjustable:
Using different frequencies allows the user to decide upon different penetration depths and resolutions
7. Data Interpretation:
Continuous survey data can be interpreted in real time or processed off-site, allowing for estimations of depth, layer thickness and dimensions of larger objects
8. Cost:
GPR is less expensive to use than radiography (X-ray)
While there are many advantages of GPR, there are also some drawbacks to be considered:
1. Performance:
The biggest difficulty for GPR lies in its use on high-conductivity materials like salt contaminated soils or those with a high clay content. Dry and rocky soils can also limit performance due to signal scattering in heterogeneous conditions
2. Expertise:
GPR surveys require expertise to design and conduct effectively, while the data output of GPR systems also requires expertise to interpret
3. Energy:
GPR systems have a relatively high level of energy consumption, which can be problematic for more extensive field surveys
The versatility of ground penetrating radar means that it has found a wide range of uses across different industries and organisations. Some typical and not-so-typical uses for GPR include:
Earth Sciences:
GPR is used by earth scientists, geologists and others for the study of bedrock, soils, ice and groundwater
Prospecting:
Although it can’t find the exact materials themselves, GPR can be used to locate natural subsurface voids that could contain gold nuggets or gems
Space Exploration:
GPR systems can be fitted to space vehicles such as the lunar rover Yutu to investigate the soil and rock properties of the Moon
Mining:
GPR is used to assist with mining by mapping structures within a borehole, with modern radar systems producing three-dimensional images from within a single borehole
Structural Analysis:
Engineers use GPR for the non-destructive analysis of structures
Utilities:
GPR is widely used for locating and checking the state of buried utilities. Where standard utility locating tools, which use electromagnetic induction, cannot locate non-conductive utilities such as plastic conduits or concrete sewers, GPR can. GPR can even be used to check the thickness of subsurface pipe walls, and locate delamination or voids.
Archaeology:
Archaeologists use GPR to map subsurface archaeological features as well as burial sites and cemeteries. GPR can also measure the depth at which objects or artefacts are located, helping to determine age through what is known as ‘time slicing’ without damaging them. The technology has also been deployed in the field of heritage to investigate the state of historical buildings
Law Enforcement:
Similar to archaeological uses, law enforcement have used GPR to locate unmarked graves or buried evidence. Police have even used GPR to monitor the movements of people within buildings, including indicating the presence of metal items that could be weapons
Military:
Military applications for GPR include detecting mines, improvised explosive devices (IEDs), unexploded ordnance and tunnel networks
Vehicle Localisation:
GPR has been shown to be effective for helping steer or localise autonomous and other vehicles, penetrating and reading the ground at speeds of up to 60mph, for example, to maintain vehicle positioning on roads and lanes during night-time snow storms. The technique has also been used to survey roads at speed
Ground penetrating radar (GPR) is used for a wide range of different applications to detect metal and non-metallic objects as well as voids and other subsurface and structural irregularities. The level of penetration for the radar is dependent on factors such as the materials being assessed, with some offering far greater rates than others. Although GPR takes expertise to use and interpret, it is a fast, safe and effective non-destructive process.
What Can Ground Penetrating Radar Detect?
Ground penetrating radar detects metallic and non-metallic objects as well as voids, underground irregularities and changes in material layers. It can measure the location, dimensions and thickness of targets with data provided quickly and safely for large survey areas.
How Much Does A GPR Cost?
GPR systems can cost thousands, with basic sets costing around £11,500 and more advanced systems with greater functionalities costing over three times as much.
How Deep Can Ground Penetrating Radar See?
The depth of penetration for GOR depends upon the scanned medium, the transmitted frequency and the radiated power of the electromagnetic waves. However, as the frequencies increase the penetration depth reduces, although this also improves resolution.
At 2.6 GHz the typical maximum penetration is 0.3m while frequencies of 400 Mhz can offer penetration depths of around 2 metres. If the frequencies are lower still (200-16Mhz) the penetration depths can reach several metres, depending on the subsurface media being scanned.
What Can Ground Penetrating Radar Not Detect?
While GPR can locate items, voids, or changes in soils, it cannot identify the specific nature of materials that are located, such as gold or precious gems.
Is Ground Penetrating Radar Accurate?
While it is an effective method of non-destructive testing, GPR is not 100% accurate. The accuracy of GPR depends upon the materials being scanned, the conditions of the scan, the frequencies and equipment being used, and the expertise of the person performing the scan.
Is Ground Penetrating Radar Safe?
Ground penetrating radar systems are safe and can be used in public areas, emitting around 1% of the power of a mobile phone signal.