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Ground Penetrating Radar isn’t perfect – Limitations of GPR

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Limitations of Ground Penetrating Radar

Ground penetrating radar (GPR) is a fantastic instrument for locating underground utilities. It uses electromagnetic waves to penetrate the ground and detect underground utilities such as pipes, cables, and conduits. It is non-invasive, fast, and accurate, making it an excellent tool for utility mapping and inspection. However, like all technologies, GPR has its limitations. In this article, we will explore the limitations of using GPR for underground utility locations.

Ground Penetrating Radar equipment being used in NSW Sydney, Australia service provided by Geoscope
Ground Penetrating Radar equipment being used on work in Sydney NSW

Most professionals in the industry could agree that Ground Penetrating Radar is great at estimating the depth of underground utilities … until it can’t.

GPR is a perfect accompaniment to electromagnetic locating methods but it does not come without its challenges. We can be honest and say that is not a perfect solution for all work sites, soil types, and ground conditions which means there are times when GPR’s effectiveness will be faced with limitations.

Ground Penetrating Radar equipment being used in NSW Sydney, Australia service provided by Geoscope
Ground Penetrating Radar equipment being used on different soil conditions

When the soil conditions and the properties of underground services looking to be found all align in the favour of Ground Penetrating Radar, it can be a magical sight. Although this is sometimes hard to fathom and sometimes leaves people scratching their heads over how on one site, GPR can find everything and on another site, GPR looks like a waste of time and a piece of junk.

When you learn about the limitations of GPR Ground Penetrating Radar and find out the ideal situations and the conditions that are less favourable for GPR, you’ll be better equipped to understand what is actually going on.

What is GPR?

To understand the limitations of GPR, would help you to know a little about what Ground Penetrating Radar is and how it works.

Limitations of Ground Penetrating Radar

Ground Penetrating Radar or GPR is a geophysical method that uses radar pulses to image the subsurface of the earth. GPR works by emitting a pulse of electromagnetic radiation into the ground and then detecting the reflections of the pulse as it travels through the subsurface. The reflections are used to create an image of the subsurface, which can be used to locate and identify underground utilities and other features.

It is often used to locate underground utilities, such as copper or cast iron water pipes, cast iron and polyethylene or nylon gas pipes, and electrical cables and duct banks.

It can also be used to locate underground structures, such as tanks, foundations and tunnels, and to identify geological features, such as bedrock and soil layers.

GPR is a non-invasive method, meaning it does not require any physical excavation or drilling to obtain subsurface data. It is a useful tool for mapping the subsurface in a quick and cost-effective manner, and it can be used in a variety of settings, including urban, rural, and remote areas but unfortunately, it is not without its limitations.

Limitations of GPR

GPR has a limited penetration depth capacity depending on the selected GPR antenna frequency

The depth of penetration of GPR is determined by the frequency of the radar’s electromagnetic waves. 

Ground Penetrating Radar equipment being used in NSW Sydney, Australia service provided by Geoscope
Ground Penetrating Radar showing frequency of radar's electromagnetic waves

Lower frequency waves, such as those in the 100 MHz range, have longer wavelengths and are able to penetrate deeper into the ground. However, these waves are also more prone to interference from the ground and may not provide as much resolution as higher-frequency waves. 

Higher frequency waves, such as those in the 1000 MHz or 1 GHz range, have shorter wavelengths and are able to provide higher-resolution images of the subsurface. However, they are not able to penetrate as deeply into the ground as lower-frequency waves.

The choice of antenna frequency depends on the specific application and the subsurface conditions.

For example, if you are trying to locate objects deeper than 3 metres, such as pipes or geotechnical investigations, a lower-frequency antenna may be more appropriate. 

On the other hand, if you are trying to locate shallow targets such as rebar, post-tension cables, and conduits, a higher-frequency antenna may be more suitable. 

When it comes to locating underground services, the sweet spot for Ground penetrating radar antenna selection is around the middle of the range which sits at 400-500MHz.

It is important to note that the penetration depth of GPR is also affected by other factors such as the type and density of the soil, the presence of metallic objects, and the moisture content of the soil.

Ground Penetrating Radar equipment being used in NSW Sydney, Australia service provided by Geoscope during night
Ground Penetrating Radar equipment being used in different subsurface conditions

In short, lower-frequency waves are able to penetrate deeper into the ground, but may not provide as much resolution as higher-frequency waves. While higher-frequency waves are able to provide higher-resolution images of the subsurface but are not able to penetrate as deeply into the ground. So, it is important to consider the trade-off between penetration depth and resolution when selecting the appropriate frequency for a GPR survey.

