uGeneral statement

uFigure captions

uNew technologies

uProduction congestion

uP-P & P-SV images

uConclusions

uAcknowledgment

uGeneral statement

uFigure captions

uNew technologies

uProduction congestion

uP-P & P-SV images

uConclusions

uAcknowledgment

uGeneral statement

uFigure captions

uNew technologies

uProduction congestion

uP-P & P-SV images

uConclusions

uAcknowledgment

uGeneral statement

uFigure captions

uNew technologies

uProduction congestion

uP-P & P-SV images

uConclusions

uAcknowledgment

Figure Captions

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New Seismic Technologies 

Two new marine seismic technologies now allow maximum source-receiver offsets of nine or 10 km:

• Super-long towed cables--If seismic data are acquired with a towed-cable option, both the hydrophone cable and the air gun source move across the target to be imaged.

• Stationary ocean-bottom-cable (OBC) or ocean-bottom-sensor (OBS) technology--If data are acquired with OBC or OBS technology, the sensors remain stationary on the seafloor as a source boat towing only an air gun array travels along the seismic line.

The distinction between stationary seafloor sensors and towed-cable sensors is important when considering the challenge of acquiring long-offset data for deep-target imaging across congested production areas. Both data-acquisition options are illustrated in Figure 1.

Example of Production Congestion 

An example of production congestion existing across some shallow-water areas of the Gulf of Mexico is shown in Figure 2. Here, a six-mile (10 km) diameter circle is positioned on the map to illustrate the difficulty of towing a six-mile cable across the area in any azimuth direction without snagging the cable on a platform, well head, or other surface-exposed facility. In contrast to the difficulty of executing towed-cable operations across this area, north-south OBC lines AA and BB and east-west OBC line CC (actual profiles used in one long-offset OBC data-acquisition program) pass within a few meters of several production platforms and other permanent facilities. 

Once OBC sensors are deployed on the seafloor, a source boat towing only a short air gun array can maneuver through the congested area with minimal difficulty. Seafloor sensors are usually deployed along profiles extending 50 to 100 km. Thus source-to-receiver offsets of 10 km and greater are easily implemented with OBC/OBS technology for purposes of super-deep imaging in areas heavily congested with production facilities.

Example of P-P and P-SV Images 

An additional appeal of OBC/OBS technology is that 4-C seismic data can be acquired, allowing targeted reservoirs to be imaged with P-SV (converted shear) wavefields as well as with conventional P-P wavefields. Towed-cable technology acquires only P-P seismic data. 

An example of P-P and P-SV images constructed from 4-C OBC data acquired with nine km maximum source-receiver offsets across one congested area on the northern shelf of the Gulf of Mexico is shown as Figure 3. The P-SV image has been time warped to match P-P image time coordinates. This time warping is a first-order depth registration of the two images based on an averaged VP/VS velocity ratio function for the area. This approximate image registration is sufficiently accurate to allow depth-equivalent geology to be identified in the two side-by-side images. 

Encircled structural features A and B are interpreted to be depth-equivalent geology. The time warping technique positions reflections A and B in time-warped P-SV image space to within 100 ms of their positions in P-P image space. 

A vertical salt structure blanks out the P-P and the P-SV images approximately midway between CDP coordinates 19,600 and 21,000. Using local seismic-measured rms velocities for depth conversion, the base of the data window in this display extends to approximately 7.5 km (~25,000 ft), the realm of super-deep gas targets. 

Features 1 through 4 on the P-SV image indicate a cyclic depositional process, which is important geologic information that is not obvious in the P-P image. Feature 5 is an example of P-SV data showing strata that are not obvious in the P-P image. Feature 6 is an example of P-P data imaging high-dip strata better than P-SV data at this location. In our experience, we have found the opposite also to be true in some instances; that is, in some settings P-SV data image high-dip structure better than P-P data.

Conclusions 

Two important conclusions can be made from these data examples:

• First, the use of OBC/OBS seismic technology is an excellent way to acquire long-offset seismic data for imaging super-deep targets across areas where there are dense congestions of production and engineering structures. In some instances, OBC/OBS technology will be the only option for acquiring long-offset seismic data across such areas.

• Second, P-SV data image much deeper than many explorationists thought would be possible. It requires evidence such as shown in Figure 3 to establish the principle that P-SV data at depths exceeding seven km (~23,000 ft) have signal quality and spatial resolution equivalent to P-P data.

In addition to the data shown in Figure 3, our investigations have documented numerous other examples where P-SV images are equal in quality and resolution to P-P images at depths of 20,000 to 26,000 ft across the Gulf of Mexico.

Acknowledgment 

The U.S. Department of Energy funded this research study; WesternGeco allowed the research team access to selected 4-C OBC multiclient seismic data.

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