Medium frequency pseudo noise geological radar

• US 6,651,739 B2
• Filed: 02/21/2001
• Issued: 11/25/2003
• Est. Priority Date: 02/21/2001
• Status: Expired due to Fees

• Current Assignee: Administrator of The National Aeronautics and Space Administration, National Aeronautics and Space Administration US Government As Represented By The Administrator of The, US Government As Represented By The Director National Security Agency
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Amini, B. Jon, Byerly, Kent A., Arndt, G. Dickey, Carl, James R.
• Primary Examiner(s): Bagnell, David
• Assistant Examiner(s): Walker, Zakiya
• Application Number:
  US09/793,817
• Publication Number:
  US 20020112855A1
• Time in Patent Office: 1,007 Days
• Field of Search: 166/254.2, 166/254.1, 166/250.03, 175/50, 324/326, 342/22, 342/25, 731/520.2
• US Class Current: 166/250.03
• CPC Class Codes: G01S 13/325 using transmission of coded signals, e.g. P.S.K. signals
• CPC Class Codes: G01S 13/885 for ground probing prospecting or detecting using electromagnetic waves G01V3/12
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005
• CPC Class Codes: Y02A 90/30 Assessment of water resources

Abstract

System and methods are disclosed for transmitting and receiving electromagnetic pulses through a geological formation. A preferably programmable transmitter having an all-digital portion in a preferred embodiment may be operated at frequencies below 1 MHz without loss of target resolution by transmitting and over sampling received long PN codes. A gated and stored portion of the received signal may be correlated with the PN code to determine distances of interfaces within the geological formation, such as the distance of a water interfaces from a wellbore. The received signal is oversampled preferably at rates such as five to fifty times as high as a carrier frequency. In one method of the invention, an oil well with multiple production zones may be kept in production by detecting an approaching water front in one of the production zones and shutting down that particular production zone thereby permitting the remaining production zones to continue operating.

Claims

1. A method for detecting interfaces in an earth formation, comprising:

generating a plurality of digital pulses;

producing said plurality of digital pulses at a carrier frequency;

transmitting said plurality of digital pulses into said geological formation from a first physical position;

detecting a received signal comprised of reflections from said geological formation due to said transmitting of said plurality of digital pulses;

over sampling said received signal such that a sampling rate is greater than five times said carrier frequency to produce a digitized received signal;

gating said digitized received signal for selecting a selected time frame of said received signal to produce a gated digitized received signal;

correlating said plurality of digital pulses with said gated digitized received signal for detecting an interface at a selected distance from said first physical position.

2. The method of claim 1, further comprising:

utilizing a carrier frequency in the range from 100 KHz to 1 MHz.

3. The method of claim 1, further comprising accumulating said plurality of digital pulses over a selected period of time prior to said step of correlating.

4. The method of claim 1, further comprising:

providing a digital to analog converter for converting said plurality of digital pulses at said carrier frequency to an analog signal prior to said step of transmitting.

5. The method of claim 4, wherein said step of oversampling further comprises oversampling said received signal such that a sampling rate is used that is from five to fifty times said carrier signal.

6. The method of claim 1, wherein said first position is within a borehole through said earth formation.

7. The method of claim 6, further comprising transmitting said plurality of digital pulses from at least one dipole antenna within said borehole.

8. The method of claim 7, wherein said producing of said plurality of digital pulses at said carrier frequency is done entirely with digital components.

9. The method of claim 8, further comprising providing a plurality of dipole antennas within a downhole tool.

10. The method of claim 1, further comprising:

transmitting said plurality of digital pulses through a non-ferrous wellbore tubular.

11. A downhole system for detecting an interface in an earth formation, said downhole system being operable in a borehole within said earth formation, said downhole system comprising:

an all digital signal generator for producing a plurality of digital pulses at a carrier frequency;

at least one dipole antenna associated with said downhole system for transmitting a transmitted signal and receiving a received signal;

an analog to digital signal converter for sampling said received signal at a sampling rate more than five times said carrier frequency to produce a received digital signal;

and a correlator for comparing said plurality of digital pulses with respect to said received digital signal.

12. The downhole system of claim 11, further comprising a digital to analog converter for converting said plurality of digital pulses at said carrier frequency to an analog signal.

13. The downhole system of claim 11, further comprising:

a plurality of dipole antennas.

14. The downhole system of claim 11, wherein said analog to digital converter has a sampling speed of from five to fifty times said carrier frequency.

