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GCMulti-Previous HitAzimuthNext Hit Seismic 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 seismic raypaths to such an extent that uniform illumination of sub-salt targets cannot be achieved with conventional seismic technology. If there is no uniform illumination of a target, a seismic 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-Previous HitazimuthNext Hit data acquisition.

 

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-Previous HitazimuthNext Hit data acquisition concept. Towed-cable data are acquired by traversing the survey area in several Previous HitazimuthNext Hit directions. This diagram shows three overlapping Previous HitazimuthNext Hit tows. Some multi-Previous HitazimuthNext Hit surveys involve as many as six Previous HitazimuthNext Hit tows.

fig02

Figure 2. (a) Narrow-Previous HitazimuthNext Hit marine data acquisition; (b) Previous HitWideNext Hit-Previous HitazimuthNext Hit marine data acquisition. 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 data example 1, Nile Delta (from Keggin and others, 2006).

fig04

Figure 4. Right, multiazimuth data 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 Previous HitazimuthNext Hit directions. In this manner, sub-salt geology is imaged with overlapping layers of data, each data layer representing a different Previous HitazimuthNext Hit in which the data-acquisition 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, data are acquired with a narrow-Previous HitazimuthNext Hit geometry that involves 10 or 12 parallel hydrophone cables spaced to form an acquisition template approximately one kilometer Previous HitwideNext Hit and perhaps 10 or 12 kilometers long. Several arrays of air guns are distributed across this cable spread.

 

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

 

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

 

Examples

 

Examples of the increased geological information provided by multi-Previous HitazimuthNext Hit seismic imaging are illustrated as Figures 3 and 4. The data 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 data comparison. The improvements in target details seen in the six-Previous HitazimuthNext Hit image are significant compared to what can be seen in the traditional single-Previous HitazimuthNext Hit image.

 

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

 

Conclusions

 

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

 

· Improve the overall signal-to-noise ratio of sub-salt data.

· Allow better removal of diffraction noise.

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

· Increase lateral resolution of data.

· Produce more accurate amplitude attributes.

· Provide better attenuation of multiples.

 

Any one of these factors is a significant improvement in seismic technology. Collectively, this list forms a compelling reason to implement multi-Previous HitazimuthNext Hit tows of Previous HitwideNext Hit-Previous HitazimuthNext Hit 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-Previous HitazimuthNext Hit acquisition and processing: SEG Expanded Abstracts, v. 25, p. 2886.

 

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

 

 

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