Methods of processing magnetotelluric signals

• US 8,055,446 B2
• Filed: 10/11/2006
• Issued: 11/08/2011
• Est. Priority Date: 10/11/2006
• Status: Expired due to Fees

• Current Assignee: Kent A. Byerly
• Original Assignee: Kent A. Byerly
• Inventors: Byerly, Kent A.
• Primary Examiner(s): TEIXEIRA MOFFAT, JONATHAN CHARLES
• Application Number:
  US11/546,164
• Publication Number:
  US 20080091354A1
• Time in Patent Office: 1,854 Days
• Field of Search: 702/6, 702/7, 702/11, 702/13, 702/14, 702/16, 702/17, 702/57, 702 65- 67, 702/70, 702/71, 702 73- 78, 702/176, 702/183, 702/187, 702189-191, 702/193, 702/197, 702/198, 324/335, 324/339, 324/345, 324/346, 324/347, 324/348, 324/350, 166/254.1, 166/254.2
• US Class Current: 702/6
• CPC Class Codes: G01V 3/082 operating with fields produced by spontaneous potentials, e.g. electrochemical or produced by telluric currents G01V3/26 takes precedence

Abstract

A method for processing magnetotelluric data to identify subterranean deposits is provided for. The data comprise the amplitude of alternating magnetic or electrical fields recorded over time in at least two vector components at one or more locations in an area of interest. Amplitude pulses are identified in the data, wherein the pulses are time segments comprising an amplitude peak meeting defined threshold criteria in at least one vector component. The pulses are filtered at a set of predetermined frequencies to separate amplitude data at each frequency from the remainder of the amplitude data in the pulses. The frequencies correspond to subterranean depths over a range of interest. Differential impedances ΔZ of the pulses at each frequency at each location are then statistically analyzed to determine a value correlated to the resistance of the earth at each frequency. The resistance values are indicative of the presence or absence of deposits at the corresponding subterranean depth at the location.

Claims

1. A method of processing magnetotelluric data to identify subterranean deposits, wherein said data comprise the amplitude of alternating magnetic or electrical fields recorded over time in at least two vector components at one or more locations in an area of interest, said method comprising:

(a) identifying amplitude pulses in said data, said pulses being time segments comprising an amplitude peak meeting defined threshold criteria in at least one said vector component;

(b) filtering said pulses at a set of predetermined frequencies to separate amplitude data at each said frequency from the remainder of the amplitude data in said pulses, wherein said frequencies correspond to subterranean depths over a range of interest; and

(c) statistically analyzing a differential impedance Δ

Z of said pulses at each said frequency at each said location to determine a value correlated to the resistance of the earth at each said frequency, the resistance being indicative of the presence or absence of deposits at the corresponding subterranean depth at the location;

wherein differential impedances of a given pulse x, Δ

Z_x(f), are defined as follows;

2. The method of claim 1, wherein said magnetotelluric data comprise the amplitude of the magnetic field recorded over time, and synthetic electric field data are generated based on the recorded magnetic field data, said synthetic electric field data being amplitude data over a range of frequencies.

3. The method of claim 2, wherein said synthetic electric field data are generated by

(a) transforming a unit electrical impulse with a Fourier transform;

(b) generating low order estimates of the frequency response of recorded magnetic pulse data; and

(c) multiplying the Fourier transform of the processed unit pulse by the low order estimates.

4. The method of claim 1, wherein said magnetotelluric data comprise the amplitude of the electric field recorded over time, and synthetic magnetic field data are generated based on the recorded electric field data, said synthetic magnetic field data being amplitude data over a range of frequencies.

5. The method of claim 1, wherein said magnetotelluric data comprise the amplitude of both the magnetic and electric fields recorded over time.

6. The method of claim 1, wherein said magnetotelluric data are recorded over time in a vertical and a horizontal component.

7. The method of claim 6, wherein said pulses have an amplitude peak meeting defined threshold criteria in their corresponding vertical vector component.

8. The method of claim 1, wherein said magnetotelluric data are recorded over time in a vertical and two horizontal components.

9. The method of claim 1, wherein said vector components are orthogonally oriented relative to each other.

10. The method of claim 1, wherein said vector components comprise a vertical component, a north-south component, and an east-west component.

11. The method of claim 1, wherein said data are obtained and said pulses are identified by receiving the amplitude of magnetotelluric signals in at least two vector components and recording said magnetotelluric signals over a plurality of time segments, said time segments being recorded in response to the detection of amplitude peaks meeting defined threshold criteria in at least one said vector component.

12. The method of claim 11, wherein said pulses have an amplitude peak meeting defined threshold criteria in a second vector component.

