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So, you’ve heard of a GPR survey before, it rings a bell. Maybe it was from a sleepy Sunday afternoon watching Time Team? I know that’s where I first heard of it.

But what exactly is it? What does it do? And how could it help you?

Well hopefully we will be able to answer all those questions and more in this blog.

What is it & how does it work?

Well firstly, what does GPR stand for? It is an acronym for Ground Penetrating Radar, although can also be known by terms such as Ground Probing Radar and Georadar.

It is a lot less destructive than it sounds, being a non-invasive geophysical technique used to examine what goes on just below the surface of the Earth.

GPR uses electromagnetic energy to do this, as all materials have different electrical properties, they can be identified by how they react to the energy directed at them.

But don’t worry! That doesn’t mean they have a strange reaction that could do weird things in the subsurface!

The energy that the GPR unit emits basically gets reflected by any different materials under the surface, and their electrical properties determine how much of this energy is reflected, and how much passes through. The contrasts between all these reflections can then show where there are different elements below the surface.

Typically, GPR units consist of two antennas: one for emitting high-frequency pulses and another for receiving the reflected signals. However, most antennas are not focused. This is because they are generally too bulky and awkward to use on site when they are.

When antennas are not focussed, they can also reflect some energy into the air / above the surface which can result in some strange readings if you are not looking for them, where above ground objects like walls and cars can be picked up. If you have an experienced surveyor carrying out the GPR survey this wouldn’t be anything to worry about, as they would be aware of the surrounding objects and make sure any irregularities are accounted for.

Alternatively, a shielded antenna can (it could be argued should), be used as these cut out any background electromagnetic ‘noise’.

Would a GPR Survey be right for me?

Before starting it is important to know whether the use of GPR would be the most effective way to carry out your survey. The range required and subsurface soil type both have an impact on the results, as do the surrounding features and site access. Contacting us to discuss your needs is the best place to start, as our surveyors can determine what the best course of action would be for you.

All materials have different electrical properties, [so] they can be identified by how they react to the energy directed at them.

GPR is used for a range of different detection requirements, and a range of different antennas that operate on different wavelengths are available depending on the application. Broadly speaking the lower the frequency output, the deeper the penetration. However this generally comes at a cost, and that cost is reduced resolution. Likewise, higher frequency antennas will give much sharper resolution but will be limited to reduced depth penetration. So unless a dual-frequency antenna is used, a compromise will need to be struck depending upon what subsurface items are being sought.

Commonly used for engineering and building applications, GPR is used to locate the spacing and depth of subsurface/buried elements like reinforcing steel, buried walls/structures and cables/utility lines. Specialist geospatial or engineering companies also sometimes use it for positioning cables or anchors, measuring rebar cover, mapping voids (such as detecting voids/deformations in concrete) and studying the soil / bedrock.

It can also be used in applications such as environmental remediation (landfill location, contaminant zones), archaeology (mapping features, artifacts & cemeteries) & the military (detection of unexploded bombs & tunnels); here at Laser Surveys we fundamentally use GPR for location of service routes, where it is used in tandem with Electromagnetic Location (EML) technology (a more sophisticated form of the more recognisable ‘cat & genny’); these two technologies together form what is currently the most comprehensive method of non-intrusive Underground Services survey.

Advantages & Disadvantages

Like anything, there are pro’s and con’s to GPR, and ultimately, whether you use it depends on whether it is right for what you want to do! So, we’ve listed the most common ones to help you get a good idea of where this technology wins the day!

Advantages

• The depth of detectable features can range from a few cm to hundreds of metres, depending on the type of antenna used. At Laser Surveys our equipment is most suitable for linear detection of features up to a depth of approximately 3.5m.

• Gives rapid collection of large amounts of continuous data compared to other methods

• Often the data can be reviewed on-site for quality control and error detection purposes using portable display screens integrated into the equipment This means if an error or anomaly occurs it can be detected and rectified on site without returning later, thus cutting costs.

• GPR has more penetrating power than other non-destructive techniques such as thermography & ultrasonic, so greater depths can be reached.

Disadvantages

• The electromagnetic energy is transmitted and received in a complex 3D cone shape, with the reflections coming from anywhere on the wave front, meaning it can be hard to identify anomalies and can lead to errors in depth estimation and interpretation

• The performance of the system can be affected by the environment that it is surveying – soil types & surroundings can create noise that can mask features below

• Without reference to further resources, it can be very difficult to identify the type of subsurface feature located; classification of individual services is particularly hard unless other information is taken into account; this makes GPR of limited value if used as a stand-alone method of service detection.

We hope this has helped you delve into the hidden world of GPR, and next time you watch Time Team (I am assured they still show repeats!) you can show off your GPR knowledge with confidence!