Patent ID: 8044819

Claim:
A method to detect the relative position of a drill bit with respect to a coal seam boundary using an electric-field borehole telemetry apparatus, that includes the steps: a) providing a measure-while-drilling apparatus that includes inclination sensors, directional sensors, logging sensors of choice and an electric-field borehole telemetry apparatus, b) within the electric-field borehole telemetry apparatus, in addition to monitoring the inclination, direction and logging parameters, monitoring one or more parameters of the electrical output of the telemetry apparatus, c) transmitting to the surface the inclination, direction and logging parameters as well as the one or more parameters of the said electrical output by means of the telemetry apparatus, d) computing the usual drilling parameters needed to guide the drill string along the intended path, e) determining from the one or more transmitted parameters of the electrical output from the said downhole apparatus parameters indicative of approaching or penetrating the coal boundary, and f) making corrections to the direction of drilling to maintain the drill string and bit in the coal seam, g) the method including: i) providing an insulating gap in lower end extent of the drill string, directly behind the drill bit, thereby to maneuver the gap to travel closely and in alignment with the bit in the coal seam, ii) applying output voltage derived from the measure while drilling apparatus to the gap maintained with the bit in the coal seam to derive a voltage difference between electrical leads provided at the upper end of the string and in the earth at a distance from said upper end of the string.