We have applied multiscale deformable-layer tomography (DLT) to build a laterally varying velocity model, using a single-offset vertical seismic profile (VSP) data set acquired for a salt proximity survey in southern Texas. The purpose of the VSP survey is to delineate the 2D salt flank using the P-wave reflections. Previous study has identified an anhydrate layer as the cap rock of the salt dome. The large impedance contrasts of this anhydrite layer generate strong downgoing P (sediment)-S (anhydrite)-P (salt) waves recorded by downhole geophones. Incidentally, the P-S-P-waves have traveltimes similar to those of the P-wave salt flank reflections, thus contaminating the imaging of the salt flank. Identifying shear-mode contamination requires an accurate velocity model of anhydrite. However, the extremely poor coverage of the single-offset VSP greatly challenges tomographic techniques to determine the lateral velocity variation. We tackle this problem using multiscale DLT, which characterizes the velocity field by a set of deformable layers. We constrain the layer velocities using the check-shot data and invert for the geometric variation. The inverted model indicates that the anhydrite layer has a “thick-thin-thick” lateral variation with offset, and the S-wave in the anhydrite layer helps in imaging the P-S-P-waves along the well track. The estimated anhydrite layer geometry is validated by the kinematic accuracies of P-waves in the data domain and P-S-P-waves in the image domain. Some in-salt dipping structures are determined by multiscale DLT as well. This field data example indicates that multiscale DLT is feasible for estimating velocities using VSP data of the single-offset situation. An accurate velocity model is the key for modeling and adaptive subtraction of the shear-mode contamination related to the salt geometry.

中文翻译：

---

通过可变形层断层扫描为单偏移距 VSP 数据建立速度模型：德克萨斯盐丘示例

我们已应用多尺度可变形层断层扫描 (DLT) 来构建横向变化的速度模型，使用为德克萨斯州南部的盐附近调查获得的单偏移垂直地震剖面 (VSP) 数据集。VSP 勘测的目的是使用 P 波反射来描绘 2D 盐翼。先前的研究已经确定了一个无水层作为盐丘的盖层。该硬石膏层的大阻抗对比产生强烈的下行 P（沉积物）-S（硬石膏）-P（盐）波，由井下地震检波器记录。顺便说一下，PSP 波的走时与 P 波盐侧反射的走时相似，从而污染了盐侧的成像。识别剪切模式污染需要精确的硬石膏速度模型。然而，单偏移距 VSP 的极差覆盖极大地挑战了断层扫描技术，以确定横向速度变化。我们使用多尺度 DLT 来解决这个问题，它通过一组可变形层来表征速度场。我们使用检查镜头数据约束层速度并反转几何变化。反演模型表明硬石膏层具有随偏移量的“厚-薄-厚”横向变化，硬石膏层中的 S 波有助于沿井道对 PSP 波进行成像。估计的硬石膏层几何形状通过数据域中的 P 波和图像域中的 PSP 波的运动精度进行验证。一些盐内浸渍结构也由多尺度 DLT 确定。这个现场数据示例表明，多尺度 DLT 对于使用单偏移量情况的 VSP 数据估计速度是可行的。准确的速度模型是对与盐几何相关的剪切模式污染进行建模和自适应减法的关键。