Converted Wave Processing

This chapter explains how datasets are rotated through arange of angles and the resulting traces are cross-correlatedat each rotation angle. The chapter contains all informationto apply one of three different types of rotation to an inputdataset, construct two types of velocity functions forconverted-wave data, apply a time-domain converted-waveDMO to the input data, stack CDP-gathered traces using astraight mean algorithm, compute the asymptotic conversionpoint (ACP) bin number for each trace in the dataset, andupdate the LIN, CDP, and TRC databases to match thesevalues.

In This Chapter

➲ Converted Wave Processing Overview 1179

➲ 2-C Rotation Analysis 1181

➲ Apply 2-C Rotation 1187

➲ Construct P-S Velocities* 1191

➲ Converted Wave DMO 1195

➲ Converted Wave Stack 1200

➲ P-S Asymptotic Binning* 1205

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Converted Wave Processing Overview1463 ProMAX Reference

Converted Wave Processing Overview

Converted wave means the conversion of a seismic wavepropagating with one type of particle motion to another.Usually this means conversion from P (compressional oracoustic) waves to S (shear) waves. P-waves have particlemotion polarized in the direction of wave propagation (alongthe ray), while S-waves have particle motion polarizedperpendicular to the direction of wave propagation.

The underlying assumption in all of the software presentedhere is that the mode conversion, from P to S, occurs at thereflection point and that there is a simple P raypath from thesource to the reflection point and a simple S raypath from thereflection point to the receiver. The properties of the S-waveenergy, especially when used with and compared to the P-wave processed data, can provide valuable information aboutlithologic and fluid properties of the reflecting horizons.

In general, in an isotropic earth, all particle motion of P-waves and mode-converted S-waves (Sv-waves) will be in theplane of wave propagation. That is, there will be no motionperpendicular to the source-receiver direction.

Anisotropic rocks can complicate this situation by changingthe polarization of the S-wave. This phenomenon, known asshear-wave birefringence, causes the S-wave to split into afast wave in the principle direction of anisotropy, and a slowwave perpendicular to this direction. If the anisotropy iscaused by fracturing, the fast direction is in the direction ofthe fractures and the slow direction is perpendicular to thefractures. The direction of fracturing can be determined byanalysis of the S-wave polarizations. This polarizationanalysis has been shown to be most reliable when done using4 data components from two perpendicular shear wavesources recorded by two perpendicular horizontal receivers.The analysis may also be done using only two recordedcomponents from converted wave data, but can not beexpected to be as reliable as when using four datacomponents. The 2-C Rotation Analysis tool can be used forthis analysis.

The Apply 2-C Rotation tool may be used to rotate the entiredataset to the principal direction of anisotropy, anddetermined using the 2-C Rotation Analysis tool. It may also

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be used to correct for recording geometry for the isotropiccase if the source-receiver direction is not the same as thegeophone orientation on the ground.

Once the data have been polarized to the desired direction,data processing may proceed in a similar manner toconventional P-wave data processing. P-wave reflectionpoints for a flat layered earth will occur at the midpointbetween source and receiver. P-to-S converted wave reflectionpoints will fall much closer to the receiver due to thedifference in velocity between the P-wave (source to reflectionpoint raypath) and the S-wave (reflection point to receiverraypath). The ratio of P-wave to S-wave velocity is often takento be about 2, although it may be much greater inunconsolidated rocks, and somewhat less in very hard rocks.For a Vp/Vs ratio of 2, the reflection point will lie about two-thirds of the way from the source to the receiver. TheAsymptotic Binning tool will modify the ProMAX databasegeometry allowing the correct time-invariant binning ofreflection points for converted waves. This allows the data tobe processed through velocity analysis, statics, and otherprestack processes which require the analysis of commonreflection point traces.

The Asymptotic Binning process is an approximation that isaccurate at relatively large reflection depths. At shallowerdepths, the reflection (conversion) point is actually closer tothe receiver. The tool Converted Wave Stack will stack thedata using a more accurate reflection point mapping in theshallower section.

Both Asymptotic Binning and Converted Wave Stack makethe assumption that the reflections result from flat layers.Dipping beds will not be imaged correctly with theseprocesses. Converted Wave DMO may be used to give a moreaccurate stack image in the presence of dip. The ConstructP-S Velocities tool may then be used to compute a velocityfield that can be used with acoustic migration programs toperform converted wave migration of the data. Any ProMAXpoststack time migration is suitable for application to datathat has been processed through Converted Wave DMO.

Reference

Shear-wave Birefringence: A new tool for evaluating fractured reservoirs Martinand Davis Geophysics: The Leading Edge of Exploration October, 1987

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