Abstract: A mining machine (4) has a vertically movable jib (6) mounted in front of it and carrying a drum shaped drilling head (12) at its forward end. The drilling head is provided with radially projecting picks which rip coal out of a coal seam (1) located between an underburden (3) and an overburden (2). A forwardly-facing directional microphone (16) is mounted on the forward end of the machine (4) between the lines M where it can monitor the resonant frequencies of the picks. Electrical circuitry mounted on the machine controls the position of the jib (6) in accordance with the relative amplitudes of a unique set of natural resonant frequencies produced by the picks during mining.