An overview of seismic azimuth for towed streamers

2,960 views
2,794 views

Published on

The Leading Edge, May 2010. Accompanies the 2009 SEG Honorary Lecture tour.

Published in: Technology, Business
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
2,960
On SlideShare
0
From Embeds
0
Number of Embeds
41
Actions
Shares
0
Downloads
108
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

An overview of seismic azimuth for towed streamers

  1. 1. ACQUISITION/PROCESSING $Q RYHUYLHZ RI VHLVPLF D]LPXWK IRU WRZHG VWUHDPHUV ANDREW LONG, PGS C onventional 3D streamer seismic processing generally ignores any azimuth component in the data. We are used to the convenience of acquiring overlapping shot gathers in straight lines, as we can sort the data into common midpoint gathers, and then exploit the power of stack in processing to attenuate random noise. The higher the fold, the more random noise we attenuate. Conventional 3D surveys are acquired using “swath”or “racetrack” vessel shooting, wherein the survey has a single line orientation (or “survey azimuth”), and a long, narrow spread of streamers are towed by a single vessel. Apart from the front of the streamers (short source-receiver offsets), most source-receiver combinations have a relatively common azimuth (the angle between their particular vector and the survey orientation, Figure 1). Thus, the subsurface geology is illuminated only from one particular shooting direction. We assume that most coherent noise types are well behaved and Figure 1. Schematic illustration for a vessel towing an array of streamers. The lower right graph shows the generalized distribution we can remove them in processing. We assume that the target of offsets and azimuths throughout a survey area, appropriate to the illumination is acceptably uniform, and we can produce clean acquisition strategy being used. The center of the circle represents zero seismic images. Most of the time these assumptions are in the offset, and the outside of the circle represents maximum offset. Each ballpark of truth and our resultant seismic data allow us to radial spoke represents the source-receiver azimuth. As seen for both achieve our exploration and appraisal objectives. the vessel diagram and the rose diagram, there is a much larger range of azimuths available for the shortest offsets than is the case for the But, what happens when these assumptions are violated? largest offsets (far end of the streamer spread), if conventional narrow- What happens when the coherent noise is too complicated azimuth (NAZ) acquisition is pursued. and too pervasive for us to interpret the underlying signal? What happens when the reservoir illumination is irregular/ discontinuous, and the seismic image is unacceptably distort- NAZ One vessel towing an array of streamers and source(s). ed? In the worst case, exploration opportunities are missed, MAZ Two or more coincident NAZ surveys with different wells are misplaced, and fortunes are lost. An increasingly survey azimuths combined in processing. popular solution to these dilemmas is to consider acquisition WAZ Typically two or more vessels used simultaneously to strategies that yield a larger distribution of azimuths and off- increase the range of azimuths and offsets available sets at each location in the survey than is the case for narrow- for each shot gather in processing. azimuth (NAZ) towed streamer acquisition. Table 1 presents a glossary of contemporary abbreviations used in this article, WATS A particular flavor of WAZ pioneered by BP. and explained in detail later. RAZ Typically a combination of MAZ and WAZ de- The catalyst for embracing these strategies has under- signed to yield the most continuous distribution of standably been a history of expensive drilling failures, notably azimuths and offsets possible with towed streamer. in the Gulf of Mexico (GOM) and the North Sea. The lessons FAZ Perfect azimuth and offset distribution at every point learned in these regions are now starting to trickle into other in the survey. Possible only in practice when the regions. source and receivers can be physically decoupled. A common barrier to successful seismic imaging is the Table 1. Azimuth acquisition strategies. presence of salt, volcanics, and carbonates in the near surface or overburden. We all understand that future (offshore) oil and gas exploration must move into more challenging envi- azimuths can be recorded for all offsets, and at all locations ronments: deeper water, subsalt areas outside the GOM, the (bins) throughout the survey area. Arctic, in unexplored areas, in areas affected by basalts, or in areas affected by thick carbonates. Most of the “easy” oil and Target illumination gas was found long ago. The