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Introduction

Depth migration algorithms are now frequently usedduring the processing of seismic data especially ingeologically complex areas. They are theoretically more fitto solve positioning problems (migration) in presence ofsubsurface velocity anomalies and steep dips. Moreoverthe final depth volume allows a better integration of themany professionals involved in E&P projects: geologists,geophysicists and engineers. In fact such an integrationshould start well before the final depth migration is run: theconstruction of the velocity model is indeed the result of aclose integration between time and depth processers,geophysicists, subsurface and structural geologists and alsoreservoir engineers (L. Pizzaferri et al., 1998).

Nowadays Pre-Stack Depth Migration (PSDM)algorithms are becoming more and more affordable, due tothe available low cost computing power. PSDM has provedindeed to be a very effective tool for better seismic imaging.The following three case histories are taken from differentgeographical and geological environments, from salttectonics, to compressional geology (overthrust), to passivemargins. The methodological approach is also different fromone case to the other. All cases are taken from the vastexperience that ENI E&P Division has gained in the lastyears, both in terms of theoretical development (proprietaryPSDM algorithms) and from exposure to a variety of differentbasins and play world wide.

1st Case History: West Africa deep waters andcomplex salt tectonics features

The study area is located in the West African ultra-deep waters of the South Atlantic Ocean, in an areacharacterized by very complex structures (fig. 1), exhibiting

Advanced PSDM Approaches for Sharper Depth Imaging

Davide Casini Ropa*, Davide Calcagni, Luigi PizzaferriENI Exploration and Production Division

Summary

Pre Stack Depth Migration must be considered the solution to many imaging problems especially inpresence of complex structures. This paper presents a series of three case histories where the proper choice ofdepth migration strategies led to a fine tuned sharp seismic image, of much superior quality with respect to the timeimage.

most of the salt tectonics features like salt walls, salt pillowsand salt diapirs. Due to its intrinsic limitations, time migrationleads to unrealistic images not only below the salt pillows,but also at salt flanks (fig. 2), in spite of the overall highquality of the time data. The PSDM project had to improvethe definition of the salt geometries and of the overallstructural setting obtained by pre-stack time migration.Kirchhoff depth migration was able to produce very goodresults in the upper sedimentary portion of the area, so itwas used to define the velocity field by means of the standardlayer stripping methodology via iterative updating of thevelocity field itself.

Not surprisingly, first-arrival Kirchhoff migrationwas not able to properly image sub-salt structures,particularly below the salt pillows; this is where wavefieldPSDM had to demonstrate its superiority. Figure 2 showsthe comparison between the prestack time section and thedepth section obtained by the wavefield migration of theentire dataset (PSPI downward continuation kernel) plus

Fig. 1: An image of the Salt top in the area (notice the isolated saltpillows). The color code is proportional to the depth.

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Monte Carlo imaging (L. Cazzola et al., 2004). The PSPIWave Equation algorithm was fully developed internally byENI geoscientists. The improvement in the definition of thecentral salt structure is quite clear; even the sedimentspinching on the left salt wall assume a completely differentshape. A comparison between a commercial Kirchhoffmigration package and the final PSDM image is shown infigure 3. Though the Kirchhoff image shows a high qualityfar from the salt pillow, the breakdown of the first arrivalassumption makes the events below the salt completelyblurred. The comparison also shows that the Wave Equationimaging exhibits a surprisingly low level of noise. The velocityfield was evaluated by means of a Kirchhoff approach and,in this case study, the resulting estimated velocity fieldadequately describes the subsurface, since events below thesalt pillows appear mostly continuous and geologicallyconsistent. An example is shown in figure 4, where the finalmigration is superimposed to the velocity field used for

Fig. 2 : An eloquent comparison of Pre-Stack Time Migration (above)and PSPI Wave Field PSDM (below).

Fig. 3 : Kirchhoff PSDM (above) and PSPI WE PSDM (below). Notice thelow level of noise reached in the PSPI results (below).

Fig. 4 : Superposition of velocity model over seismic image provided byPSPI Wave Equation code.

migration.

