Wavefield separation is a crucial step in suppressing crosstalk artifacts and improving imaging quality in elastic reverse time migration (ERTM). In isotropic elastic media, the most popular wavefield separation technique is the Helmholtz decomposition. However, the Helmholtz decomposition produces pure-mode wavefields with incorrect amplitudes, phases, and physical units. In addition, when we implement multi-shot ERTM using pure-mode wavefields, polarity changes in the converted wave can destroy reflection events. To remedy these problems, we modify the Helmholtz decomposition and apply a scalar imaging condition to obtain scalar images of ERTM with accurate amplitudes and phases. Instead of using the P- and S-wave decoupled elastic wave equations, we produce vector P- and S-wavefields through the following two steps. First, we correct the amplitudes and phases of the pure-mode wavefield during the Helmholtz decomposition. Second, we obtain vector P- and S-wavefields using a wavefield composition method. Using the separated vector wavefields, we adopt a scalar imaging condition for ERTM. These operations enable us to produce migrated images directly with the correct amplitudes, phase, and physical units. Numerical results demonstrate that the proposed ERTM provides high-quality migrated images. The method is also feasible and sufficiently robust for imaging complex subsurface structures.


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