efforts required to replace the Most people can intuitively understand that the shooting world’s reserves must inevitably increase, and seismic meth- direction in a 3D streamer survey will influence the target il- ods such as multi-azimuth (MAZ), wide-azimuth (WAZ), lumination. So, it is a natural progression to understand that and rich-azimuth (RAZ) will inevitably become more com- shooting the same survey area in two or more different direc- monplace. Note that full-azimuth (FAZ) seismic is achievable tions (survey azimuths) will provide complementary target only when the source(s) can be physically decoupled from the illumination. The combination of these surveys will collec- receiver spread, such as land or seafloor 3D seismic, and all tively have better overall target illumination, i.e., the sum 512 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  2. 2. ACQUISITION/PROCESSING Figure 2. Prestack depth migration (PSDM) comparison of (left) one azimuth from a conventional NAZ survey versus (right) three azimuth cubes processed to output one MAZ cube. (Courtesy PGS) of the parts gives a better result. Provided that the two-way survey azimuths could be individually stacked with a com- times (TWT) to all events are close to identical at coincident mon 3D velocity model, and then the three stack cubes could locations throughout each azimuth cube, their summation be summed with minimal error after prestack depth migra- (a multi-azimuth, or MAZ, stack) will provide a clearer im- tion of each azimuth cube. age of the reservoir. The universal benefits of MAZ surveys Note that MAZ or WAZ acquisition and processing will are twofold: higher signal-to-noise content and better lateral normally be contemplated because target illumination and/ resolution (i.e., event continuity and character). Figure 2 is or coherent noise issues seriously degrade the seismic imaging an example from Varg Field in the North Sea. process when conventional narrow-azimuth (NAZ) acquisi- MAZ surveys can either be pursued by acquiring two or tion and processing is pursued. As a consequence of the in- more new survey azimuths, or by “upgrading” an existing 3D herent data quality challenges, standard velocity picking is of- (NAZ) survey with a new (NAZ) survey, typically acquired ten difficult, and only prestack time migration will be robust in the orthogonal direction to the existing survey. The lat- enough to tolerate the associated errors in the velocity model. ter option is attractive for areas where a reasonable under- Having said that, if there is evidence of azimuthally depen- standing of the exploration targets already exists. Thus, it is dent velocity effects, either because of lateral velocity gradi- possible to identify locations for quite small, target-specific ents, or in the worst case because of azimuthal anisotropic 3D surveys that will allow an improved (MAZ) seismic im- effects, then an independent velocity model must be derived age of each specific target. The acquisition methodology is for each azimuth cube prior to MAZ summation. This will conventional: a single vessel towing a streamer spread. The affect project turnaround. It has become common to build processing methodology is mostly conventional: standard ve- a common reference velocity model for each azimuth cube locity picking and model building, time-domain migration, during the first pass of velocity picking, and then higher- etc. The first-pass velocity picking will examine whether there order velocity corrections are picked independently for each are azimuthally dependent differences in the stacking veloci- azimuth cube, usually including VTI anisotropic corrections. ties for each survey azimuth. In the example of Figure 2 (Gaus The ultimate scenario is to apply prestack depth migration and Hegna, 2003), it was established that three overlapping with TTI anisotropic velocity corrections (for tilted strata May 2010 The Leading Edge 513 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  3. 3. ACQUISITION/PROCESSING Figure 3. Rose diagrams for a three-azimuth MAZ survey (left), a WAZ survey (center), and a RAZ survey (right). A MAZ survey combines two or more NAZ surveys acquired by a single vessel, but with different survey azimuths. A WAZ survey uses two or more vessels to acquire a larger range of azimuths for each shot. Conventional WAZ shooting acquires parallel lines with one survey azimuth. A RAZ survey applies a WAZ vessel configuration to a MAZ shooting template, and acquires the most continuous range of azimuths for all shots. Refer also to Figure 5. with transversely variant anisotropy), but that is conceivable The power of azimuth stack only if an accurate velocity model can be built, and if the ef- Random noise