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We show examples of application of the GPR method in Geotechnics. The examples presented were supplied by Geomega Company. The data records were interpreted by Pierre Streicher.
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Step 1: Detection of networks buried under a paved road
Figure shows a radar section and its interpretation. It concerns the detection of networks buried under a paved road, prior to the drilling of a directional well. At a depth of 1 m, there is a telecommunication cable (PTT), which in places is buried deeper near a trench. This trench, in which a large diameter pipe and two smaller-diameter pipes are present, is made of a more absorbent material which attenuates the reflection on the cable.
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Step 2: Detection of geological features.
A GPR study was conducted in order to specify the location and extent of cavities that had been discovered during reconnaissance drilling. The section shown on the top was recorded with a source antenna emitting an electromagnetic pulse with a central frequency of 100 MHz. On the right-hand side of the image, we follow the stratification and the echo linked to the contrast between the limestone and the shaly layers. On the left-hand side, a particularly clear and marked echo corresponds to the cavity of an old exploitation chamber which has not been back-filled.
The section shown on the bottom is an example of detection of geological features under a railroad tunnel. In spite of the irregularity of the measurement surface (presence of railroad ties) and the difficulty of going through a very heterogeneous medium on the surface (ballast), the radar recording, using a low-frequency antenna (300 MHz), permits one to obtain information to a depth which, in this case in gneiss, is as much as 10 meters. At the surface, and still buried in the original signal, punctual and regularly spaced anomalies correspond to the railroad ties (sleepers). In the shallows, the ballast base presents a distinct quasi continuous horizon. Deeper, the radar image of the ground shows marked echoes related to fracturing.
