Geophysics and Geophysical Exploration (지구물리와물리탐사)

Korean Society of Earth and Exploration Geophysicists (한국지구물리·물리탐사학회)
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Maximising the lateral resolution of near-surface seismic refraction methods

천부 탄성파 굴절법 자료의 수평 분해능 최대화 연구

  • Palmer, Derecke (School of Biological, Earth and Environmental Sciences, The University of New South Wales)
  • Published : 2009.02.28
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Abstract

The tau-p inversion algorithm is widely employed to generate starting models with most computer programs, which implement refraction tomography. This algorithm emphasises the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases which are indicative of shear zones. This study demonstrates the failure of the tau-p inversion algorithm to detect or define a major shear zone which is 50m or 10 stations wide. Furthermore, the majority of refraction tomography programs parameterise the seismic velocities within each layer with vertical velocity gradients. By contrast, the Generalized Reciprocal Method (GRM) inversion algorithms emphasise the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is confirmed by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, an analysis of the shot record amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. The major conclusion reached in this study is that while all seismic refraction operations should aim to provide as accurate depth estimates as is practical, those which emphasise the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D traverses in which the structural features can be recognised from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features such as faults which do not display any intrinsic variation or 'signature' in seismic velocities.

굴절법 토모그래피를 구현하는 대부분의 컴퓨터 프로그램은 타우-피 역산 알고리즘을 이용하여 초기 모델을 생성한다. 타우-피 역산 알고리즘은 지층의 수직 분해능에 초점을 맞추기 때문에 전단 영역의 존재를 지시하는 탄성파 속도의 감소와 같은 수평적인 변화를 탐지하는데 실패하는 경우가 자주 발생한다. 본 연구에서는 타우-피 역산 알고리즘이 50미터 혹은 10개 측점 너비의 주요 전단 영역을 탐지하거나 정의하는데 실패하는 사례를 보여준다. 그럼에도 불구하고 대다수의 굴절법 토모그래피 프로그램들이 각 지층의 수직 속도 구배로 탄성파 속도를 매개화한다. 이와는 달리, 일반상반성방법(Generalized Reciprocal Method; GRM) 역산 알고리즘은 개별 지층의 수평 분해능을 강조한다. 본 연구에서는 GRM 역산 알고리즘을 이용하여 50미터 폭의 전단 영역을 성공적으로 탐지하고 정의하는 사례를 보여준다. 전단 영역의 존재는 2차원 선두파 진폭분석과 이후의 3차원 굴절법 탐사의 일환으로 수행된 몇 개의 근거리 직교 탄성파 탐사에 의해 확인된다. 또한. 송신원 기록 진폭분석 결과는 풍화대에서 수직 속도 구배보다는 속도역전이 발생하는 것을 보여준다. 결론적으로 말하면, 모든 탄성파 굴절법 탐사가 실용적으로 정확한 심도추정 결과를 제공하는 것을 목적으로 하면서도 개별 지층의 수평 분해능을 강조하는 기법들이 지질환경공학적인 응용에 더 유용한 결과를 생성한다는 것이다. 향상된 수평 분해능의 장점은 구조적 특징이 탄성파 속도의 변화 크기로부터 인식될 수 있는 2차원 트래버스(tracverse)로 얻어질 수 있다. 또한, 3차원 탐사로부터 얻어진 공간 패턴은 탄성파 속도에서는 고유한 변화나 징후를 보이지 않는 단층과 같은 구조적 특징의 인식을 가능하게 한다.

Keywords

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