Ground-penetrating radar surveys

A ground-penetrating radar survey is a geophysical survey that does not break the measured materials. The survey is based on electromagnetic waves sent to the road, soil or water on a radio wave frequency. Some of the sent waves return back to the recording unit when encountering an electric barrier layer, such as the surface of a different structural layer, and some of them continue until the next layer. When this is done as a continuous survey, it is possible to get a continuous profile, i.e. a vertical cross-section, of the measured target. The frequency range used in the survey is 10–3000 MHz.

This survey is used on roads and streets to determine the thicknesses of the current structural layers, and partly also to determine their quality. This information is utilized generally in structural improvement planning and quality control for a newly constructed road.

In soil studies, it is used to determine soil types and rockheads or water tables. In water system studies, the status and soil type of the bottom of a watercourse can be determined with this survey.

Roads and streets are measured with a 1–2,5 GHz air-coupled antenna (pavement radar) with a depth range of c. 0.8 m and with a 400–600 MHz ground-coupled antenna (structural radar) which can reach a depth of c. 3.5 m depending on the ground soil. During the survey, the targets are video recorded, and accurate coordinate information and travel information is collected for accurate locationing of the survey result.

The ground radar survey result is used to determine the thicknesses of the different structural layers, the soil type of the ground soil and discontinuities in the structure, such as mass replacements. The results can also show the location and position of steel nets installed inside the road as well as point-based targets, such as drums, pipes and cables.

This survey is also often used for quality control of the pavement and the construction of the whole structure. This includes determining the air void in the pavement, the thickness and position of the constructed layers and possible mass replacements or mass displacements.

Based on the ground radar survey result, sampling can be targeted to problem spots or discontinuities. With the help of the sampling, the interpretations of layer thicknesses and soil type definitions can also be confirmed.