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许多井间地震走时的层析成象解释方法是将震源和接收器之间的射线路径近似为直线,而不管射线路径上速度结构的不均匀性效应。当速度没有多大变化时,这样的近似是有效的。在许多地区观察到的速度横向变化为10—20％,甚至更多,这引起了射线相当大的弯曲。有些工作已经考虑了这种非线性效应,但在应用于井间地震问题时,还没有出现许多成功的实例。在这里我们提出了一个基于二维射线追踪的迭代反演方法,并成功地应用于野外资料。这个解释方法是由迭代射线追踪和更新速度模型来实现的。在每次迭代中,通过一个线性方程组将资料和由射线追踪得到的当前模型的走时差值与未知的速度扰动量联系起来。用阻尼最小二乘法求得这个速度扰动量,并因此更改模型。这个迭代过程一直进行到合成走时和野外资料间的拟合达到给定的误差范围之内,或者直到走时拟合不再有改进为止。我们把这个方法应用于结晶岩中井间试验所得的野外资料(Wong,1983)。地震图的频率范围是1—6.6kHz,允许速度结构的分辨率达到几米的数量级,所得的速度图象与其它地质和地球物理数据符合得很好。
Many well-known tomographic interpretation of seismic transit times approximates the ray path between the source and receiver as a straight line, regardless of the inhomogeneity effect of the velocity structure on the ray path. Such an approximation is valid when the speed does not change much. The observed lateral velocity variation in many regions is 10-20% or more, which causes considerable beam bending. Some work has considered this non-linear effect, but many successful examples have not yet appeared when applied to cross-well seismic issues. Here we present an iterative inversion method based on two-dimensional ray tracing, which has been successfully applied to field data. This method of interpretation is achieved by iterative ray tracing and updating of the velocity model. In each iteration, a linear system of equations is used to relate the travel time difference between the data and the current model tracked by the ray to an unknown amount of velocity perturbation. Obtain this velocity perturbation using the damping least squares method and change the model accordingly. This iterative process continues until the fit between the synthesized travel time and the field data reaches a given error range, or until the travel time fit no longer improves. We apply this method to the field data obtained from interwell experiments in crystalline rocks (Wong 1983). The seismogram has a frequency range of 1-6.6 kHz, allowing the resolution of the velocity structure to reach the order of a few meters, and the resulting velocity image is in good agreement with other geophysical and geophysical data.