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-azimuth data acquisition.
Figure 1 – Multi-azimuth data acquisition concept. Towed-cable data 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.
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 data, each data layer representing a different azimuth 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.
Figure 2 – (a) Narrow-azimuth marine data acquisition; (b) Wide-azimuth 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.
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-azimuth geometry that involves 10 or 12 parallel hydrophone cables spaced to form an acquisition 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 data-acquisition scheme, illustrated in figure 2b, involves multiple vessels that generate wider-azimuth data in a single tow. Here the center vessel tows a narrow-azimuth data-acquisition 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 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 azimuth circle, and there is a greater likelihood that uniform target illumination is achieved.
Examples of the increased geological information provided by multi-azimuth seismic imaging are illustrated as figures 3 and 4.
Figure 3 – Left, multi-azimuth data example 1, Nile Delta, (from Keggin and others, 2006).
The data in figure 3 come from a deep-water 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-azimuth image are significant compared to what can be seen in the traditional single-azimuth image.
Figure 4 – Right, multi-azimuth data example 2, Gulf of Mexico (from Michell and others, 2006).
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-azimuth technology is used are impressive.
Industry interest in multi-azimuth 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-azimuth data, multi-azimuth 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-azimuth tows of wide-azimuth arrays to define sub-salt drilling targets.