on NAZ data is attenuated by the brute force fort is justifiable. As noted, quite simplistic MAZ processing of stacking across a range of source-receiver offsets in each flows have nevertheless proven to be successful in many areas. common midpoint (CMP) gather. The improvement in sig- It is intuitively understandable that if a particular subsur- nal-to-noise content is proportional to the square root of the face location is illuminated by two surveys acquired in differ- fold of stack. But, we must usually make simple assumptions ent survey azimuths, the associated seismic wavefields must when attempting to remove coherent noise in processing. travel through different parts of the near surface. This is in For example, assuming that there is a robust and systematic fact a double-edged sword. On one hand, localized distor- difference in the moveout of primary signal versus noise on tions of the seismic wavefield in the near surface will be “aver- CMP gathers. Or, that the noise can be robustly described aged out” by the MAZ stack, effectively achieving a solution by a linear, parabolic, or hyperbolic moveout model. Or, that to problems beyond the scope of normal processing. On the the traveltime of the noise can be predicted using a simple other hand, these different travel paths may result in small Earth model, and real noise can thus be attenuated by adap- vertical and lateral mispositioning of some events between tive subtraction of estimated noise. Surface-related multiple (coincident) azimuth cubes. So, the MAZ summation process elimination (SRME) predicts multiples from the data itself, may in fact degrade the resolution somewhat. The solution is assuming that all subsurface reflections occur in a vertical most likely a combination of careful trim statics and aniso- plane below the streamers, and direct arrivals, refraction and tropic depth migration, but this is an area of ongoing R&D. various exotic modes of coherent noise are not present in the Furthermore, some azimuths may yield localized data areas data. If any of these assumptions are violated, the SRME that contain incoherent events and/or excessive noise, and subtraction process may in fact introduce or exaggerate will penalize the “good” data yielded in those locations on noise. Likewise, near-surface scattering, focusing and defo- other azimuth cubes when summed into the MAZ stack. On- cusing, and various other transmission effects are generally going research (Manning et al., 2008) is developing methods not acknowledged by the various methods used in an effort to build a suite of attributes for each azimuth cube that can be to attenuate coherent noise in processing. used to reliably discriminate between “good” and “bad” data Industry experience in recent years, however, backed by samples. Then, on a trace-by-trace and sample-by-sample ba- 3D modeling studies, demonstrates that if a range of source- sis, only those samples from each azimuth cube that would receiver azimuths is recorded in addition to a range of source- usefully contribute to a coherent output stack event are used receiver offsets, then even the most complex coherent noise in the MAZ stack. When pursued in concert with accurate can be attenuated by the brute force of stack. Figure 4 is a prestack depth migration of each azimuth cube, these meth- conceptual demonstration of this point. A simple 3D model ods will optimize the resolution and quality of MAZ data. was built with one reflecting interface below two point dif- The first-order benefit of MAZ to data quality is improved fractors. The top row of Figure 4 shows a 3D CMP gather, target illumination. To fully exploit the power of stacking where both axes are offset (in the x and y directions), and across different azimuths for attenuating complex coherent the two-way traveltime surfaces of the diffractions are shown. noise, however, the acquisition geometry must acquire a larg- Four 2D profiles are then extracted in the second row of Fig- er and more continuous set of offsets and azimuths per shot ure 4 to simulate acquisition in different azimuths and spatial than is the case for NAZ or MAZ surveys. As now discussed, locations. A normal moveout (NMO) correction has been this leads to the design of WAZ surveys. applied. While the arrival time of the primary reflection is 514 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  4. 