Elastic wavefield separation methods and their applications in reverse time migration
Wu Xiao1,3, Liu Yang1,2,3, Cai Xiaohui4
1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum(Beijing), Beijing 102249, China; 2. Faculty of Petroleum, Karamay Campus of China University of Petroleum(Beijing), Karamay, Xinjiang 834000, China; 3. CNPC Key Laboratory of Geophysical Prospecting, China University of Petroleum(Beijing), Beijing 102249, China; 4. Institute of Geotechnical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China
Abstract：The elastic wavefield separation is a key technique for elastic reverse time migration (RTM),which is mainly based on Helmholtz decomposition.In this paper,we analyze and summarize elastic wavefield separation methods,and classify them into two kinds:one is direct separation and the other is indirect separation.The former obtains P-and S-wavefields by directly calculating the divergence and curl of the original wavefield.This kind includes the space-domain methods and the wavenumber-domain methods.The latter obtains P-and S-wavefields at the next time using recursively the separated P-and S-wavefields of the previous time.We further introduce the basic principles of three space-domain methods,a wavenumber-domain method and an indirect separation method.We adopt a homogenous model,a layered model and the Marmousi model to demonstrate the effectiveness and efficiency of these five methods.The two conventional space-domain methods have the least computational complexity,but the separation results do not preserve the amplitude.The indirect separation method is effective and the calculation is moderate,but there are energy anomalies at velocity interfaces.The improved space-domain method has the best separation effect (but usually needs larger computational cost),followed by the wavenumber-domain method.Finally,the influences of these five methods on the elastic RTM results are tested with the Marmousi model,which indicate both the improved space-domain method and the wavenumber-domain method have the satisfactory imaging results.
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