*2.2. Principle of Reverse Time Migration*

The principle of SH shear wave reverse time migration (RTM) used in this paper is briefly introduced below:

Reverse time migration based on the solution of two-way wave equation is the most advanced pre-stack depth migration imaging. After an accurate velocity model is obtained, SH shear wave reverse time migration imaging (SH-RTM) generally includes three steps [32]:


$$I(\mathbf{x}, z) = \int\_0^T \mathcal{S}(\mathbf{x}, z, t) \mathcal{R}(\mathbf{x}, z, t) dt \tag{1}$$

where *S*(*x*, *z*, *t*) is the source wavefield obtained by forward modeling, *R*(*x*, *z*, *t*) is the receiver wavefield obtained by reverse continuation simulation at the same time under the same velocity model, and *t* is the total propagation time.

It can be seen from Equation (1) that the final result of reverse time migration is affected by the accuracy of the source wave field *S*(*x*, *z*, *t*) and the receiver wave field *R*(*x*, *z*, *t*). Therefore, in order to obtain accurate imaging of the target area, it is necessary to make good use of high-quality SH shear wave data and apply them to reverse time migration.