4. ACQUISITION/PROCESSING identical on all four CMP gath- ers, the arrival time and shape of the diffraction events are dif- ferent. Note that these noise events are coherent across the horizontal axis, which is offset. Each of the four CMP gathers is then stacked to yield the four stack traces in the lower row of Figure 4. We now see that the noise events have effectively been randomized, so if the four stack traces are then stacked again, now across the azimuth domain, they will be attenuated as random noise events. In other words, azimuth stack is able to treat coherent noise in the offset domain as random, and thus, re- move it. MAZ surveys increase the range of azimuths throughout the survey area by adding con- ventional NAZ surveys (refer also to Figure 3). The power of azimuth stack is exploited dur- ing the final summation of each azimuth cube. Experience shows that MAZ acquisition and pro- cessing is unable, however, to re- solve the challenging noise prob- Figure 4. 3D synthetic modeling study used to demonstrate the ability of azimuth stack to attenuate lems in areas such as the GOM. complex coherent noise events. Refer to the text for detailed explanation. The top row represents 3D Extra measures must be taken CMP gathers, the middle row represents 2D CMP split-spread gathers extracted from the top row, and to attenuate the myriad types the bottom row represents the (offset) stack of the four gathers in the middle row. (After Widmaier et al., of complex noise that cannot 2002.) be addressed in processing. The solution is wide-azimuth (WAZ) acquisition. WAZ surveys the maximum crossline offset (and maximum source-receiver historically use two or more vessels to acquire a larger range azimuth range) can be robustly expanded with new acquisi- of azimuths within each shot gather. As a consequence, WAZ tion at a later date if required. While several processing steps surveys have a first-order benefit of coherent noise attenuation must obviously be modified from conventional 3D (NAZ) from the power of azimuth stack, and, as opposed to MAZ processing, WATS processing is robust overall, allowing the surveys, the second-order benefit arises from improved target use of established tools to incrementally progress from a start- illumination. Figure 5 shows the most popular implementa- ing position of noisy data and a highly inaccurate velocity tion of WAZ, named wide-azimuth towed streamer (WATS) model, to better velocity models and correspondingly higher by BP. The WATS vessel configuration simulates split-spread quality final data. Depth migration is a typical requirement acquisition, and acquires a survey two or more times using due to the acquisition geometry used, further reinforcing the conventional straight line “swath” or “racetrack” shooting. critical necessity of being able to build a robust processing Each survey version deploys the streamer and source vessel(s) flow from a starting platform of poor data. in a fixed geometry, referred to as a “tile.” The first tile has the minimum lateral source-streamer separation (close to zero), Putting the pieces together and each succeeding tile increases this lateral offset by a dis- So, where do you start when “conventional” 3D acquisition tance equal to the width of the streamer spread. Thus, two and processing consistently fails in a given location? Note the tiles of acquisition simulate acquisition with a streamer spread implicit acceptance that money has already been squandered two times as wide as the one actually being towed; three tiles in a series of failed efforts. Is it possible to make a compel- of acquisition simulate acquisition with a streamer spread ling story to skip the misery, and execute a successful MAZ three times as wide as the one actually being towed, and so or WAZ survey early in the exploration program, maybe on. The source positions are exactly repeated for each tile, so even from day one? In established areas such as the GOM, 516 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  5. 5. ACQUISITION/PROCESSING Figure 5. Wide-azimuth towed streamer acquisition (WATS), the BP-led flavor of WAZ. Two “tiles” of acquisition are shown, such that the effective streamer spread used to complete the survey is twice the physical width of the actual streamer spread used during each tile of survey acquisition. Increasing the crossline source-receiver offsets increases the range of source-receiver azimuths within each shot used for processing. Figure 6. Wave-equation PSDM comparison of conventional NAZ data (left) versus WATS data (right) over a complex detached salt feature in the Gulf of Mexico. Two tiles of WATS acquisition were used. The profound improvement in image quality and resolution on the right primarily arises because of the power of azimuth stack to attenuate complex coherent noise events, and also because of better target illumination. (Courtesy of PGS) WAZ (typically WATS) acquisition has become standard shows a depth slice from a 10,000+ km2 XWATS survey in practice. Exploration WATS (XWATS) with two tiles can be the GOM. Conventional NAZ data is