System and method for using time-distance characteristics in acquisition, processing, and imaging of t-CSEM data

• US 20060203613A1
• Filed: 02/21/2006
• Published: 09/14/2006
• Est. Priority Date: 02/18/2005
• Status: Active Grant

• Current Assignee: BP Corporation North America Incorporated (BP PLC)
• Original Assignee: BP Corporation North America Incorporated (BP PLC)
• Inventors: Jilek, Petr, Xia, Ganyuan, Thomsen, Leon, Allegar, Norman C., Johnson, Daniel, Dellinger, Joseph A.
• Granted Patent: US 7,502,690 B2
• Time in Patent Office: Days
• US Class Current: 367/38
• CPC Class Codes: G01V 3/083 Controlled source electromagnetic [CSEM] surveying
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

There is provided herein a system and method of acquiring, processing, and imaging transient Controlled Source ElectroMagnetic (t-CSEM) data in ways that are similar to those used for seismic data. In particular, the instant invention exploits the time-distance characteristics of t-CSEM data to permit the design and execution of t-CSEM surveys for optimal subsequent processing and imaging. The instant invention illustrates how to correct t-CSEM data traces for attenuation and dispersion, so that their characteristics are more like those of seismic data and can be processed using algorithms familiar to the seismic processor. The resulting t-CSEM images, particularly if combined with corresponding seismic images, may be used to infer the location of hydrocarbon reservoirs.

Claims

1. A method of processing EM data for use in geophysical exploration of a predetermined volume of the earth containing structural and stratigraphic features conducive to the generation, migration, accumulation, or presence of hydrocarbons, comprising the steps of:

(a) acquiring an EM survey that covers at least a portion of said predetermined volume of the earth, said survey comprising a plurality of EM traces;

(b) selecting at least one of said EM traces;

(c) selecting a plurality of different frequencies;

(d) selecting a value of a parameter Q, wherein said parameter Q is a quality factor related to a transmission of EM data;

(e) correcting each of said at least one selected EM traces for at least one of attenuation and dispersion at each of said selected plurality of frequencies according to said parameter Q, thereby producing a plurality of processed EM traces; and

,

(f) writing at least a portion of said processed EM traces to computer storage.

2. A method according to claim 1, wherein said parameter Q is approximately equal to ½

.

3. A method according to claim 1, wherein step (e) comprises the steps of:

(e1) selecting a reference frequency ω

₀.

(e2) for each of said selected EM traces,

(i) selecting one of said plurality of frequencies, wherein said selected frequency is represented by ω

,

(ii) correcting said selected EM trace for attenuation and dispersion according to the following formula;

exp[+ω

R₀/2QV_phs+iω

(R₀/V_phs−

R₀/V₀)],

 

where, R₀ is a length of a travel path from an EM source of said selected EM trace to a receiver sensing said EM source,

 

where V₀≡

V_phs(ω

₀)=√

{square root over (2ω

₀ρ

/μ

)},

 

where

$V_{\mathrm{phs}}=\sqrt{\frac{2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\rho <br><br>}}{\mathrm{\mu <br><br>}}},$

 

where ρ

is a subsurface resistivity,

 

where μ

is a subsurface magnetic permeability, and,

(iii) performing steps (i) and (ii) above for each of said selected plurality of frequencies, and,

(e3) performing at least step (e2) for each of said selected EM traces, thereby producing a plurality of processed EM traces.

4. A method according to claim 1, wherein said computer storage is selected from a group consisting of a magnetic disk, a magnetic tape, an optical disk, a magneto-optical disk, RAM, and non-volatile RAM.

5. A method according to claim 1, further comprising the step of:

(h) viewing at least a portion of said processed EM traces on a display device.

6. A device adapted for use by a digital computer wherein a plurality of computer instructions defining the method of claim 1 are encoded,

said device being readable by said digital computer,

said computer instructions programming said digital computer to perform said method, and,

said device being selected from the group consisting of computer RAM, computer ROM, a PROM chip, flash RAM, a ROM card, a RAM card, a floppy disk, a magnetic disk, a magnetic tape, a magneto-optical disk, an optical disk, a CD-ROM disk, or a DVD disk.

7. A method according to claim 1, wherein said EM data traces are selected from a group consisting of CSEM data traces, f-CSEM data traces, and t-CSEM data traces.

