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GCMulti-Azimuth Previous HitSeismicNext Hit for Sub-Salt Targets*

Bob Hardage1

Search and Discovery Article #40381 (2009)

Posted January 14, 2009

 

*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, December, 2008, and entitled “Results Shine for New Technology”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.

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

 

General Statement

One technical barrier plaguing hydrocarbon exploration is the inability to see geologic targets below distorted salt layers that span large areas of numerous depositional basins. The complex geometrical shapes of most salt bodies distort trajectories of Previous HitseismicNext Hit raypaths to such an extent that uniform illumination of sub-salt targets cannot be achieved with conventional Previous HitseismicNext Hit technology. If there is no uniform illumination of a target, a Previous HitseismicNext Hit image of that target cannot be correct. A new technology that addresses this problem of non-uniform illumination of sub-salt targets is a concept called multi-azimuth Previous HitdataNext Hit Previous HitacquisitionNext Hit.

 

uGeneral statement

uFigures

uImaging methods

uExamples

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigures

uImaging methods

uExamples

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigures

uImaging methods

uExamples

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigures

uImaging methods

uExamples

uConclusions

uReferences

 

 

Figures

fig01

Figure 1. Multi-azimuth Previous HitdataNext Hit Previous HitacquisitionNext Hit concept. Towed-cable Previous HitdataNext Hit are acquired by traversing the survey area in several azimuth directions. This diagram shows three overlapping azimuth tows. Some multi-azimuth surveys involve as many as six azimuth tows.

fig02

Figure 2. (a) Narrow-azimuth Previous HitmarineNext Hit Previous HitdataNext Hit Previous HitacquisitionNext Hit; (b) Wide-azimuth Previous HitmarineNext Hit Previous HitdataNext Hit Previous HitacquisitionNext Hit. Source Boat 2 may be removed in areas where there are congested production facilities, or it may be moved to travel behind Source Boat 1 near the tail-end of the cable spread.

fig03

Figure 3. Left, multiazimuth Previous HitdataNext Hit example 1, Nile Delta (from Keggin and others, 2006).

fig04

Figure 4. Right, multiazimuth Previous HitdataNext Hit example 2, Gulf of Mexico (from Michell and others, 2006).

Imaging Methods

 

As shown in Figure 1, a hypothetical salt trend is imaged with three cable tows that traverse the area in three different azimuth directions. In this manner, sub-salt geology is imaged with overlapping layers of Previous HitdataNext Hit, each Previous HitdataNext Hit layer representing a different azimuth in which the Previous HitdataNext Hit-Previous HitacquisitionNext Hit template moves across the geologic target area. The objective is to create a uniform illumination of any target that is below the image-distorting salt layer. There are several options for the geometrical configuration of the source/cable system that is towed along each of these traverses:

 

· One possibility is shown as Figure 2a. In this option, Previous HitdataNext Hit are acquired with a narrow-azimuth geometry that involves 10 or 12 parallel hydrophone cables spaced to form an Previous HitacquisitionNext Hit template approximately one kilometer wide and perhaps 10 or 12 kilometers long. Several arrays of air guns are distributed across this cable spread.

 

· A second Previous HitdataNext Hit-Previous HitacquisitionNext Hit scheme, illustrated in Figure 2b, involves multiple vessels that generate wider-azimuth Previous HitdataNext Hit in a single tow. Here the center vessel tows a narrow-azimuth Previous HitdataNext Hit-Previous HitacquisitionNext Hit system, but its companion source vessels increase the source-to-receiver azimuth aperture by a factor of three or more compared to the azimuth range of the system described by Figure 2a.

 

If this wide-azimuth concept is used to acquire the overlapping Previous HitdataNext Hit layers in Figure 1, the azimuths of the raypaths arriving at each subsurface imaging point are almost uniformly distributed around the complete 360-degree azimuth circle, and there is a greater likelihood that uniform target illumination is achieved.

 

Examples

 

Examples of the increased geological information provided by multi-azimuth Previous HitseismicNext Hit imaging are illustrated as Figures 3 and 4. The Previous HitdataNext Hit in Figure 3 come from a deepwater area of the Nile Delta where a thick, rugose anhydrite layer complicates the imaging of deeper targets. One of the target objects below this image-distorting layer is shown in this Previous HitdataNext Hit comparison. The improvements in target details seen in the six-azimuth image are significant compared to what can be seen in the traditional single-azimuth image.

 

The example in Figure 4 is across Mad Dog Field in the Gulf of Mexico. The improvements in Previous HitdataNext Hit quality and in image detail when multi-azimuth technology is used are impressive.

 

Conclusions

 

Industry interest in multi-azimuth Previous HitseismicNext Hit technology is growing because the technique creates such dramatic improvements in the images of complex, hard-to-see, sub-salt targets. Both theory and Previous HitdataNext Hit-processing tests have shown that compared to single-azimuth Previous HitdataNext Hit, multi-azimuth Previous HitdataNext Hit can:

 

· Improve the overall signal-to-noise ratio of sub-salt Previous HitdataNext Hit.

· Allow better removal of diffraction noise.

· Create a more uniform illumination of targets below layers that distort raypath distributions.

· Increase lateral resolution of Previous HitdataNext Hit.

· Produce more accurate amplitude attributes.

· Provide better attenuation of multiples.

 

Any one of these factors is a significant improvement in Previous HitseismicNext Hit technology. Collectively, this list forms a compelling reason to implement multi-azimuth tows of wide-azimuth arrays to define sub-salt drilling targets.

 

References

 

Keggin, J., T. Manning, W. Rietveld, C. Page, E. Fromyr, and R. van Borselen, 2006,

Key aspects of multi-azimuth Previous HitacquisitionTop and processing: SEG Expanded Abstracts, v. 25, p. 2886.

 

Michell, S., E. Shoshitaishvili, D. Chergotis, J. Sharp, and J. Etgen, 2006, Wide azimuth streamer imaging of mad dog; have we solved the subsalt imaging problem?: SEG Expanded Abstracts, v. 25, p. 2905.

 

 

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