The sonic echo test is a survey method consisting in the measurement of the reflection speed of compression waves, to assess the integrity of a pole.
The method was conceived in Holland during the 70s, as instrument to check the quality of pre-fabricated foundation poles in concrete, extensively used in that country. Due to the regularity of the surfaces of the prefabricated poles, the sonic echo test could be used with a high level of reliability. For this type of test, a compression wave is propagated until the base of the pole and is reflected towards its head. The compression wave is generated with an impact on the pole head and the signal recorded by a geophone is displayed in a graph in real time.
If discontinuities are present in the pole such as variations of the section or cracks, these cause reflections. If the discontinuities are fairly significant, such as for example a complete fracture of the concrete, these cause an almost complete fracture, thus preventing to detect the base of the pole.
In order to execute the test properly, the pole head must be accessed.
The geophone is placed on the concrete surfaced, properly prepared, through a coupling material and by placing on top a load of a few kg. When the head of the pole is hit in a selected point, the impact enables the acquisition of the signal produced by the geophone, which is shown immediately on the screen.