overwhelmed by com- acquired over large areas quite efficiently, thus enabling the plex noise associated with the myriad of salt morphologies in confident identification of exploration prospects (Figure 6). the survey area. Quite a significant amount of this noise is If image quality is still inadequate for appraisal or produc- attenuated by the first tile of WATS acquisition, and two tiles tion purposes, new WATS can be acquired to “upgrade” the are viewed as meeting exploration objectives. In other words, data set, repeating the shot locations, but using even larger a low-cost entry to regional WAZ acquisition was possible. If crossline source-receiver offsets to increase the range of azi- required, more intensive WAZ acquisition of larger azimuths muths available for each shot going into processing. Figure 7 can be intelligently pursued later on a localized scale. May 2010 The Leading Edge 517 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  6. 6. ACQUISITION/PROCESSING Figure 7. PSDM depth slice at 1.65 km from the 10,000+ km2 Crystal WATS survey in the Gulf of Mexico. Only one WATS tile has been used to create this preliminary image, yet the cancellation of complex coherent noise modes in comparison to conventional NAZ acquisition is already highly encouraging. (Courtesy of PGS) Similarly, MAZ is becoming more common at the explo- length, etc., can be derived, but ray tracing is fundamentally ration scale in the Mediterranean, where the highly reflective unable to model localized 3D noise characteristics, so quanti- and rugose Messinian anhydrite layer creates overwhelming tative data quality versus acquisition geometry analysis is im- diffraction multiples, and target illumination is irregular. possible. Instead, 3D finite-difference modeling is required to MAZ is a proven solution in the Mediterranean, but is un- replicate the acquisition of an entire 3D survey with different able to satisfactorily attenuate the coherent noise modes in acquisition geometry/strategy, and each data volume must be the GOM, where WAZ is necessary. Most of the time, how- taken through a full 3D processing and imaging sequence. ever, there is no precedent for MAZ or WAZ being known Such efforts require vast computational resources and many solutions to seismic challenges in a given location, so we must months of time to execute (Regone, 2007). look harder at how we plan new 3D surveys. Part of the solu- The ultimate scale of the projected asset development will tion to better 3D surveys involves the combination of time- be important when determining whether investment is justi- consuming modeling exercises. More importantly, MAZ and fied in new MAZ or WAZ surveys, whether those MAZ or WAZ surveys need to be viewed as realistic options, rather WAZ surveys should be designed to be “upgradeable” from than unlikely or exotic luxuries. If the various stakeholders meeting exploration to eventually meeting production objec- can engage in a transparent and energetic dialog related to all tives, and what project timeframe is acceptable. Ultimately, the geophysical and commercial challenges involved in the the presurvey planning dialog may require close collaboration survey area, they may be able to reduce the geophysical and between many different technical and commercial disciplines. commercial options to a small subset that can be tested and In the most challenging scenarios, of course, MAZ or quantified in a reasonable timeframe, and the merits of MAZ WAZ alone will not suffice, and both target illumination and or WAZ acquisition and processing may become compelling complex noise attenuation must be optimized. This introduc- quite quickly in some locations. es the notion of rich-azimuth (RAZ), which is a generic com- There are two main 3D modeling tools at our disposal promise to full-azimuth (FAZ), and applies to the fact that a for evaluating NAZ versus MAZ versus WAZ surveys. Both larger range of azimuths than MAZ or WAZ are acquired for require a reasonably detailed 3D geological model (not always all offsets, rather than describing a specific acquisition style achievable before a survey), and both are time- and resource- or strategy. While large-scale seafloor FAZ seismic remains consuming. 3D ray tracing will produce various kinematic at- prohibitively expensive and inefficient, RAZ represents an tributes related to the illumination of target horizons. Survey achievable solution with towed-streamer vessel operations. parameters such as optimal shooting direction, survey geom- In the methodology published by Howard (2007) of BHP etry versus illumination density, streamer length, recording Billiton Petroleum, RAZ applies a WAZ vessel configuration 518 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  7. 