GPR can't see through wet soils

Ground Penetrating Radar equipment being used in semi-wet soil NSW Sydney, Australia service provided by Geoscope
Ground Penetrating Radar equipment used on wet soil

If you are familiar with attenuation, it is the process by which the energy of a GPR signal is reduced as it travels through the ground. 

Attenuation occurs due to several factors such as absorption, scattering, and dispersion

Absorption refers to the process by which the energy of the GPR signal is absorbed by the soil and converted into heat. Scattering refers to the process by which the energy of the GPR signal is scattered in all directions as it travels through the soil. 

Dispersion refers to the process by which the GPR signal is slowed down and its wavelength is changed as it travels through the soil.

Soils that have high levels of water or clay content tend to have high attenuation and can significantly reduce the strength of a GPR signal. As a result, the signal may not be able to penetrate as deeply into the ground or may not be able to provide as much resolution as in soils with lower attenuation. In addition, the attenuated GPR signal may not be able to reflect back into the GPR receiver, making it difficult to obtain a clear image of the subsurface.

It is important to note that the attenuation of GPR signals can be reduced by using higher frequency antennas, as these waves are less affected by the soil. However, as mentioned earlier, higher-frequency antennas are not able to penetrate as deeply into the ground as lower-frequency antennas.

It can only provide a two-dimensional image of the subsurface dependent on the selected GPR make and model

Ground Penetrating Radar equipment showing 2D images in a project in NSW Sydney, Australia service provided by Geoscope
GPR showing 2D image of the subsurface

By only seeing data in a singular view, it is easy to misinterpret the data or potentially not be able to see what’s below

Many GPR systems are designed to only provide a two-dimensional (2D) image of the subsurface, which means that they can only detect objects that are parallel to the ground surface. 

This is because the GPR antenna is typically positioned on the surface of the ground and emits a single beam of electromagnetic waves that travel directly downwards. The return signals are then collected by the antenna and processed to create a 2D image of the subsurface.

While 2D images can be useful for certain applications, such as utility mapping, they may not be sufficient for more complex or detailed subsurface investigations in less-than-ideal soil conditions.

There are GPR systems that can provide 3D images of the subsurface but are relatively limited as well as they are typically more expensive and require more advanced equipment and software.

3D images can be created by using a multi-antenna array GPR, in which multiple antennas are positioned at different angles and heights, and the data is collected and processed to create a 3D image of the subsurface. This allows us to get more details, subsurface structure, and dimensions of objects.

Using software and strategic 2D grid scans, 3D Ground Penetrating Radar data can be created to give the ability to slice an area and view depth slices over the x and y-axis. 

It’s important to note that the 2D images obtained from GPR still can provide valuable information on the depth and location of subsurface features and utilities, but for more complex tasks or greater precision 3D imaging may be required.

There are many different GPR systems on the market, each with its own unique features and capabilities. Some manufacturers of GPR systems that provide 2D images and also the ability to use the data to create 3D slices include:

  • GSSI
  • Malå GeoScience MALÅ 
  • IDS / Leica 
  • Impulse Radar
  • Proceq

It is worth noting that there are many other models and brands that provide 2D and 3D GPR imaging, these examples are just a sample of what’s available in the market, and it is important to research and consider the specific needs of your project before choosing a GPR system.

Ground Penetrating Radar can not see through metal

Ground Penetrating Radar equipment not being used in metallic soils in a project in NSW Sydney, Australia service provided by Geoscope
Ground Penetrating Radar equipment only being used on non-metallic surface

Ground Penetrating Radar (GPR) uses electromagnetic waves to detect objects and structures below the surface of the ground. Metal objects, such as rebar in concrete or buried metal pipes, can disrupt or reflect these waves, making it difficult or impossible for the radar to see through them. This is because Metal surfaces reflect the waves instead of absorbing them as soil or rock do.

To explain further, we have technical terms in the industry such as the dielectric constant of metals, permittivity, and Radial Distribution of Potential or RDP. 

The dielectric constant (also known as the relative permittivity) of a material is a measure of how easily an electric field can penetrate the material. In general, materials with a higher dielectric constant are more polarisable which means their electrons are more easily rearranged in response to an electric field.

For most metals, the dielectric constant is very low, typically around 1. This is because the electrons in a metal are held in a “sea” that is free to move throughout the material, rather than being localized in specific bonds like they are in most other materials. As a result, it is difficult to polarise the electrons in a metal and the dielectric constant is low.

