GPR Detection – What are the limits of GPR?

Before beginning construction activities that may disturb the ground such as excavation, cutting, digging, etc. It is essential to know if there are any underground utilities. It is essential to know whether or not there are underground utilities such as electrical cables, water lines, sewer lines, etc. GPR (Ground Penetrating Radar) is a tool that detects objects buried in the ground using electromagnetic signals.

GPR survey or GPR detection is a very efficient method to obtain the depth and location of buried objects. GPR has some limitations: depth of penetration, size of the objects, efficiency directly related to the soil conditions, etc.

GPR survey: What is GPR?

Ground-penetrating radar is used in many areas to observe man-made and natural features. GPR detects metallic and non-metallic pipes, power lines, conduits, water lines, rebar and post-tensioning cables within concrete.

Ground-penetrating radar waves are equal to those of a cell phone or Wi-Fi network, while x-rays require a 50-foot clearance before being used for safety reasons. In general, ground-penetrating radar is the most cost-effective option and the fastest method of testing concrete.

The principle of using radio waves to determine internal ground structures has been known for a long time. Among the first works in this field, the use of radio echo sounders to determine the thickness of ice layers in Antarctica and the Arctic and to measure the thickness of glaciers is undoubtedly the greatest success. Ground-penetrating radar detection in non-glacial areas was initiated in the early 1970s. The first achievements focused on work on permafrost soils.

What are the limits of GPR?

How deep can you see? This is the most common question asked of ground-penetrating radar vendors. Even though the physical principles are well known, most new surveyor users are not aware that there are fundamental physical limits.

Many people believe that the penetration of GPR is limited by the equipment. This is true in a way, but the depth of the search is essentially determined by the equipment itself and any improvement in instrumentation will not overcome the fundamental physical limitations.

The minimum size of the element that can be detected

The minimum size of a service that can be detected using GPR depends on the resolution of the radar, which is related to the frequency and depth that needs to be surveyed. As a general rule, the detection by GPR of services whose diameter is at least equal to 10% of the depth (i.e. services with a diameter of 10 cm buried at 1 m depth). This rule is not fixed, it varies considerably depending on the soil.

It is necessary to find a service in multiple locations, multiple times, in order to track its path and positively identify it as a service and not as clutter. Surveys of larger areas provide a better context for this decision process, while small areas make it more difficult to safely locate a buried utility.

Surface conditions

The condition and type of the surface being surveyed will affect the limits of GPR. If a surface is reinforced concrete with dense bars, the signal will suffer from scatter. Rocky or landscaped ground will give the same result. This greatly reduces the user’s ability to identify potential targets. The same is true if the surface is completely saturated due to a period of rain or if the surface is made of coarse gravel.

For a survey on public services. It is these limitations of the surveyor that make the full range of tools available to a surveyor so important. GPR survey is extremely important in order to have accurate results.

Soil type and moisture

The most important geophysical factor in the limitations of GPR is the soil type. Highly conductive soils, such as clay, exhibit high signal attenuation. This greatly reduces the effective penetration depth of a given antenna frequency.

We are often asked how deep your GPR can see? This depends on the choice of antenna for a given application. A low frequency antenna like an 80 MHz will have a high potential penetration (40m). A high frequency antenna, such as a 750 MHz, will have a much lower penetration (3.5m). The trade-off is that low frequency antennas have a lower resolution. In real terms, this means that you will only identify larger targets at depth.

Any given frequency will have an effective depth at which it can work. However, this depth is based on ideal conditions, much like the miles per gallon published by automobile manufacturers. The actual depth of penetration is determined by site conditions, particularly soil type, and often cannot be determined until after the survey.

Conclusion

The limits of ground-penetrating radar are only those of the imagination and the availability of adequate instruments. Today, GPR detection is applied in a variety of fields, including locating underground utilities, searching for buried landmines and unexploded ordnance, mine site assessment, forensic investigations, and measuring the thickness and quality of snow and ice, to name a few. Contact us or call us now to benefit from our GPR detection services.