The depth of the drill should be at least equal to that presumed of the foundation (5 metres).

A plastic tube with an internal diameter of 40 millimetres is placed into the drill ; it is then sealed to the ground over its whole length. This injection is to be performed with a cement slurry by means of a plunge tube (between the plastic tube and the ground).

The tube has to be stopped at its base by a strong stopper and it has to be filled with water.

The water and the cement injection are meant to allow a good sonic bond between the reception probe and the ground.

The injection has to be achieved with painstacking care : the fact is, a bad tube to ground bond does not allow the sound wave to reach the reception probe.