VSP Data Imaging Investigation

Today there is a requirement of processing complex underground structure with high resolution in oil and gas exploration and development. To meet the demand, now VSP is becoming a powerful technology which could solve the problem of subtly imaging underground structure and finely describe the oil and gas reservoir. As the seismic equipment performance gradually improved, VSP data collection is becoming more and more perfect. As a result, effective VSP data imaging method turns into one of many advanced research hotspots. This paper describes VSP observation system and the characteristics of the wave field, and has done some research and exploration in numerical simulation of VSP data, wave field separation and reverse-time migration.This paper adopted finite difference numerical simulation VSP data, used Taylor series expansion deducing finite difference scheme of Wave equation with second order in time and high order in space, and discussed the stability conditions and seismic source function. The paper detailedly analyses the principle and effect of perfectly matched layer absorbing boundary conditions working on the artificial boundary reflections interfere with the absorption and attenuation. Aiming at the dispersive problem of finite difference method numerical simulation, flux correct transport method was applied to effectively suppress it.VSP data contains abundant wave field information, the energy difference between up-going wave and down-going wave brings challenge to the processing work. By analyzing the disadvantages and limitations of traditional wave field separation method, the f-k filter and median filter method were optimized respectively. The optimized f-k filter method solve the rieber mixed problem of traditional method output wave field. And the optimized median filter menthod breaks the limitation of traditional method using single filter direction and single filter window, it develops the precision of separating wave fields.Based on the two-way wave equation, the reverse-time migration(RTM) has become the most accurate migration method so far. The paper introduces the calculation process of reverse-time migration in detail, aiming at the boundary and storage problem, the optimized storage way has been proposed. According to the principle of random boundary conditions to do the computation instead of storing data, the optimized storage way realizes the target of greatly saving storage without bringing in the noise, ensures the precision of migration profile.On imaging condition application of reverse-time migration, this paper analyzes and compares the principle and application of three methods which are the cross-correlation imaging conditions, normalization imaging conditions and ratio of upgoing over downgoing wavefield amplitudes. For suppressing the low frequency noise interference, the Laplacian noise suppression method is used to improve the accuracy in imaging profile.Through the comprehensive research and analysis, the paper indicated that high precision reverse-time migration could validly image VSP data and has advantage on nicely depicting small faults, large angle structure and the local area around the well. As a new seismic explaration technology, VSP data imaging method has profound significance and broadapplication prospects.