With the right pattern and tools, illumination of the reservoir, NPT and survey times are greatly improved.
David Hill, WesternGeco; Mark Thompson and Marianne Houbiers, StatoilHydro
A definition of the American English idiom “go around in circles” is “to engage in excited but useless activity.” In the world of offshore seismic data acquisition, however, acquiring seismic data via a circular shooting pattern has been something of a Holy Grail since the early 1980s. It was theorized then that a circular seismic shooting pattern would greatly improve imaging of complex geologic features, in particular circular salt dome flanks and related faulting. The circular acquisition geometries would enable a wider range of azimuths to be acquired and better define and characterize subsurface features.
But shooting circular surveys offshore failed to gain traction with the oil and gas industry because marine acquisition technology at the time could not provide accurate steering and streamer positioning, and because the industry was not capable of processing these kinds of data.
Fast-forward 25 years or so, and technology and knowledge have caught up—now, new, Full-AZimuth (FAZ) Coil Shooting from a single-vessel is commercial. In this instance, “going around in circles” becomes a useful activity.
Commercial projects have shown that this new method is an efficient and cost-effective means of acquiring seismic data, with a 360° range of azimuths across the full offset range over a survey area. A chief benefit is that the technique accomplishes this with only one vessel, eliminating the need for deploying multiple source and streamer vessels. That makes the technique an ideal choice for acquiring small- to medium-sized datasets or for FAZ surveys in remote areas, where it may not be feasible to mobilize several seismic vessels simultaneously.
MAZ, WAZ, RAZ METHODS
It is widely accepted that multi- and wide-azimuth (MAZ, WAZ) seismic methods deliver superior subsurface imaging and improved attenuation of coherent noise and multiples in comparison with conventional Narrow-AZimuth (NAZ) methods.
But both MAZ and WAZ methods feature shooting geometries based solely on straight lines, whether with a single vessel shooting a survey multiple times with one vessel (MAZ), or multiple vessels in parallel, enabling a larger crossline offset (WAZ) leading to a larger azimuthal contribution and improved imaging. However, the parallel geometries inherent in these methods mean that line turns reduce efficiency, resulting in significant NonProductive Time (NPT)—sometimes reaching as much as 50%. In circumstances involving multiple vessels with wide streamer spreads, turning time usually exceeds three hours per sail line. That could add up to several weeks of NPT for a large survey.
Recent studies suggested that wider crossline offsets resulted in better images and that the best images came from data over a complete 360° range of azimuths, resulting in sampling dubbed FAZ, which is achieved by coil-pattern shooting. An early effort at Rich-AZimuth (RAZ) sampling involved a combination of MAZ and WAZ methods targeting a subsalt illumination challenge in Shenzi Field in the deepwater Gulf of Mexico. Multiple vessels were deployed in parallel in three different sailing directions and then in triangular paths around the edge of the survey area, Fig. 1. Some useful data were recorded during the turns, thereby enhancing productivity and boosting the coverage area at incremental cost.
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Fig. 1. Preplot sail lines for the Shenzi RAZ survey. Image courtesy of BHP Billiton, Hess Corp and Repsol-YPF.
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That underscored what appeared to be a game-changing technology opportunity: Efficiently acquiring data while turning could reduce or even eliminate NPT while delivering the richest data samples in the most cost-effective way. In other words, coil-pattern shooting offers an affordable way to deliver FAZ results with a single vessel. The coverage pattern for various acquisition geometries underscores the case, Fig. 2.
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Fig. 2. Comparison of acquisition geometries.
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HOW IT WORKS
The initial challenge in implementing this new method was to find a way to accurately record seismic data while turning. In older, conventional data acquisition, positioning of in-sea equipment was inaccurate during turns. At the same time, streamer steering technology—a must for circular shooting with multiple streamers—was not feasible until recently. And most seismic data processing strategies were based on straight parallel lines, simple isotropic stacking and migration velocity fields.
WesternGeco introduced the Q-Marine* single-sensor marine seismic system in 2000. With the system, Q-Fin* automated steering devices precisely control the streamers’ depth and lateral position, allowing the streamers to remain separated and laterally displaced during turns. The system employs deflectors mounted close to the edge of the outer streamers. Conventional systems, on the other hand, deploy deflectors outside the streamer spreads resulting in a bigger turn radius.
Strong cross-currents can increase noise levels when towing streamers in a curve. Using advances in electronics and fiber optic networks, the system is able to record densely placed single sensors. This enables the geophysicist to sample the signal and noise wavefield adequately so that they can be separated. With sufficient noise samples, it is capable of suppressing the noise with targeted signal processing techniques while preserving signal integrity. This effective noise removal method, part of the digital group forming process, enables the acquisition of high-quality seismic data in poor weather or during circular streamer towing through strong currents.
