Method and apparatus for using non-linear spectral fitting procedures on gamma-ray spectral data

• US 5,369,578 A
• Filed: 05/20/1993
• Issued: 11/29/1994
• Est. Priority Date: 08/07/1990
• Status: Expired due to Term

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Roscoe, Bradley A., Stoller, Christian
• Primary Examiner(s): Weinhardt, Robert A.
• Application Number:
  US08/064,622
• Time in Patent Office: 558 Days
• Field of Search: 364/422, 364/498, 364/571.03, 364/571.04, 250/256, 250/260, 250/262, 250/270
• US Class Current: 702/8
• CPC Class Codes: G01V 5/06 for detecting naturally radioactive minerals
• CPC Class Codes: G01V 5/101 and detecting the secondary Y-rays produced in the surrounding layers of the bore hole

Abstract

A method and an apparatus for decomposing a gamma spectrum representative of an unknown material for determining the contribution of each constituent postulated to constitute the material, wherein an energy spectrum of the gamma rays issued from the material is formed, as well as a composite spectrum made up of individual standard spectra of the postulated constituents and comprising elemental yields being representative of the proportion of the corresponding constituents; the best fit between the measured spectrum and the composite spectrum is determined by modifying simultaneously at least one elemental yield and at least one parameter representative of the conditions under which the measured spectrum and the composite spectrum have been obtained. The best fit may be based on any non linear least squares search for a global minimum of X² =(S Y-U)^T W(S Y-U), where "U" is a matrix representing the measured spectrum, "S" is a matrix representing the composite spectrum, "Y" is a matrix representative of the elemental yields and "W" is a weight matrix. The non linear fitting method used may be e.g. a gradient search or the Marquardt method.

Claims

1. A method for decomposing a gamma ray spectrum representative of an unknown material in order to determine the respective gamma ray contribution of each constituent postulated to constitute said material, comprising:

measuring gamma rays of said unknown material by detecting and counting the gamma rays issued from said material;

forming an energy spectrum of said measured gamma rays;

establishing a composite spectrum constructed of individual standard spectra of said postulated constituents and comprising elemental yields being representative of the fraction of the corresponding constituents in said composite spectrum;

determining the best fit between said measured spectrum and said composite spectrum by simultaneously varying at least two parameters from a set of parameters which describe the transformation of the measured and composite spectra from channel numbers to each spectrum'"'"'s physical domain, said parameter set containing gain, offset, resolution, resolution degradation, background subtraction and non-linear gain parameters; and

determining from the best fit and a best composite spectrum associated with said best fit the relative fractions of the gamma ray spectra associated with the corresponding constituents of said material or one or more of the parameters describing the spectral transformation from channel numbers to the physical domain.

2. The method according to claim 1 wherein said best fit is a non linear least squares search for a global minimum of X² =(S Y-U)^T W(S Y-U), where "U" is a matrix representing said measured spectrum, "S" is a matrix representing said composite spectrum, "Y" is a matrix representative of said elemental yields and "W" is the weight matrix.

3. The method according to claim 2 wherein said search is a gradient search.

4. The apparatus according to claim 3 wherein said best fit is a non linear least squares search for a global minimum of X² =(S Y-U)^T W(S Y-U), where "U" is a matrix representing said measured spectrum, "S" is a matrix representing said standard spectra, and "Y" is a matrix representative of said elemental yields and "W" is the weight matrix.

5. The apparatus according to claim 4 wherein said search is a gradient search.

6. The method according to claim 1 wherein said material includes earth formation surrounding a borehole.

7. The method according to claim 6 further comprising irradiating said formation with neutrons of sufficient energy to interact with the atoms of said material constituents and said detecting step comprises detecting said gamma rays resulting from the interactions between said neutrons and said constituents atoms.

8. The method according to claim 6 wherein said detecting step comprises detecting gamma rays issued from radioactive tracers incorporated in said formation.

9. The method according to claim 6 wherein said detecting step comprises detecting said gamma rays resulting from the natural radioactivity of said earth formation.

10. The method according to claim 1 wherein the step of determining the best fit comprises minimizing the chi-square function defined as:

##EQU17## where "m" is the number of elemental energy intervals;

"Vⁱ " is the variance of the content of each energy interval in said measured spectrum, with Vⁱ =1/Wⁱ ; and

"Eⁱ " is the error in the fit, with ##EQU18## where "n" is the number of said constituents;

"S" is the matrix representative of said standard spectra;

"Wⁱ " is the weight of the content of each energy interval of the measured spectrum;

"Y" is a vector whose components are the elemental yields; and

"U" is the measured spectrum;

said minimizing step comprising multiplying the derivative of X² with respect to at least one parameter and related to the background by a factor "K" representative of the fact that the background spectrum presents a statistical nature.

11. The method according to claim 10 wherein said factor K=1-c, where ##EQU19## where "b" is the background subtraction factor;

"y_k " is the elemental yield; and

"B" is the background spectrum.

