Solution for Improving the identification of Low Relief Carbonate Reefs in Shallow Water Areas

Pre-stack depth migration using high precision velocity model is an effective tool to restore the real structures under the variation of lateral interval velocity. Due to lots of seismic data in shallow water area with low coverage and large minimum offset, it is difficult to use shallow seismic data to invert the correct velocity, which will cause large velocity error to the targets when calculating deep velocity. Meanwhile, the frequency bandwidth of carbonate reefs is narrow owing to the absorption of muddy seabed and formation. How to improve the imaging accuracy and resolution of the carbonate reefs by the limited seismic data becomes the key to the reservoir research and subsequent drilling. Two of the key challenges for velocity model building in the shallow water area with the interest targets of low relief carbonate reefs are a heterogeneous overburden, such as small scale low velocity channels and the presence of high velocity carbonates, which result in strong velocity contrasts. Two model building techniques have been presented in this paper to address these challenges. FWI has been shown to be highly effective in resolving overburden heterogeneity and high-precision tomography allows us to preserve sharp contrasts in our velocity models, TTI QPSDM solved the imaging accuracy of low relief carbonate reefs and broaden the frequency band. Full waveform inversion has certain requirements on the accuracy of initial velocity model. The greater the velocity model error, the more serious the FWI local minimum caused by cycle skipping. On the other hand, the interval velocity model converted from RMS velocity with Dix formula lacks velocity details in shallow layers due to the small acquisition coverages causing hard to get accurate velocity from seismic data, and shallow logging data are not usually recorded by reason of limited investment and less target interest. How to get relative accurate initial velocity model, high quality shot gathers with rich low frequency and signal to noise ration improvement is important points in this paper.