Critical reflection illumination analysis

Jun Cao1 and Joel D. Brewer1

Abstract

Poor imaging is frequently observed in many subsalt regions, making the subsalt stratigraphy interpretationand prospect evaluation challenging. We propose a critical reflection illumination analysis to evaluate subsaltillumination in areas where high-velocity contrasts create illumination and imaging shadows. Critical reflectionoften occurs at the base or flank of salt bodies. If critical reflection occurred, continued iterations of processingand imaging would generate little, if any, improvement in imaging results. Similarly, increasing the offset/azi-muth of the acquisition would offer limited or no advantage. We introduce the critical reflection illuminationmap and illumination rose diagram to efficiently and effectively evaluate the probability of critical reflection forthe target. This analysis can help avoid expensive processing, imaging, and acquisition efforts for areas that arein the critical/postcritical reflection regime. Critical reflection illumination analysis can also be applied to otherhigh-velocity contrast scenarios.

IntroductionExtensive exploration for more than a century has

left remaining global reserves in more complex andchallenging geologic and geophysical settings. In recentyears, the industry has developed many advanced seis-mic acquisition, processing, and migration technologiesto improve imaging in seismically challenging regions,such as the subsalt area in the deep water of the Gulf ofMexico (GOM). New acquisition configurations such aswide azimuth (WAZ), multi-azimuth, rich azimuth, long-offset streamers, and ocean-bottom systems (e.g.,Howard and Moldoveanu, 2006; Keggin et al., 2006;Michell et al., 2006; Beaudoin and Ross, 2007; Brice,2011) have been proposed to improve illumination,and thus imaging, compared to conventional narrow-azimuth acquisition. On the processing and migrationside, 3D surface-related multiple elimination (SRME)helps mitigate artifacts generated by multiples, andanisotropic reverse time migration (RTM) (e.g.,Fletcher et al., 2009; Zhang and Zhang, 2009) overcomesthe limitations of ray-based and one-way wave equationbased isotropic migration. These advanced acquisition,processing, and migration technologies have improvedimaging in many areas, but the subsalt imaging problemremains.

We often observe poorly imaged subsalt sedimentstructures close to the base of salt body (BOS), eventhough the BOS above them may be well imaged, asshown in Figure 1 for the benchmark SEG/EAGE saltmodel imaging and in Figure 2 for a deep-water GOMsubsalt imaging example. The poorly imaged structure

may be steep (Figure 1) or relatively flat (Figure 2).When interpreters see images similar to Figure 2, theymay ask where the horizon goes, and whether or howwe can better image the missing part. Factors that mayproduce poor imaging include limited acquisition aper-ture, limited migration aperture, and inaccuracy of themigration model and migration operator. The image inFigure 2 is generated by the most accurate imaging cur-rently available, RTM, of synthetic data using the truemodel. We use a much wider migration aperture thanthe industry standard. We increase the acquisition off-set from standard WAZ (7 km × 4 km) to a maximum of14 km along both inline and crossline directions, but theimage shows no improvement in the wipeout zone.The well-imaged and unimaged parts actually containthe same dip, but different overburden BOS geometries.Analysis of the wave propagation between BOS and tar-get horizon shows that the critical reflection phenome-non at the BOS is the dominant factor producing thepoor subsalt imaging. This phenomenon is created bythe relative geometry/angle between the subsalt struc-ture and the overlying BOS rather than the dip of thestructure or the dip of the BOS itself, in addition to thevelocity contrast at the BOS. When the reflected wavefrom the target structure is postcritical at the BOS, thetransmitted/head wave has little chance of reachingsurface streamers with strong energy. In that case, addi-tional processing and/or imaging iterations may not im-prove the imaging of that subsalt target, and increasingthe offset or azimuth range of the acquisition systemalso has little impact on improving the image quality.

1ConocoPhillips. E-mail: Jun.Cao2@ConocoPhillips.com; Joel.D.Brewer@ConocoPhillips.com.Manuscript received by the Editor 5 March 2013; revised manuscript received 6 May 2013; published online 26 July 2013. This paper appears in

INTERPRETATION, Vol. 1, No. 1 (August 2013); p. T57–T61, 7 FIGS.http://dx.doi.org/10.1190/INT-2013-0031.1. © 2013 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved.

