Elastic Reverse Time Migration And Extract Angle Domain Common Image Gathers

Prestack reverse time migration(RTM),which exploits the two-way wave equation,is capable of accurately repositioning various seismic events without dip angle limitation.However,real Earth media allows P-and S-waves to propagate simultaneously,migration schemes in acoustic approximation may be susceptible to S-wave,which will decrease the quality of migration images.Elastic RTM,which utilizes the two-way elastic wave equation,has the capability to accurately imaging P-and S-wave seismic events and produce multi-wave reflectivity images which will provide more rich subsurface information and reduce blind area of P-wave exploration.Hence,elastic RTM with multi-component seismic data has great potential and development prospecting.This paper investigates and implements elastic RTM and extracts angle domain common image gathers.This article employs the velocity-stress elastic wave equation to reconstruct the source and receiver wavefields using staggered-grid infinite difference scheme.To decrease the crosstalk artifacts in migration images for elastic RTM,this paper decomposes the extrapolated elastic wavefields into pure wave modes and apply the imaging condition to extract subsurface reflectivity images.The wavefield decomposition can be implemented by conventional Helmholtz theory or the decoupled elastic wave equation.To mitigate the polarity reversal of converted images in conventional decomposition method,this paper proposes a polarity correction scheme.For the decoupled wavefields,this paper applies an inner product imaging condition to obtain reflectivity images with explicit physical meaning.Moreover,the stress image of P-wave can supplement elastic imaging.For the implementation of 3D elastic RTM,this paper presents a modified excitation imaging condition with low memory and computation costs.This article estimates the propagation angle of P-wave resorted to the Poynting vector in the space-time domain.By introducing the local reflector information,this article effectively computes the incident angle of P-wave.During the application of imaging condition in elastic RTM,this article maps the imaging values into the angle domain by estimated incident angle information and produce the angle domain common image gathers.Finally,this article investigates the elastic and viscoelastic least-square reverse time migration(LSRTM).Numerical examples have shown that 1)two wavefield decomposition schemes are capable of extracting pure modes;2)the polarity correction scheme proposed in this article can effectively mitigate polarity reversal problem and greatly enhance imaging quality;3)the angle domain common image gathers in elastic RTM can accurately be extracted;4)elastic and viscoelastic LSRTM has the ability to improve image resolution and suppress the crosstalk artifacts;5)2D and 3D elastic RTM has the potential to retrieve high-quality subsurface reflectivity images.