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A mud hammer is driven by the flow of drilling fluid to generate pressure pulses. Timing and/or amplitude of the pulses are altered to encode data by applying electromagnetic forces to a movable member of the mud hammer. In an example embodiment the movable member carries one or more magnets and ele

A mud hammer is driven by the flow of drilling fluid to generate pressure pulses. Timing and/or amplitude of the pulses are altered to encode data by applying electromagnetic forces to a movable member of the mud hammer. In an example embodiment the movable member carries one or more magnets and electromagnetic forces are applied to the movable member by one or more electromagnets. The mud hammer may also generate electrical power that may be applied to charge batteries and/or drive downhole electrical apparatus.

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1. A mud pulse telemetry method comprising: operating a downhole pulser in a drill string to generate pressure pulses by flowing drilling fluid through the drill string, the flowing drilling fluid causing oscillating motion of a movable member of the pulser, the movable member comprising a hammer fo

1. A mud pulse telemetry method comprising: operating a downhole pulser in a drill string to generate pressure pulses by flowing drilling fluid through the drill string, the flowing drilling fluid causing oscillating motion of a movable member of the pulser, the movable member comprising a hammer for generating drilling fluid pulses within a bore of the drill string;altering motion of the movable member by applying electromagnetic forces to the movable member to alter one or both of the intensity and timing of the pressure pulses according to telemetry data. 2. A method according to claim 1 wherein altering motion of the movable member comprises allowing a magnetic field from one or magnets in the movable member to induce electrical currents in a plurality of coils and varying a load applied to each of the plurality of coils. 3. A method according to claim 2 wherein the coils are spaced apart longitudinally and extend around the movable member the movable member moves along an axis concentric with the coils. 4. A method according to claim 3 wherein the one or more magnets comprise a plurality of magnets angularly spaced apart around the movable member. 5. A method according to claim 4 wherein the plurality of magnets are aligned parallel to the axis. 6. A method according to claim 2 wherein the one or more magnets comprise a plurality of magnets angularly spaced apart around the movable member and the plurality of coils are angularly spaced apart around an axis along which the movable member moves. 7. A method according to claim 6 wherein for each of the plurality of magnets there is a corresponding one of the plurality of coils. 8. A method according to claim 7 wherein the plurality of magnets alternate in polarity. 9. A method according to claim 8 wherein controlling the motion of the movable member comprises driving electrical currents through the coils of the plurality of coils. 10. A method according to claim 9 comprising selecting polarities of the currents such that magnetic fields produced by the coils of the plurality of coils alternate in orientation. 11. A method according to claim 2 comprising applying electrical currents induced in the coils to charge a battery. 12. A method according to claim 1 comprising detecting the pressure pulses at a location removed from the downhole pulser and extracting the telemetry data from the detected pressure pulses. 13. A method according to claim 12 wherein the location removed from the downhole pulser is a location where the drill string emerges from a surface of the earth. 14. A method according to claim 1 wherein the movable member is located in a bore of a drill collar and applying electromagnetic forces to the movable member comprises passing electrical current through one or more electromagnets in a wall of the drill collar. 15. A method according to claim 1 wherein altering the motion of the movable member comprises altering a frequency of oscillation of the movable member. 16. A method according to claim 1 comprising tracking one or both of a position of the movable member and a velocity of the movable member and timing application of the electromagnetic forces to the movable member based on one or both of the tracked position and the tracked velocity of the movable member. 17. A method according to claim 16 wherein the position or the velocity of the moveable member is tracked based on a pressure measurement of downhole drilling fluid. 18. A method according to claim 16 further comprising applying the motion of the moveable member to generate electrical power, and wherein the position or the velocity of the moveable member is tracked based on either the current or the voltage of the generated electrical power. 19. A method according to claim 1 further comprising applying the motion of the movable member to generate electrical power. 20. A method according to claim 19 wherein altering the motion of the movable member comprises timing the generation of electrical power. 21. A method according to claim 19 wherein generating electrical power comprises allowing a magnetic field from one or more magnets carried by the movable member to induce an electrical current in a coil located adjacent to the movable member. 22. A method according to claim 1 comprising applying the generated electrical power to charge a battery. 