Enhanced sonde recognition

• US 20070085698A1
• Filed: 10/14/2005
• Published: 04/19/2007
• Est. Priority Date: 10/14/2005
• Status: Active Grant

• Current Assignee: Radiodetection Limited
• Original Assignee: Radiodetection Limited
• Inventors: Royle, John
• Granted Patent: US 7,310,584 B2
• Time in Patent Office: Days
• US Class Current: 340/870.100
• CPC Class Codes: G01V 3/26 operating with magnetic or electric fields produced or modified either by the surrounding earth formation or by the detecting device with electromagnetic waves G01V3/30

Abstract

A detector for locating a sonde includes a plurality of antennas, an analogue to digital converter 11 and a digital signal processor to isolate the magnetic signal produced by the sonde. The digital signal processor includes a phase feedback loop 21 to allow the digital processing unit to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde.

Claims

1. A detector for detecting an oscillating magnetic signal produced by a sonde, the detector comprising:

a plurality of antennas for detecting electromagnetic signals with a plurality of frequencies comprising the magnetic signal produced by the sonde, each antenna having an analogue output;

an analogue to digital converter for sampling the analogue output of the antennas to produce a digital output; and

digital processing means, for processing the digital output from the analogue to digital converter to isolate the magnetic signal produced by the sonde, wherein the digital processing means comprises a phase feedback loop to allow the digital processing means to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde.

2. A detector as claimed in claim 1, wherein the digital processing means comprises frequency shifting means for shifting the digital output from the analogue to digital converter in the frequency domain so that the frequency spectrum of the magnetic signal produced by the sonde is shifted to DC.

3. A detector as claimed in claim 2, wherein the frequency shifting means comprises mixing means.

4. A detector as claimed in claim 3, wherein the mixing means comprises a homodyne mixer.

5. A detector as claimed in claim 1, wherein the digital processing means comprises a filter having a frequency response of a sinc^m filter, where m≧

2, to produce a sinc^m filtered output.

6. A detector as claimed in claim 5, wherein the filter is a finite impulse response filter.

7. A detector as claimed in claim 6, wherein the finite impulse response filter comprises N,m taps, having coefficients a_m,n, where:

$a_{m,n}=\underset{k=n}{\overset{n+N}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>a_{m-1,k};<br><br>$ $a_{0,n}=0;<br><br>$ $a_{0,N·<br><br>m}=1;<br><br>$ $N=\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{number}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{samples}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{in}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{aperture}<br><br>\underset{\_}{.}$

8. A detector as claimed in claim 5, wherein the sinc^m filtered output is passed to a downsampler which downsamples every N samples to produce one sample.

9. A detector as claimed claim 1, wherein the digital processing means comprises a low-pass filter to isolate the magnetic signal produced by the sonde.

10. A detector as claimed in claim 9, wherein the bandwidth of the low-pass filter is less than 2.0 Hz at −

130 dB.

11. A detector as claimed in claim 9, wherein the bandwidth of the low-pass filter is less than 1.0 Hz at −

135 dB.

12. A detector as claimed in claim 9, wherein the bandwidth of the low-pass filter is less than 0.5 Hz at −

140 dB.

13. A detector as claimed in claim 12, wherein the analogue to digital converter comprises a delta-sigma analogue to digital converter.

14. A detector as claimed in claim 12, the detector comprising a frequency spectrum analyzer having frequency bins and an output, wherein the frequency bins of the frequency spectrum analyzer are substantially aligned with a fundamental frequency and a plurality of the harmonic frequencies of the magnetic signal produced by the sonde.

15. A detector as claimed in claim 14, wherein the frequency bins are adapted to follow frequency variation of the magnetic signal produced by the sonde.

16. A detector as claimed in claim 14, wherein the output from the spectrum analyser analyzer is passed to a matched filter which produces a rank correlation coefficient.

17. A detector as claimed in claim 16, wherein the output from the matched filter is passed to an arbitrator which decides whether the sonde is detectable by the detector or not.

18. A detector as claimed in claim 14, wherein the frequency spectrum analyzer comprises a complex discrete Fourier transform.

19. A detector as claimed in claim 14, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 20 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

20. A detector as claimed in claim 14, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 10 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

21. A detector as claimed in claim 14, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 5 mHz of the fundamental frequency of the magnetic signal produced by the sonde

22. A detector as claimed in claim 14, the output of the frequency spectrum a analyzer being comprised of measurements of the fundamental frequency and harmonic frequencies of the magnetic signal produced by the sonde and measurements of noise, allowing a signal to noise ratio to be calculated for the fundamental frequency and harmonic frequencies, wherein the signal to noise ratio for at least 4 of the harmonic frequencies is greater than 6 dB.

