Layer stripping of shear-wave splitting in marine PS waves
Layer stripping of shear-wave splitting in marine PS waves
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DOI：
10.1111/j.1365-246x.2008.04060.x
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发表时间：
2009-03
影响因子：
2.8
通讯作者：
R. Haacke;G. Westbrook;S. Peacock
中科院分区：
文献类型：
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作者：
R. Haacke;G. Westbrook;S. Peacock
分裂S波的特性可用于推断：（1）地球的应力和应变状态;（2）水力传导率的方向依赖性;（3）响应于动态过程（如地震周期）而发生的岩体中孔隙流体压力的微小变化。分裂S波的测量在浅（<1000 m海底）海洋沉积物中特别有用，其中S波分裂来自方位角弹性各向异性，通常是由于存在近垂直对齐的裂缝。在这里，我们提出了一种方法来测量少量的S波分裂海洋P到S模式转换波，并说明了技术与数据从海底地震仪（OBS）部署在西斯瓦尔巴大陆坡。该分析适用于修改后的版本的阿尔福德旋转和层剥离技术开发的零偏移距S波源和PS波，在反射器进行模式转换，是接近海底的覆盖水深相比。当地震记录在平面内和平面外分量上都包含相干信号时，层剥离技术能够将S波分裂与P波速度各向异性和反射层倾角的影响解耦，P波速度各向异性和反射层倾角影响下行P波，射线路径的一部分。数据中S波分裂的量很小，然而，并且我们发现，这对于分析来说比层剥离理论的已知理论限制（例如使用有限偏移源）引起更大的实际问题。对于示例数据的分析，我们开发了一些必要的程序，以减轻低信噪比水平。这些措施包括使用广泛的炮点接收器方位角来产生数据冗余，识别和拒绝不良测量的方法，以及预测层剥离方法，该方法通过分析最大限度地减少了由逐层结果中的分散引起的误差传播。对于主周期约为30 ms的PS波，我们发现该技术能够测量4层或5层的S波分裂，精度约为0.5 ms。如果S波分裂量大于这些数据，则成功处理的层的数量将增加，对于这些数据，在450 m深度间隔内，总累积S波分裂约为10 ms。测量了快分裂S波的方位，精度约为15 μ m。我们的研究结果给出了一个S波速度各向异性的1- 2%，在最浅的25-30米的海底，这意味着存在一个差异的水平应力，或非常接近，海底。在整个调查部分的S波分裂积累的速度是一致的预测一组平行的，充满流体的裂纹与裂纹密度约0.015。发现最高150 m处的快S波方向为75 ± 15 °，然后顺时针漂移到190-210 °的方位角。快S波偏振方向的顺时针漂移意味着近地表的一组裂缝（可能与海底坡度有关）和构造成因的深层沉积物中的另一组裂缝之间的优势转移。方位各向同性的区域占据了两组几乎正交的裂纹之间的间隔，在该区域中，产生弹性各向异性的机制可以相互抵消。从口译
SUMMARY The properties of split S waves can be used to infer: (1) the state of stress and strain in the Earth; (2) the directional dependence of hydraulic conductivity and (3) small changes in pore-fluid pressure in the rock mass that occur in response to dynamic processes, such as the earthquake cycle. Measurements of split S waves are particularly useful in shallow (<1000 m subseabed) marine sediments, where S-wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near-vertical aligned cracks. Here we present a method of measuring small amounts of S-wave splitting in marine P-to-S mode-converted waves, and illustrate the technique with data from an ocean-bottom seismometer (OBS) deployed on the west Svalbard continental slope. The analysis applies a modified version of the Alford rotation and layer-stripping technique developed for zero-offset S-wave sources and treats PS waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth. When the seismic record contains coherent signal on both the in-plane and out-of-plane components, the layer-stripping technique is capable of decoupling the Swave splitting from the effects of P-wave velocity anisotropy and reflector dip that influence the downgoing, P wave, part of the ray path. The amount of S-wave splitting in the data is small, however, and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer-stripping theory (such as use of a finite-offset source). For the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal-to-noise levels. These include using a wide range of shot-receiver azimuths to generate data redundancy, methods of identifying and rejecting poor measurements, and a predictive layer-stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer-by-layer results. With the PS waves of the example data, which have a dominant period about 30 ms, we find the technique is capable of measuring S-wave splitting to a precision of about 0.5 ms for 4 or 5 layers. The number of layers successfully treated would increase if the amount of S-wave splitting were larger than in these data, for which the total cumulative S-wave splitting was about 10 ms over a 450 m depth interval. The orientation of the fast split S wave was measured with a precision of about 15 ◦ . Our results give an S-wave velocity anisotropy of 1–2 per cent in the shallowest 25–30 m subseabed that implies the presence of a differential horizontal stress at, or very close to, the seabed. The S-wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel, fluid-filled cracks with crack-density about 0.015. The fast S wave was found to be oriented at 75 ± 15 ◦ for the uppermost 150 m, before drifting clockwise to an azimuth of 190–210 ◦ . The clockwise drift in the fast S-wave polarization direction implies a transfer of dominance between one set of cracks in the near surface (probably related to the slope of the seabed) and a different set in deeper sediments of tectonic origin. A zone of azimuthal isotropy, where the mechanisms that produce the elastic anisotropy may cancel each other out, occupies the interval between the two almost orthogonal sets of cracks. From interpretation