基于倾角-偏移距域道集的绕射波成像

Diffraction imaging using dip-angle and offset gathers

地震绕射波是地下非连续性地质体的地震响应,绕射波成像对地下断层、尖灭和小尺度绕射体的识别具有重要的意义.在倾角域共成像点道集中,反射波同相轴表现为一条下凸曲线,能量主要集中在菲涅耳带内,绕射波能量则比较发散.由于倾角域菲涅耳带随偏移距变化而存在差异,因此本文提出一种在倾角-偏移距域道集中精确估计菲涅耳带的方法,在各偏移距的倾角域共成像点道集中实现菲涅耳带的精确切除,从而压制反射波.在倾角-偏移距域道集中还可以分别实现绕射波增强,绕射波同相轴相位校正,因此能量弱的绕射波可以清晰地成像.在倾角域共成像点道集中,反射波同相轴的最低点对应于菲涅耳带估计所用的倾角,因此本文提出一种在倾角域共成像点道集中直接自动拾取倾角场的方法.理论与实际资料试算验证了本文绕射波成像方法的有效性.

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 2 D 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 structureoriented 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 dipangle 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.