基于平面波加速的VTI介质最小二乘逆时偏移

地震各向异性集中表现为速度各向异性,势必影响地震波运动学特征.传统声波逆时偏移(RTM)和最小二乘逆时偏移(LSRTM)没有考虑介质各向异性特征,导致反射波不能正确归位、同相轴出现扭曲及寻优速度慢或不收敛等,VTI介质逆时偏移(VTI-RTM)矫正了声波成像的不足,但仍存在低频干扰严重、中深部成像不佳、振幅保持差等缺陷.为此,本文首先实现了VTI介质最小二乘逆时偏移(VTI-LSRTM)方法,为了节省I/O及内存需求并提高效率,进一步引入平面波编码技术,提出了一种基于平面波加速的VTI介质最小二乘逆时偏移(VTI-PLSRTM)策略.在此基础上开展了简单模型及复杂Marmousi模型成像试验,并与标准逆时偏移剖面对比表明:本方法能够校正各向异性造成的相位畸变,且在迭代中自动压制串扰及低频噪声、补偿中深部能量,是一种兼具质量与效率的保幅成像策略;对速度误差的敏感性测试说明该方法需要相对正确的偏移速度及Thomsen参数模型.

Fast least-squares reverse time migration based on plane-wave encoding for VTI media

Seismic anisotropy of the earth medium is primarily manifested as velocity anisotropy,which certainly affects the kinematic properties of seismic waves propagating inside the earth. The traditional acoustic reverse time migration (RTM) and least-squares reverse time migration (LSRTM) do not account for this distortion of phases caused by strong anisotropy,which can lead to defocusing of migration images,the warp of events and the slow searching efficiency or non-convergence following the increase of iterations. Although reverse time migration for vertical transverse isotropic media (VTI-RTM) is capable of making up these defects,the low-frequency noise in shallow section,poor imaging due to energy loss in mid-deep part,imbalanced amplitude and others still exist in migration imaging processing. To correct for this drawback,this paper uses a linearized inversion method applicable for vertical transverse isotropic media firstly,denoted as VTI-LSRTM. The key points of this method include two parts:① We use a linearized anisotropy quasi-acoustic modeling operator for forward modeling during the least-squares iterations. ② Gradient direction of the misfit function is derived using the adjoint-state method for back propagating the residual wavefields. Then for saving the input/output and storage demands and improving the efficiency of inversion,we further introduces the plane-wave encoding technique to the inversion framework of VTI media and presents the strategy of fast least-squares reverse time migration with plane-wave encoding for VTI media. Numerical tests on synthetic dataset with a simple model and a complex Marmousi model prove that,① the merit of the proposed method surpasses standard acoustic RTM and LSRTM when the recorded data had strong anisotropy effects,and ② the crosstalk introduced by plane-waves and the low-frequency noise from itself of two-way wave propagator can be suppressed automatically,and ③ the advantage of this approach compared with conventional VTI-LSRTM is that it both produces high quality images with better balanced amplitudes in mid-deep parts and higer resolution & SNR & fidelity similar to VTI-LSRTM and decreases the computational cost significantly. The sensitivity tests for background velocity errors reveal that the liability of this method is the requirement for relatively accurate migration velocity and Thomsen parameter models.