Seismic sources that generate waves from deep within the earth, where the waves are of controlled character (shear or pressure in controlled directions), and that are coherent at frequencies of a plurality of hertz, are useful in seismic exploration. This is because the reflection of seismic waves from a geological strata depends upon the frequency of the waves and the distance from the source as a function of e.sup.f/c where f is frequency and c is the speed of sound. One technique for generating longitudinal shear waves from deep within a borehole, is to place a hydraulic actuator within the borehole, with the cylinder of the actuator clamped to the borehole wall, and to drive the actuator from a hydraulic source at the surface of the earth. Considerable power can be lost in transmitting high-pressure hydraulic fluid through a long depth of borehole, and it is difficult to closely control oscillations of high-pressure hydraulic fluids. A system which enabled the generation of a variety of seismic waves using an apparatus of relatively moderate cost located deep within a borehole, would be of considerable value.