An Improved Vicinity Ray Tracing in Two-Dimensional Laterally Inhomogeneous Velocity Structure

於二維側向不均勻速度構造之修正鄰近波缐追跡法

陳朝輝(C. H. Chen) ； 施東峰(T. F. Shih)

《Terrestrial, Atmospheric and Oceanic Sciences》 3卷4期 (1992/12) Pp. 483-501

摘要

鄰近波線追跡(VRT)是基於高頻近似而於具有側向速度變異構造內用以計算介質中之波場分佈。VRT具有多項優點，諸如給予明確之波束寬度，計算較迅速等。但此法中所提出各波線對測站貢獻的走時之近似計算法並不能通用於所有波線。即當波線於測站與震源間入射時，此法失敗。因此在本研究中，基於平行波線的觀念，發展了測站走時計算的修正方法，而可適用於所有波線。本研究並以數值計算，實際測試VRT法及修正走時計算的精確度與適用性。其結果顯示走時修正法雖引進較多的近似估計，但誤差並不會扭曲合成波動記錄；以及VRT法能適用於同時存在多個Caustic區和強烈側向速度變異構造，並保持其穩定性和準確性。此外，在二維速度構造中，我們將計算波前基本動態性質的過裎，改以由波線追跡系統中直接求取。此項過程的改變可節省大量計算時間而仍保有相同的準確度。

並列摘要

We have developed a modified vicinity ray tracing (VRT) method that has improved the travel time approximations and strengthened the theoretical base of the original one. The vicinity ray tracing (Kim and Cormier, 1990) method is a high-frequency asymptotic procedure to compute the wavefield in laterally inhomogeneous velocity structures. However, in calculating travel times the original VRT method only considers the central rays on the far side of the station but misses those on the side near the source. The modified version extends the travel time approximation to the rays on both sides of the station. This is facilitated by assuming that the ray paths near the surface in the vicinity of the station are parallel to each other. The modified approach has been examined with a variety of velocity models. For the model with a low velocity layer, the synthetic seismograms demonstrate that the dependence of the amplitudes on the source-receiver distance in the shadow zone is as clear as that illustrated using a classical Gaussian Beam Method. When strong lateral heterogeneity and thus multiple caustic regions are present, the synthetic wave field is still shown to be stable. As another improvement, the calculation of wavefront parameters is furnished in kinematic ray tracing system without invoking Kim and Cormier's dynamic VRT system. This alternative procedure can save large computational time without sacrificing the accuracy in synthetic seismograms.