12. The method according to claim 1 further including the step of deriving from said relative fractions of the gamma ray spectra which provide the best fit, the fraction of each of said constituents.

13. A method for decomposing a gamma ray spectrum representative of an unknown material in order to determine the respective gamma ray contribution of each constituent postulated to constitute said material, comprising;

measuring gamma rays of said unknown material by detecting and counting the gamma rays issued from said material;

forming an energy spectrum of said measured gamma rays;

establishing a composite spectrum constructed of individual standard spectra of said postulated constituents and comprising elemental yields being representative of the fraction of the corresponding constituents in said composite spectrum;

determining the best fit between said measured spectrum and said composite spectrum by simultaneously varying at least one elemental yield and at least one parameter from a set of parameters which describe the transformation of the measured and composite spectra from channel numbers to each spectrum'"'"'s physical domain, said parameter set containing gain, offset, resolution, resolution degradation, background subtraction and non-linear gain; and

determining from the best fit and a best composite spectrum associated with said best fit, the relative fractions of the gamma ray spectra associated with the corresponding constituents of said material or one or more of the parameters describing the spectral transformation from channel numbers to the physical domain.

14. The method according to claim 13 wherein said best fit is a non linear least squares search for a global minimum X² =(S Y-U)^T W(S Y-U), where "U" is a matrix representing said measured spectrum, "S" is a matrix representing said composite spectrum, "Y" is a matrix representative of said elemental yields and "W" is the weight matrix.

15. The method according to claim 14 wherein said search is a gradient search.

16. The method according to claim 13, wherein said material includes earth formation surrounding a borehole.

17. The method according to claim 16 further comprising irradiating said formation with neutrons of sufficient energy to interact with the atoms of said material constituents and said detecting step comprises detecting said gamma rays resulting from the interactions between said neutrons and said constituents atoms.

18. The method according to claim 16 wherein said detecting step comprises detecting gamma rays issued from radioactive tracers incorporated in said formation.

19. The method according to claim 13 wherein the step of determining the best fit comprises minimizing the chi-square function defined as:

##EQU20## where "m" is the number of elemental energy intervals;

"Vⁱ " is the variance of the content of each energy interval in said measured spectrum, with Vⁱ =1/Wⁱ ; and

"Eⁱ " is the error in the fit, with ##EQU21## where "n" is the number of said constituents;

"S" is the matrix representative of said standard spectra;

"Wⁱ " is the weight of the measured spectrum;

"Y" is a vector whose components are the elemental yields; and

"U" is the measured spectrum;

said minimizing step comprising multiplying the derivative of X² with respect to at least one parameter and related to the background by a factor "K" representative of the fact that the background spectrum presents a statistical nature.

20. The method according to claim 19 wherein said factor K=1-c, where ##EQU22## where "b" is the background subtraction factor "y_k " is the elemental yield;

and "B" is the background spectrum.

21. The method according to claim 13 further including the step of deriving from said elemental yields which provide the best fit, the fraction of each of said constituents.

22. The method of claim 13 wherein said energy spectrum formation further comprises forming a background energy spectrum of said measured gamma rays and said best fit determination is between the measured, background and composite spectra.

23. An apparatus for decomposing a gamma ray spectrum representative of an unknown material in order to determine the respective gamma ray contribution of each constituent postulated to constitute said material, comprising;

means for measuring gamma rays of said unknown material by detecting and counting the gamma rays issued of said material;

means for forming an energy spectrum of said measured gamma rays;

means for establishing a composite spectrum constructed of elemental standard spectra of said postulated constituents and comprising elemental yields being representative of the fraction of the corresponding constituent in said composite spectrum;

means for determining the best fit between said measured spectrum and said composite spectrum by simultaneously varying at least two parameters from a set of parameters which describe the transformation of the measured and composite spectra from channel numbers to each spectrum'"'"'s physical domain, said parameter set containing gain, offset, resolution, resolution degradation, background subtraction and non-linear gain parameters; and

means for determining from the best fit and a best composite spectrum associated with said best fit, the relative fractions of the gamma ray spectra associated with the corresponding constituents of said material or one or more of the parameters describing the spectral transformation from channel numbers to the physical domain.

24. The apparatus according to claim 23 further comprising means for deriving from said relative fractions of the gamma ray spectra which provide the best fit, the fraction of each of said constituents.

25. The apparatus according to claim 23 wherein said best fit is a non linear least squares search for a global minimum of X² =(S Y-U)^T W(S Y-U), where "U" is a matrix representing said measured spectrum, "S" is a matrix representing said standard spectra, "Y" is a matrix representative of said elemental yields and "W" is the weight matrix.

26. The apparatus according to claim 25 wherein said search is a gradient search.

27. The apparatus according to claim 23 wherein said unknown material comprises earth formation surrounding a borehole.

28. The apparatus according to claim 27 further comprising means for irradiating said formation with neutrons of sufficient energy to interact with the atoms of said formation constituents and said means for detecting are designed to detect the gamma rays resulting from the interactions between the neutrons and the constituents atoms.

