Low Frequency Ground Probing Radar (GPR)

Engineering and Environmental Applications and Examples

Why use GPR

A ground probing radar (GPR) survey is often the only geophysical technique that can be used in engineering and environmental applications on busy urban and industrial sites. It is capable of working through a wide variety of surface materials both inside and outside buildings and structures. Typical locations can range from car parks, highways and waste ground through to floors of factories, basements and sub-structures.

How it works

The radar system works by transmitting a pulse of radiowaves at a set central frequency into the material being examined. A receiver is set up to scan for returns from the transmitter. Low frequencies GPR penetrate to deeper depths that high frequency radar, the lower the frequency the greater the depth but at the expense of resolution. The receiver is configured to record approximately 64 readings per point; this stacked data is then averaged to produce a single reliable reading, reducing the effect of spurious data and background 'noise'. Due to the antennas being unshielded, the presence of nearby buildings may be picked up in the radar data.

The success of GPR relies on the variability of different materials in the transmission of radiowaves. Contrasts in the dielectric constant of materials result in variations in the amplitude of the radiowaves returning to the receiver. These variations form anomalies within the mean level of ground response.

Surveying techniques

Low frequency GPR is often used in conjunction with high frequency radar, as the high frequency, although having less depth penetration has a greater resolution and therefore can identify possible services that may interfere with the low frequency GPR. Subsequently, selected traverses are surveyed; collecting deep penetrating, relatively noise free data.

Case study

A previously derelict building was proposed for redevelopment with the knowledge of a railway tunnel running underneath the building. The objective of the survey was to determine the depth of cover to the rail tunnel to allow load-bearing calculations to be made.