7. ACQUISITION/PROCESSING Figure 8. (left) Illustration of the azimuth versus offset behavior when traveling in a circle. The locomotive sees the “end” of the nearest carriage and the “side” of the farthest carriage. Likewise, narrow azimuths are recorded for the shortest offsets when sailing a vessel in a circle, and larger azimuths are recorded for the longest offsets. (right) Illustration of the common-midpoint (CMP) trajectory for a single circle. There is zero overlap between offsets for any CMP location. A large circle maps the shortest-offset CMP locations, and a much smaller circle maps the longest-offset CMP locations. to a MAZ shooting template (Figure 3). Thus, “established” contrast to the relative simplicity and uniformity of 3D ac- processing flows for MAZ and WAZ surveys are exploited to quired with straight lines, the data acquired from coarsely meet the data quality objectives. By pursuing several steps to overlapping circles have highly irregular fold and azimuth mitigate nonproductive acquisition time and also pursuing coverage, presenting unique challenges to data sorting and efficient processing strategies, the Shenzi RAZ survey with regularization, velocity picking and model building, and three survey azimuths in the GOM took less than twice the imaging. Many NAZ processing algorithms will require time that might have been needed to collect a high-quality quite fundamental modifications to be applicable. It will no NAZ survey with three obstructions to undershoot and a typ- longer be possible to use conventional midpoint-based it- ical 40% infill requirement. Again, an appropriate stakehold- erative velocity picking and imaging strategies. Large trace er dialog must drive the scope of geophysical survey planning volumes will need to be manipulated for each step of the when such investments are necessary. new processing strategy, requiring significant computational resources. One perspective may view this style of acquisition Straight lines versus circles as a long-awaited catalyst to embrace “true” 3D processing An important cost with streamer WAZ is the cost of oper- and imaging methods. Another less favorable perspective ating several vessels each day. Cole and French (1984) first may instead view true 3D processing and imaging as a nec- proposed shooting in circles more than two decades ago; the essary remediation to compensate for the irregular azimuth idea is that the vessel “shoots broadside back into its own and offset sampling from bin-to-bin. For small 3D survey streamers” and is thus able to acquire wider azimuths for the areas with a roughly circular survey shape, however, shooting far offsets using only one vessel. The method, however, is in circles will be require less survey days than conventional far from simple, both in terms of acquisition and processing. WATS shooting in straight lines. Less time will be spent As illustrated in the right side of Figure 8, a vessel sailing in by the vessel outside the survey area doing line changes, so a circle acquires a broad circle of midpoints; the near-offset the survey duration will be shorter, despite the small daily midpoints are defined by a circle similar in size to that sailed production rates in terms of square kilometers acquired. For by the vessel, and the far-offset midpoints are defined by a surveys larger than a few hundred square kilometers, how- much smaller circle. As also illustrated in Figure 8, the near ever, the higher daily production rates from WATS shooting offsets will be acquired with narrow azimuths, and the far will overtake circle shooting WAZ in efficiency. Due to the offsets will be acquired with larger azimuths (increasing with highly discontinuous azimuth distribution from bin-to-bin streamer length). Collectively, the irregular midpoint and (arising from the coarse move-up between circles), it is dif- azimuth coverage from a single circle of acquisition presents ficult to categorize circle shooting as MAZ, WAZ, or RAZ. several challenges, compounded when the move-up between Indeed, a common rose diagram cannot be built for all loca- circles is coarse. The left side of Figure 9 shows the near- tions throughout the survey area, as is the case for the MAZ offset fold coverage and the right side of Figure 9 shows the and WAZ methods discussed earlier. full-offset fold coverage for an area of overlapping circles. In Overall, the final parameterization for any survey that 520 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  8. 