8. A method according to claim 1, wherein step (e) comprises the steps of:

(e1) correcting each of said at least one selected EM traces for attenuation and dispersion at each of said selected plurality of frequencies according to said parameter Q, thereby producing a plurality of corrected EM traces, and,

(e2) processing said corrected EM traces with at least one seismic imaging algorithm, thereby producing a plurality of processed EM traces.

9. A method according to claim 8, wherein each of said at least one seismic imaging algorithms is selected from a group consisting of muting, deconvolution, wavelet shaping, statics, velocity analysis, time-offset correction, NMO, frequency filtering, pre-stack imaging, pre-stack migration, DMO, stacking, gain correction, post-stack imaging, post-stack migration, AVO, and attribute generation.

10. A method according to claim 1, comprising the further step of:

(g) interpreting said processed EM traces for a purpose of exploration for hydrocarbons within said predetermined volume of the earth.

11. A method of collecting and processing EM data for use in geophysical exploration within a predetermined volume of the earth containing structural and stratigraphic features conducive to the generation, migration, accumulation, or presence of hydrocarbons, comprising the steps of:

(a) collecting an EM survey that images at least a portion of said predetermined volume of the earth, said survey comprising a plurality of EM traces;

(b) selecting at least one of said EM traces, each of said selected EM traces having a source-receiver distance associated therewith, and each of said selected EM traces having a plurality of EM samples organized in natural time associated therewith;

(c) selecting at least one time-distance processing algorithm;

(d) applying said at least one time-distance processing algorithm to said selected EM traces and said samples associated therewith using said at least one time-distance processing algorithm at least according to said associated source-receiver distances and said selected EM trace samples, thereby producing a plurality of processed EM traces; and

,

(e) writing at least a portion of said processed EM traces to computer storage.

12. A method according to claim 11, wherein said at least one time-distance processing algorithm is selected from a group consisting of velocity filtering, muting in a window, f-k filtering, NMO, DMO, time migration, depth migration, velocity estimation, tomography, and, tau-p filtering.

13. A method according to claim 11, wherein step (d) comprises the steps of:

(d1) selecting a plurality of different frequencies,

(d2) selecting a value of a parameter Q, wherein said parameter Q is a quality factor related to a transmission of EM data,

(d3) correcting each of said at least one selected EM traces for at least one of attenuation and dispersion at each of said selected plurality of frequencies according to said parameter Q, thereby producing a corresponding plurality of corrected EM traces, each of said corrected EM traces having a plurality of corrected samples associated therewith, and,

(d4) processing said corrected EM traces and said associated corrected samples using said at least one time-distance processing algorithm at least according to said associated source-receiver distances, thereby producing a plurality of processed EM traces.

14. A method according to claim 13, wherein said parameter Q is approximately equal to ½

.

15. A method according to claim 11, wherein said computer storage is selected from a group consisting of a magnetic disk, a magnetic tape, an optical disk, a magneto-optical disk, RAM, and non-volatile RAM.

16. A method according to claim 11, further comprising the step of:

(h) viewing at least a portion of said processed EM traces on a display device.

17. A device adapted for use by a digital computer wherein a plurality of computer instructions defining the method of claim 11 are encoded,

said device being readable by said digital computer,

said computer instructions programming said digital computer to perform said method, and,

said device being selected from the group consisting of computer RAM, computer ROM, a PROM chip, flash RAM, a ROM card, a RAM card, a floppy disk, a magnetic disk, a magnetic tape, a magneto-optical disk, an optical disk, a CD-ROM disk, or a DVD disk.

18. A method according to claim 11, wherein said EM data traces are selected from a group consisting of CSEM data traces, f-CSEM data traces, and t-CSEM data traces.

19. A method of geophysical exploration within a predetermined volume of the earth, wherein is provided an EM survey that images at least a portion of the predetermined volume of the earth, said EM survey containing at least two EM traces therein, comprising the steps of:

(a) selecting at least one of said EM traces;

(b) selecting a plurality of frequencies;

(c) correcting each of said at least one selected EM traces for attenuation and/or dispersion at each of said selected plurality of frequencies, thereby producing a plurality of processed EM traces;

(d) processing said EM traces with at least one seismic imaging algorithm, thereby producing a plurality of seismic processed EM traces; and

,

(e) writing at least a portion of said seismic processed EM traces to computer storage.

20. A method according to claim 19, wherein step (d) comprises the step of:

(d1) processing said EM traces with at least one seismic imaging algorithm, wherein said at least one of said at least one seismic imaging algorithms is a time-distance algorithm, thereby producing a plurality of seismic processed EM traces.

