Study On Forward Modeling Of Seismic Wavefields In Complex Media

Seismic numerical modeling plays an important role in seismic prospecting. Based on modeling principles, the main methods can be divided into two kinds: ray-tracing method and wave equation method.Ray-tracing is hard to accomplish for the reflection wave in complex structures with complicated surface. This paper draws some conclusions in the study of trial-and-error method, shortest path ray-tracing method and constant gradient velocity ray-tracing method. Firstly, the propagation directions of transmitted rays and reflective rays are determined and presented, and the trial-and-error method is applied into the models with complex structure and surface. Some digital models show that the algorithm suits complex media and is stable. However, trial-and-error method can hardly work when strong velocity contrast exists, and it is hard to reach the global minimum traveltime point in multiple arrival times. The shortest path ray-tracing method is studied in the paper which aims to solve the problem. Based on the graphic structure and Huygens'Principle, the shortest raypaths and the reflective wave raypaths are calculated, and the precision is analyzed. Example shows that the shortest path ray-tracing method can adapt to strong velocity variation and can get the global minimum traveltimes, but it can only be used for two-point ray-tracing and has a problem of ray bifurcation. Following the basic work of Langan's constant gradient velocity method, the ray-tracing paths can be derived by solving a set of analytical expressions with a mesh grid method. Combined with Snell's Law, the reflection wave raypaths are determined. Layer coordinates are needed when calculating the reflection wave raypaths, but the general Layer coordinates description method does not suit complex media with rough surface or reverse faults. In this paper, a linear connection method and segmental architecture description method are put forward to make the description of layer coordinates convenient. And we obtain good results when used in constant velocity gradient method. Unlike the common ray methods, the Gaussian Beams method is valid even in caustic region, critical region, etc. In this paper, the Gaussian Beam method is used in complex media and the model testing results show this method is good.Ray tracing method can be achieved faster than wave equation method, but it cannot reveal the dynamic characteristics of wavefields. Thus, the staggered-grid high-order finite difference method in inhomogeneous media is studied in this paper, and it is applied to 2-D acoustic wave, visco-acoustic wave, elastic wave and visco-elastic wave in complex media model. The 3-D acoustic wave and elastic wave with the staggered-grid high-order finite difference method is researched and the result is tested with simple layered model. Some key problems including boundary condition, stability and dispersion in finite difference method are analyzed. Transparent boundary, paraxial approximations absorbing boundary and PML absorbing boundary are all presented. Some stable conditions by former scholars are analyzed and re-tested. Also, some parameters which lead to dispersion in acoustic wave finite difference are quantitatively analyzed theoretically.Based on CRP theory, ray tracing method can be applied to survey geometry design. Through fold number analyzing of every trace, the geometry parameters can be optimized. Meanwhile, illumination analysis is used the use of wave equation in survey geometry design. In the end, a forward modeling software is presented.