A simple, quick introduction to... Ground Penetrating Radar.

What is GPR?

Ground penetrating radar (GPR) is an advanced, non-invasive sub-surface imaging technique that typically uses short pulses of electromagnetic energy to ‘see’ into the ground.  GPR can image through soil, concrete, tarmac, rock, wood, ice and even water.  It is quick, easy to use and inexpensive in comparison to other investigation methods.  It is capable of probing down to a few tens of metres (depending on the system type & ground conditions) and provides the user with a ‘cross-sectional’ image of the sub-surface.

GPR is very good at determining the shape, size, extent and depth of a buried object.  It can be used to find pipes, foundations, voids, trenches, geological layers, re-inforcing bars, etc.  It works best in dry ground environments but can also work well in wet, saturated materials.  It does not work very well in saline conditions and through dense clays.

GPR is used by a wide range of people including:

• Civil Engineers & NDT Archaeologists
• Geologists
• Geotechnical Engineers
• Glaciologists
• Forensic Investigators
• Environmental Scientists
• Hydrologists

In fact, anyone who needs to investigate buried features.  It is particularly popular for utility detection and roadway evaluation.

GPR in action

As the GPR system is moved across the surface, pulses of electromagnetic energy are directed into the ground from a transmitting antenna.  The energy travels though the ground where buried objects reflect/scatter part of the energy back to the surface, which is picked up by the GPR’s receiving antenna.  These returning signals are converted in to a GPR section (or radargram) which is displayed on a computer screen in real-time.  The operator then uses knowledge, skill and experience to assess the data and interpret the features seen in the section.

Surveys are usually conducted along profile lines (to produce 2D sections). However, any number of closely spaced lines can be combined to create a 3D volume of data.  GPR systems are designed to use different antenna frequencies (typically 20-1500MHz).

Low frequency systems (20-250MHz) penetrate the deepest, but only image larger objects.  These tend to be used for hydrological, glacial and geological work.  Higher frequency systems (>250MHz) see smaller objects but don’t penetrate as deep. These are more commonly used for civil engineering and NDT applications.

GPR is perfectly safe to use but, as with any site technique, appropriate heath and safety risk assessments must be made before carrying out an investigation.  To use a GPR system in the EU, operators need to have an appropriate and up-to-date licence, be adequately trained, responsible and follow a code of practice.  Operators must ensure that their surveys do not interfere with any vital telecommunications systems (e.g., aircraft, hospitals, defense, etc).