By overcoming these hurdles, coil-pattern shooting enables a single vessel to acquire data continuously, eliminating NPT during line changes, and to deliver richly sampled FAZ data with reduced noise and multiple attenuation. Adequate sampling in azimuth ensures that the dataset can be split into a range of azimuths needed for building anisotropic velocity models and analyzing fractures. In addition, circular geometries allow the acquisition of near-offset data over an entire survey area—a capability conventional multi-vessel WAZ surveying lacks. Acquiring near offsets helps better image the seabed and shallow overburden, while sampling the water bottom means that more efficient Surface-Related Multiple Elimination (SRME) models can be built. Therefore, coil-pattern shooting enables even more effective use of processing algorithms to remove noise and enhance resolution.
EARLY EXPERIENCE
The first field test of the coil-pattern shooting method was conducted in the Gulf of Mexico in April 2007 over an area previously covered by a parallel WAZ survey. It confirmed the feasibility of sailing in circles while keeping streamers constantly separated and positioning receivers accurately.
Subsequently, a synthetic modeling exercise was conducted to compare coil-pattern shooting and a four-vessel WAZ survey with regard to the effectiveness of 3D multiple attenuation. Coil-pattern shooting proved more effective, due to achieving double the number of shots acquired over the same area, wider azimuth distribution and better near offsets. The simulation also showed that a coil-pattern shooting survey would require 61 days to record 321,706 shots vs. the four-vessel WAZ survey at 62 days and 160,000 shots.
The second field test occurred in December 2007 in the Black Sea over deepwater acreage. The area covered offered complex geology that presented tough imaging problems—for example, prospects in limestone reef and shale sequences overlying volcanic structures—and an extremely short turnaround.
The survey entailed rotating nine circular coils around a fixed point, resulting in the pattern resembling the petals of a dahlia, Fig. 3. The results were deemed favorable in comparison with those a large conventional streamer survey conducted the year before, showing improved imaging of sub-carbonate reef structure in the presence of complex water-bottom multiples, Fig. 4.
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Fig. 3. Programed coverage pattern for Black Sea field test.
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Fig. 4. Comparison of NAZ 3D survey and coil-pattern shooting test results from Black Sea field test.
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The first commercial implementation of the technique occurred offshore Indonesia in August–September 2008. It followed a modeling study that compared coil-pattern shooting with MAZ and with a conventional NAZ survey with four shooting directions; the study concluded that the technique could best illuminate the target. The survey was completed in 49 days, compared with 60 days projected for a three-azimuth MAZ survey and 75 days for a four-azimuth survey, Fig. 5.
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Fig. 5. Comparison of acquisition duration for three single-vessel geometries considered for survey off Indonesia.
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HEIDRUN MODELING
Coil-pattern shooting acquisition would undergo its biggest test to date in a collaboration between StatoilHydro and WesternGeco over the Heidrun oil and gas field in the Norwegian Sea.
Although Heidrun, in 1,150 ft of water, has been producing since the mid-1990s, imaging problems owing to complex geology have inhibited its full development. Heidrun is in a setting that is intensely faulted, and the main reservoir is highly compartmentalized. Securing robust illumination of the subsurface is complicated by a dome-shaped feature thought to be a salt diapir. Also, there are some high-impedance limestone banks of seep deposits surrounding the dome. In certain areas of the fields, faults and dip directions are obscured.
Given the complexity of Heidrun geology, StatoilHydro decided to include a coil-pattern shooting option in a 2008 simulation that studied whether a WAZ or MAZ approach would resolve its imaging problems.1 The study compared a conventional NAZ approach and WAZ methods with various different configurations that included coil-pattern shooting.
The velocity and density models StatoilHydro and WesternGeco designed for the study preserved not only the key features that characterize the field, but also the areas prone to unclear imaging, Fig. 6. The study simulated the three acquisition geometries and migrated the data with the same velocity model used in initial modeling, Fig. 7.
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Fig. 6. Heidrun depth-velocity model.
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Fig. 7. Simulations for proposed Heidrun study of acquisition geometry options.
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The simulation found that the coil-pattern shooting technique provided more consistency and accuracy in amplitudes and offered a greater range of azimuths and higher fold of coverage, fewer artifacts, improved noise suppression and enhanced multiple attenuation, Fig. 8.
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Fig. 8. Heidrun modeled horizon amplitudes.
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HEIDRUN FIELD TEST
To validate the conclusions of the modeling effort, StatoilHydro ordered a coil-pattern shooting survey over Heidrun patterned roughly after the dahlia pattern used in the Black Sea test, but with 18 intersecting coils, double the number used in the Black Sea test, Fig. 9.2
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Fig. 9. Heidrun Field test acquisition geometry.
