There are more than 64 million kilometres of roads in the world, all of different age and condition, some paved and some unpaved. The road infrastructure is important for innumerable reasons and needs to be maintained to a certain level to ensure safe access, both for private and public transportation.
Today, most inspections and maintenance plans are based on invasive investigations with drilling or sampling of the roadbed at discrete locations; this gives precise information but only at those points. With a growing road network requiring maintenance due to age and use, the need for information on its condition is increasing. Geophysical methods can then be a suitable solution to create a more comprehensive picture of the internal structure of our roads.
The depth and resolution that can be achieved depends on the frequency of the transmitted electromagnetic pulse. A higher frequency results in higher data resolution, whilst a lower frequency results in better depth penetration. What frequency to use depends on the application, and what resolution and depth is required. For road embankment mapping (with total thickness and internal layers) frequencies around 450 MHz to 2.3 GHz are quite convenient. If the geology underneath the road are of interest as well, frequencies of 100 to 250 MHz can be used.
Most often it is efficient to use two antenna frequencies or more at the same time, providing high resolution for thin asphalt layers with a high frequency antenna and a good overview of the total road embankment with a lower frequency antenna. As seen in the table below resolution changes with antenna frequency but this is also true for an increasing depth, meaning that objects need to be larger, at depth, to be identified.
Table 1. GPR antenna frequency and estimated resolution and depth.
Frequency is not the only factor that determines the achievable depth and resolution; the ground through which the signal is sent, also determines the quality of the collected data. Resistive soils such as sand and gravel, as well as rocks, are usually suitable for GPR surveys, whilst less appropriate soils usually contain conductive materials, for example, clay. Soils saturated with water can also be problematic for GPR surveys.
GPR measurements for road applications are most often done by pulling the antennas in a cart along the ground, which can be efficient for smaller areas. For longer investigation lines, Guideline Geo also provides a convenient cart, the RoadCart, for holding 1-3 antennas behind a vehicle. The MALÅ MIRA HDR can be an efficient tool if wider and very precise investigations are needed.
During measurements, the position of the GPR antenna is tracked by an external or internal GPS for precise positioning of your investigation results.
Suitable applications to solve with GPR within the area of roads are:
The results from a GPR investigation on roads is rather straightforward. Data can be analysed in different types of software (for example ReflexW and GPR-Slice) where the data is processed and interpreted according to application. Layer data is most often presented as X, Y and Z, where X and Y gives the positions and Z the depth to the interpreted layer. This data can be viewed superimposed on maps with easy color scheme representing the layer thickness interpretation. The example image shows a road investigation of asphalt. As seen in the mid parts of the profile, asphalt has been added on several occasions.