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Technical paper

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Additional seismic data processing and/or acquisi-tion can be expensive and may not provide helpfulinformation for imaging and interpreting the target inthe postcritical regime. Therefore, the subsalt targetillumination related to the critical reflection providesimportant information for imaging and interpretation.Standard ray-based or one-way wave equation-basedillumination tools have been used to analyze the subsaltillumination (e.g., Xie et al., 2006; Liao et al., 2009), butthese methods propagate the waves through the modelbetween the target and the sources and receivers. Raytheory and one-way wave equation have inherent diffi-culty in models with a complex sharp velocity contrast,for example, the sediment-salt interface. One-way waveequation based methods also bear the inherent diffi-culty of propagating wide-angle waves accurately. Inthis paper, we propose an effective localized analysis toevaluate the subsalt illumination specifically related to

the critical reflection phenomena at the BOS, namely,critical reflection illumination analysis. This analysiscan also be used in other high-velocity overburden sce-narios, such as in the presence of basalt or carbonates.

Critical reflection illumination analysisIn this paper, we use 2D diagrams to illustrate the

idea for simplicity, but all analyses are performed in3D. Assuming the wave impinges the BOS with incidentangle θi (Figure 3), if the incident angle is equal to orlarger than the critical angle, which is determined bythe salt and sediment velocities at the incident location,most of the incident energy to the BOS is reflected awayfrom the salt interface, where the transmitted/headwave has little chance to reach the sensors abovethe BOS.

In critical reflection illumination analysis, the propa-gation of seismic energy is calculated from each pointof interest along respective trajectories, satisfyingSnell’s law for arriving and departing wave paths(Figure 4). For each pair of trajectories, if any trajectoryis critical or postcritical at the BOS, the illuminationcontributed by that wave pair to the target point willbe zero; otherwise, its contribution is counted. Thisprocess is repeated at the target point for waves withdifferent incident angles and azimuth angles. The illumi-nation contributions from all the wave paths are thensummed to give the final critical reflection illuminationstrength at that location.

Figure 2. A representative subsalt 3D image. The circled areashows the poor imaging of a relatively flat target under a well-imaged BOS.

Figure 3. Diagram illustrating how energy propagatesupward through the boundary between the sediment and saltbody.

Figure 4. Diagram illustrating the undertaken analysis forone wave pair from a point on a target horizon. The symbolsθi and θc are the incident angles and critical angles, respec-tively at the BOS for the two wave pairs.

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Figure 1. The model and migrated image of the 2D SEG/EAGE salt model (after Cao and Wu, 2009). The circled areashows the poor imaging of a steep structure under a well-im-aged BOS.

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The described procedure provides the basis for twotypes of critical reflection illumination analysis. Thefirst type focuses on the critical reflection illuminationfor a given target horizon. A horizon map can be gen-erated with this analysis to illustrate the illuminationstrength for each location of the horizon. From this,we can determine the locations of the postcritical areasthat will be hard to image. The second analysis showscritical reflection illumination strength for a given tar-get location. It generates a dip-azimuth domain illumi-nation rose diagram to show the illumination strengthfor all possible target structural orientations (or dip-azimuth combinations) at the given location. The rosediagram also provides the degree of precritical or post-critical for the current interpreted horizon at the ana-lyzed point.

Critical reflection illumination analysis helps avoidexpensive but insignificant iterations of processing,imaging, and acquisition for areas in the postcriticalregime. Favorable features of the proposed illuminationanalysis include the following aspects. First, wave onlypropagates in the smoothly varying subsalt sediment, asopposed to typical illumination analysis methods withwave propagating through the complex salt body,which is inherently difficult for those widely used ray-based and one-way wave equation based methods. Raytracing is used in this implementation because raytheory is valid and accurate to describe the wavepropagation in this smoothly varying area. Second, thisanalysis is efficient, which allows for an interactive and

iterative analysis to evaluate various interpretationscenarios that could include different target horizons,BOS geometries, and velocity models.

ResultsWe apply the critical reflection illumination analysis

to a deep-water GOM model with a salt body. Figure 5shows the critical reflection illumination map for oneinterpretation of the subsalt target horizon. In themap, red means completely precritical and blue meanscompletely postcritical. In the area of interest, the illu-mination map shows a narrow (blue) band of the post-critical area bounded by the white dashed polygon.Scattered postcritical areas also exist in the down-dipregion. Figure 6 shows the root mean square image am-plitude on the horizon generated by 3D RTM using datawith a maximum offset of 14 km. The poorly imagedareas, including those detailed isolated down-dippatches, match well with the postcritical areas shownin the illumination map in Figure 5. Most of the lightgreen image amplitude within the white polygon andadjacent to the top of the polygon shown in Figure 6is not extracted from the image of the horizon, but fromthe adjacent BOS image because the horizon and BOSare so close in those areas. This indicates that thesepostcritical areas will be hard to image with acousticseismic using sources and receivers above the BOS,even using data with very long offset and wide azimuth.

