多层垂直对称轴横向各向同性介质精确走时计算

给出了计算多层垂直对称轴横向各向同性（VTI）介质精确射线路径和走时的方法,所用的体波相速度公式、群速度公式和Snell定律都是严格的显式解析公式. 任意基本波的射线路径和走时计算问题都可以转化成一个等效的透射问题,再用文中的公式来计算,具体实现方法用一个多次波和一个首波的实例给出. 最后分别用精确公式和Thomsen近似公式计算了相同模型相同基本波的走时曲线. 比较两者计算结果可发现, 近似公式反复使用会使误差积累,同时揭示了近似公式适用范围的局限性,强调了使用近似公式需要注意其适用范围的重要性.      

Three-dimensional geometrical ray theory and modelling of transmitted seismic energy of data from the Nevada Test Site

This paper presents a geometrically based algorithm for computing synthetic seismograms for energy transmitted through a 3-D velocity distribution. 3-D ray tracing is performed to compute the traveltimes and geometrical spreading (amplitude). The formulations of both kinematic and dynamic ray-tracing systems are presented. The two-point ray-tracing problem is solved by systematically updating the initial conditions and adjusting the ray direction until the ray intersects the specified endpoint. The amount of adjustment required depends on the derivatives of the position with respect to the given starting angles between consecutive rays. The algorithm uses derivatives to define the steepest-descent direction and to update the initial directions. The convergence rate depends on the complexity of the model.
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity.     

Modelling and inversion of single-ended refraction data from the shot gathers of multifold deep seismic reflection profiling—an approach for deriving the shallow velocity structure

A multifold crustal-scale deep seismic near-vertical reflection profile generates a large number of single-ended shot gathers, which provide redundant data sets because of overlapping coverage of the shallow refractors. We present an approach for deriving the shallow velocity structure by modelling and inversion of single-ended seismic refraction first arrival traveltime data. We apply this method to a data set acquired with a 12-km long spread with 100 m spacing of shots and receivers, of the Neoproterozoic Marwar basin in the NW Indian shield. The approach is shown to be quite successful for delineating the shallow refractor depths, steep dips and velocities, even in the absence of regular reverse refraction profiles. The study reveals two-layered sedimentary formations, Malani volcanics and a complicated basement configuration of the Marwar basin, and provides a measure of resolution and uncertainty of the estimated model parameters. A seismic section of the near-trace gather is found to be qualitatively consistent with the derived structural features of the basin. The relative highs and lows, observed in the Bouguer gravity profile, further corroborate the derived velocity model. The present approach can be especially useful in offshore areas and elsewhere, where the single-ended multifold seismic profiles are the only available data sets.     

基于井数据约束的高精度层析速度反演

测井数据是一种准确可靠的先验信息。在层析反演方程组中加入井数据信息一定程度上提高了反演精度和效率,但仍无法精确描述微幅构造地质体。针对这一特点,笔者提出了基于井数据约束的高精度层析速度反演方法。首先进行常规的井约束层析速度反演,假定测井处的网格速度不更新,建立井约束反演方程组,更新速度场,得到初步的偏移成像结果;然后在此基础上利用测井速度信息约束偏移深度,对更新后的速度场进行速度分析及精细化建模,提高速度建模精度,同时减少了反演迭代更新次数,提高了反演效率。模型试算及实际资料处理结果表明,该方法具有更高的反演精度和计算效率。     

大地电磁测深与地震初至波走时交叉梯度反演

在地球物理勘探过程中,观测数据有限且存在误差,导致反演结果存在非唯一解。为了解决非唯一性问题,笔者研究了交叉梯度数学理论,实现了用交叉梯度约束2D 大地电磁和地震初至波走时的联合反演,并建立了2个模型。通过各自模型实验和不同模型间的对比,证明了交叉梯度联合反演比单独反演修复几何形态和物性参数的效果好,且交叉梯度与岩石物性无关;利用得到的交叉梯度图进一步证明了联合反演模型相似度高,也从交叉绘图中得到联合反演的物性关系更好的结论,反向证明了联合反演的正确性。