A strict integration between depth imagers and thegeologists and geophysicists acquainted with the complexarea proved to be one of the winning factors in the successfulcompletion of the study together with the use of theproprietary PSDM algorithm.

2nd Case history: enhancement of seismicresolution

The study concerned an actually producing field,located in the Adriatic Sea, at a water depth of 50 m. Thereservoir is a turbiditic sequence of Pliocene age. A total of12 thin layers of gas bearing sands spread over a totalthickness of 400 m, the top being at around 1300 m bsl. The

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trap is mainly structural: a typical anticline with Apenninetrend, having origin from an overthrust. The closure in onedirection (NE) is clearly delimited by the thrust fault whiletowards NW it is defined by a clear syncline.

The real problem is the closure towards SE, wherethe available 3D seismic in time did not show very clearevidences of low angle reverse faults that supposedlycrossed all the structure subdividing it in many separatedcompartments.

A well drilled on what was supposed to be a flankturned out to be just the opposite: the dips at the well didnot fit at all the dips taken from the time seismic, they were inthe opposite direction.

After a few 2D tests, it was decided of solving theproblem by a full 3D PSDM approach. The results clearlyshowed that the depth volume solved most of the structuraluncertainties and was able to match pretty well either depthsand dips of the reflectors.

The depth image was of much superior quality (fig.5), improving the continuity of the seismic events, theinterpretability of formational tops and the definition of thefault patterns, leading to immediate positive consequenceson the reserve estimate and the development plan.

In this case the code used was a Kirchhoff TrueAmplitude (KTA) fully developed internally. This codeguarantees the amplitude preservation of events being ableto regularize the illumination in the angle domain, bothscattering and dip angles (V. Lipari et al., 2004).

The KTA approach has also reached its fully maturitywith the implementation of anisotropy (N. Bienati et al., 2005)

3rd Case history: how 3D PSDM can reduce theexploration risk

This case history illustrates how a high riskexploration lead has been matured into a drillable prospectby improving 3D seismic imaging and ultimately reducingrisk associated with trap definition (P. Bocca et al., 2005).

The Vesta prospect, northeast of East Swan-1, islocated within exploration permit AC/P-21 in the Vulcan Sub-basin, Timor Sea, offshore Australia. It is a large tilted faultblock complex on a central high in the Swan Graben, andlies about 50 km southwest of the Jabiru and Challis fields,

Fig. 5: Enhancement of seismic resolution after depth migration (3DPSTM time above and 3D PSDM depth below). Notice the goodfit of the dipmeter data after PSDM.

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equally distant from the Skua and Puffin fields, which arelocated 30 km to the south and southwest.

The 3D PSDM Onnia reprocessing has addressedVesta seismic uncertainties that based on 3D PSTM Onniainterpretation were largely attributed to the lack of reflectorcontinuity and to the presence of multiples over the SwanGraben. The low signal to noise ratio (S/N) and the lack ofreliable velocity picking in time domain severely impactedon the depth conversion approach and on the trap definition.

3D Pre-Stack Depth Migration (PSDM)reprocessing was conducted in 2003 on a portion of the

Fig. 7: Close-up of main fault footwall, comparison between PSTM(above) and PSDM (below). Notice how the geometry of thefootwall reflectors can be better resolved with PSDM processing.

Fig. 6: Comparison between PSTM (above) and PSDM (below). Thedepth image displays better continuity of the reflectors and offerssuperior image of the main bounding fault of the graben.

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Onnia 3D seismic cube, located in exploration permit AC/P-21, Timor Sea.

The main objective of the reprocessing was to obtainthe best seismic depth image and the most realistic structuralreconstruction of the subsurface in order to mitigate the riskfactors associated with trap definition (trap retention andtrap efficiency). This represents one of the main challengesfor oil exploration in the area.

The 3D PSDM methodology was chosen as themost appropriate imaging tool to define the correctsubsurface geometry and fault imaging through the use ofan appropriate velocity field. An integrated approach tobuilding the final velocity model was adopted, with asubstantial contribution from the regional geological model.

