AVO and Devonian reef exploration revisited:The role of multiples

HAN-XING LU and LARRY LINESMemorial University of Newfoundland

St. John ‘s, Newfoundland

0ur paper “AVO and Devonian reef exploration: Difficul-ties and possibilities” (TLE, August 1995) contained ques-tions and suggestions about the effects of multiples onamplitude variation with offset in carbonate reef exploration.The basic question is: “How will interbed multiple reflectionsalter the AVO reflections from Devonian reefs as originallymodeled by the Knott-Zoeppritz equations?” Due to the dif-ficulty and importance of Devonian reef exploration, we be-lieve that the effect of multiples on AVO in carbonatesdeserves further attention. We have investigated AVO effectsusing more well logs and seismic data from various Albertareef plays in order to test our preliminary conclusions, and wecompare reflection models of “primaries only” with those of“primaries plus multiples.” For investigations on real data,our attention then focused on whether the AVO effects of

Figure 1. (a) AVO model response for NMO corrected“primaries only” Nisku Formation reflections at a reefalwell. Trace spacings are 225 m between offsets O-1800 m.Reef reflections are at a depth of about 1700 m (left). AVOmodel response for “primaries plus multiples” using thesame model parameters (right). Traces from thereflectivity model are computed up to the critical angle.Both sets of synthetic seismograms were generated usinga wavelet extracted from the real data. (b) AVO amplitudeevent picks for the synthetic seismograms of Figures lashow little difference between amplitudes of both models.

range-limited stacks reflects the behavior of “primaries only”or “primaries plus multiples” models.

Our motivation for using AVO to detect differences be-tween porous carbonate and shales depends mainly on Pois-son’s ratio differences between shales and carbonates.According to Hilterman’s AVO equation (TLE, August 1995),Poisson’s ratio contrasts should have a significant effect onamplitudes as source-receiver offset increases. (AlthoughHilterman’s equation was the basis for our study, our AVOmodeling made extensive use of more general equations suchas the Knott-Zoeppritx relations and reflectivity modeling.)Our use of P-wave and S-wave velocity logs suggest Pois-son’s ratio for carbonates of 0.28-0.30 and values as high as0.40 for shales. Modeling computations are based on veloc-ity models created by the blocking of P and S-wave data.

Range-limited stacks. Due to the noisy scatter in AVO valuesfor these reef reflection data, we use range-limited stacks as ad-vocated by Gadallah (Reservoir Seismology, PennWell, 1993).

Figure 2. (a) AVO model response for NM0 corrected“primaries only” Nisku formation reflections at a basinalwell (left). AVO model response for “primaries plusmultiples” (right). (b) AVO amplitude event picks for thesynthetic seismograms of Figure 2a show significantdifferences between amplitudes for the two models.“Primaries only” has an AVO increase while “primariesplus multiples” shows a decrease.

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Figure 3. Offset limited stacked seismic section for ahasinal well. The square is the location of the Niskuformation. (a) Stack of near offsets (33-917 m). (b) Stackof far offsets (917-1800 m).

The stacking procedure will enhance signal-to-noise whilesacrificing some resolution. The simplest situation involvesnear-range and far-range stacks.The near-range and far-rangedistances for stacking are guided by AVO modeling and byour objective of using the maximum possible range in ouranalysis. Thus far we have noticed that the model-guidedrange-limited stacks tend to differentiate between hasinalshales and reefal carbonates.

For the data sets investigated. hasinal shale/limestone re-flectors tend to cause an amplitude increase in far-offsetstacks relative to the near-offset stacks, while porousdolomite/limestone reflectors tend to show a decrease. Thisbehavior is in agreement with Hilterman’s equation relatingPoisson’s ratio contrasts to AVO.

Results. Using the models based on P-wave and shear-wavewell logs, we see that inclusion of multiples has differingAVO effects - even for wells that are less than 1000 m apart.For a reefal well in Figure 1, we see that the AVO effects aresimilar whether we compute primaries only or include mul-tiples. Figure la shows the seismic responses computed bythe reflectivity method and Figure I b shows a graph of thepicked normalized amplitude values as a function of offsetfor the “primaries only” and “primaries plus multiples” mod-els. There is a great deal of similarity between the curves -suggesting that multiples for this model do not play a crucialrole. As predicted, by real data behavior, range-limited stacksof real data show a decrease with AVO, as in our originalpaper.

On the other hand, the AVO effects for a basinal well inFigure 2 shows a marked difference between “primariesonly” and “primaries plus multiples” for the model responsesand the amplitude picks. For the case of “primaries only”there is a substantial increase in AVO (as in our first paper)whereas the “primaries plus multiples” response shows aslight decrease in AVO. For the range-limited stacks, it is in-teresting to note which of these two model responses is mir-rored by the real seismic data near the basinal well.

As shown hy Figure 3, range-limited stacks of real datanear the basinal shale well show an AVO increase as predictedhy the “primaries only” synthetic. This behavior agrees with

the original data results in our first paper. The similarity be-tween the range-limited stack AVO and the “primaries only”models is probably due to the fact that stacking has enhancedthe energy of the primaries over the range in question whileattenuating much of the multiple energy. Therefore, our “pri-maries only” model is appropriate. On the other hand, if onewere to examine AVO effects on a trace-by-trace basis beforeany multiple suppression methods were used. the “primariesplus multiples” should be more appropriate. Although thisleaves some intriguing questions regarding AVO, it suggeststhat the original real data analysis based on “primaries only”modeling and range-limited stacking is valid.

Conclusions. Our conclusions for the recent studies on De-vonian reefs do not deviate significantly from those in the firstpaper. It appears that in some cases the inclusion of multiplescan have a significant effect on AVO. However, each caseshould be individually evaluated by AVO modeling of pri-maries and multiples. Furthermore, our range-limited stackedresults tend to behave like the “primaries only” models. IE

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