The DHTG borehole geophone has been designed to satisfy the increasing demands of a highly specialised market. The probe manufactured entirely in stainless steel, the fiberglass case support and the field proven electronics makes the DHGT a reliable and robust field instrument. Its main characteristic is the clamping system, consisting of the progressive bending of an harmonic steel spring aside the probe. A powerful continuous current motor, mounted inside the probe and controlled by the surface electronics, pushes a piston fixed to one end of the stainless steel spring. When the piston comes back to its starting position, the progressive bending of the spring clamps the probe to the opposite wall. Pushing out the piston again unclamps the probe, which can then be easily pulled out of the hole.