23. A method according to claim 19 comprising applying the generated electrical power to drive altering motion of the movable member. 24. A method for operating a mud hammer, the method comprising: providing a hammer for generating drilling fluid pulses within a bore of a drill string, at least one magnet coupled to the hammer, an electromagnet located to generate a variable magnetic field at the magnet and a power source connected to drive the electromagnet;driving motion of the hammer under the combined influence of a flow of drilling fluid through the bore and the variable magnetic field and controlling the variable magnetic field to alter the motion of the hammer to generate pulses in the drilling fluid, the pulses encoding data. 25. A method according to claim 24 wherein the data is encoded in the frequency of the pulses. 26. A method according to claim 24 wherein the data is encoded in the amplitude of the pulses. 27. A mud hammer comprising: a hammer movable relative to a fluid port in a bore of a drill string section to generate drilling fluid pulses within the bore;a magnet;a coil located near the magnet;a power source connected to energize the coil to generate a variable magnetic field at the magnet;wherein: one of the magnet and the coil is coupled to the hammer; andthe power source comprises a control circuit configured to receive a signal encoding data; and the control circuit is configured to control the variable current through the coil to alter motion of the hammer to generate drilling fluid pulses encoding the data. 28. A mud hammer according to claim 27 wherein the magnet comprises a plurality of magnets angularly spaced apart around the hammer. 29. A mud hammer according to claim 28 wherein neighbouring ones of the magnets are opposite in polarity. 30. A mud hammer according to claim 29 wherein the coil comprises one of a plurality of coils that are angularly spaced apart around an axis along which the hammer is movable. 31. A mud hammer according to claim 30 wherein for each of the plurality of magnets there is a corresponding one of the plurality of coils. 32. A mud hammer according to claim 31 comprising a power supply controllable to drive electric currents through the coils, the power supply connected so as to cause magnetic fields of adjacent ones of the coils to have opposite polarities. 33. A mud hammer according to claim 30 comprising a control circuit configured to selectively connect each of the plurality of coils to an electrical load. 34. A mud hammer according to claim 27 comprising a polarity reversing switch coupled between the power source and the coil. 35. A mud hammer according to claim 34 wherein the polarity-reversing switch comprises an H-bridge circuit. 36. A mud hammer according to claim 1 comprising a switching network configured to selectively connect the coil to the power supply, a load or another coil. 37. A mud hammer according to claim 36 wherein the switching network is configured to selectively connect the coil to one of a plurality of loads. 38. A mud hammer according to claim 27 wherein the coil is coupled to the hammer. 39. A mud hammer according to claim 27 wherein the magnet is coupled to the hammer. 40. A mud hammer according to claim 29 wherein the coil is mounted within a wall of the section of drill string. 41. A mud hammer according to claim 39 wherein the coil comprises a wire coil extending around the circumference of the bore. 42. A mud hammer according to claim 27 wherein the coil is coupled to provide electrical power to an electronic component. 43. A mud hammer according to claim 42 wherein the electronic component comprises a battery. 44. A mud hammer according to claim 42 where in the electronic component comprises an electromagnetic telemetry system. 45. A mud hammer according to claim 27 comprising a shoulder located to restrict movement of the hammer in a downhole direction. 46. A mud hammer according to claim 27 comprising a shoulder located to restrict movement of the hammer in an uphole direction. 47. A mud hammer according to claim 45 wherein the shoulder is mounted to the section of drill string. 48. A mud hammer according to claim 45 wherein the shoulder defines the fluid port. 49. A mud hammer according to claim 27 comprising a biasing means configured to bias the hammer toward a position in which the fluid port is less restricted. 50. A mud hammer according to claim 27 comprising a biasing means configured to bias the hammer toward a position in which the fluid port is more restricted. 51. A mud hammer according to claim 49 wherein the hammer defines a cavity, and wherein the biasing means is located in the cavity. 52. A mud hammer according to claim 27 wherein the hammer is cylindrical and the axis of the hammer is substantially collinear with the axis of the section of drill collar. 53. A mud hammer according to claim 52 wherein: the hammer comprises a keying feature that engages a corresponding alignment feature in the section of drill collar;the keying feature maintains the axis of the hammer in a substantially collinear relationship with the axis of the section of drill collar; andthe keying feature prevents the rotation of the hammer relative to the section of drill collar. 54. A mud hammer according to claim 52 wherein: the hammer comprises a plurality of projections;the projections extend away from the hammer and abut an inner surface of the bore; andthe projections maintain the axis of the hammer in a substantially collinear relationship with the axis of the bore. 55. A mud hammer according to claim 52 wherein an uphole end of the hammer is tapered.