A method of and apparatus for determining the precise position of the drill bit on the bottom of a long drill string in a deep borehole in the earth, during a drilling operation. The method comprises placing a plurality of geophones in a three-dimensional array near the surface of the earth, above the expected position of the drill bit in the earth, causing at least a low energy seismic source to be initiated near the drill bit in the earth, repeating the source a number of times, and determining at the surface of the earth the times of initiation of each of the seismic waves. Responsive to the known times of initiation, stacking each of the repeated geophone signals from each of the geophones, for each of the repetitions of the source, whereby each of the stacked signals will be in-phase with those that resulted from earlier and later repetitions of the source. Several embodiments of an improved downhole seismic source are described, and an improved type of three-dimensional array.