GPR for river dyke - GSB

Figure 4: Layout of measuring on Prottasha bridge where a pair of 1.2 m antennas are used
4 Test results
NGI has designed two types of tests for this on site training of GSB personnel. One is the
indoor test to detect re-bars in the concrete and brick wall etc. The other is the outdoor test
on a river dyke. In the indoor test, horn (Transmitting & Receiving) antennas of length 10 cm
have been used (see Figure 3). The outdoor tests use two antennas of length 1.2 m: one for
transmitting the signal and the other for receiving the signal. Both antennas are tied with a
rope with 1 m separation between them (see Figure 4).
4.1 Indoor test results
Figure 5 shows the test result obtained while the radar antennas moved on a concrete floor
(Figure 3). A figure like Figure 5 is called as a radargram. The horizontal axis of the figure is
the trace number or the measurement number. In this test we move 2 cm to perform one
measurement. Therefore in the figure, each horizontal unit is 2 cm. The vertical axis is the
travelling time. In the figure the red arrow points to the time reference corresponding to the
ground surface. The blue arrow points to the time when the reflection from the bottom of the
floor concrete block arrives. The travelling time difference between the reflections from the
ground surface and the bottom of the concrete plate is about t = 2.5 ns. If we assume the
wave speed in the concrete is v =14 cm/ns. The thickness of the concrete block is s = v t/2
= 17.5 cm. According to theory, the reflection from a point target appears as a hyperbolic
curve in a radargram. In the radargram, we find many hyperbolas (yellow arrows in the
figure), which are the reflections from the re-bars inside the concrete floor.