An apparatus is provided for assessing the location of a
drill bit underground. The apparatus includes an acoustic sound generator that is driven by the drilling mud supplied to the
drill bit. The sound generator a characteristic string of pulses, which may be termed a signature or key. The key is repeated over and over. Monitors (i.e., sensors) at the surface listen for this key. The key is distorted by the inconstant
angular velocity of the
drill bit. Thus the observed data do not precisely match the key. On the basis of numerical algorithms, a digitally revised reference
signal or key, is identified to map the known reference key onto the
best fitting observed data. The correction factors are then applied to map the modified reference key onto the data observed at other sensors of an array of sensors mounted on the surface. By determining the phase shift and
travel time of the signals at the various sensors, and having determined the speed of
wave propagation in the geological media, the position of the bit, or a fairly close approximation thereof, may be obtained. The correction factors applied to the reference key may also tend to permit the actual rotational speed of the
drill bit to be determined.