Ground Penetrating Radar (GPR) is a technology by which it is possible to non-destructively look beneath the surface and determine information about the below-surface environment.
For further information please see GPR Training Module 2.1 Introduction to GPR
Ground Penetrating Radar (GPR) works by transmitting a signal into the ground and waiting for reflections which it receives. By calculating the time delay between sending the transmitted signal and receiving the reflections it is possible to calculate the depths of the features detected.
The GPR signal is reflected by any changes in the electromagnetic properties of the medium it is travelling through. This means that GPR can be used to detect most types of underground features including: air voids, water, metal, concrete, and layers in the ground (geology). The greater the contrast between the electromagnetic characteristics of the materials the greater the size of the reflected signal will be. Conversely, subtle changes in material characteristics will result in smaller reflections, making some features more difficult to detect.
Ground Penetrating Radar (GPR) is a fully non-destructive technology which is relatively efficient to deploy. GPR can scan areas of a surface to determine the presence and locations of features beneath that surface. It can resolve both metallic and non-metallic features including air voids and the presence of water. GPR surveys can be applied to many ground materials including soil and concrete and used to determine information about structures.
If you need to locate or map a sun-surface feature, or determine information about how a structure was built, Ground Penetrating Radar is likely to be able to provide the answers.
For further information please see: GPR Training Module 3.2 Reflections and Target Depths
Ground Penetrating Radar can be used to perform a variety of GPR surveys including:
The maximum penetration depth of a Ground Penetrating Radar varies depending on the frequency of the GPR being used and the material the ground is made up of. Typical GPR penetration can be in the region of:
Maximum penetration is also dependent on the ground material, wet clay and soils with high salinity or concrete containing fly ash will provide very poor soils for GPR.
For further information please see: GPR Training Module 3.3 GPR Penetration
Higher frequency GPR have a higher resolution and can see between reinforcing bars and detect objects with smaller diameters whilst lower frequency GPR have less resolution.
As a rule of thumb, GPR can resolve objects with a diameter of 10% of the depth at which they are detected. That means that an object with a 10cm diameter could be detected at depths of up to 1m. This is a simple rule of thumb and under most conditions GPR performance can exceed this.
For further information please see: GPR Training Module 3.4 GPR Resolution
Whilst a very useful survey technique, GPR is subject to some limitations. Please see our Limitations of GPR page for more information.
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.