**1. Introduction**

Currently, the main means of oil-gas exploration are mainly based on the P-wave (compressional wave). A series of mature technologies developed for P-wave exploration, including data acquisition, data pre-processing, pre-stack time migration, and pre-stack depth migration, have made great progress and are still making progress. However, with the extension of exploration to low permeability, deep and unconventional reservoirs and the increasingly complex exploration targets, relying solely on P-wave technology is facing more and more challenges. Through an analysis of the limitations of P-wave exploration and the potential of shear wave exploration, Gou et al. [1] proposed the effectiveness and applicability of SH shear wave (horizontally polarized shear wave) exploration technology and conducted SH shear wave data acquisition and processing tests in an exploration area in Western China [2].

Generally speaking, the resolution of seismic exploration is a quarter of the wavelength, which depends on the velocity and frequency. As the wave velocity of subsurface media is objective and certain, the resolution can only be improved by increasing the frequency. In terms of tight reservoirs or deep carbonate reservoirs, the P-wave velocity can be higher than 5000 m/s, and the effective frequency will be reduced to about 30 Hz with the attenuation of deep high-frequency components, which is very unfavorable for the identification of deep thin layers [3,4]. The further improvement of the effective frequency band in the deep layer will face great challenges in source excitation. If the frequency band is consistent, the wave speed of the shear wave is usually about half that of the P-wave,

**Citation:** Zhou, C.; Yin, W.; Yang, J.; Nie, H.; Li, X. Reverse Time Migration Imaging Using SH Shear Wave Data. *Appl. Sci.* **2022**, *12*, 9944. https://doi.org/10.3390/app12199944

Academic Editors: Guofeng Liu, Zhifu Zhang and Xiaohong Meng

Received: 9 August 2022 Accepted: 30 September 2022 Published: 3 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and the resolution can reach twice that of the P-wave. Shear-wave exploration is one of the effective ways to improve the resolution [5–7].

In recent years, with the rapid development of computer calculation, reverse time migration has gradually become an industrial applied imaging technology used to solve the imaging of various complex structures. Compared with that of the P-wave, the shear wave velocity is low and the frequency band is wide, such as carbonate rocks (Takougang et al., 2020) [8], which can improve the resolution of the formation; the shear wave propagation is only related to the rock frame, which can be used to accurately construct the subsurface structure. In terms of the near offset, the shear wave has a higher signal-to-noise ratio than the P-wave, so it has higher inversion precision [9–16]. To improve the accuracy and efficiency of shear-wave pre-stack depth migration, it is necessary to apply a high-precision finite-difference scheme to shear wave seismic imaging on the basis of the low-speed characteristics of the shear wave, and this method needs to maintain good accuracy of spatial difference and time difference under the condition of a large spatial grid and time step. Thus, migration algorithm selection is one of the subjects discussed in this paper.

Since the mid-1990s, reverse time migration has been applied to multi-component wave seismic data excited from an elastic-wave source [17–20], and it has overcome calculation problems and interference artifacts in P- and shear wave simulation. In recent years, most studies have focused on the P- and shear wave decoupled method to separate P- and shear waves [21–28]. In these studies, there is no case of the direct use of shear wave data for reverse time migration, but the elastic wave field was used to separate P- and shear waves or decoupled P- and shear wave fields obtained from the elastic-wave equation. This paper will focus on the direct use of SH shear wave source data for reverse time migration imaging and together with corresponding procedures for velocity analysis and velocity model building.

The study area, Sanhu, referred to in this paper, is located in the Qaidam Basin. The reservoir in the area is mainly consisted of thin sand-mudstone interbed and unconsolidated sandstone reservoirs, and this area is the largest Quaternary biogas production area discovered in China. Affected by the reservoir gas, the reservoir imaging is seriously blurred and distorted. To determine the characteristics of the study area, in this paper, the SH shear wave velocity model in the depth domain is established by using the SH shear wave data in the survey, and the subsurface reflecting interface in this area is imaged by using the SH shear wave reverse time migration imaging technology [29,30]. During depth domain velocity model building, we obtain the P-to-SH shear wave velocity ratio in the area from the time domain P-wave and SH shear wave imaging results, which is used to constrain the overall P- and SH shear wave depth domain velocity model building.
