For every position of the reception probe, a sound pulse is broadcast by a stroke a hammer gives to the foundation or to the structure carried by the foundation.

The hammer is fitted with a precise triggering device which causes the measurement to start as soon as it strikes the foundation.

Then, the sound wave propagates through the foundation.

The CS 97 unit records the signal received by the reception probe in the tube. The duration between the hammer stroke and the first arrival of the sound wave on the reception probe is measured.

The probe is taken 0.5 m lower down again, a new hammer stroke is given, and so on ...

The measurement terminates when the probe has reached the bottom of the tube.

If two operators make the measurement, it takes no more than about 25 minutes to wholly test a 20 metre deep drill.