Module 2.4: How GPR Works Eternally

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In this example the Ground Penetrating Radar is positioned on the ground and is not moving. The GPR sends a pulse into the ground and then switches to listen mode.

The pulse travels into the ground until it encounters something which would cause a reflection (such as a pipe or a rock).

Some of the energy is reflected back towards the GPR (some of the energy will be reflected in other directions and be lost, and some will continue on… but at least some of the energy will be reflected back towards the GPR).

The GPR has been measuring the time delay since the signal was transmitted (shown on the A-Scan or Amplitude Scan), while no signal is being received it shows no response.

When the energy is received by the GPR it shows as an increase in amplitude – the amplitude returns to zero again to wait for the next reflection.

In this case, it can be seen that there are two targets in the ground because the targets are at different distances from the GPR, but the GPR is only able to record the time delays between transmitting the signal and receiving the pulses and in practice has no idea where they are.

At this stage a GPR is not very useful and in order to become useful and be able to locate objects it needs to add another dimension to the data it collects.

In this example the Ground Penetrating Radar is positioned on the ground and is not moving. The GPR sends a pulse into the ground and then switches to listen mode.

The pulse travels into the ground until it encounters something which would cause a reflection (such as a pipe or a rock).

Some of the energy is reflected back towards the GPR (some of the energy will be reflected in other directions and be lost, and some will continue on… but at least some of the energy will be reflected back towards the GPR).

The GPR has been measuring the time delay since the signal was transmitted (shown on the A-Scan or Amplitude Scan), while no signal is being received it shows no response.

When the energy is received by the GPR it shows as an increase in amplitude – the amplitude returns to zero again to wait for the next reflection.

In this case, it can be seen that there are two targets in the ground because the targets are at different distances from the GPR, but the GPR is only able to record the time delays between transmitting the signal and receiving the pulses and in practice has no idea where they are.

At this stage a GPR is not very useful and in order to become useful and be able to locate objects it needs to add another dimension to the data it collects.

Making GPR useful

As with the ship mounted example on the previous page ‘KB GPR Training Module 1.4: How does radar work’. It is necessary to add another dimension to the Ground Penetrating Radar data in order to make it useful, and we can do that by moving the antenna.

  • It would be impractical to rotate the antenna, so instead we can move the GPR across the ground.
  • The GPR is moved forward, after approximately 2.25cm (this interval will vary between different GPR) the wheel sends a signal to the control box.
  • The control box tells the transmitter to generate and send a pulse, and immediately starts to record the time delay.
  • The signal travels into the ground and some of the energy is reflected back.
  • The reflected signal is collected by the receiving antenna and amplified in the receiver before it travels up to the control box.
  • A response is plotted on an A-Scan with a given time delay.
  • This time the A-Scan is represented with the time delay shown vertically and instead of seeing a signal (often called a wiggle view) the size (amplitude) of the received signal is shown by increased contrast of the black or white with grey representing no reflection.
  • The GPR continues to move forwards and this process is repeated every 2.25cm.
  • The series of A-Scans are plotted side by side.
  • This view is called a B-Scan (Brightness Scan), with the reflections shown as areas of increased brightness against a grey background.

The GPR has now become useful, it is not only able to detect objects present in the ground but also to provide an approximate depth (based on the time delay) and location on a two dimensional image, which represents a ‘section’, ‘cross-section’ or ‘transect’ of the ground exactly of the path along which the GPR was pushed. That is referred to as a B-Scan or ‘brightness scan’ in radar terminology.

This is a simplified description of how a GPR works, in real life there are many other considerations some of which we will cover in the next modules.

Our Most Common Questions

Frequently Asked Questions

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What can a Ground Penetrating Radar survey detect?

With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

Will GPR compromise safety on my site?

GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

Is a GPR survey 100% accurate?

While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

Is GPR equipment difficult to use?

The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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