Several examples demonstrated that the 3D PSDMreprocessing significantly improved the seismic image andthus the confidence in the interpretation, contributingstrongly to the definition of the exploration targets. Thisimprovement can be observed in fig. 6 that highlights thebetter continuity of the reflectors and the higher resolutionobtained using the KTA PSDM. Another striking example isshown in fig. 7.

The interpretation of the new seismic data hasresulted in a new structural picture in which higherconfidence in seismic imaging has improved fault correlation.This has enabled better structural definition at the MiddleJurassic Plover Formation level that has reduced thecomplexity of the large Vesta Prospect, in the centre of theSwan Graben to the northwest of East Swan-1. Improvedunderstanding of the fault reactivation mechanism and thestructural elements of the trap (trap integrity) were eventuallyincorporated in the prospect risking.

In the Swan Graben 3D PSDM has proved to be avery powerful instrument capable of producing significantimpact on the exploration even in an area with a complexgeological setting and a fairly poor seismic data quality.

Conclusions

The examples shown in this paper clearlydemonstrate the great impact that a PSDM processing canhave on the seismic image, producing a more focused image,with sharper details and an increased resolution.

Time processing is not a secondary part of depthimaging: the standard or sophisticated processing sequencestargeted only to obtain the best time data may not be suitable

for a good depth processing. A close cooperation betweentime and depth processors must always be implemented.

It is also clear that a reliable depth image requires agood definition of the velocity field in the subsurface. Indeedthe geologists give an important contribution in the validationand updating of the velocity layers, especially in areas withpoor signal-to-noise ratio.

It must also be stressed the fact that during thedepth imaging process one need to take into account allavailable ‘a priori’ data (well depths, sonic logs, check shotdata, all suite of logs). This is especially true during thedefinition of the initial velocity model.

Velocity gradients are of utmost importance and evenstacking velocities can help in defining an initial guess orinitial gradient map. All the examples that have been presentedshow that depth imaging is an interpretative process thatrequires the active presence and strong interaction ofgeoscientists with different backgrounds. They must applynot only their own specialistic knowledge but also encouragediscussion and creativity. This is the only way of producingnot only a better focused seismic image but also an improvedunderstanding of the geology of the area of interest that, atthe end, represents the real objective of such projects.

Finally each project needs appropriate tools and itis responsibility of the depth imaging experts the correctchoice of the best methodological approach to each individualproject, always optimizing the cost-benefit ratio.

Acknowledgments

The authors would like to thank ENI Explorationand Production Division management for permission topublish this paper.

The authors are also grateful to many of their colleagues atENI E&P Division in Milan for their co-operation.

References

L. Pizzaferri, E. Delfini, G. Francescato, K. Barsoum; 1998; DepthImaging of a Complex Structure in an Offshore DeltaicEnvironment; I Simposio Internacional De GeofisicaMinera, II Conferencia Latinoamericana de Geofisica,Congreso Venezolano de Geofisica; October 25- 29, 1998;Caracas; Venezuela.

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L. Cazzola, L. Pizzaferri, L. Ratti, G. Cardone, E. Bonomi; 2004; Anexample of Wave Field Depth Migration and Monte CarloImaging in West Africa Deep Waters; 74 th SEGConvention; October 10-15, 2004; Denver (CO); USA.

V. Lipari, C. Andreoletti, G. Bernasconi, N. Bienati, L. Cazzola, G.Drufuca; 2004; Some Practical issues related to Migrationin the Angle Domain; 74 th SEG Convention; October 10-15, 2004; Denver (CO); USA.

N. Bienati, C. Andreoletti, A. Melis, P. Cibin, L. Pizzaferri, D.Calcagni; 2005; 3D Anisotropic Depth Migration for VTIMedia: Theory and Case Histories; InternationalPetroleum Technology Conference (IPTC); November 21–23, 2005; Doha; Qatar.

P. Bocca, L. Fava, E. Stolf; 2005; 3D Pre-stack Depth Migration:A tool for reducing Exploration risk in Swan Graben, TimorSea; APPEA Journal 2005, pag. 421-438.