To reduce exploration costs, E&P companies have sought, and seismic companies have delivered, improvements in 3-D acquisition productivity. However, image resolution, most notably in the crossline direction, previously has been compromised. An approach being posited by Polarcus can eliminate cost-quality compromise and provide E&P companies with a solution that addresses both productivity and quality.

In terms of seismic spatial sampling for 3-D imaging, the subsurface imaging grid is a function of the relative spatial geometry between sources and receivers on the surface. For towed streamer acquisition, a subsurface line is sampled midway between the surface position of each source and each streamer. Therefore, sets of subsurface lines are acquired on each pass of a multisource and multistreamer survey vessel.

Traditionally, 3-D towed streamer surveys have used two source arrays fired at alternate shot-point locations. For any streamer configuration, the subsurface line sampling is then one-fourth of the streamer separation. Using this technique, the spacing between subsurface lines is controlled by the streamer separation. For example, 100-m (328-ft) separation produces 25-m (82-ft) subsurface line spacing, and 50-m (164-ft) streamer spacing produces 12.5-m (41-ft) subsurface line spacing.

Within the last few years, large footprint surveys have increased in areas where quick acquisition turnaround is essential to meet regulatory requirements. This is driven by environmental concerns, climatic limitations or to meet license work commitments. This demand from E&P companies has led to seismic contractors offering more streamers and larger separations. Over the past decade, the trend in the number of streamers has progressed from 10 to 16 or more. Simultaneously, streamer separation has increased from 100 m to 200 m (656 ft). As streamer separation increases, so does the crossline sampling interval from 25 m for 100-m separations, to 50 m for 200-m separations. This can lead to lower resolution imaging in areas of complex structural geology and aliasing of multiples and other types of coherent noise.

Multisource acquisition
Polarcus is revisiting the concept of using multiple sources to increase the crossline sampling from towed streamer spreads. For example, using three source arrays produces subsurface line spacings that are one-sixth of the streamer separation compared to one-fourth for dual sources. Figure 1 shows the relationship between crossline sampling vs. streamer separation for dual- and triple-source configurations.

From the data, it is evident that the increased crossline sampling from triple-source configurations is significant and grows as streamer separations increase.

Figure 2 shows that the 25-m crossline sampling produced by dual sources and 100-m streamer separations can be achieved with 150-m streamer separations and triple sources, increasing efficiency by 50% for 12-streamer spreads. The efficiency benefit for a 12-m by 150-m streamer spread with triple sources holds even when compared to a 16-m by 100-m streamer spread with dual sources.

When the triple-source geometry is coupled with de-blending of overlapping records from dense inline shooting, the bin fold can be maintained at near the same levels as dual sources while providing 50% more surface shots and subsurface source-to-receiver ray paths.

Polarcus can extend the multisource overlap shooting concepts to acquire very dense crossline sampling with conventional streamers. In a recent test, the company demonstrated a 6.25-m (20.5-ft) crossline sampling using a five-source configuration. This type of acquisition is not necessarily aimed at increased operational productivity but more to specific projects where complex structures in the near surface need to be resolved to allow accurate imaging of the underlying hydrocarbon deposits.

By leveraging modern-day acquisition technologies of source and receiver/recording systems and advanced data processing, Polarcus has demonstrated that using more than two sources in dense in-line overlapping shot mode can enhance geophysical data quality while maintaining the high productivity of large streamer spreads.

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