Prestack Elastic Generalized-screen Migration for Multicomponent Data

Abstract

The multicomponent seismic exploration has been demonstrated as an effective technology for various objectives such as direct hydrocarbon exploration, identification of pore fluid and reservoir characterization, because both S and P waves are recorded. For computational efficiency, however, migration methods based on the acoustic wave equation, which represents the ideal propagation of P waves only in homogeneous and isotropic media, are still used to image the multicomponent data in many cases. To apply acoustic wave equation migration to multicomponent data, preprocessing is required that separates P waves from S waves or vice versa. However, because of the difficulty of separating P or S waves from multicomponent data, the remaining undesired mode-reflected waves cause distortion in the migrated image. In this study, I developed an efficient prestack depth migration based on one-way elastic wave equations with an elastic generalized-screen (EGS) propagator, which effectively describes the behavior of elastic waves with the mode conversion at the interfaces. The EGS propagator was designed through the modification of the scalar generalized-screen (SGS) propagator, which efficiently computes the wave propagation in media with lateral velocity variations. We expanded the vertical slowness term up to the 2nd order to obtain a high level of accuracy for the migration of complex subsurface structures. The developed EGS migration algorithm can produce P and S migrated images without preprocessing for the separation of P and S waves before migration because the mode separation operator is included in the propagator, while other migration algorithms require the mode-separated input data. To verify the validity of our EGS migration, we performed numerical experiments using synthetic multicomponent data sets generated from several velocity models. Reflectors were imaged at their correct positions, and spurious events due to the other modes were minimized in both PP and PS migrated images. By comparing the result of the EGS migration to that of a scalar one-way wave equation migration for the synthetic data from a simple two-layer velocity model with a plane wave source, we confirmed that the EGS migration preserves the reflection amplitude better than the scalar one-way wave equation migration in the migrated image. The gas reservoir, which was not depicted clearly because of the scattering in the P-migrated image, was clearly identified in the PS migrated image because the S wave is not affected by the existence of gas in pores. In addition, the PP and PS images by EGS migration for the complex Marmousi-2 and SEG/EAGE salt models contain far fewer spurious events due to the other mode than does the scalar wave equation migration, although the scalar wave equation migration uses mode-separated data as its input data.|다성분 탄성파 탐사는 P파 외에 S파도 기록하기 때문에 저류층의 영상화를 비롯한 탄화수소 직접탐지, 공극 유체의 판별, 저류층 특성 분석 등에 효과적인 기술이다. 하지만 다성분 탄성파 자료의 구조보정에 있어서 아직까지 대부분의 경우 계산상의 효율성을 이유로 등방성 매질에서의 P파 전파만을 표현하는 음향 파동방정식을 사용하고 있다. 이 경우 구조보정 적용 전에 P파만을 또는 S파만을 따로 분리하는 전처리 과정을 거쳐야 한다. 그러나 P파와 S파를 완벽하게 분리하는 것은 어려우며, P파와 S파가 완벽하게 분리되지 않은 자료를 사용할 경우 남아있는 다른 모드의 반사파에 의해 구조보정 영상에 많은 왜곡이 발생할 수 있다. 본 연구에서는 지하 매질의 경계면들을 전파하면서 P파와 S파 상호간의 모드 전환을 거치는 탄성파의 거동을 효율적으로 표현할 수 있는 elastic generalized-screen (EGS) 전파자를 이용한 단방향 파동방정식 중합 전 심도 구조보정 기법을 개발하였다. EGS 전파자는 수평적 속도 변화가 존재하는 매질에서 파의 전파를 빠르게 계산할 수 있는 기존의 scalar generalized-screen 기법에 탄성 파동방정식을 적용하고 전파자의 수직 느리기항을 2차까지 확장하여 복잡한 구조의 매질에서 보다 높은 정확도의 파동장을 계산할 수 있도록 고안되었다. 