Time-lapse seismic reservoir monitoring is used as a way to gain insight into subsurface processes. Yet, the application of standard 4D technology onshore faces challenges, such as high cost, significant environmental footprint and, consequently, relatively infrequent surveys. As part of the Otway Project Stage 3 CO₂ injection study, continuous automated borehole-based monitoring using distributed acoustic sensing has been paired with permanently deployed surface sources, referred to as surface orbital vibrators, as a way to monitor the spreading CO₂ plume. The injection of 15,000 tonnes of CO₂ in a saline reservoir at a depth of 1550 m is monitored using five boreholes instrumented with enhanced sensitivity fibre optic cables and nine surface orbital vibrators, creating an array of 45 well–source pairs. The data are processed with an offset vertical seismic profiling processing workflow developed to address key challenges of the continuous distributed acoustic sensing acquisition using surface orbital vibrators. The processing flow includes deconvolution with a source sweep recorded by a pilot geophone installed below the surface orbital vibrators. A second deconvolution with a wavelet estimated from direct arrivals compensates for the difference between distributed acoustic sensing measurements and the pilot geophone as well as near-surface variations. Image quality is noted to be best for short offsets and decreases with increasing offsets and well deviations. As surface orbital vibrators generate unique sweeps in two rotation directions, further processing is applied to stack these rotation signals together, which further improves the images. The resulting 2D transects of each well–source pair visually provide good illumination of the subsurface, suggesting continuous monitoring of the spreading CO₂ plume should be possible with some further tuning of the processing workflow for time-lapse repeatability.

中文翻译：

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使用分布式声学传感器和表面轨道振动器获取的多井偏移垂直地震剖面数据的处理：CO2CRC Otway 项目案例研究的第 3 阶段

延时地震储层监测被用作深入了解地下过程的一种方式。然而，标准 4D 技术在陆上的应用面临着挑战，例如成本高、环境足迹显着以及因此相对不频繁的调查。作为 Otway 项目第 3 阶段 CO ₂注入研究的一部分，使用分布式声学传感进行连续自动钻孔监测与永久部署的表面源（称为表面轨道振动器）配对，作为监测扩散的 CO ₂羽流的一种方式。注入 15,000 吨 CO ₂在一个 1550 米深处的盐水储层中，使用五个配备增强灵敏度光纤电缆和九个表面轨道振动器的钻孔进行监测，创建了一个由 45 个井源对组成的阵列。数据使用偏移垂直地震剖面处理工作流程进行处理，该工作流程旨在解决使用表面轨道振动器进行连续分布式声学传感采集的关键挑战。处理流程包括使用安装在地表轨道振动器下方的引导地震检波器记录的震源扫描去卷积。使用从直接到达估计的小波进行的第二次解卷积补偿了分布式声学传感测量与引导地震检波器以及近地表变化之间的差异。注意到图像质量对于短偏移是最好的，并且随着偏移和井斜的增加而降低。由于表面轨道振动器在两个旋转方向上产生独特的扫描，因此应用进一步处理将这些旋转信号叠加在一起，从而进一步改善图像。由此产生的每个井源对的 2D 横断面在视觉上为地下提供了良好的照明，表明对 CO2 扩散的持续监测₂羽应该可以通过对处理工作流程进行一些进一步调整以实现延时可重复性。