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.

Abstract: A geophone apparatus capable of detecting both horizontal shear and compressional seismic waves has a pair of geophones mounted such that, when in use, said geophones will be inclined at an angle of about 45.degree. in opposite directions from vertical. The output signals generated by the two geophones are summed to yield a compressional wave trace and/or are subtracted one from the other to yield a horizontal shear wave trace.