23. A detector as claimed in claim 14, wherein the signal to noise ratio for at least 8 of the harmonic frequencies is greater than 6 dB.

24. A detector as claimed in claim 14, wherein the signal to noise ratio for at least 16 of the harmonic frequencies is greater than 6 dB.

25. A method for detecting an oscillating magnetic signal produced by a sonde, the method comprising:

detecting electromagnetic signals with a plurality of frequencies comprising the magnetic signal produced by the sonde;

sampling the detected signal to produce a digital output;

processing the digital output with a digital signal processor to isolate the magnetic signal produced by the sonde;

converting the isolated signal to its magnitude and phase measurements; and

feeding back the phase of the isolated signal to allow the processor to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde.

26. A method as claimed in claim 25, wherein the processing comprises shifting the frequency of the digital output in the frequency domain so that the frequency spectrum of the magnetic signal produced by the sonde is shifted to DC.

27. A method as claimed in claim 26, wherein the frequency shifting comprises mixing.

28. A method as claimed in claim 27, wherein the mixing comprises homodyne mixing.

29. A method as claimed in claim 25, wherein the processing comprises filtering the frequency shifted signal with a filter having a frequency response of a sinc^m filter, where m≧

2, to produce a sine^m filtered output.

30. A method as claimed in claim 29, wherein the filter is a finite impulse response filter.

31. A method as claimed in claim 30, wherein the finite impulse response filter comprises N.m taps, having coefficients a_m,n, where:

$a_{m,n}=\underset{k=n}{\overset{n+N}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>a_{m-1,k};<br><br>$ $a_{0,n}=0;<br><br>$ $a_{0,N·<br><br>m}=1;<br><br>$ $N=\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{number}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{samples}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{in}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{aperture}<br><br>\underset{\_}{.}$

32. A method as claimed in claim 29, wherein the sinc^m filtered output is passed to a downsampler which downsamples every N samples to produce one sample.

33. A method as claimed in claim 32, wherein the downsampled signal is passed to a low-pass filter to isolate the magnetic signal produced by the sonde.

34. A method as claimed in claim 33, wherein the bandwidth of the low-pass filter is less than 2.0 Hz at −

130 dB.

35. A method as claimed in claim 33, wherein the bandwidth of the low-pass filter is less than 1.0 Hz at −

135 dB.

36. A method as claimed in claim 33, wherein the bandwidth of the low-pass filter is less than 0.5 Hz at −

140 dB.

37. A method as claimed in claim 25, wherein the sampling is performed by a delta-sigma analogue to digital converter.

38. A method as claimed in claim 25, the magnetic signal having a fundamental frequency and harmonic frequencies, the method comprising analyzing the frequency spectrum of the magnetic signal with a frequency spectrum analyzer having frequency bins and an output, wherein the frequency bins of the frequency spectrum analyzer are substantially aligned with the fundamental frequency and a plurality of the harmonic frequencies of the magnetic signal produced by the sonde.

39. A method as claimed in claim 38, wherein the frequency bins follow frequency variation of the magnetic signal produced by the sonde.

40. A method as claimed in claim 38, wherein the output from the spectrum analyzer is passed to a matched filter which produces a rank correlation coefficient.

41. A method as claimed in claim 40, wherein the output from the matched filter is passed to an arbitrator which decides whether the sonde is detectable by the detector or not.

42. A method as claimed in claim 38, wherein the frequency spectrum analyzer comprises a complex discrete Fourier transform.

43. A method as claimed in claim 38, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 20 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

44. A method as claimed in claim 38, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 10 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

45. A method as claimed in claim 38, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 5 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

46. A method as claimed in claim 38, the output of the frequency spectrum analyzer being comprised of measurements of the fundamental frequency and harmonic frequencies of the magnetic signal produced by the sonde and measurements of noise, the method comprising measuring the signal to noise ratio of the fundamental frequency and harmonic frequencies, wherein the signal to noise ratio for at least 4 of the harmonic frequencies is greater than 6 dB.

47. A method as claimed in claim 38, wherein the signal to noise ratio for at least 8 of the harmonic frequencies is greater than 6 dB.

