Fiber optic distributed acoustic sensors (DAS) are becoming a widely used tool for seismic sensing. Here we examine recordings of two subsurface chemical explosions, DAG-1 and DAG-3, each of which was about one metric ton (TNT equivalent), that were recorded from a helical fiber installed in two boreholes 80 m away from the source location. Several clear phases including the initial P wave, a weak S wave, and a surface reflected P wave are observed on the helical DAS data. We estimate a velocity model using arrival times measured from the fiber. The DAS waveform data were compared with colocated accelerometers at specific depths in both frequency and time domains. The spectra of the DAS data matched spectra estimated from the accelerometer records. Comparisons of observed waveform shape between the accelerometer records and the fiber measurements (strain-rate) show reasonable agreement except for the data near the event depth. The DAS data and the accelerometer agreed in relative amplitudes but we had difficulties in matching absolute amplitudes, possibly due to errors in metadata. Synthetic strain-rate waveforms were calculated using a 2D wavenumber algorithm and matched the waveform shape and relative amplitudes. In general, DAS is effective at recording strong ground motions at high spatial density. Comparison of the synthetic seismograms with observed data indicate that the waveforms are not consistent with a pure isotropic explosion source and that the observed S waves originate from very near the source region.

中文翻译：

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模拟记录在分布式光纤传感器上的地下爆炸

光纤分布式声学传感器 (DAS) 正在成为一种广泛使用的地震传感工具。在这里，我们检查了两个地下化学爆炸的记录，DAG-1 和 DAG-3，每个大约一公吨（TNT 当量），它们是从安装在距源位置 80 m 的两个钻孔中的螺旋光纤记录的。在螺旋 DAS 数据上观察到几个清晰的相位，包括初始 P 波、弱 S 波和表面反射 P 波。我们使用从光纤测量的到达时间来估计速度模型。DAS 波形数据在频域和时域的特定深度与共置加速度计进行了比较。DAS 数据的光谱与根据加速度计记录估计的光谱相匹配。除了事件深度附近的数据外，加速度计记录和光纤测量（应变率）之间观察到的波形形状的比较显示出合理的一致性。DAS 数据和加速度计在相对幅度上一致，但我们很难匹配绝对幅度，可能是由于元数据中的错误。使用二维波数算法计算合成应变率波形，并匹配波形形状和相对幅度。一般而言，DAS 可有效记录高空间密度下的强烈地面运动。合成地震图与观测数据的比较表明波形与纯各向同性爆炸源不一致，并且观测到的 S 波起源于非常靠近源区域。