8. ACQUISITION/PROCESSING Figure 9. (left) Near-offset (0–1 km) CMP fold coverage modeled for a survey of overlapping circles. Note the highly irregular fold in the central area of full coverage. (right) Full-offset (0–6 km) CMP fold coverage modeled for a survey of overlapping circles. Again, note the irregular fold everywhere. Conventional CMP processing is not applicable for this type of acquisition. Circle diameter = 6.5 km, circle overlap = 1.2 km, bin size = 12.5 × 12.5 m. Figure 10. (left) Shot gather from the simultaneous firing of the front and rear source vessels in a WATS vessel configuration (refer to Figure 5). (center) PSDM image of WATS data used with conventional source shooting. (right) PSDM image of WATS data used with simultaneous source firing. There is no discernible difference between conventional and simultaneous source shooting when appropriate processing is applied. There are WAZ efficiency advantages associated with simultaneous shooting and WATS acquisition. (Courtesy of PGS) pursues broader azimuth sampling is of course influenced by categories: front and rear source vessel (the WATS version), the daily vessel cost(s), the survey duration, the turnaround of and all source vessels at the front. The WATS approach si- the processing phase, and whether the final data product can multaneously acquires both positive and negative offsets be reasonably expected to meet the survey objectives. (equivalent to a split spread) for each line sailed (Figure 5), so the overall shooting strategy is identical to “conventional” The future of WAZ 3D using racetrack or swath shooting. If strong feathering Efficiency and cost are obviously the major obstacles to WAZ affects the streamer spread, the near-offset coverage for the surveys becoming mainstream outside the GOM. While rear source vessel will become irregular, and in the worst most historical attention has been given to the streamer case, shallow data quality may be compromised. If all source spreads being towed (wider is usually cheaper), much atten- vessels are at the front of the streamer spread, the near-offset tion is now given now to the source strategies used. WAZ coverage will be more regular, but only positive offsets will vessel configurations for straight-line shooting fall into two be recorded for each sail line. Therefore, each sail line must May 2010 The Leading Edge 521 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  9. 9. ACQUISITION/PROCESSING Figure 11. Summary chart for the relationships between target illumination, the attenuation of coherent noise, and better azimuthal sampling. Rich-azimuth (RAZ) is a combination of multi-azimuth (MAZ) and wide-azimuth (WAZ) acquisition strategies, and provides the most smoothly varying and continuous offset and azimuth coverage throughout a survey area. Refer to Figure 3. be acquired in both directions (i.e., twice) if both positive ter image quality too. The idea has its foundations in acous- and negative offsets are required (yielding better azimuthal tic physics; it was first published by Guus Berkhout of Delft coverage). Thus, the choice of vessel configuration and shoot- University in 1992 and is now referred as “blended” shots ing strategy is ultimately influenced by many operational, (Berkhout, 2008). If very small and low-cost source vessels geophysical, and geological considerations. can be developed, the idea may have application in the fu- The main evolution in source strategy for WAZ acquisi- ture. Refer also to Vermeer (2009). tion is the use of “simultaneous” shooting. In an idea already GOM experience suggests that frequencies above ap- known for two decades from vibroseis experience onshore, proximately 30 Hz are not required in imaging to satisfy source arrays at the front and rear source vessels in a WATS most subsalt exploration objectives, but this is also a decision configuration are fired together, yielding the shot gather style based upon computational cost. The imaging algorithms re- in the left side of Figure 10. Historically, such data would quired for the vast WAZ data sets generally operate in the generally be regarded as being unacceptably contaminated by frequency domain, and so cost is directly proportional to “interference noise”, and would be unusable. Methods have the frequency range being imaged. Larger frequency ranges evolved, however, where the two shot contributions can be will inevitably be required by WAZ data sets, but emphasis discriminated based upon the dip components of the data, upon the operational benefits of deeper streamer towing will residual noise can be attenuated, and the final image is com- understandably persist. PGS released the dual-sensor Geo- parable to that acquired with conventional shooting (Fromyr Streamer technology in 2007, enabling deep streamer towing et al., 2008, refer to the right side of Figure 10). Once the to yield richer low- and high-frequency signal content than methodology matures, significant