起伏地表二维粘弹复杂介质瑞雷波全波形反演研究

Based on linear viscoelastic media, this research will first implement heterogeneous rotated staggered grid higher-order finite-difference modeling and numerical simulations in the presence of irregular topography, which can provide synthetic prediction wavefields for viscoelastic Full Waveform Inversion (FWI) of Rayleigh waves in strong lateral variations. Second, precise source wavelet can be estimated by advanced Virtual Real Source (VRS) method, which can greatly reduce source signature effects on FWI of Rayleigh waves. Third, well-behaved misfit functions will be designed based on novel seismic Envelope Inversion (EI), which can effectively overcome obstacles issued by cycle skipping, significantly mitigate high nonlinearity and multimodality of objective functions, and greatly reduce the dependence on the initial model for full waveform inversion of Rayleigh waves. Thus, long-wavelength large-scale background velocity models can be reconstructed by EI for full waveform inversion of Rayleigh waves, which can greatly improve convergence and robustness for FWI of Rayleigh waves. Fourth, high-precision and high-resolution S-wave velocity models can be obtained, fine description ability on small geologic structures can be improved, adaptability of Rayleigh waves for complicated subsurface environments and laterally heterogeneous anomalies including irregular topography can be enhanced by multiscale full waveform inversion of Rayleigh waves, which can fully exploit full recorded kinematic and dynamic information in wavefields. This research will present new theories and its methodologies on Rayleigh waves from 2D viscoelastic full waveform inversion of Rayleigh waves in the presence of significant topographic fluctuations in strong lateral variations, will widen conventional theories of Rayleigh waves, obtain new findings, and will promote the progress of high-precision Rayleigh wave exploration.

本项目基于线性粘弹介质，通过起伏地表复杂介质瑞雷波非均匀旋转交错网格高阶有限差分正演模拟，为瑞雷波全波形反演提供快速高精度理论波场；通过先进的虚实源对比法为瑞雷波全波形反演提取精确地震子波，有效降低震源特性对全波形反演的影响；对波场分离后的纯瑞雷波通过新颖的地震包络反演策略构建良好行为的目标函数，充分利用地震包络中丰富的低频信息使目标函数表面形态连续光滑，有效克服全波形反演周波跳跃问题，大大降低目标函数的强烈非线性、多极值性和反演过程对初始模型的依赖性，显著提高全波形反演收敛性和稳健性；通过瑞雷波多尺度全波形反演充分利用全波场运动学和动力学特征，进行高精度和高分辨率横波速度建模，增强瑞雷波在起伏地表和横向非均匀介质中勘探适应性，提高其对复杂构造和较小异常体的精细刻画能力，由此构建起伏地表粘弹复杂介质瑞雷波全波形反演理论及方法，获得新的发现，拓宽现有瑞雷波理论范畴，推动瑞雷波高精度勘探发展。

由于现有瑞雷波研究主要集中在水平地表弹性水平层状介质模型，利用瑞雷波频散曲线反演获得一维横波速度剖面和其它岩土力学参数。因此导致现有瑞雷波存在对更接近实际地质结构的粘弹复杂介质反演精度低、在复杂介质出现时频散曲线提取和模式认定困难、反演水平分辨率低和对起伏地表横向非均匀复杂介质难以适用等主要问题。.鉴于此，本项目基于线性粘弹介质模型，通过起伏地表复杂介质瑞雷波非均匀旋转交错网格高阶有限差分正演模拟技术，为瑞雷波全波形反演提供了快速高精度理论波场；通过先进的虚实源对比法为瑞雷波全波形反演提取了精确地震子波，有效降低了震源特性对全波形反演的影响；对波场分离后的纯瑞雷波通过新颖的地震包络反演策略构建了良好行为的目标函数，充分利用了地震包络中丰富的低频信息使目标函数表面形态更加连续光滑，有效克服了全波形反演周波跳跃问题，大大降低了目标函数的强烈非线性、多极值性和反演过程对初始模型的依赖性，显著提高了全波形反演的收敛性和稳健性；通过瑞雷波多尺度全波形反演充分利用了全波场运动学和动力学特征，进行了高精度和高分辨率横波速度建模，有效增强了瑞雷波在起伏地表和横向非均匀介质中勘探适应性，显著提高了其对复杂构造和较小异常体的精细刻画能力，由此构建了起伏地表粘弹复杂介质瑞雷波全波形反演理论及方法，拓宽了现有瑞雷波理论范畴，推动了瑞雷波高精度勘探发展。