基于VTI介质的地震初至波层析静校正方法

Tomography static corrections by seismic first break based on VTI media

  • 摘要: 为了改善复杂近地表区地震数据的静校正效果,开展了基于横向各向同性(Vertical Transverse Isotropy,VTI)介质的层析反演速度建模及目标驱动静校正方法研究。正演模拟中将各向异性参数引入旅行时线性插值(Linear Travel-time Interpolation,LTI)射线追踪算法,建立了VTI介质中初至波走时的计算式,迭代反演采用最小二乘正交分解(Least Square QR decomposition,LSQR)算法,并且在反演过程中加入正则化约束因子。静校正处理中分别计算基准面静校正量和基于模型的初至波剩余静校正量,并对2种静校正量进行融合计算与应用对比,分析其对构造形态和成像精度的影响。结果表明:随着偏移距增加,介质各向异性效应对地震波场传播的影响逐渐增大,考虑各向异性影响时在VTI介质中计算的初至波走时具有更高的精度;加入正则化约束的LSQR反演算法能有效提高反演解的稳定性,基于VTI介质的层析反演可获得更为精细的近地表速度模型。基准面静校正将地震数据校正到统一水平面,主要解决长波长静校正问题,确保构造形态的正确成像;剩余静校正则针对高程剧烈变化的影响,弥补基准面静校正之后的残余静校正量,解决短波长静校正问题,从而进一步提升成像精度。理论数值计算和实际资料处理验证了研究方法的有效性和正确性,该方法能获得更为准确的近地表速度和野外静校正量,有效解决了复杂近地表对地震资料造成的静态时移问题,有利于高分辨率地震成像。

     

    Abstract: In order to improve the static correction effect of seismic data in complex near-surface area, the tomography velocity modeling and target-driven static correction method based on vertical transverse isotropy (VTI) medium were studied. The forward simulation introduces the anisotropic parameters into the linear travel-time interpolation (LTI) ray-tracing algorithm, and establishes the formula for calculating the first breaks travel time in VTI media. The iterative inversion adopts the least square QR decomposition (LSQR) algorithm and incorporated regularization constraint factors in the inversion process. In the static correction processing, the quantities of datum plane static correction and the model-based residual static correction were calculated separately, and the two kinds of statics were combined and compared, then the target driven static correction analysis was carried out. The results show that the influence of anisotropy on seismic wave propagation increases with the increase of offset, and when considering the influence of geological anisotropy, the first break travel time calculated in VTI media has higher accuracy. The LSQR inversion algorithm with regularization constraints can effectively improve the stability of the inversion solution, and the tomography inversion method based on VTI medium can obtain a more precise near-surface velocity model. The datum statics corrects seismic data to the unified horizontal plane, which mainly solves the problem of long wavelength statics and ensures the correct imaging of structural form. The residual statics is to compensate for the residual static correction after the datum plane static correction, solve the short-wavelength static correction problem, and further improve the imaging accuracy. Theoretical numerical calculation and actual data processing results verify the correctness and effectiveness of the proposed method, which can obtain more accurate near surface velocity models and field static correction quantities, effectively solving the static time shift problems caused by complex near surface seismic data, which is beneficial for high-resolution seismic imaging.

     

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