Assessment of resolution and accuracy of the Moving Window Cross Spectral technique for monitoring crustal temporal variations using ambient seismic noise
Assessment of resolution and accuracy of the Moving Window Cross Spectral technique for monitoring crustal temporal variations using ambient seismic noise
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DOI：
10.1111/j.1365-246x.2011.05074.x
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发表时间：
2011-08-01
影响因子：
2.8
通讯作者：
Brenguier, F.
中科院分区：
文献类型：
--
作者：
Clarke, D.;Zaccarelli, L.;Brenguier, F.
通过分析地球的地震响应及其随时间的演变，可以监测地壳弹性行为的时间变化。这种分析在与从环境地震噪声的互相关中重建地震格林函数相结合时特别有趣，它绕过了对地震事件发生的依赖所施加的限制。事实上，由于地震噪声是连续记录的，不依赖于震源，因此这些互相关函数可以类似于在每个台站放置连续重复的双源的记录，并且可以用来提取地震速度变化的观测值。然而，这些变化通常非常小：大约为0.1%。这种准确性只能通过分析完全重建的波形来实现，包括后来的散射到达。我们重点研究了移动窗口互谱分析方法，该方法具有在频域中工作的优点，可以清楚地定义相干信号在相关函数中的带宽。我们将该方法应用于微震噪声互相关中，考察了该方法的敏感性。微震噪声互相关已经被微小的合成速度变化所扰动，并且被随机污染。我们提出了阈值信噪比，超过这个阈值，就可以可靠地观察到这些扰动。这些值是互相关收敛的指标，因此在确定微震噪声记录的长度时，可以将其用作准则，这些记录在可以用于移动窗口互谱技术的监测之前是必需的。
Temporal variations in the elastic behaviour of the Earth's crust can be monitored through the analysis of the Earth's seismic response and its evolution with time. This kind of analysis is particularly interesting when combined with the reconstruction of seismic Green's functions from the cross-correlation of ambient seismic noise, which circumvents the limitations imposed by a dependence on the occurrence of seismic events. In fact, because seismic noise is recorded continuously and does not depend on earthquake sources, these cross-correlation functions can be considered analogously to records from continuously repeating doublet sources placed at each station, and can be used to extract observations of variations in seismic velocities. These variations, however, are typically very small: of the order of 0.1 per cent. Such accuracy can be only achieved through the analysis of the full reconstructed waveforms, including later scattered arrivals. We focus on the method known as Moving-Window Cross-Spectral Analysis that has the advantage of operating in the frequency domain, where the bandwidth of coherent signal in the correlation function can be clearly defined. We investigate the sensitivity of this method by applying it to microseismic noise cross-correlations which have been perturbed by small synthetic velocity variations and which have been randomly contaminated. We propose threshold signal-to-noise ratios above which these perturbations can be reliably observed. Such values are a proxy for cross-correlation convergence, and so can be used as a guideline when determining the length of microseismic noise records that are required before they can be used for monitoring with the moving-window cross-spectral technique.