On our previous GPR training page ‘KB GPR Training Module 2.4: How GPR works externally’ we ended with a simplified example showing how a Ground Penetrating Radar (GPR) is used to produce a two-dimensional image which represents a cross section through the ground, exactly of the path along which the GPR was pushed. We discovered that the cross section is made up of a series of one dimensional radar scans called Amplitude Scans (or A-Scans) which are displayed side by side to produce a two-dimensional image.
The technical term for that two-dimensional image (which shows a ‘cross-section’, ‘section’ or ‘transect’ of the ground) is ‘B-Scan’ which stands for ‘Brightness Scan’. A brightness scan is a plot of distance travelled forwards (measured in meters) against time delay which is used to calculate depth (measured in nano seconds ns), with the amplitude (size) of the received signal shown by increased contrast, or brightness of the white and black lines against a grey background.
In our previous example, we represented the electromagnetic wave travelling from the Ground Penetrating Radar into the ground as a vertical line, however, this is not correct… an electromagnetic signal from a GPR doesn’t travel vertically down into the ground – it travels in a cone. This means a GPR will detect objects before (and after) it passes immediately above them.
Lets take a look at two simple representations which show what happens when a GPR is pushed across a point target (which could represent a stone or any other anomaly including a utility), and a linear target which remains at a constant depth throughout the scan (such as a layer).
When a Ground Penetrating Radar is pushed towards a point target, it will illuminate that target with its radar pulse before it arrives. Some energy from the electromagnetic pulse will be reflected back towards the GPR (other energy may be reflected elsewhere and lost).
The reflected signal arrives at the GPR and is plotted on the first A-Scan showing a reflection at a given time delay.
This A-Scan is represented on the display as a vertical line with a point of increased contrast at the time delay at which the reflected energy was received.
The radar continues to travel towards the target.
After about 2.25cm, another pulse will be sent by the GPR, this will again illuminate the target, and again some energy will be reflected and travel back to the antenna.
As with the previous pulse, a second vertical line will be plotted with the reflection shown as a point of increased contrast. Because the GPR is closer, the point representing the reflection will be shown with a lower time delay (or closer to the surface).
This process is repeated until the GPR is directly above the target. Here, the reflection will be shown at its shallowest point.
As the GPR travels away from the target, the time delay between sending the pulse and the reflected energy being received by the GPR will steadily increase until the target is no longer illuminated by the GPR’s electromagnetic pulse.
This process creates the shape referred to as a hyperbola, which allows a GPR user to recognise the point in the ground where a point target is present and locate the centre of the top of the target by locating the apex of the hyperbola.
When a Ground Penetrating Radar is pushed above a linear target such as a layer within the ground, the GPR works in exactly the same way but this time the target remains at a relatively constant time delay with respect to the GPR.
The GPR sends a signal and when the signal meets the layer interface some of the energy is reflected back. That energy travels back to the GPR where it is collected by the antenna and a point of increased contrast is shown on the A-Scan at a given time delay.
The GPR moves forwards 2.25cm and anther pulse is sent into the ground, that energy travels down until it hits the same interface and again some of the energy is reflected back. The reflected energy is collected by the GPR antenna and displayed as a point of increased contrast with a similar time delay.
This process is repeated as the GPR travels forwards, with each of the reflections displayed on the B Scan at a relatively constant time delay.
The B-Scan image will show a linear target such as a layer as a line of white and black against a grey background.
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.
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.
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.
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.