48. A method as claimed in claim 38, wherein the signal to noise ratio for at least 16 of the harmonic frequencies is greater than 6 dB.

49. A computer program in a computer readable medium for detecting an oscillating magnetic signal produced by a sonde in an electromagnetic signal with a plurality of frequencies comprising the magnetic signal produced by the sonde, the computer program comprising:

sampling the electromagnetic signal to produce a digital output;

processing the digital output to isolate the magnetic signal produced by the sonde;

converting the isolated signal to its magnitude and phase measurements; and

feeding back the phase of the isolated signal to allow the processing to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde.

50. A computer program as claimed in claim 49, wherein the processing comprises shifting the frequency of the digital output in the frequency domain so that the frequency spectrum of the magnetic signal produced by the sonde is shifted to DC.

51. A computer program as claimed in claim 50, wherein the frequency shifting comprises mixing.

52. A computer program as claimed in claim 51, wherein the mixing comprises homodyne mixing.

53. A computer program as claimed in claim 49, wherein the processing comprises filtering the frequency shifted signal with a filter having a frequency response of a sinc^m filter, where m≧

2, to produce a sinc^m filtered output.

54. A computer program as claimed in claim 53, wherein the filter is a finite impulse response filter.

55. A computer program as claimed in claim 54, wherein the finite impulse response filter comprises N.m taps, having coefficients a_m,n, where:

$a_{m,n}=\underset{k=n}{\overset{n+N}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>a_{m-1,k};<br><br>$ $a_{0,n}=0;<br><br>$ $a_{0,N·<br><br>m}=1;<br><br>$ $N=\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{number}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{samples}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{in}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{the}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{aperture}<br><br>\underset{\_}{.}$

61. A computer program as claimed in claim 49, wherein the sampling is performed by a delta-sigma analogue to digital converter.

62. A computer program as claimed in claim 49, the magnetic signal having a fundamental frequency and harmonic frequencies, the computer program comprising analyzing the frequency spectrum of the magnetic signal with a frequency spectrum analyzer having frequency bins and an output, wherein the frequency bins of the frequency spectrum analyzer are substantially aligned with the fundamental frequency and a plurality of the harmonic frequencies of the magnetic signal produced by the sonde.

63. A computer program as claimed in claim 62, wherein the frequency bins follow frequency variation of the magnetic signal produced by the sonde.

64. A computer program as claimed in claim 62, wherein the output from the spectrum analyzer is passed to a matched filter which produces a rank correlation coefficient.

65. A computer program as claimed in claim 64, wherein the output from the matched filter is passed to an arbitrator which decides whether the sonde is detectable by the detector or not.

66. A computer program as claimed in claim 62, wherein the frequency spectrum analyzer comprises a complex discrete Fourier transform.

67. A computer program as claimed in claim 62, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 20 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

68. A computer program as claimed in claim 62, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 10 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

69. A computer program as claimed in claim 62, wherein one of the frequency bins of the frequency spectrum analyzer is aligned within 5 mHz of the fundamental frequency of the magnetic signal produced by the sonde.

70. A computer program as claimed in claim 62, the output of the frequency spectrum a analyzer being comprised of measurements of the fundamental frequency and harmonic frequencies of the magnetic signal produced by the sonde and measurements of noise, the method comprising measuring the signal to noise ratio of the fundamental frequency and harmonic frequencies, wherein the signal to noise ratio for at least 4 of the harmonic frequencies is greater than 6 dB.

71. A computer program as claimed in claim 62, wherein the signal to noise ratio for at least 8 of the harmonic frequencies is greater than 6 dB.

72. A computer program as claimed in claim 62, wherein the signal to noise ratio for at least 16 of the harmonic frequencies is greater than 6 dB.

56. A computer program as claimed in 53, wherein the sinc^m filtered output is passed to a downsampler which downsamples every N samples to produce one sample.

57. A computer program as claimed in claim 56, wherein the downsampled signal is passed to a low-pass filter to isolate the magnetic signal produced by the sonde.

58. A computer program as claimed in claim 57, wherein the bandwidth of the lowpass filter is less than 2.0 Hz at −

130 dB.

59. A computer program as claimed in claim 57, wherein the bandwidth of the low-pass filter is less than 1.0 Hz at −

135 dB.

60. A computer program as claimed in claim 57, wherein the bandwidth of the low-pass filter is less than 0.5 Hz at −

140 dB.

73. (canceled)

74. (canceled)