Scalar PP and PS imaging of elastic RTM by wavefield decoupling method

To overcome the crosstalk between different wave modes in elastic RTM, we need separate the forward and inverse wavefield into P- and S-wave. The wavefields decoupling method can maintain the dynamic characteristics of separated wavefield, so it is already widely used in elastic RTM. However, these decomposed wavefields are vectors, and thus, we must consider how to retrieve scalar PP and PS images from these vectors. For this purpose, we derive a scalar P-wave equation from the velocity-stress relationship for PP imaging. The phase and amplitude of this scalar P-wave are consistent with the theoretical scalar field. Therefore, this scalar P-wave can be exploited to perform PP imaging directly, and the PP imaging result can retain the wavefield dynamic characteristics. For PS imaging, it is difficult to calculate an accurate scalar S-wave. However, we also propose a scalar PS imaging method which divides the PS image into energy and sign components according to the geometric relationship between the wavefield vibration and propagation directions. These PP and PS imaging methods can maintain the correct phase and amplitude information while eliminating polarity reversal phenomena. Several numerical models are used to verify the robustness and effectiveness of our method.