15. The downhole system of claim 11, wherein said carrier frequency is selectable and may be selected to be less than 1 MHz.

16. The downhole system of claim 11, wherein said carrier frequency is between 100 KHz and 1 MHz.

17. The downhole system of claim 11, further comprising:

a gate control for selecting a portion of said received signal related to a selected distance from said borehole.

18. The downhole system of claim 11, further comprising:

a digital accumulator operable for accumulating said plurality of digital pulses prior to said step of correlation.

19. A system for detecting an interface in a geological formation, comprising:

a transmitter operable for producing a plurality of digital pulses into said geological formation at a selectable carrier frequency from at least 500 KHz to 1 MHz;

a receiver for receiving a received signal comprised of reflections of said plurality of digital pulses from said geological formation;

an analog to digital converter for said receiver having a sampling speed at least five times faster than said carrier frequency to produce a digitized received signal; and

a correlator for comparing at least a portion of said digitized received signal with said plurality of digital pulses.

20. The system of claim 19, wherein said system is operable for selectively transmitting at a frequency from 100 KHz to over 1 MHz.

21. The system of claim 19, further comprising a gate for selecting a time interval of said digitized received signal.

22. The system of claim 19, further comprising an accumulator for storing said digitized received signal for use by said correlator.

23. The system of claim 19, further comprising a digital to analog converter for said transmitter.

24. The system of claim 19, wherein said analog to digital converter has a sampling speed at least from five to fifty times said carrier frequency.

25. The system of claim 19, further comprising at least one dipole antenna.

26. The system of claim 19, further comprising a non-ferrous wellbore tubular through which said transmitter transmits said plurality of digital pulses.

27. The system of claim 19, further comprising one or more antennas positioned within a borehole.

28. The system of claim 19, wherein said transmitter is operable for producing said plurality of digital pulses at said carrier frequency entirely with digital components.

29. The system of claim 19, further comprising a digitally controllable filter for adjusting a frequency response of said received signal.

30. The system of claim 19, further comprising a wellbore tubular with an opening therein for receiving an antenna.

31. A downhole system for detecting an interface in a geological formation, said downhole system being operable in a borehole within said geological formation, said downhole system comprising:

an all digital signal generator for producing a plurality of digital pulses at a carrier frequency;

at least one dipole antenna associated with said downhole system for transmitting a transmitted signal and receiving a received signal;

an analog to digital signal converter for sampling said received signal at a sampling rate more than five times said carrier frequency to produce a received digital signal as well as to obtain proper range measurement accuracy and resolution for said interface;

and a pseudo-noise (PN) correlator for comparing said plurality of digital pulses with respect to said received digital signal.

32. A system for detecting an interface in a geological formation, comprising:

a transmitter operable for producing a plurality of digital pulses into said geological formation at a selectable carrier frequency from at least 500 KHz to 1 MHz;

a receiver for receiving a received signal comprised of reflections of said plurality of digital pulses from said geological formation;

an analog to digital converter for said receiver having a sampling speed at least five times faster than said carrier frequency to produce a digitized received signal and to obtain proper range measurement accuracy and range measurement resolution for said interface;

and a pseudo-noise (PN) correlator for comparing at least a portion of said digitized received signal with said plurality of digital pulses.

33. A downhole system for detecting a movable liquid body interface in a geological formation, said downhole system being operable in a borehole within said geological formation, said downhole system comprising:

an all digital signal generator for producing a plurality of digital pulses at a carrier frequency;

at least one dipole antenna associated with said downhole system for transmitting a transmitted signal and receiving a received signal;

a means for producing a received digital signal as well as increasing the accuracy and resolution of detecting said movable liquid body interface in said geological formation;

and a pseudo-noise (PN) correlator for comparing said plurality of digital pulses with respect to said received digital signal.

34. A system for detecting a movable body liquid interface in a geological formation, comprising:

a transmitter operable for producing a plurality of digital pulses into said geological formation at a selectable carrier frequency from at least 500 KHz to 1 MHz;

a receiver for receiving a received signal comprised of reflections of said plurality of digital pulses from said movable liquid body interface in said geological formation;

a means for producing a received digital signal as well as increasing the accuracy and resolution of detecting said movable liquid body interface in said geological formation;

and a pseudo-noise (PN) correlator for comparing at least a portion of said digitized received signal with said plurality of digital pulses.