13. The method of claim 1, wherein said data are filtered by transforming the data from the amplitude-time domain to the magnitude-phase/frequency domain using a fast Fourier transform, thereby defining frequency windows, Δ

f, which correspond to particular depths at a particular location.

14. The method of claim 1, wherein the differential impedance of amplitude at each frequency is based on the impedances of a vertical channel and at least one horizontal channel.

15. The method of claim 1, wherein a differential impedance Δ

Z of said pulses at each said frequency is analyzed by;

(a) statistically analyzing a differential impedance of each pulse over Δ

F;

(b) eliminating pulses in which the analyzed differential impedances do not satisfy predetermined threshold criteria, thereby generating a subset of pulses I for each Δ

F; and

(c) statistically analyzing a differential impedance of each pulse in subset I over Δ

F.

16. The method of claim 1, wherein the differential impedance Δ

Z of said pulses at each said frequency is analyzed by;

(a) determining an average differential impedance of each pulse over Δ

F;

(b) eliminating pulses in which the average differential impedances do not satisfy predetermined threshold criteria, thereby generating a subset of pulses I for each Δ

F; and

(c) determining an average differential impedance for pulses in subset I over Δ

F.

17. The method of claim 1, further comprising displaying said resistivity values.

18. A method of transforming data representing magnetotelluric fields above an area of interest on the earth to values representing the resistivity of the earth at various depths below said area of interest, wherein said data comprise the amplitude of alternating magnetic or electrical fields over time in at least two vector components at one or more locations in said area of interest, said method comprising:

(a) identifying amplitude pulses in said data, said pulses being time segments comprising an amplitude peak meeting defined threshold criteria in at least one said vector component;

(b) filtering said pulses at a set of predetermined frequencies to separate amplitude data at each said frequency from the remainder of the amplitude data in said pulses, wherein said frequencies correspond to subterranean depths over a range of interest; and

(c) statistically analyzing differential impedance Δ

Z of said pulses at each said frequency at each said location to determine said values correlated to the resistance of the earth at said subterranean depths in said area of interest, the resistance being indicative of the presence or absence of deposits at the corresponding subterranean depth at the location;

wherein differential impedances of a given pulse x, Δ

Z_z(f), are defined as follows;

19. The method of claim 18, wherein said method further comprises displaying said resistivity values for visual analysis and enhanced interpretation thereof.

20. The method of claim 18, wherein said magnetotelluric data comprise the amplitude of the magnetic field recorded over time, and synthetic electric field data are generated based on the recorded magnetic field data, said synthetic electric field data being amplitude data over a range of frequencies.

21. The method of claim 18, wherein said magnetotelluric data comprise the amplitude of the electric field recorded over time, and synthetic magnetic field data are generated based on the recorded electric field data, said synthetic magnetic field data being amplitude data over a range of frequencies.

22. The method of claim 21, wherein said synthetic electric field data are generated by

(a) transforming a unit electrical impulse with a Fourier transform;

(b) generating low order estimates of the frequency response of recorded magnetic pulse data; and

(c) multiplying the Fourier transform of the processed unit pulse by the low order estimates.

23. The method of claim 18, wherein said magnetotelluric data comprise the amplitude of both the magnetic and electric fields recorded over time.

24. The method of claim 18, wherein said magnetotelluric data are recorded over time in a vertical and a horizontal component.

25. The method of claim 18, wherein said magnetotelluric data are recorded over time in a vertical and two horizontal components.

26. The method of claim 25, wherein said pulses have an amplitude peak meeting defined threshold criteria in their corresponding vertical vector component.

27. The method of claim 25, wherein said pulses have an amplitude peak meeting defined threshold criteria in a second vector component.

28. The method of claim 18, wherein the differential impedance of amplitude at each frequency is based on the impedances of a vertical channel and at least one horizontal channel.

29. The method of claim 18, wherein a differential impedance Δ

Z of said pulses at each said frequency is analyzed by;

(a) statistically analyzing a differential impedance of each pulse over Δ

F;

(b) eliminating pulses in which the analyzed differential impedances do not satisfy predetermined threshold criteria, thereby generating a subset of pulses I for each Δ

F; and

(c) statistically analyzing a differential impedance of each pulse in subset I over Δ

F.

30. The method of claim 18, wherein the differential impedance Δ

Z of said pulses at each said frequency is analyzed by;

(a) determining an average differential impedance of each pulse over Δ

F;

(b) eliminating pulses in which the average differential impedances do not satisfy predetermined threshold criteria, thereby generating a subset of pulses I for each Δ

F; and

(c) determining an average differential impedance for pulses in subset I over Δ

F.