이방성을 고려한 4차원 탄성파 모니터링 자료의 구조보정 기술개발
Development of 4D Seismic Monitoring Tool Considering Anisotropic Media based on Pseudo-acoustic Wave Equation
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초록
이산화탄소 지중저장의 대상이 되는 퇴적분지에 대한 등방성 탄성파 영상화 및 모니터링 기술 적용 시 전파 방향에 따른 속도 차이를 유발하는 지층의 이방성이 탐사 자료의 영상화 결과를 왜곡시킬 수 있다. 탄성파 이방성 중 지층에서 흔하게 나타나는 특성은 횡등방성이다. 본 연구에서는 횡등방성을 고려한 시간경과 탄성파 지층모델에 대하여 유사 음향파동방정식 기반의 참반사보정 기술을 제안한다. 횡등방성이 강한 지역에서 4차원 탄성파 모니터링 시, 이방성을 고려하지 않은 파동방정식을 이용하면 탄성파 이방성에 의해 오프셋 거리에 따른 반사점의 깊이에 차이가 생겨 실제반사점의 깊이보다 영상화 된 반사점의 깊이가 높게 관측되는 모니터링 결과 해석의 부정확성을논의하고, 타원형 이방성을 가정한 역시간 구조보정 기반의 모니터링 알고리듬 개발을 통해 이를극복할 수 있음을 확인한다. 복잡한 3차원 지질구조를 가정한 가상의 4차원 탄성파 모니터링을 통해 제안하는 알고리듬을 적용하여 오프셋 거리에 따른 반사점의 깊이 오차를 수정하여 정확성을높였고, 타원형 이방성과 횡등방성의 알고리듬을 비교하여 무엇이 더 효율적인지 논의한다.
Abstract
Seismic anisotropy can be an obstacle when we interpret the seismic data from the seismic survey and monitoring in the sedimentary basins. In this study, we discuss the inaccuracy of the conventional isotropic monitoring algorithm in strong VTI media in causing reflectivity point differences because it offsets differences without considering anisotropic media. To mitigate its shortcomings, we propose a reverse-time migration algorithm for the 4D seismic data regarding vertically transverse isotropic (VTI) media and confirm that this proposed anisotropic monitoring algorithm based on elliptical anisotropy can properly correct the monitoring results. Finally, we improve the accuracy of reflectivity point depth by adapting the anisotropic equation in the pseudo-acoustic wave equation and controlling the offset distance. We compare the efficiency between elliptical anisotropy and VTI of the proposed monitoring algorithm using the synthetic monitoring data from a complex 3D geological model.
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