cost savings in acquisition conventional streamers at conventional depths. Sub-Messin- efficiency will result, larger azimuths will be recorded for each ian data examples in the Mediterranean are surprisingly close shot, and seafloor seismic may even become affordable. in quality to some MAZ results acquired with conventional Rather than merely viewing simultaneous shooting as a streamers, so the optimum WAZ configuration in terms of strategy with marginal data compromises in the pursuit of final data quality may be superwide, dual-sensor streamer significant cost savings, there are theoretical reasons to con- spreads towed deep, complemented by an array of small and sider shooting with an array of source vessels as enabling bet- low-cost source vessels. 522 The Leading Edge May 2010 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/
  10. 10. ACQUISITION/PROCESSING Conclusions Manning, T., C. Page, S. Hall, J. Keggin, B. Barley, W. Rietveld, E. A progression in survey effort and investment has been de- Fromyr, and R. van Borselen, 2008, Leveraging the value of multi- scribed that is intended to provide an overview of how the azimuth seismic (MAZ) through MAZ-stack: 70th Conference acquisition of larger azimuths throughout a marine 3D sur- and Exhibition, EAGE, Extended Abstracts. vey area will improve data quality under various scenarios. Regone, C., 2007, Using 3D finite-difference modeling to design Figure 11 provides a convenient schematic summary. Based wide-azimuth surveys for improved subsalt imaging, Geophysics, upon the convenience of established narrow-azimuth (NAZ) 73 (4), X4–X6. processing methodologies, the comparative benefits of multi- Vermeer, G., 2009, Wide-azimuth towed-streamer data acquisition azimuth (MAZ) and wide-azimuth (WAZ) survey strategies and simultaneous sources, The Leading Edge, 950–958. can be contrasted in terms of benefits to target illumination Widmaier, M., J. Keggin, S. Hegna, and E. Kjos, 2002, The use of or the attenuation of complex coherent noise. It is assumed multi-azimuth streamer acquisition for attenuation of diffracted that NAZ acquisition and processing is unable to meet the multiples, 72nd Annual International Meeting, SEG, Expanded data quality objectives in areas affected by geological barri- Abstracts, 89–92. ers to successful seismic imaging. The first-order benefit of Acknowledgments: This article is based upon the 2009 SEG MAZ is improved target illumination, and the second-order Honorary Lecture for South Pacific, titled “Multi-azimuth and benefit is coherent noise attenuation. In contrast, the first- Wide-azimuth Seismic: Foundations, Challenges, and Opportuni- order benefit of WAZ is coherent noise attenuation, and the ties.” The content is deliberately generalized. I thank several people second-order benefit is improved target illumination. Rich- for sharing their time and resources during the lecture prepara- azimuth acquisition delivers the ultimate combination of tion, in particular Carl Regone and James Keggin from BP, Mike improved target illumination and coherent noise attenuation Howard from BHP, and Steve Campbell and Graham Pound from for towed-streamer surveys, but will be reserved for the most PGS. The viewpoints expressed here are those of the author, and not challenging scenarios to data quality. necessarily of PGS or any other company. It is encouraging to report that the azimuth story is evolv- ing rapidly, and technology solutions to improved efficiency Corresponding author: andrew.long@pgs.com are being continuously delivered. The geophysical benefits of improved azimuth sampling are transparent and irrefutable, so this subject will have a high profile out- side the “established” regions for oil and gas exploration and develop- ment in the near future. References Berkhout, A.J., 2008, Changing the mindset in seismic data acquisition, The Leading Edge, 27, 924–938. Cole, R.A. and W.S. French, 1984, Three- dimensional marine seismic data ac- quisition using controlled streamer feathering: 54th Annual International Meeting, SEG, Expanded Abstracts, 293–295. Fromyr, E., G. Cambois, R. Loyd, and J. Kinkead, 2008, Flam – A simultane- ous source wide-azimuth test: 78th Annual International Meeting, SEG, Expanded Abstracts, 2821–2825. Gaus, D. and S. Hegna, 2003, Improved imaging by pre-stack depth migra- tion of multi-azimuth towed streamer seismic data: 65th Annual Conference and Exhibition, EAGE, Extended Ab- stracts. Howard, M., 2007, Marine seismic sur- veys with enhanced azimuth coverage: Lessons in survey design and acquisi- tion, The Leading Edge, 28, 480–493. May 2010 The Leading Edge 523 Downloaded 05 May 2010 to 65.127.193.41. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/

×