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The Heidrun survey was conducted during a four-day span with ten 4.5-km-long streamers separated by 75 m, with a coil radius of 5.6 km. This effort yielded FAZ high-fold data over a 2.6-sq-km target area along with enough surrounding aperture to produce 3D migrated sections comparable to conventional seismic data acquired previously.
Results showed minimal noise and multiple energy remaining in the data, yielding clear images. The coil-pattern shooting sections also produced better definition of faults in the reservoir section and clearer imaging of the structure’s flanks when compared with conventional survey data, Fig. 10.
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Fig. 10. Coil-pattern comparison with a conventional NAZ 3D survey (left).
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The effort was not without its challenges for the survey team, as the Heidrun tension-leg platform and two loading buoys were in the way, forcing some modifications in the original survey design to avoid any equipment from entering the safety zones around these objects.
In short, the Heidrun Field test confirmed the predictions of the modeling study, giving strong support to the concept as a viable and cost-effective approach for seismic acquisition offshore Northern Europe. Prior to this effort, all WAZ surveys had been conducted in the Gulf of Mexico with multiple vessels and receivers; Heidrun confirmed the viability of using the technique with one vessel in the North Sea.
BENEFITS OF COIL SHOOTING
The chief advantage of the technique is the capability for continuous recording. In practice, there will be some line-change time. Also, as with parallel towed streamer surveys, there will also be a need to perform some infill shooting—but less with coil-pattern shooting. Figure 11 shows that the major benefit is the increase in shooting time from 56% to 87% of available recording time.
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Fig. 11. Improvement in shooting time.
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Figure 12 displays the acquisition time for coil-pattern shooting compared with three-vessel WAZ and three-azimuth MAZ. While three-vessel WAZ acquires data faster than the other two methods, it is roughly double the cost per day compared with single-vessel operations. In addition, the technique is more productive than three-azimuth MAZ up to about 2,200 sq km and more effective for areas smaller than 400 sq km.
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Fig. 12. Comparison of acquisition durations.
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The coil method requires vessels that are capable of tight turning. Currently, continual tweaks are made to the survey design, exploring the best ways to arrange circles. As the number of coil-pattern shooting surveys grows, so too will the various arrangements of circular patterns. Using a single vessel also reduces the environmental impact of the seismic operation.
Ultimately, this method can contribute to improving the quality of imaging in complex geological settings, including subsalt, presalt, subbasalt and subcarbonate. It enables the latest enhanced-azimuth imaging techniques to be employed in locations more remote and on smaller targets than ever before. 
LITERATURE CITED
1 Houbiers, M., Arntsen, B., Thompson, M., Hager, E., Brown, G. and D. Hill, “Full azimuth seismic modelling at Heidrun,” presented at the PETEX conference, London, Nov. 25−27, 2008.
2 Houbiers M. and M Thompson: “Full azimuth field trial at Heidrun,” presented at the EAGE conference, Amsterdam, June 2009. Houbiers, M., Garten, P., Thompson, M., Straith, K. R. and A. Smalø Moen, “Marine full azimuth field trial at Heidrun,” to be presented at the SEG conference, Houston, October 2009.
ACKNOWLEDGMENT
The authors thank StatoilHydro ASA for permission to publish this work.
* Trademarks of WesternGeco.
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THE AUTHORS
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David Hill, Applied Geophysics Manager at WesternGeco in Gatwick, England, joined the company in 2000. He is responsible for global geophysical support for WesternGeco. Previously, he worked with Amoco UK for 10 years as an operations geophysicist and designed, acquired and processed 2D and 3D seismic surveys. He received a BSc degree (Hons) in physics and geophysics from the University of Liverpool, England.
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Mark Thompson is Team Leader for Acquisition and Imaging within the Increased Oil Recovery Research Program at the StatoilHydro Research Centre in Trondheim, Norway. Previously, he worked on development of 4C processing and acquisition technologies, and on time-lapse seismic processing and modeling. He began his career in 1989 as a field geophysicist with Western Geophysical. He earned a BSc degree in applied earth science from Kingston Polytechnic and an MSc degree in petroleum geology and geophysics from Imperial College, both in London.
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Marianne Houbiers works as a Research Geophysicist on seismic modeling and imaging within the Increased Oil Recovery Program at the StatoilHydro Research Centre in Trondheim, Norway. She joined StatoilHydro in 2006. Previously, she worked as a survey methodologist in the methods departments of Statistics Netherlands and Statistics Denmark. She holds MSc and PhD degrees in theoretical physics from Utrecht University, The Netherlands.
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