We measured anisotropic seismic properties of schists of the Homestake Formation located at a depth of 1478 m in the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, U.S.A. We deployed a 24 element, linear array of three-component geophones in an area the Homestake Mine called 19-ledge. An airless jackhammer source was used to shoot two profiles: (1) a walkaway survey to appraise any distance dependence and (2) a fan shot profile to measure variations with azimuth. Slowness estimates from the fan shot profile show a statistically significant deviation with azimuth with the expected 180° variation with azimuth. We measured P wave particle motion deviations from data rotated to ray coordinates using three methods: (1) a conventional principal component method, (2) a novel grid search method that maximized longitudinal motion over a range of search angles, and (3) the multiwavelet method. The multiwavelet results were computed in two frequency bands of 200–600 Hz and 100–300 Hz. Results were binned by azimuth and averaged with a robust estimation method with error bars estimated by a bootstrap method. The particle motion results show large, statistically significant variations with azimuth with a 180° cyclicity. We modeled the azimuthal variations in compressional wave speed and angular deviation from purely longitudinal particle motion of P waves using an elastic tensor method to appraise the relative importance of crystalline fabric relative to fracturing parallel to foliation. The model used bulk averages of crystal fabric measured for an analogous schist sample from southeast Vermont rotated to the Homestake formation foliation directions supplied by SURF from old mine records. We found with average crustal crack densities crack induced anisotropy had only a small effect on the observables. We found strong agreement in the travel time data. The observed amplitudes of deviations of P particle motion showed significantly larger variation than the model predictions and a 20° phase shift in azimuth. We attribute the inadequacies of the model fit to the particle motion data to inadequacies in the analog rock and/or near receiver distortions from smaller scale heterogeneity. We discuss the surprising variability of signals recorded in this experimental data. We show clear examples of unexplained resonances and unexpected variations on a scale much smaller than a wavelength that has broad implications for wave propagation in real rocks.

中文翻译：

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直接在霍斯特克矿山进行P波各向异性测量：波在大陆壳中传播的含义

我们在美国南达科他州黑山的桑福德地下研究设施（SURF）中测量了深度为1478 m的霍姆斯特克地层片岩的各向异性地震特性。霍姆斯特克矿区一个叫做19-ledge的区域。使用无气风炮源来拍摄两个剖面：（1）一次走人调查，以评估任何距离相关性；（2）扇形剖面来测量方位角的变化。根据扇形剖面的慢度估计值显示出，与方位角的统计偏差显着，预期的方位角会发生180°变化。我们使用三种方法测量了从旋转到射线坐标的数据到P波粒子运动的偏差：（1）传统的主成分方法，（2）一种新颖的网格搜索方法，该方法可以在搜索角度范围内最大化纵向运动，（3）多小波方法。多小波结果是在200–600 Hz和100–300 Hz的两个频带中计算的。通过方位角对结果进行分类，并使用鲁棒估计方法对结果进行平均，并使用自举方法估计误差条。质点运动结果显示，具有180°周期性的方位角具有统计学上的重大变化。我们使用弹性张量方法对压缩波速度和P波纯纵向粒子运动的角度偏差的方位角变化进行了建模，使用弹性张量法评估了晶体织物相对于平行于叶裂的相对重要性。该模型使用从佛蒙特州东南部类似片岩样品中测得的晶体织物的体积平均数，该片岩样品旋转至SURF根据旧矿记录提供的霍姆斯特克岩层页岩化方向。我们发现，以平均地壳裂缝密度而言，裂缝诱发的各向异性对可观测物的影响很小。我们在旅行时间数据中发现了很强的一致性。观测到的P粒子运动偏差幅度显示出比模型预测大得多的变化，并且方位角相移20°。我们将适用于粒子运动数据的模型不足之处归因于模拟岩石中的不足之处和/或较小规模异质性导致的近接收器变形。我们讨论了记录在该实验数据中的信号的令人惊讶的可变性。