We investigated temporal variations of seismic wave velocity associated with hydraulic fracturing using Green's functions computed from ambient noise data. In October and November of 2014, we set up a broadband array at a shale play site inside the Sichuan basin where a pilot horizontal drilling and hydraulic injections were conducted. We first computed cross‐correlation functions using continuous data recorded by 21 three‐component broadband sensors deployed around the treatment well. We then employed a running window correlation‐based coda wave interferometry technique to measure apparent velocity changes from the daily Green's functions of all the station pairs in the frequency range of 1 to 3 Hz. We found significant velocity changes right after the hydraulic fracturing, which exhibited a clear direction‐dependent pattern. S wave velocity along raypaths parallel to the well trajectory showed a clear increase while those perpendicular exhibited a small decrease. The anisotropic changes in seismic velocity observed here were also confirmed from surface wave horizontal particle motion data. By comparing our observations with normal stress changes calculated with a half‐space elastic model, we speculate that stress changes induced by the hydraulic fracturing were likely to be responsible for the observed anisotropic changes in seismic velocity. Our results suggest that time‐lapse seismic imaging with ambient noise data provides a promising probe for monitoring geomechanical changes related to exploitation of unconventional oil and gas resources.

中文翻译：

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西南页岩游动场中水力注入引起的速度变化由表面波尾波和极化数据揭示

我们使用根据环境噪声数据计算出的格林函数研究了与水力压裂有关的地震波速度的时间变化。2014年10月和2014年11月，我们在四川盆地内的一个页岩游乐区建立了一个宽带阵列，在那里进行了水平钻井和水力喷射试验。我们首先使用部署在治疗井周围的21个三分量宽带传感器记录的连续数据来计算互相关函数。然后，我们采用了基于运行窗口相关性的尾波干涉测量技术，在1至3 Hz的频率范围内，根据所有电台对的每日格林函数测量视在速度变化。在水力压裂之后，我们发现了明显的速度变化，表现出清晰的方向相关模式。沿平行于井眼轨迹的射线路径的S波速度显示出明显的增加，而垂直于井眼轨迹的S波速度显示出较小的减小。从表面波水平粒子运动数据也证实了这里观测到的地震速度的各向异性变化。通过将我们的观察结果与使用半空间弹性模型计算的法向应力变化进行比较，我们推测由水力压裂引起的应力变化很可能是引起地震速度各向异性变化的原因。我们的结果表明，具有环境噪声数据的延时地震成像为监测与非常规油气资源开采有关的岩土力学变化提供了有希望的探针。