21. A method according to claim 20, wherein said time-distance processing algorithm is selected from a group consisting of muting, NMO, DMO, time migration, depth migration, tau-p filtering, velocity estimation, tomography, and f-k filtering.

22. A method according to claim 20, wherein step (c) comprises the steps of:

(c1) selecting a value of a parameter Q, wherein said parameter Q is a quality factor related to a transmission of EM data, and,

(c2) correcting each of said at least one selected EM traces for attenuation at each of said selected frequencies according to said parameter Q, thereby producing a plurality of processed EM traces.

23. A method according to claim 22, wherein said parameter Q is approximately equal to ½

.

24. A method according to claim 21, wherein step (c) comprises the steps of:

(c1) selecting a reference frequency ω

₀,

(c2) for each of said selected EM traces,

(i) selecting one of said plurality of frequencies, wherein said selected frequency is represented by ω

,

(ii) correcting said selected EM trace for attenuation and dispersion according to the following formula;

exp[+ω

R₀/2QV_phs+iω

(R₀/V_phs−

R₀/V₀)],

 

where, R₀ is a length of a travel path from said selected EM trace to an EM source giving rise to said selected EM trace,

 

where V₀≡

V_phs(ω

₀)=√

{square root over (2ω

₀ρ

/μ

)},

 

where

$V_{\mathrm{phs}}=\sqrt{\frac{2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\rho <br><br>}}{\mathrm{\mu <br><br>}}},$

 

where ρ

is a subsurface resistivity, and,

 

where μ

is a subsurface magnetic permeability, and,

(iii) performing steps (i) and (ii) above for each of said selected plurality of frequencies, and,

(e3) performing at least step (e2) for each of said selected EM traces, thereby producing a plurality of processed EM traces.

25. A method according to claim 21, wherein said computer storage is selected from a group consisting of a magnetic disk, a magnetic tape, an optical disk, a magneto-optical disk, RAM, and non-volatile RAM.

26. A method according to claim 21, further comprising the step of:

(g) viewing at least a portion of said processed EM traces on a display device.

27. A device adapted for use by a digital computer wherein a plurality of computer instructions defining the method of claim 21 are encoded,

said device being readable by said digital computer,

said computer instructions programming said digital computer to perform said method, and,

said device being selected from the group consisting of computer RAM, computer ROM, a PROM chip, flash RAM, a ROM card, a RAM card, a floppy disk, a magnetic disk, a magnetic tape, a magneto-optical disk, an optical disk, a CD-ROM disk, or a DVD disk.

28. A method according to claim 21, wherein said EM data traces are selected from a group consisting of CSEM data traces, f-CSEM data traces, and t-CSEM data traces.

31. A method according to claim 21, further comprising the step of:

(f) viewing at least a portion of said processed EM traces on a display device.

29. A method of geophysical exploration for hydrocarbons within a predetermined volume of the earth, wherein is provided an EM survey that images at least a portion of the predetermined volume of the earth, said EM survey containing at least two EM traces therein, comprising the steps of:

(a) selecting at least one of said EM traces;

(b) selecting a plurality of frequencies;

(c) selecting a reference frequency ω

₀,

(d) choosing one of said plurality of selected frequencies, wherein said chosen frequency is represented by ω

,

(e) correcting said selected EM trace for attenuation and dispersion according to

exp[+ω

R₀/V_phs+iω

(R₀/V_phs−

R₀/V₀)],

 

where, R₀ is a length of a travel path from said selected EM trace to an EM source giving rise to said selected EM trace,

 

where V₀≡

V_phs(ω

₀)=√

{square root over (2ω

₀ρ

/μ

)},

 

where

$V_{\mathrm{phs}}=\sqrt{\frac{2<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\rho <br><br>}}{\mathrm{\mu <br><br>}}},$

 

where ρ

is a subsurface resistivity, and,

 

where μ

is a subsurface magnetic permeability,

(f) performing steps (d) and (e) above for each of said selected plurality of frequencies, thereby creating a processed EM trace; and

,

(g) using said processed EM trace to explore for hydrocarbons within the predetermined volume of the earth.

(i) processing said EM traces with at least one seismic imaging algorithm, thereby producing a plurality of seismic processed EM traces; and

,

(e) writing at least a portion of said seismic processed EM traces to computer storage.