In other terms, the dielectric constant of metal is infinite as the net electric field inside the metal is zero. This means that radar energy, better pictured as a ray of light, can not pass through the metal no matter what frequency it is pulsing at.

Another term we have is permittivity, also known as the electric constant. This is a measure of a material’s ability to store electric energy in an electric field. It is related to a material’s dielectric constant, which is a measure of how easily an electric field can penetrate the material. In general, materials with a higher permittivity can store more electric energy than materials with a lower permittivity.

When it comes to Ground Penetrating Radar (GPR) it is important to consider that the permittivity of a material is one of the factors that affect the ability of GPR signals to penetrate material and detect subsurface objects or structures. High-permittivity materials tend to store and reflect more GPR signals than low-permittivity materials like metal. Therefore, GPR is more effective in penetrating low-permittivity materials such as soils and some rocks.

In simpler terms, when GPR energy comes into contact with metal objects, the energy is reflected greater than it would with plastic materials and higher RDP. Therefore results in getting clear data on buried utilities, underground services and metal objects such as tanks. 

By having clear data and knowing what metal reflection looks like we can make better deductions from our interpretations of what is buried below.

It requires a skilled operator to interpret the data

Ground Penetrating Radar service provided by Geoscope in SydnetyNSW
Skilled Ground Penetrating Radar Operator interpreting data on-site

GPR also requires a skilled operator to interpret the data and distinguish between different types of utilities. In some cases, additional methods may be required to accurately locate and identify a particular utility. This is because the data can be complex and difficult to understand. 

The image produced by GPR is a series of radar reflections, which can be affected by a variety of factors, such as the type of subsurface materials, the presence of other utilities or structures, and the depth of the features.

To accurately interpret the data, the operator must be familiar with the principles of GPR and how it is affected by different subsurface conditions. They must also be able to recognise the characteristic signatures of different types of utilities such as the difference between a copper pipe and a plastic pipe, or foundations, and distinguish them from other types of features.

The skill and experience of the operator are important factors in the accuracy and effectiveness of a GPR survey. It is important to choose a qualified and experienced operator to ensure the best possible results.

Other limitations of GPR

Here are some additional limitations of GPR

  • GPR is not effective at detecting non-metallic utilities, such as plastic pipes or fibre optic cables in soils with similar dielectric properties.
  • GPR signals can be reflected or absorbed by different materials in the subsurface, which can make it difficult to accurately interpret the data.
  • GPR surveys can be time-consuming and labour-intensive, especially if the area to be surveyed is large or has a lot of interference from other utilities or structures.
  • GPR equipment can be expensive to purchase and maintain.
  • GPR surveys can be affected by weather conditions, such as heavy rain or extreme heat, which can impact the quality of the data.
  • GPR surveys can be disrupted by underground power cables or other sources of electromagnetic interference.

Overall, Ground Penetrating Radar (GPR) is an important tool for locating underground utilities because it can provide detailed and accurate information about the subsurface without requiring any physical excavation or drilling. This can save time and money and minimise the disruption to the surface.

Ground Penetrating Radar equipment being used in NSW Sydney, Australia service provided by Geoscope
GPR locating buried utilities that are not visible on the surface

GPR is particularly useful for locating buried utilities that are not visible or accessible at the surface, such as pipes and cables that are buried under pavement or covered by other structures. It is also useful for identifying the depth and orientation of utilities, which is important for accurately planning and executing any excavation or construction work.

In addition to its practical applications, using GPR for utility locating is also a safer option compared to traditional methods that involve physically digging up the ground because GPR does not expose workers to the risks of excavation, such as cave-ins, collisions with buried utilities, or exposure to hazardous materials, however powerful the GPR is, as I have said, it still has its own limitations and it is important to understand these limitations when planning and conducting a GPR survey for utility locating. 

By knowing the limitations, you can plan your survey accordingly and make informed decisions about the equipment and techniques to use. 

Additionally, It is also important to be aware that the GPR alone does not guarantee a complete utility locate and it should be used in conjunction with other locate methods for the best results.

If you want to know more about GPR, may it be how to use GPR this article is for you! 

Ground Penetrating Radar – A GPR Expert’s Guide

This article is an expert’s guide on how Ground Penetrating Radar works in locating underground utilities.
Article

But if you are interested to know the opposite of GPR limitations which  are the advantages of using it, kindly refer to this article:

Advantages of Ground Penetrating Radar in Utility Locating

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Article

Finally, if you are looking to avail our GPR Services, click this link — Ground Penetrating Service by Geoscope Locating or simply directly fill out the form below. 

Patricia Cupiado

Patricia Cupiado

Contributor Author of this article

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