Depending on the conventions in your country, Ground Penetrating Radar (GPR) is also known as Surface Penetrating Radar (SPR), Ground Probing Radar and Impulse Radar. However, in the UK the most commonly used term is Ground Penetrating Radar or GPR and these will be the standard terms used throughout this website.
Ground Penetrating Radar is a geophysical method that uses radar pulses to produce images of the below ground environment or subsurface. GPR can be used in a variety of environments and is able to detect any underground feature with different electromagnetic properties to its surrounding media. Some examples of features that can be detected by GPR include:
Although it is often regarded as a new technology, GPR has been around for a long time. A quick search online and you will find references to Ground Penetrating Radar research as early as 1904, but the first commercial GPR system was built by GSSI in 1974. By selling and promoting an ‘off the shelf’ system into universities and research intuitions around the world, GSSI sparked a new series of research and development projects and it is from this point when modern GPR history really begins.
By the 1990s, at least GSSI, Mala Geoscience, and Sensors and Software were producing commercially available GPR.
In the 90’s the rapid development and miniaturisation of personal computers (and the invention of laptops) allowed more data to be collected and stored on-site. It also made possible for that information to be post processed and visualised in increasingly different ways back in the office. About this time IDS released their first multi-channel, multi-frequency GPR onto the market.
In 2002, Mala opened GPR to the ‘mass market’ when they released the Easy Locator, the first affordable and easy to use GPR. The Easy Locator sold in large numbers and a few years later, keen to differentiate, IDS released the Detector Duo, the first dual frequency easy to use GPR.
The market has moved on considerably since the Easy Locator and the Detector Duo, but, Mala’s Easy Locator HDR and IDS Opera Duo (the successors to those original products), are two of the big selling GPR available today. There are also a lot of other products in the same class, including by GSSI and Sensors and Software, both of whom continue to manufacture GPR to this day.
Ground Penetrating Radar is a very adaptable technology which can be used for a lot of different applications, some of these include:
GPR use in Archaeology usually refers to the detection of buried remains, foundations and ditches which allows resources to be focused for intrusive investigation.
A road survey (also called a pavement survey) or railway ballast survey using GPR will be primarily focused on the detection of layers and ‘ballast fouling’, which refers to the mixing of materials at the layer interfaces and silting of granular fills which causes their mechanical properties to change.
The main applications for GPR within concrete surveys include the detection and mapping of rebar depth, location and distribution, detection of post tension cables, the thickness of the slab, identifying voids and delamination, and the presence of moisture.
Because it works by detecting changes in electrical properties, GPR is one of the ideal technologies for detecting underground voids. Some examples of GPR surveys to detect voids include chimney flue surveys (voids within walls), surveys to detect underground basements and storage areas, badger surveys (detection of the tunnels), and detection of voids beneath concrete often caused by the base material being eroded by water.
GPR use in forensics is usually in the context of detecting bodies, buried weapons and hidden compartments (voids) in walls. A related survey is a GPR survey to detect graves and empty spaces within graveyards.
Use of GPR to detect and map the locations of underground infrastructure including pipes, cables and sewers. This is one of the simpler applications for GPR and by far the largest area today.
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.