Diffraction imaging using dip-angle and offset gathers

作者：Li ZhengWei^*; Zhang JianFeng; Liu Wei

来源：Acta Geophysica Sinica, 2018, 61(4): 1447-1459.

DOI：10.6038/cjg2018K0583

• Diffraction
• Fresnel zone
• Dip-angle gather
• Imaging
• Resolution
• Dip-angle estimation

Diffractions are the seismic response of geologic discontinuities, such as faults, pinchouts and small scattering objects, in general all objects which are small compared to seismic wavelength. Diffraction imaging has a great potential to detect these structural details beyond the classical Rayleigh limit of half a seismic wavelength. However, diffractions are essentially treated as noise in conventional seismic processing sequence, such as Normal Move-Out and stack. Moreover, the energy of diffractions is typically one or two orders of magnitude weaker than that of specular reflections. Therefore, the separation of diffractions is a challenging issue. @@@ The dip-angle domain is appropriate to differentiate the events between diffraction and reflection. In 2D migrated dip-angle gathers, diffraction events can exhibit flatting under correct migration velocity while reflecting events have concave shapes. We obtain the dip-angle gathers using prestack time migration (PSTM). The angle in the dip-gathers generated by PSTM is an apparent dip-angle instead of the real one in inhomogeneous media. However, the diffraction and reflection events exhibit the same characteristics in the apparent dip-angle gathers as in the dip-angle gathers, which allows us to separate diffractions and reflections efficiently. @@@ We attenuate specular reflections by muting the Fresnel zone in the dip-angle gathers. As the Fresnel zone in the migrated dip-angle gathers changes with offset, we propose a method to estimate the Fresnel zone in the dip-angle and offset gathers, by which we can precisely mute the Fresnel zone in the migrated dip-angle gathers of different offset to separate diffractions and reflections. Moreover, we enhance the diffraction amplitudes and correct the phase of diffractions in the dip-angle and offset gathers. @@@ Construction of the reflector dip field is of great importance for the precise estimation of the Fresnel zone in the dip-angle and offset gathers. Reflector dips are usually derived from conventional migrated stacked sections in depth by using local coherent analysis or structure oriented filter. However, the existence of diffractions may decrease the accuracy and stability of these traditional methods. In the dip-angle gathers, the stationary points of reflection curves occur at the dip angle of reflectors. We propose a method to construct the dip field using the dip angle gathers directly. @@@ In numerical examples, we first test the proposed diffraction PSTM with synthetic data. We generate diffractions by small-scale faults and small-scale, low-magnitude velocity variations along interfaces. We can see that all diffraction points are imaged correctly. No residual reflection events can be found. Further, we test our method on a real data set. We observe certain and uncertain faults in the migrated section of conventional PSTM while clear diffraction images at the corresponding locations in the migrated section of diffraction PSTM.

• 出版日期2018-4
• 单位中国科学院大学; 中国科学院地质与地球物理研究所; 中国科学院地质与地球物理研究所兰州油气资源研究中心