30. A method according to claim 29, wherein said computer storage is selected from a group consisting of a magnetic disk, a magnetic tape, an optical disk, a magneto-optical disk, RAM, and non-volatile RAM.

32. A device adapted for use by a digital computer wherein a plurality of computer instructions defining the method of claim 29 are encoded,

said device being readable by said digital computer,

said computer instructions programming said digital computer to perform said method, and,

said device being selected from the group consisting of computer RAM, computer ROM, a PROM chip, flash RAM, a ROM card, a RAM card, a floppy disk, a magnetic disk, a magnetic tape, a magneto-optical disk, an optical disk, a CD-ROM disk, or a DVD disk.

33. A method according to claim 29, wherein said EM data traces are selected from a group consisting of CSEM data traces, f-CSEM data traces, and t-CSEM data traces.

34. A method of conducting an unaliased EM survey to acquire data suitable for use in geophysical exploration of a predetermined volume of the earth containing structural and stratigraphic features conducive to the generation, migration, accumulation, or presence of hydrocarbons, comprising the steps of:

(a) selecting a subsurface target within said predetermined volume of the earth;

(b) determining at least one EM velocity proximate to said subsurface target, said at least one EM velocity being representative of a velocity of an EM wave propagation proximate to said subsurface target;

(c) determining an approximate dip of said subsurface target;

(d) using at least said at least one EM velocity and said approximate dip of said subsurface target to determine at least one of an EM source-receiver near offset in said EM survey, an EM source-receiver far offset in said EM survey, a number of EM traces in said EM survey, an EM trace spacing in said EM survey and a discrete sampling interval in natural time, thereby designing an unaliased EM survey;

(e) collecting EM data traces according to said designed EM survey; and

,

(f) using said EM data traces for the exploration, appraisal, development, or surveillance of hydrocarbons within said predetermined volume of the earth.

35. A method according to claim 34, wherein said EM data traces are selected from a group consisting of CSEM data traces, f-CSEM data traces, and t-CSEM data traces.

36. A method according to claim 34, further comprising the step of:

(g) writing at least a portion of said processed EM data traces to computer storage.

37. A method according to claim 36, wherein said computer storage is selected from a group consisting of a magnetic disk, a magnetic tape, an optical disk, a magneto-optical disk, RAM, and non-volatile RAM.

38. A device adapted for use by a digital computer wherein a plurality of computer instructions defining the method of claim 34 are encoded,

said device being readable by said digital computer,

said computer instructions programming said digital computer to perform said method, and,

said device being selected from the group consisting of computer RAM, computer ROM, a PROM chip, flash RAM, a ROM card, a RAM card, a floppy disk, a magnetic disk, a magnetic tape, a magneto-optical disk, an optical disk, a CD-ROM disk, or a DVD disk.

39. A method of geophysical exploration for hydrocarbons beneath the surface of the earth, comprising the steps of:

(a) selecting a subsurface target;

(b) situating at least one EM receiver proximate to said subsurface target;

(c) positioning an EM source within sensing ranging of at least one of said at least one EM receivers;

(d) activating said EM source according to a predetermined pattern, wherein said predetermined pattern is selected from a group consisting of a pseudo-random series of short binary pulses and a frequency sweep over a predetermined frequency range;

(e) sensing said activated EM source via said at least one of said EM receivers, thereby obtaining at least one EM trace;

(f) processing said at least one EM traces according to said predetermined pattern, thereby acquiring CSEM data suitable for use in geophysical exploration beneath the surface the earth.

40. A method of geophysical exploration for hydrocarbons within a predetermined volume of the earth according to claim 39, wherein step (f) comprises the steps of:

(f1) obtaining a pilot signal representative of said predetermined pattern, and,

(f2) cross correlating said pilot trace with said at least one of said EM traces, thereby acquiring CSEM data suitable for use in geophysical exploration within said predetermined volume of the earth.

41. A method of geophysical exploration for hydrocarbons within a predetermined volume of the earth according to claim 39, wherein said pilot signal representative of said predetermined pattern is obtained from a near-field receiver situated proximate to said EM source, said near-field receiver at least for sensing said activated EM source.

42. A method of geophysical exploration for hydrocarbons within a predetermined volume of the earth according to claim 39, wherein further comprising the step of:

(g) writing at least a portion of said processed EM data traces to computer storage.

43. A method according to claim 39, wherein said at least one EM trace is selected from a group consisting of a CSEM trace, an f-CSEM trace, and a t-CSEM trace.