29. The apparatus according to claim 24 wherein said detecting means are designed to detect the gamma rays resulting from the natural radioactivity of said earth formation.

30. The apparatus according to claim 23 wherein the step of determining the best fit further comprises a means for minimizing the chi-square function defined as:

##EQU23## where "m" is the number of elemental of energy intervals;

"Vⁱ " is the variance of the content of each of said energy interval in the measured spectrum, with Vⁱ =1/Wⁱ ; and

"Eⁱ " is the error in the fit, with;

##EQU24## where "n" is the number of said constituents;

"S" is the matrix representative of said standard spectra;

"U" represents said measured spectrum;

"Y" represents said elemental yields; and

"W" represents the weight matrix;

said minimizing means comprising means for multiplying the derivative of X² with respect to at least one parameter related to the background, by a factor "K" representative of the fact the background spectrum presents a statistical nature.

31. The method of claim 23 wherein said means for forming an energy spectrum further comprises a means for forming a background energy spectrum of said measured gamma rays and said means for determining best fit is between the measured, background and composite spectra.

32. An apparatus for decomposing a gamma ray spectrum representative of an unknown material in order to determine the respective gamma ray contribution of each constituent postulated to constitute said material, comprising:

means for measuring gamma rays of said unknown material by detecting and counting the gamma rays issued of said material;

means for forming an energy spectrum of said measured gamma rays;

means for establishing a composite spectrum constructed of elemental standard spectra of said postulated constituents and comprising elemental yields being representative of the fraction of the corresponding constituent in said composite spectrum;

means for determining the best fit between said measured spectrum and said composite spectrum by simultaneously varying at least one elemental yield and at least one parameter from a set of parameters which describe the transformation of the measured and composite spectra from channel numbers to each spectrum'"'"'s physical domain, said parameter set containing gain, offset, resolution, resolution degradation, background subtraction and non-linear gain parameters; and

determining from the best fit and a best composite spectrum associated with said best fit, the relative fractions of the gamma ray spectra associated with the corresponding constituents of said material or one or more of the parameters describing the spectral transformation from channel numbers to the physical domain.

33. The apparatus according to claim 32, further comprising means for deriving from elemental yields which provide the best fit, the fraction of each of said constituents.

34. The apparatus according to claim 32 wherein said unknown material comprises earth formation surrounding a borehole.

35. The apparatus according to claim 34 further comprising means for irradiating said formation with neutrons of sufficient energy to interact with the atoms of said formation constituents and said means for detecting are designed to detect the gamma rays resulting from the interactions between the neutrons and the constituents atoms.

36. The apparatus according to claim 34 wherein said detecting means are designed to detect the gamma rays resulting from the natural radioactivity of said earth formation.

37. The apparatus according to claim 32 wherein the means for determining the best fit further comprising a means for minimizing the chi-square function defined as:

##EQU25## where "m" is the number of elemental of energy intervals;

"Vⁱ " is the variance of the content of each of said energy interval in the measured spectrum, with Vⁱ =1/Wⁱ ; and

"Eⁱ " is the error in the fit, with;

##EQU26## where "n" is the number of said constituents;

"S" is the matrix representative of said standard spectra "U" represents said measured spectrum;

"Y" represents said elemental yields;

"W" represents the weight matrix;

said minimizing means comprising means for multiplying the derivative of X² with respect to a least one parameter related to the background, by a factor "K" representative of the fact the background spectrum presents a statistical nature.

38. The method of claim 32 wherein said energy spectrum formation further comprises forming a background energy spectrum of said measured gamma rays and said best fit determination is between the measured, background and composite spectra.

39. A method for decomposing a gamma ray spectrum representative of an unknown material in order to determine the respective gamma ray contribution of each constituent postulate to constitute said material, comprising:

measuring gamma rays of said unknown material by detecting and counting the gamma rays issued from said material;

forming an energy spectrum and a background energy spectrum of said measured gamma rays;

establishing a composite spectrum constructed of individual standard spectra of said postulated constituents and comprising elemental yields being representative of the fraction of the corresponding constituents in said composite spectrum;

determining the best fit between said measured spectrum, said background specrum and said composite spectrum by simultaneously varying at least two parameters from a set of parameters which describe the transformation of the measured, background and composite spectra from channel numbers to each spectrum'"'"'s physical domain, said parameter set containing gain, offset, resolution, resolution degradation, BSF, element yields and non-linear gain parameters; and

determining from the best fit and an associated best composite spectrum the relative fractions of the gamma ray spectra associated with the corresponding elemental constituents of said material or one or more of the parameters describing the spectral transformation from channel numbers to the physical domain.

40. The method of claim 39 wherein in the best fit determination step at least one elemental yield is varied and at least one other paramater is simultaneously varied.

41. The method of claim 39 further including the step of deriving from said relative fractions of the gamma ray spectra which provide the best fit, the fraction of each of said constiuents.