The Research On VSP Reverse-time Migration Method

Vertical Seismic Profiling is a kind of new seismic exploration technology which was developed rapidly in recent decades. It observes the seismic wave field inside the well, namely the underground vertical study of wave field distribution, so it has many unique advantages than the traditional surface seismic section observation, also have many new uses. But, the special observation system of detection in wells is often not very appropriate for imaging of structure near the wells. VSP data is usually in the form of common shot gathers, the incident wave and reflected wave field need to be processed when migration, so it is more complex than the ground observation data to be migrated, for the latter can be assumed as the reflection model for detonation, the wave equation may be simplified to single-travel equation, whereas the VSP data is not able to do so. Under the condition of the ground observation, the downward continuation method is known to all, lateral continuation may be adopted under the condition of VSP observation naturally. But, the problem is that the velocity is changeable within the scope of the detector array of VSP. And horizontal geophone array is often considered as downward continuation in the same medium speed, so the difficulty increasedThe reverse-time migration is considered to be the best way to image the complex underground structures in the industry. But it requires a lot of storage space. As the rapid development of the parallel computers and storage devices in recent years, reverse time migration is possible to be realized.Firstly, the paper uses the method of high-order staggered grid finite difference to accurately simulate VSP wave field, combined with perfectly matched layer (PML), to absorb boundary reflection. Then, extrapolates the VSP full wave field and separated up going wave field in reverse-time. Finally, images the extrapolated data according to the imaging conditions. Furthermore, discusses the illusion problems occurred in the symmetric position of the well when does VSP reverse-time migration by separately imaging of single-shot data for near and far diffractions besides the well. Moreover, in order to show the necessity of wave field separation to migration, migrations of up-going only and full wave field are accomplished for a graben model and a semi-circular uplift model.42pictures,2tables,39references.