A
telemetry tool is placed in a
drill string adjacent to a
drill bit. The
telemetry tool includes position and direction sensors, a power supply, a
signal receiver and a
signal emitter. An external base unit, may be mounted on the surface at some distance from the
wellhead. The external base unit generates, and transmits into the
geological formation, a
carrier signal. The
carrier signal may be a DC
carrier signal, and it may be a variable DC
signal.The base unit also has a receiving unit, which may have the opposite
DC voltage to that of the carrier signalemitting unit. The
geological formation provides a large current path between the
carrier current generating unit and the downhole end of the
drill string. The
carrier current, or some portion thereof, is received at the
telemetry unit. The
drill string defines a relatively
low resistance conductor for the return signal. Thus a circuit is established from the base unit, through the
geological formation to the
telemetry unit, and back up the
drill string to end back at the base unit.The
telemetry unit superimposes a time varying
signal on the
carrier current. The time varying signal includes a
recognition sequence, followed by a data string. The data string may include information pertaining to
compass direction,
azimuth dip, rotational speed, acceleration in any of three axes, and so on.The base unit strips the signal off the carrier current, and reads the code. This
system may tend to permit the telemetry tool to operate at relatively low power. The relatively
higher power carrier current is provided by the base unit.