Processing methods for time division CSEM data

Patent metadata

Field	Value
Publication number	US-10209386-B2
Application number	US-201314404204-A
Country	US
Kind code	B2
Filing date	Jun 27, 2013
Priority date	Aug 30, 2012
Publication date	Feb 19, 2019
Grant date	Feb 19, 2019

Abstract

Method for inverting, in the frequency domain (42), controlled source electromagnetic survey data (41) acquired using a time-division compound waveform made up of sub-sequences of different base waveforms, for example square waves of different frequencies. A windowed Fourier decomposition method is used, with the window size and shape designed in consideration of the sub-sequences. The window length may be twice the length of the compound waveform, or more. Alternatively the window length may be comparable to the sub-sequence length, or slightly less. Window shapes include cos2, rectangular, and triangular. The method addresses the problem of unknown arrival times for each sub-sequence, and also transition transients that occur between sub-sequences.

First claim

The invention claimed is:

1. A method for controlled-source electromagnetic prospecting comprising:
repeatedly transmitting into a subsurface electromagnetic energy having a time-division source waveform, which is a compound waveform made by combining two or more sub-sequences of base waveforms having different primary or fundamental frequencies, wherein in each sub-sequence, the base waveform for that sub-sequence is repeated a plurality of times, and furthermore wherein in an i th sub-sequence, the base waveform is repeated at a frequency f i , with f 1 being the lowest frequency of the f i , and wherein each f i is an integer multiple of f 1 with jumps of no larger than twice f 1 ;
obtaining electromagnetic data resulting from the repeated transmission;
decomposing the data into frequency components using a windowed decomposition method involving one of: (i) Fourier transformation, (ii) a correlation-based equivalent to Fourier transformation, or (iii) isolating a single frequency component by modeling the single frequency component to include (a) exponential amplitude change, increasing or decreasing depending on whether source-receiver offset is, respectively, decreasing or increasing, and (b) linear phase delay,
wherein said decomposing is performed using a computer and is designed to deal with not knowing arrival times of particular sub-sequences in the time-division waveform;

imaging, with a computer, the frequency components and generating a data volume of electrical resistivity of earth structures that returned electromagnetic waves to receivers that recorded the electromagnetic data; and
prospecting for hydrocarbons at a location derived from the data volume of the electrical resistivity as being indicative of a presence of hydrocarbons.

2. The method of claim 1 , wherein the arrival time uncertainty is dealt with by one of:
using a small decomposition window, being a time window with length comparable to the length of a single sub-sequence but less than the length of the entire time-division waveform;
using a larger decomposition window, being a time window having a length and shape that are determined to ensure an equal number of appearances of each sub-sequence within the window.

3. The method of claim 2 , wherein the larger window is used, and it is tapered.

4. The method of claim 3 , wherein the window is one of a group consisting of a cos 2 window, a rectangular window, and a triangular window.

5. The method of claim 3 , wherein the window of the windowed decomposition method is of a length equal to an even integer multiple of the length of the time-division compound waveform.

6. The method of claim 3 , wherein the time-division compound waveform is designed such that consecutive occurrences of any particular sub-sequence are in phase with each other.

7. The method of claim 3 , wherein each sub-sequence has a time duration that is an integer multiple of the reciprocal of a selected fundamental frequency.

8. The method of claim 2 , wherein the decomposition window is applied to the obtained electromagnetic data at a plurality of different times, and each frequency component is obtained from composite data of the different windows.

9. The method of claim 8 , wherein the windows applied at different times include overlapping windows.

10. The method of claim 2 , wherein the small decomposition window is used, and the small decomposition has a length less than or equal to the length of a sub-sequence representing a particular frequency selected for extraction in the decomposition.

11. The method of claim 10 , wherein the window is applied a plurality of times in an overlapping manner, and those with peak amplitude responses are identified and used to obtain amplitude or phase values, or both, for one or more of the frequency components.

12. The method of claim 11 , wherein the peak amplitude responses are identified by tracking expected arrivals using a tracking algorithm.

13. The method of claim 10 , wherein the window length is less than the sub-sequence length by an amount determined in consideration of reducing inclusion of transition transients that occur between sub-sequences.

14. The method of claim 2 , wherein the window is rectangular and not tapered.

15. The method of claim 1 , wherein the windowed decomposition method comprises taking a Fourier transform of the obtained electromagnetic data, window-by-window.

16. The method of claim 1 , wherein the windowed decomposition method comprises said correlation-based equivalent to Fourier transformation, and the correlation-based equivalent is correlation with a full-sequence sinusoid or other waveform having a fundamental frequency equal to a frequency of interest, and repeating the correlation for each other frequency of interest.

17. The method of claim 1 , wherein the windowed decomposition method comprises said correlation-based equivalent to Fourier transformation, and the correlation-based equivalent is correlation with a reference waveform designed to collapse a selected frequency component while moving other frequency components to a different time.

Assignees

• Hornbostel Scott C
• Ou Shangli
• Exxonmobil Upstream Res Co

Inventors

• Hornbostel Scott C
• Ou Shangli

Classifications

• G01V3/083 Primary

  Controlled source electromagnetic [CSEM] surveying · CPC title

• G01V2003/086

  Processing · CPC title

• G01V3/38

  Processing data, e.g. for analysis, for interpretation, for correction · CPC title

• G01V3/12 Primary

  operating with electromagnetic waves {(operating with millimetre waves G01V8/005)} · CPC title

• G01V3/02

  operating with propagation of electric current · CPC title