Walkaway VSP cannot obtain accurate velocity field, as it asymmetrically reflects ray path and provides uneven coverage to underground target, thereby presenting issues related to imaging quality. In this study, we propose combining traveltime tomography and prestack depth migration for VSP of an angle-domain walkaway, in a bid to establish accurate two-dimensional and three-dimensional (3D) velocity models. First, residual curvature was defined to update velocity, and an accurate velocity field was established. To establish a high-precision velocity model, we deduced the relationship between the residual depth and traveltime of common imaging gathers (CIGs) in walkaway VSP. Solving renewal velocity using the least squares method, a four-parameter tomographic inversion equation was derived comprising formation dip angle, incidence angle, residual depth, and sensitivity matrix. In the angle domain, the reflected wave was divided into up- and down-transmitted waves and their traveltimes were calculated. The systematic cumulative method was employed in prestack depth migration of a complex surface. Through prestack depth migration, the offset-domain CIGs were obtained, and dip angle was established by defining the stack section horizon. Runge—Kutta ray tracing was employed to calculate the ray path from the reflection point to the detection point, to determine the incident angle, and to subsequently calculate the ray path from the reflection point to the irregular surface. The offset-domain residual depths were mapped to the angle domain, and a new tomographic equation was established and solved. Application in the double complex area of the Tarim Basin showed the four-parameter tomographic inversion equation derived in this paper to be both correct and practical and that the migration algorithm was able to adapt to the complex surface.

中文翻译：

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用于复杂表面上角域走动的垂直地震剖面的行进时间层析成像和叠前深度偏移

步入式VSP无法获得准确的速度场，因为它不对称地反射光线路径并为地下目标提供不均匀的覆盖范围，从而带来与成像质量有关的问题。在这项研究中，我们建议结合行进时间层析成像和叠前深度偏移进行角域走动的VSP，以建立准确的二维和三维（3D）速度模型。首先，定义残留曲率以更新速度，并建立准确的速度场。为了建立高精度的速度模型，我们推导了走人式VSP中残差深度与常用成像道集（CIG）的行进时间之间的关系。使用最小二乘法求解更新速度，推导了包括地层倾角，入射角，剩余深度和灵敏度矩阵。在角域，将反射波分为上下传输波，并计算其传播时间。在复杂表面的叠前深度偏移中采用了系统的累积方法。通过叠前深度偏移，获得了偏移域的CIG，并通过定义叠层剖面水平线确定了倾角。使用Runge-Kutta光线跟踪来计算从反射点到检测点的光线路径，确定入射角，并随后计算从反射点到不规则表面的光线路径。将偏移域的剩余深度映射到角度域，建立并求解了新的层析成像方程。