The Source Extension Of OBS Data Based On Virtual Source Method

The virtual source method (VSM) is a method that was proposed by AndreyBakulin and Rodney Calvert in2006(Bakulin, A., and R. Calvert,2006) to solve theproblems of imaging through complex overburden. The method uses time-reversallogic to create a new downward-continued data set with virtual sources (VS’s) at thegeophone locations. Time reversal focuses energy that passes through the overburdeninto useful primary energy for the VS. This method can covert the seismic datarecorded in the boreholes with sources on the surface to the data which sources andreceivers are both in the boreholes. Using this method we can image the subsurfaceunder complex overburden with no velocity model about the overburden.In this paper, we apply the VSM method to the OBS data in the purpose ofextension the sources on the ocean surface to the bottom of the sea (or any point)below sea level, at the same time increase the density of virtual sources and the shootlines, thus we can improve the image quality of OBS by increasing fold of the unitsCDP. This paper summarizes the results of previous studies and describes thedevelopment and research status of VSM. We solve the implementation problems inthe process of using VSM by simulation experiments, such as the maximum allowedsource spacing to avoid causing the spatial aliasing, edge effect and spurious eventscaused by uncompleted source aperture.On the basis of learning studied in previous work, VSM method was realized byC++language, and using two synthetic model with complex overburden to verify thescientific validity of the method. At the same time, this paper presents a way to applyVSM to increase the density of OBS data and solves some problems faced in theimplementation process, such as the elimination of the low-frequency information inthe OBS data, sources aperture position changing with different purposes depth, theprinciples of determining the size of source aperture. The algorithm is successfullyapplied to increase density of VS’s and VS lines.The sources on the sea surface areremoved to the target depth of700m below the sea. The original OBS data with sevenshoot lines and25m shot spacing is increase to18shot lines with shot spacing of12.5meters, which greatly increased the density of seismic shot, so as to achieve the aim toincrease the fold of unit CDP, improving the quality of velocity analysis. In the lastpart, we get the image of subsurface using the new virtual sources gathers, in contrastto the image of original OBS data. We can see that the reflection events of shallowsubsurface are more clearly than the image of original OBS data. In some extreme, theusing of VSM has improved the imaging of OBS data. Indicating the effectiveness ofthe method; the reflection interface of seabed is still not continuous. I think we cancombine the VSM with mirror-imaging to improve the continuity of reflectioninterface of seabed in the future.