Converted-waves imaging condition for elastic reverse-time migration

Polarity changes in converted-wave images constructed by elastic reverse-time migration causes destructive interference after stacking over the experiments of a seismic survey. We derive a simple imaging condition for converted waves imaging designed to correct the image polarity and reveal the conversion strength from one wave mode to another. Our imaging condition exploits pure Pand S-modes obtained by Helmholtz decomposition. Instead of correlating components of the vector S-mode with the scalar P-mode, we exploit the entire S wavefield at once to produce a unique image. We generate PS and SP image using geometrical relationships between the propagation directions for the P and S wavefields, the reflector orientation, and the S-mode polarization direction. Compared to alternative methods for correcting PS and SP images polarity reversal, our imaging condition is simple and robust and does not add significantly to the cost of elastic reverse-time migration.