As a borehole depth increases, sonde methods eventually become ineffective (depending on the soil characteristics). For deeper bores, the most common tracking method uses a “wireline” to connect a tracking mechanism behind the drill head to the surface. This wireline connection presents difficulties because the connection must be maintained as new sections of drill rod are added. The tracking mechanism itself typically uses magnetic field sensors to determine the orientation of the drill string (just behind the drill bit) with respect to the earth’s magnetic field. Knowing this orientation and the distance traveled between measurements, allows the full drill path trajectory to be established.