다성분 자료의 구조보정 시 입력 자료를 P파와 S파 파동장으로 먼저 분리 한 후 사용하는 다른 기법들과는 달리 EGS 구조보정 기법은 모드 분리 연산자를 전파자에 포함시킴으로써 다성분 자료를 P파와 S파로 분리할 필요 없이 바로 구조보정 입력으로 사용하여 P파 및 S파 영상 단면을 생성할 수 있다. 본 연구에서 개발한 EGS 구조보정의 검증을 위하여 몇 가지 모델로부터 생성한 모드 분리되지 않은 합성 탄성파 다성분 자료를 입력자료로 EGS 구조보정 기법을 적용해 본 결과, PP 및 PS 단면에서의 반사면들이 정확한 위치에 영상화되었으며 다른 모드에 의한 이미지 왜곡도 최소화되었음을 확인할 수 있었다. 두 층으로 구성된 단순한 속도 모델에 평면파 음원을 이용하여 생성한 자료의 EGS 구조보정 결과 스칼라 파동방정식 단방향 구조보정기법에 비해 영상화된 반사파 진폭이 비교적 정확하게 보존되는 것을 보였고, P파 산란으로 인해 P파 영상만으로는 정확한 규명이 어려웠던 가스층 구조를 공극 내 가스에 영향을 받지 않는 PS 영상단면을 통해 효과적으로 규명할 수 있었다. 또한, 매우 복잡한 지하구조를 모사한 Marmousi-2 모델과 SEG/EAGE 암염 모델로부터 생성된 다성분 탄성파 자료에 음향 파동방정식 기반 구조보정과 EGS 구조보정을 적용하여 PP 및 PS 영상을 비교한 결과, 미리 P파와 S파를 분리한 다성분 자료에 음향 파동방정식 기반 구조보정을 적용한 영상에 비해 EGS 구조보정을 적용한 영상에서 서로 다른 모드의 파에 의한 영상왜곡이 많이 감소되었음을 확인하였다.; The multicomponent seismic exploration has been demonstrated as an effective technology for various objectives such as direct hydrocarbon exploration, identification of pore fluid and reservoir characterization, because both S and P waves are recorded. For computational efficiency, however, migration methods based on the acoustic wave equation, which represents the ideal propagation of P waves only in homogeneous and isotropic media, are still used to image the multicomponent data in many cases. To apply acoustic wave equation migration to multicomponent data, preprocessing is required that separates P waves from S waves or vice versa. However, because of the difficulty of separating P or S waves from multicomponent data, the remaining undesired mode-reflected waves cause distortion in the migrated image. In this study, I developed an efficient prestack depth migration based on one-way elastic wave equations with an elastic generalized-screen (EGS) propagator, which effectively describes the behavior of elastic waves with the mode conversion at the interfaces. The EGS propagator was designed through the modification of the scalar generalized-screen (SGS) propagator, which efficiently computes the wave propagation in media with lateral velocity variations. We expanded the vertical slowness term up to the 2nd order to obtain a high level of accuracy for the migration of complex subsurface structures. The developed EGS migration algorithm can produce P and S migrated images without preprocessing for the separation of P and S waves before migration because the mode separation operator is included in the propagator, while other migration algorithms require the mode-separated input data. To verify the validity of our EGS migration, we performed numerical experiments using synthetic multicomponent data sets generated from several velocity models. Reflectors were imaged at their correct positions, and spurious events due to the other modes were minimized in both PP and PS migrated images. By comparing the result of the EGS migration to that of a scalar one-way wave equation migration for the synthetic data from a simple two-layer velocity model with a plane wave source, we confirmed that the EGS migration preserves the reflection amplitude better than the scalar one-way wave equation migration in the migrated image. The gas reservoir, which was not depicted clearly because of the scattering in the P-migrated image, was clearly identified in the PS migrated image because the S wave is not affected by the existence of gas in pores. In addition, the PP and PS images by EGS migration for the complex Marmousi-2 and SEG/EAGE salt models contain far fewer spurious events due to the other mode than does the scalar wave equation migration, although the scalar wave equation migration uses mode-separated data as its input data.