Figure 5. Critical reflection illumination map for a subsalttarget horizon in a deepwater GOM model. The blue areas in-dicate the completely postcritical regions, i.e., the shadowzones of imaging.

Figure 6. Root mean square image amplitude on the horizongenerated by RTM using data with a maximum offset of 14 kmfor the same horizon as in Figure 5. Most of the light greenimage amplitude within the white polygon and adjacent tothe top of the polygon is not extracted from the image ofthe horizon but from the adjacent BOS image because thehorizon and BOS are so close in those areas.

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The difficult areas might be imaged adequately by theVSP data (e.g., Zhuo and Ting, 2010) or through con-verted wavepath migration (e.g., Wu et al., 2010).

Figure 7 displays critical reflection illumination rosediagrams in the structure dip-azimuth domain at tworepresentative subsalt locations on the horizon. Onelocation is well-imaged under a relatively flat BOS(Figure 7a). The illumination shows that the currenthorizon interpretation at this location (cross in thefigure) lies completely in the precritical regime. Theother location is not imaged under a very steep BOS(Figure 7b). The rose diagram shows that the current

horizon interpretation at this location (cross in thefigure) lies completely in the postcritical regime, whichcauses the poor image. The illumination rose diagramalso indicates that even the current interpretation atthis location has large uncertainty, e.g., �30° of changein the dip and azimuth, this location will still not be im-aged. The examples seem consistent with a traditionalview: good image under relatively flat BOS and poorimage under steep BOS. In reality, the illuminationand imaging is determined by the relative 3D geometrybetween the target horizon and overlying BOS. We canclearly see this from Figure 7b: A horizon with thesame dip but with a different azimuth (40° instead of160°) lies in the good illumination regime.

ConclusionsWe propose an efficient and effective localized criti-

cal reflection illumination analysis to evaluate subsaltillumination related to critical reflection phenomena,which can occur at the base or flank of salt bodies andcause poor imaging in many subsalt areas. The pro-duced illumination map and illumination dip-azimuthrose diagram provide effective tools to evaluate theprobability of critical reflection for the target. The pro-posed analysis can then help avoid additional process-ing, imaging, and acquisition efforts, which may notprovide the desired improvement of the target imagingand interpretation. Application of this analysis to adeepwater GOM field shows its impact to the business.This analysis can also be applied to other high-velocitycontrast scenarios.

AcknowledgmentsThe authors thank the editor Yonghe Sun, associate

editor Hongliu Zeng, reviewer Ru-Shan Wu and twoanonymous reviewers for their valuable comments thatimproved this manuscript. We gratefully thank ourcolleagues Charles Mosher, Donald Ashabranner, andErik Keskula for the helpful discussions, Ramses Mezaand Meimei Tang for providing the model and horizonused in the example, Xianhuai Zhu and Sanjay Sood forthe modeling and imaging work, and Brian Macy, JerryYuan, and Jianchao Li for the helpful discussions on raytracing. We appreciate editing by Charles Mosher, AmySharp, and Heather Shelly that improved the manu-script. We also thank Bradley Bankhead, Erik Keskula,Ken Tubman, and Jason Lore, the management ofConocoPhillips, and its partners Anadarko Petroleum,Marathon Oil, Cobalt International Energy, and VenariResources for supporting and giving permission to pub-lish this work.

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Figure 7. Critical reflection illumination rose diagrams indip-azimuth domain at two representative subsalt locations.(a) A well-imaged location under a relatively flat BOS.(b) A poorly imaged location under a steep BOS. The crossrepresents the dip and azimuth of the current interpretationat the analyzed location.

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Jun Cao received a B.Sc. and M.Sc. in geophysics fromPeking University and a Ph.D. (2008) in geophysics fromthe University of California at Santa Cruz. He is a seniorresearch geophysicist in ConocoPhillips. His researchinterests include seismic illumination analysis, imaging,inversion, and wave propagation. He is a member ofSEG and EAGE.

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