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Imaging condition for elastic RTM

Polarity changes in converted-wave images constructed by elastic reverse-time migration causes destructive interference after stacking over the experiments of a seismic survey. The polarity reversal is due to the fact that S-mode polarization changes as a function of direction, and therefore the imaged reflectivity reverses sign at normal incidence. 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 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 reverse-time migration. Several numerical examples demonstrate the effectiveness of our new imaging condition in simple and complex models.