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GCEqualizing Wavelets Produced by Different Seismic Sources*

 

Bob Hardage1

 

Search and Discovery Article #40894 (2012)

Posted March 26, 2012

 

*Adapted from the Geophysical Corner column, prepared by the authors, in AAPG Explorer, March, 2012, and entitled "When All Previous HitDataNext Hit Are Not Created Equally". Editor of Geophysical Corner is Satinder Chopra ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director. AAPG©2012

 

1 Bureau of Economic Geology, The University of Texas at Austin ([email protected])

 

General Statement

Tidewater areas can be difficult places to acquire consistent-quality seismic Previous HitdataNext Hit, because different sources have to be used across exposed land surfaces than what are used across shallow-water areas. Typically, explosives are used in shot holes in the onshore portion of a tidewater prospect, whereas environmental regulations may require that an air-gun source be used in shallow-water areas.

These two seismic sources produce different Previous HitbasicNext Hit wavelets – and profiles produced with explosives and air guns rarely tie in an optimal manner at common image coordinates without using wavelet-shaping algorithms to create equivalent reflection character across targeted intervals.

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General statement

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General statement

Figures

Example

Conclusion

 

 

 

 

 

 

 

 

 

General statement

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Conclusion

 

 

 

 

 

 

 

 

 

General statement

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Example

Conclusion

Figure Captions

Figure 1. This shallow-water profile was recorded with two different sources – shot-hole explosives and air guns. A portion of the image produced by shot-hole explosives is shown to the left of station 165; a portion of the image produced from the air-gun Previous HitdataNext Hit is shown to the right. The interpreter preferred the Previous HitdataNext Hit generated by the shot-hole explosives. Cross-equalization operators that converted the phase and frequency spectra of the air-gun Previous HitdataNext Hit to the phase and frequency spectra of the shot-hole Previous HitdataNext Hit were calculated at station 153.

Figure 2. The same Previous HitdataNext Hit exhibited on Figure 1 after cross-equalization operators determined at station 153 are applied along the complete air-gun source profile to create equivalent Previous HitbasicNext Hit wavelets in both the air-gun and shot-hole Previous HitdataNext Hit. The wavelet character at junction point 165 is now almost seamless. When this type of cross-equalization operator is applied to extensive grids of intersecting seismic profiles acquired with different sources, the consistent imaging wavelets produced in the overlapping Previous HitdataNext Hit allow a better interpretation of subsurface geology to be done across a prospect area.

Example

An example of using an explosive source and an air-gun source across a Louisiana tidewater area is documented as Figure 1 and Figure 2. This shallow-water test line was recorded twice because, at this location, explosive sources were allowed.

For one profile, the source was a 30-pound (13.6-kilogram) charge positioned at a depth of 135 feet (41 meters) at each source station. For the second Previous HitdataNext Hit acquisition along the same profile, the source was an array of four air guns with a combined volume of 920 in³, and eight air-gun pops were summed at each source station.

Considerable Previous HitprocessingNext Hit effort was expended to make the final reflection character identical on each test line. The Previous HitdataNext Hit illustrated as Figure 1 show the results of the Previous HitdataNext Hit Previous HitprocessingNext Hit. The frequency content of the two profiles is approximately the same, but wavelet character is not identical at the junction point (station 165). In this instance, the interpreter responsible for this prospect decided that the reflection character expressed by the explosive source was preferred rather than the wavelet response shown by the air-gun source.

The challenge was that in neighboring tideland areas, regulations required that an air-gun source be used in water-covered areas – shot-hole explosives could not be used in shallow water as they had been across this initial test site, and a method had to be developed that would allow air-gun-source Previous HitdataNext Hit to be used in conjunction with explosive-source Previous HitdataNext Hit acquired across adjacent exposed-land areas.

Said another way, the problem was to create a Previous HitbasicNext Hit wavelet in air-gun-generated Previous HitdataNext Hit that was equivalent to the Previous HitbasicNext Hit wavelet embedded in explosive-source Previous HitdataNext Hit. This type of problem has to be solved by Previous HitdataNext Hit-Previous HitprocessingNext Hit procedures, not by Previous HitdataNext Hit-acquisition techniques.

An approach used by many Previous HitdataNext Hit processors to ensure that equivalent Previous HitbasicNext Hit wavelets exist in two seismic profiles acquired with different sources is to calculate numerical cross-equalization operators that convert the phase and frequency spectra of source A to be equivalent to the phase and frequency spectra of source B.

This technique was applied to the tidewater seismic Previous HitdataNext Hit illustrated on Figure 1 by using Previous HitdataNext Hit from the image trace at station 153 to calculate cross-equalization operators that converted the phase/frequency spectra of the air-gun Previous HitdataNext Hit to the spectra of the explosive-source Previous HitdataNext Hit. The result is exhibited as Figure 2. The wavelet character of the profiles now agrees better at the tie point so that common horizons, Previous HitsequenceNext Hit boundaries, and facies character can be interpreted on both profiles with greater confidence.

Conclusion

The example discussed here is from a tidewater area where operating and environmental constraints forced different sources to be used on land-based and water-based seismic lines. The concept of numerical equalization of the Previous HitbasicNext Hit wavelets embedded in any grid of intersecting 2-D (or 3-D) Previous HitdataNext Hit, however, applies to a variety of onshore and offshore areas where people have access to overlapping legacy seismic Previous HitdataTop that have been acquired by different companies at different times and with different energy sources.

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