Methods of processing magnetotelluric signals

• US 20080091354A1
• Filed: 10/11/2006
• Published: 04/17/2008
• Est. Priority Date: 10/11/2006
• Status: Active Grant

• Current Assignee: Kent A. Byerly
• Original Assignee: Kent A. Byerly
• Inventors: Byerly, Kent A.
• Granted Patent: US 8,055,446 B2
• Time in Patent Office: Days
• 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

Methods for processing magnetotelluric data to identify subterranean deposits are 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

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;

$\mathrm{\Delta <br><br>}<br><br><br><br>Z_x<br><br>\left(f\right)=\left(A_n<br><br>\frac{E_{x,n}<br><br>\left(f\right)}{H_{x,n}<br><br>\left(f\right)}+A_{n+1}<br><br>\frac{E_{x,n+1}<br><br>\left(f\right)}{H_{x,n+1}<br><br>\left(f\right)}+\mathrm{\dots <br><br>}+A_N<br><br>\frac{E_{x,N}<br><br>\left(f\right)}{H_{x,N}<br><br>\left(f\right)}\right)-\left(A_m<br><br>\frac{E_{x,m}<br><br>\left(f\right)}{H_{x,m}<br><br>\left(f\right)}+A_{m+1}<br><br>\frac{E_{x,m+1}<br><br>\left(f\right)}{H_{x,m+1}<br><br>\left(f\right)}+\mathrm{\dots <br><br>}+A_M<br><br>\frac{E_{x,M}<br><br>\left(f\right)}{H_{x,M}<br><br>\left(f\right)}\right)$

where

Δ

Z_x(f) is the differential impedance of pulse x;

E_x,n(f) through E_N(f) and E_x,m(f) through E_x,M(f) are the recorded or synthetic electric fields for vector components n through N and m through M of pulse x;

H_n(f) through H_x,N(f) and H_x,m(f) through H_x,M(f) are the recorded or synthetic magnetic field for vector components n through N and m through M of pulse x;

$\frac{E_{x,n}<br><br>\left(f\right)}{H_{x,n}<br><br>\left(f\right)}<br><br><br><br>\mathrm{through}<br><br><br><br>\frac{E_{x,N}<br><br>\left(f\right)}{H_{x,N}<br><br>\left(f\right)}<br><br><br><br>\mathrm{and}<br><br><br><br>\frac{E_{x,m}<br><br>\left(f\right)}{H_{x,m}<br><br>\left(f\right)}<br><br><br><br>\mathrm{through}<br><br><br><br>\frac{E_{x,M}<br><br>\left(f\right)}{H_{x,M}<br><br>\left(f\right)}$

are the impedances for vector components n through N and m through M of pulse x; and

A_n through A_N and A_m through A_M are complex scaling factors applied to the impedances for vector components n through N and m through M of pulse x.

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 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.

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

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

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

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

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

9. The method of claim 5, wherein said pulses have an amplitude peak meeting defined threshold criteria in their corresponding vertical vector component.

10. The method of claim 1, wherein said data are obtained and said pulses 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.

11. The method of claim 10, wherein said pulses have an amplitude peak meeting defined threshold criteria in a second vector component.

12. 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.

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 for collecting magnetotelluric signals, said method comprising:

(a) receiving magnetotelluric signals in at least two vector components;

(b) detecting amplitude peaks meeting defined threshold criteria in at least one said vector component; and

(c) recording said magnetotelluric signals in all vector components over a defined time segment in response to said detection of amplitude peaks.

19. The method of claim 18, wherein said signals are recorded in response to detection of amplitude peaks meeting defined threshold criteria in all said vector components.

20. The method of claim 18, wherein said at least two vector components include a vertical and a horizontal vector component.