Vertical seismic profiling (VSP) can provide more abundant seismic wavefield information and better seismic data with high resolution and high quality for the complex underground geological structures compared with surface seismic data. Reverse time migration (RTM) method possesses significant advantages for the accurate identification of complex geological structures, and it’s considered to be the most accurate imaging method at present. Therefore, we develop a variable density acoustic RTM method which is applicable for VSP data to enhance the recognition capability of complex geological structures, and we also discuss different aspects of this proposed imaging method. Firstly, to effectively improve the modeling precision of seismic wavefields, the wavefield extrapolation of our VSP RTM method is realized by using an optimal staggered-grid finite difference (SFD) method to solve the variable density acoustic wave equation, because this optimal SFD method uses the least square (LS) method to optimize the objective function established by the time–space domain dispersion relation to estimate its difference coefficients. In other words, the time–space domain LS-based SFD method has higher numerical simulation accuracy for seismic modeling. Secondly, to effectively reduce the boundary reflections and storage requirements of our VSP RTM method, we adopt the PML absorbing boundary and the effective boundary storage strategy in the process of wavefield extrapolation. Finally, to strengthen the quality and precision of VSP RTM results, the depth imaging profile of a shot is calculated by the normalized cross-correlation imaging condition of sources which can effectively eliminate the source effects on RTM results, and Laplace filtering is applied to eliminate the imaging noises in final RTM results effectively. The imaging results of different models show the effectiveness of our RTM method for VSP data, and it can more accurately identify the complex underground geological structures compared with the RTM method for conventional surface seismic data.

与地面地震资料相比，垂直地震剖面(VSP)可以为复杂的地下地质构造提供更丰富的地震波场信息和更好的高分辨率、高质量的地震资料。逆时偏移(RTM)方法对于复杂地质构造的准确识别具有显着优势，被认为是目前最准确的成像方法。因此，我们开发了一种适用于 VSP 数据的变密度声学 RTM 方法，以增强复杂地质结构的识别能力，并且我们还讨论了该成像方法的不同方面。一是有效提高地震波场建模精度，我们的 VSP RTM 方法的波场外推是通过使用最佳交错网格有限差分 (SFD) 方法求解变密度声波方程来实现的，因为这种最佳 SFD 方法使用最小二乘 (LS) 方法来优化目标函数由时空域色散关系建立，估计其差异系数。换言之，基于时空域LS的SFD方法对地震建模具有更高的数值模拟精度。其次，为了有效降低我们的 VSP RTM 方法的边界反射和存储要求，我们在波场外推过程中采用了 PML 吸收边界和有效边界存储策略。最后，为了加强 VSP RTM 结果的质量和精度，通过归一化源的互相关成像条件计算出炮点的深度成像轮廓，可以有效消除源对RTM结果的影响，并应用拉普拉斯滤波有效消除最终RTM结果中的成像噪声。不同模型的成像结果表明了我们的RTM方法对VSP数据的有效性，与常规地面地震数据的RTM方法相比，它可以更准确地识别复杂的地下地质结构。