三维复杂地形近地表速度估算及地震层析静校正

在地表一致性模型的基础上提出一种可适用于宽线剖面、弯曲测线、传统的二维和目前广泛使用的三维地震观测.在地形及近地表低降速带地质结构复杂的探区，低降速带厚度及速度估算的精度是静校正处理的关键.本研究根据三维地震观测的初至走时数据，利用最小平方与QR分解相结合的算法，在三维空间重建近地表低降速带速度模型，根据重建速度模型实现了静校正长波长分量与短波长分量的同步计算.分析了复杂的近地表低降速带模型初至波的性质，在观测值的自动拾取以及理论值的计算中充分考虑了可能成为初至波的直达波、折射波和反射波的利用，提高了低降速带速度模型反演的精度.在初至走时观测数据的拾取中，本研究采用分形算法克服了初至波波形差异以及折射波相位反转导致的拾取误差，实现了三维初至拾取的大规模全自动化运算.在射线路径与初至波理论走时的计算中，本研究采用一种计算量与模型复杂程度无关的三维射线追踪方法，该方法以最小走时射线路径保证了与观测数据有同等意义的初至波的射线追踪及理论走时的计算.野外实际资料的处理结果表明了方法的有效性.

ESTIMATION OF NEAR-SURFACE VELOCITY AND SEISMIC TOMOGRAPHIC STATIC CORRECTIONS

A static correction approach of 3 D tomographic inversion is presented based on surface consistent model. This method can be applied to wide line profiles,crooked lines. as well as conventional 2 D and 3 D seismic survey geometries. The accurate estimation of the thickness and velocities in shallow low velocity zones is vital to proper static correction in rough topography and complicated low velocity geological structure zones. We use a least squares approach in conjunction with QR decomposition to reconstruct 3 D velocity model from the actual first break times obtained from 3 D data,and then calculat concurrently the long and short wavelength components of static correction based on the reconstructed velocity images. We analyze characteristics of the first break traveltime in complicated low velocity layers. To improve the inversion accuracy of velocity model,the various first break times from direct,reflection and refraction waves are considered for determining the observed traveltimes and the theoretical model calculation. The fractal algorithm is applied to determining the first breaks,which overcomes the error caused by the differences of wavelet shapes and the leg jump of refractions. The method is capable of selecting large volume of first breaks automatically. The ray paths and traveltimes are calculated with a 3 D ray tracer which does not require more CPUs for complicated geological models. This method can determine the ray paths associated with minimum traveltimes regardless of the wave modes. The results indicate that the 3 D tomographic static correction method is more effective for field data.