Abstract: A string of sondes is designed to be lowered down a borehole by means of a cable in order to detect the arrival of seismic waves therein, the sondes including retractable anchor means that are hydraulically actuateable and that are connected by a hydraulic circuit to a common source of hydraulic pressure. The string includes means for putting said hydraulic circuit to ambient pressure, said means being connected to said cable and being adapted to act on said hydraulic circuit in order to cause said anchor means to be retracted when a traction force exceeding a selected value is exerted on said cable.