一种用于走时层析成像的改进算法

走时层析成像是根据首波走时重建声波、地震波或电磁波波速的层析技术，其所用到的射线路径依赖于波速分布。本文提出了一个依据费马原理寻找射线路径的新方法。费马原理指出，从发射点到接点的射线首波的走时极小，因此寻找射线路径的问题是一个最优化问题。     

Inversion for interface structure using teleseismic traveltime residuals

An inversion method is presented for the reconstruction of interface geometry between two or more crustal layers from teleseismic traveltime residuals. The method is applied to 2-D models consisting of continuous interfaces separating constant-velocity layers. The forward problem of determining ray paths and traveltimes between incident wave fronts below the structure and receivers located on the Earth's surface is solved by an efficient and robust shooting method. A conjugate gradient method is employed to solve the inverse problem of minimizing a least-squares type objective function based on the difference between observed and calculated traveltimes. Teleseismic data do not accurately constrain average vertical structure, so a priori information in the form of layer velocities and average layer thicknesses is required. Synthetic tests show that the method can be used to reconstruct interface geometry accurately, even in the presence of data noise. Tests also show that, if layer velocities and initial interface positions are poorly chosen, lateral structure is still recoverable. The inversion method was applied to previously published teleseismic data recorded by an in-line array of portable seismographs that traversed the northern margin of the Musgrave Block, central Australia. The solution based on interface parametrization is consistent with models given by other studies that used the same data but different methods, most notably the standard tomographic approach that inverts for velocity rather than interface structure.     

Traveltimes for infrasonic waves propagating in a stratified atmosphere

The tau– p method of Buland & Chapman (1983) is reformulated for sound waves propagating in a stratified atmosphere under the influence of a height-dependent wind velocity profile. For a given launch angle along a specified azimuth, the ray parameter is redefined to include the influence of the horizontal wind component along the direction of wave propagation. Under the assumption of negligible horizontal wind shear, the horizontal wind component transverse to the ray propagation does not affect the direction of the wave normal, but displaces the reference frame of the moving wavefront, thus altering the observed incidence azimuth. Expressions are derived for the time, horizontal range, and transverse range of the arriving waves as a function of ray parameter. Algorithms for the location of infrasonic wave sources using the modified tau– p formulation in conjunction with regional atmospheric wind and temperature data are discussed.     

Seismic reflection traveltimes in two-dimensional statistically anisotropic random media

Velocity estimation remains one of the main problems when imaging the subsurface with seismic reflection data. Traveltime inversion enables us to obtain large-scale structures of the velocity field and the position of seismic reflectors. However, as the media currently under study are becoming more and more complex, we need to know the finer-scale structures. The problem is that below a certain range of velocity heterogeneities, deterministic methods become difficult to use, so we turn to a probabilistic approach. With this in view, we characterize the velocity field as a random field defined by its first and second statistical moments. Usually, a seismic random medium is defined as a homogeneous velocity background perturbed by a small random field that is assumed to be stationary. Thus, we make a link between such a random velocity medium (together with a simple reflector) and seismic reflection traveltimes. Assuming that the traveltimes are ergodic, we use 2-D seismic reflection geometry to study the decrease in the statistical traveltime fluctuations as a function of the offset (the source–receiver distance). Our formulae are based on the Rytov approximation and the parabolic approximation for acoustic waves. The validity and the limits are established for both of these approximations in statistically anisotropic random media. Finally, theoretical inversion procedures are developed for the horizontal correlation structure of the velocity heterogeneities for the simplest case of a horizontal reflector. Synthetic seismograms are then computed (on particular realizations of random media) by simulating scalar wave propagation via finite difference algorithms. There is good agreement between the theoretical and experimental results.