Research On Application Of Seismic Tomography In The Gas Hydrate Bearing Strata In Velocity Modeling

Gas hydrate is a kind of new green energy which has great potential, and is widely distributed in the continental margin and high latitude permafrost.The differences of physical properties between gas hydrate layers and surrounding rocks makes it possible to apply geophysical methods in the exploration. The bottom simulating reflector (BSR) and velocity anomaly are the most important geophysical response characteristics. However,BSR can only help to verify the existence of the gas hydrate, and indicate the bottom of the gas hydrate layer.lt makes it impossible for pepole forecast the distribution and content of the hydrate.Thus,it is important to find some more effective ways.Velocity is one of the most important physical parameters of underground sedimentary layers and it is very closely related with the porosity and saturation etc.Therefore, accurate estimate the velocity of underground sedimentary layer is the key to the evaluation and prediction of gas hydrate resources. There are several velocity modeling methods such as the stack velocity analysis, the migration velocity analysis and the seismic tomography. Compared to the former two,the seismic tomography can build more precise model of underground medium velocity, better reveal the subsurface structure, geological structure and reservoir characteristics.This paper mainly studies the reflection wave traveltime tomography, the process is as follows:(1)Reflection traveltime data picking up. Travel time data is the basis of the tomography, aiming at the target area with complex geological conditions, the special geological body development situation, using multi-domain joint pickup and diffraction wave suppressing ideas, improve the precision and accuracy of the pickup. (2)Establishment and discretization model. Considering the reflection traveltime tomography is not only to determine the formation velocity variations in vertical and horizontal direction, but also determine the deposited layer interface of ups and downs. Thus, the paper uses irregular grids to discrete medium model,to ensure the precision of calculation. (3)Ray tracing forward simulation. Ray tracing is an important part of traveltime tomography for the calculation of traveltime and ray path. The paper summarizes previous methods, and uses the method which is based on the irregular grid unit, GMM wavefront expansion and bilinear traveltime interpolation to apply to the reflected wave ray tracing, ensuring a high efficiency in high precison at the same time. (4) The theory of inverse problem. As for the inversion, the paper chooses the damped LSQR method because of its strong anti-interference. For the uneven coverage of rays and data, the strategy used is combined with the regularization equation and for the disunity velocity and reflector depth two parameters, the paper presents a normalization to deal with this question. These measures can alleviate the pathological degree of inversion equation, and can improve the imaging accuracy.This paper also developes 2D reflection traveltime tomography imaging software,with reflection traveltime picking, model establishment,forward and inversion calculation and graphic display function. Design several differents seabed sediment layers theoretical models and synthesized seismic records of these model test of seismic tomography method and software. Under the constraint of logging data, applies the seismic tomography method in a certain area of a seismic line, determining along the line velocity model.