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首页> 外文期刊>Pure and Applied Geophysics >Small Scale Earthquake Mechanisms Induced by Fluid Injection at the Enhanced Geothermal System Reservoir Soultz (Alsace) in 2003 using Alternative Source Models
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Small Scale Earthquake Mechanisms Induced by Fluid Injection at the Enhanced Geothermal System Reservoir Soultz (Alsace) in 2003 using Alternative Source Models

机译：使用替代震源模型，2003年增强地热系统储层苏尔兹（阿尔萨斯）注水引起的小规模地震机制

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The geothermal reservoir at Soultz-sous-Forêts is a valuable natural laboratory for understanding the mechanisms of microearthquakes generated during stimulations and circulation tests. An ongoing effort currently exists regarding the retrieval of mechanisms aimed to indicate the type of fracturing of the rock massif. As a default, a moment tensor description has been applied. Nevertheless, the retrieval of the mode of fracturing still remains ambiguous. Recent studies indicate a prevailing shear slip but, rarely, a non-shear pattern has also been observed. The moment tensor, used today as a universal tool for descriptions of the mechanism, captures general balanced dipole sources. However, in the case of small-scale earthquakes, the moment tensor need not always be reliably determined. In an effort to fit the data, there may be notable non-shear components caused by the low quality of input data. Constraining the source model to directly determine a simpler one is convenient for describing the physical phenomena expected for a particular focus. An opening of new fractures can be described, to a first approximation, by a tensile crack, optionally combined with a shear slip. Such an alternative model is called a shear-tensile crack (STC) source model. The combination is practical, and can be used to both identify events that reflect purely mode-I (tensile) failure and to determine the dilation angle of the fracture undergoing shear. The latter is particularly important in enhanced geothermal system reservoirs such as Soultz, where shear-related dilation is believed to be the primary mechanism underpinning permeability creation during stimulation injections. We performed a synthetic case study by simulating seismic data as recorded by the actual seismic array installed at Soultz-sous-Forêts. Synthetic P and S amplitudes for several shear-tensile source models were inverted for several types of station coverage. The analysis explored how results were influenced by mislocation, mismodeling, and noise contamination of data. In most cases, the orientation of the mechanism was well resolved. Determination of shear vs. non-shear content within the mechanism was more difficult. From all of the factors influencing resolution that we explored, the quality of the monitoring system (the number of stations and their distribution with respect to the focus) and noise contamination were of the highest impact. The STC source model yielded considerably less spurious non-shear fracture components than the moment tensor. From the bulk of the seismicity recorded during the stimulation in 2003, we concentrated on the first phase of the injection when only a single borehole at the site was stimulated. We processed thirteen small earthquakes with magnitudes larger than 1.4 that were not treated in previous studies. We determined that their source mechanisms were dominantly pure shear slips on pre-existing faults, just as the earthquakes investigated earlier were. The results were also in agreement with the stress pattern from in situ measurements.
机译：Soultz-sous-Forêts的地热储层是一个有价值的自然实验室，用于了解在增产和循环测试过程中产生的微地震的机理。当前正在进行有关检索机制的工作，这些机制旨在指示岩石地块的破裂类型。默认情况下，已应用矩张量描述。然而，压裂方式的检索仍然是模棱两可的。最近的研究表明存在普遍的剪切滑移，但是很少观察到非剪切模式。如今，矩张量用作描述机理的通用工具，它捕获了一般的平衡偶极子源。然而，在小规模地震的情况下，矩张量不必总是可靠地确定。为了拟合数据，可能会由于输入数据质量低而引起明显的非剪切成分。约束源模型以直接确定一个更简单的模型，对于描述特定焦点预期的物理现象很方便。可以通过拉伸裂纹（可选地结合剪切滑移）来大致描述新裂缝的出现。这种替代模型称为剪切拉伸裂纹（STC）源模型。该组合是实用的，并且可用于识别仅反映I型（拉伸）破坏的事件并确定经历剪切的裂缝的扩张角。后者在增强的地热系统储层（例如Soultz）中尤为重要，据信与剪切有关的膨胀是增产注入过程中支撑渗透性产生的主要机制。我们通过模拟安装在Soultz-sous-Forêts的实际地震阵列记录的地震数据进行了综合案例研究。对于几种类型的台站覆盖范围，将几种剪切拉伸源模型的合成P和S振幅反转了。该分析探讨了错误的位置，错误的建模以及数据的噪声污染如何影响结果。在大多数情况下，该机制的定位已得到很好的解决。确定机理中的剪切与非剪切含量比较困难。从我们探索的影响分辨率的所有因素来看，监视系统的质量（站点的数量及其相对于焦点的分布）和噪声污染的影响最大。与力矩张量相比，STC源模型产生的伪非剪切断裂分量要少得多。从2003年增产过程中记录的大部分地震活动中，我们只注视了该位置的一个单一井眼时的注水第一阶段。我们处理了13场大于1.4的小地震，这些地震在以前的研究中没有得到处理。我们确定它们的震源机制主要是在已存在断层上的纯剪切滑动，就像之前调查的地震一样。结果也与现场测量的应力模式一致。

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