A process for controlling the direction of advance of a generally horizontal borehole extended through a subterranean coal-bearing formation by a rotary drilling operation. As the drill string is rotated in the hole, the advance of the drill bit is guided along a designated path by repeatedly deflecting...http://www.google.fr/patents/US4471843?utm_source=gb-gplus-shareBrevet US4471843 - Method and apparatus for rotary drill guidance
