基于两种波场分离方法的弹性波逆时偏移对比研究

弹性波逆时偏移是一种以矢量波理论为基础的深度域偏移算法,该算法在波场传播过程中能够正确处理波形能量的转换,保持地震波的动力学和运动学特征.然而,传统弹性波逆时偏移由于成像时纵横波未分离,导致串扰噪声的产生,降低成像精度.为压制弹性波逆时偏移成像串扰噪声,本文对基于Helmholtz分解的波场分离与基于解耦方程的波场分离方法进行研究和分析,并测试对比相应的弹性波逆时偏移成像效果.均匀各向同性介质波场分离测试表明,基于Helmholtz分解分离波场的振幅与相位均发生改变,需要额外校正;而基于解耦方程的波场分离方法可以保持原始波场相位与振幅特征不变.Marmousi截断模型测试表明,基于Helmholtz分解分离波场的弹性波逆时偏移成像振幅与相位发生改变,且存在极性反转现象;基于解耦方程波场分离方法的弹性波逆时偏移成像保持原始波场矢量特征,无极性反转现象,成像效果较为理想.

Comparative study on elastic reverse time migration based on two methods of wavesfield separation

Elastic wave reverse time migration is a depth domain migration algorithm based on vector wave theory.It preserves the vector characteristics of elastic waves,and can also correctly handle the conversion of wave energy during propagation,keeping the dynamics and kinematics characteristics of seismic waves,which is more in line with the actual propagation process of seismic waves.However,the conventional elastic wave reverse time migration causes the generation of crosstalk noise due to the fact that the P wave and S wave are not separated during the imaging progress,so that the imaging accuracy is damaged.In order to suppress crosstalk noise,wavefield separation is proposed to extract pure wave profile.The commonly used wavefield separation methods are the wavefield separation based on Helmholtz decomposition and the wavefield separation based on decoupling equation.In this paper,the separation results of the two wavefield separation methods are compared and analyzed,and the corresponding elastic wave reverse time migration imaging effect is tested.Two wavefield separation methods are tested for homogeneous isotropic media,and the test results show that the amplitude and phase of the separated wavefield obtained by wavefield separation based on Helmholtz decomposition change,which need to additional correction,and wavefield separation based on decoupling equation can keep the phase and amplitude characteristics of the original vector wavefield unchanged.The elastic wave reverse time migration test for Marmousi truncated model shows that the amplitude and phase of reverse time migration imaging based on Helmholtz decomposition wavefield separation change,and there is polarity reversal phenomenon,which need to be corrected.Aiming at the problem of polarity reversal,the correction method in the common shot domain is studied.Based on the decoupled equation,the elastic wave reverse time migration imaging results maintain the original wavefield vector characteristics,and has ideal imaging accuracy without polarity reversal phenomenon.