Apparatus for signalling within a borehole while drilling

Claim 1

A down-hole signal transmitter for a mud-pulse telemetry system, the transmitter comprising: (a) a sealed, elongate casing adapted to be incorporated in a drill string when the transmitter is installed down-hole; (b) an electrical generator disposed in a mud-free environment within the casing for supplying power to measuring instrumentation down-hole; (c) a turbine having an annular impeller surrounding the casing and mounted so as to be rotatable in the mud flow passing along the drill string; (d) a rotatable member within the casing magnetically coupled to the impeller through the wall of the casing to be driven thereby and mechanically coupled to impart driving torque to the electrical generator; (e) torque control means within the casing coupled to the rotatable member for controlling the load applied to the impeller by the rotatable member to vary the torque required to drive the impeller between a first value corresponding to a first state of the control means and a second value corresponding to a second state of the control means, whereby, in a given mud flow, the impeller is driven by the mud flow at a first rotational speed when the control means is in the first state and at a second rotational speed when the control means is in the second state; and (f) signalling means coupled to the control means and operative to change the state of the control means between the first state and the second state in response to a change in state of an electrical input signal representative of data to be signalled to the surface, whereby the rotational speed of the impeller is caused to vary between the first and second rotational speeds to transmit a modulated pressure signal in the mud flow in response to input of a varying electrical data input signal to the signalling means and the resulting modulated pressure signal is detectable at the surface and convertible into an electrical data output signal representative of the measurement data.