Fractures within the earth control rock strength and fluid flow, but their dynamic nature is not well understood. As part of a series of underground chemical explosions in granite in Nevada, we collected and analyzed microfracture density data sets prior to, and following, individual explosions. Our work shows an ∼4-fold increase in both open and filled microfractures following the explosions. Based on the timing of core retrieval, filling of some new fractures occurs in as little as 6 wk after fracture opening under shallow (<100 m) crustal conditions. These results suggest that near-surface fractures may fill quite rapidly, potentially changing permeability on time scales relevant to oil, gas, and geothermal energy production; carbon sequestration; seismic cycles; and radionuclide migration from nuclear waste storage and underground nuclear explosions. INTRODUCTION The time required for rock fractures to fill and seal is poorly constrained, particularly in shallow crustal environments. Fracture apertures, which are reduced by filling and sealing, are important to oil, gas, and geothermal energy production (Hippler, 1993; Boles et al., 2004; Dempsey et al., 2013), carbon sequestration (Shukla et al., 2010), and radionuclide migration from nuclear waste storage (Ticknor et al., 1989; Moreno and Neretnieks, 1993) and underground nuclear explosion detection, monitoring, and characterization (Jordan et al., 2015). While fracture filling is often assumed to be on “geologic time scales” (e.g., Lander and Laubach, 2015), some studies have suggested that sealing on the order of 101–104 yr could better explain the timing of seismic cycles in tectonically loaded faults (e.g., Blanpied et al., 1992; Byerlee, 1993; Chester et al., 1993). Other fieldscale studies have suggested even faster sealing rates of months to years, based on transient changes in seismic velocities (Tadokoro and Ando, 2002; Hiramatsu et al., 2005; Li, 2006) and permeabilities (Rojstaczer and Wolf, 1992; Claesson et al., 2007; Xue et al., 2013; Wästeby et al., 2014) following earthquakes, but the mechanism by which these changes occur (i.e., how fractures might be closing) is not observed with these techniques. Laboratory studies have shed light onto the potential processes by which fractures can seal, including crack filling due to mineral precipitation at temperatures of 150–1000 °C (Morrow et al., 1981, 2001; Moore et al., 1994; Brantley et al., 1990; Tenthorey et al., 2003), removal of asperities through dissolution/reprecipitation (Beeler and Hickman, 2004; Gratier et al., 2013; Aben et al., 2017), or asperity contact yielding without fluids (Dieterich and Kilgore, 1994), which can lead to reduced permeability (e.g., Morrow et al., 1981; Moore et al., 1994). However, the time scales and relative importance of each of these processes are not well quantified at crustal conditions, particularly in low-temperature, near-surface conditions. Improving our understanding of fracture filling and sealing rate requires isolating a single deformation event. Earthquake prediction is so difficult that it precludes direct comparison of pre-earthquake fracture properties with a time series of postearthquake fracture properties. Thus, the Source Physics Experiment (SPE), a series of underground chemical explosions in granite in Nevada, provided a unique opportunity to collect core before and after a (nontectonic) damage event. METHODS As part of an effort to understand the role of damage on seismic signatures of underground chemical explosions (Snelson et al., 2013), core was collected prior to any chemical explosions (“pre-ex,” U15n) and after each of the larger two explosions (“post-ex,” U15n#10, U15n#12, and U15n#13; Table 1; locations in Fig. 1). These explosions were conducted in the quartz monzonite member of the Climax Stock granite, located on the arid Nevada National Security Site. Air foam, and not drilling mud, was used during drilling. U15n core was collected first, and then the borehole was enlarged to enable emplacement of the explosives. U15n#10 core was collected 6 wk after the SPE-2 explosion; U15n#12 and U15n#13 cores were collected 18 and 21 wk, respectively, after the SPE-3 explosion, and more than a year after SPE-2. Cores U15n#10 and U15n#12 were drilled at an angle of ∼30° from vertical (to ensure intersection with the explosive source region), while U15n and U15n#13 were drilled vertically (Fig. 1). All core was delivered to and stored at the U.S. Geological Survey (USGS) Data Center and Core Library in Mercury, Nevada. Samples for microstructural analyses were selected from intervals approximately every 6 m along each core, and three mutually perpendicular thin sections were made from each sample. These are the same thin sections used to define the extent of damage by Swanson et al. (2018), and more information about thin section preparation and orientations can be found there and in the GSA Data Repository1. For each thin section, transgranular microfractures were characterized as open or filled (see Data Repository Table DR1, Figure DR1, and text for classification criteria), and their *E-mail: emswanson@lanl.gov 1GSA Data Repository item 2019399, detailed methodology, including a figure showing open versus filled microfractures; a table with the data presented here; and a plot showing the fraction of filled microfractures, is available online at http://www.geosociety.org/datarepository/2019/, or on request from editing@geosociety.org. †Current address: Sandia National Laboratories, Albuquerque, New Mexico 87185, USA. Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46957.1/4848234/g46957.pdf by guest on 02 November 2019 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America respective linear densities were measured using the same technique utilized by Swanson et al. (2018) and described in detail in the Data Repository. For each sample (i.e., depth), the microfracture density values for each of the three orientations of thin sections were compared, and the largest microfracture density was chosen for comparison, to facilitate identification of potentially oriented damage. The core was not oriented, but a discussion of orientations of mesoscale fractures can be found in Swanson et al. (2018). To determine if fracture fill was pulverized host rock or newly precipitated material, we collected images on a few select samples using an FEI Inspect scanning electron microscope (SEM), using a 15 kV beam. Preparation included sputter coating the samples with gold as the conductive layer. Elemental analyses were performed using energy dispersive spectra (EDS). Elements were identified using EDAX EDS software and quantified using a standardless ZAF quantification, which accounts for effects due to atomic number (Z), absorption (A), and fluorescence (F). We use the term “filled” to describe fractures with visible amounts of material in them, in contrast with the term “sealed” used in previous studies (e.g., Wilson et al., 2003; Mitchell and Faulkner, 2009), to avoid the implication that these fractures are impermeable or have recovered their strength. However, fracture fill is a step toward sealing, and some permeability loss and/or strength recovery may have occurred (relative to open fractures), and with time, these fractures may seal completely. RESULTS We compared open microfracture density to depth for all samples (Fig. 2A). In the pre-ex core, U15n, open microfracture densities were all relatively low, with fewer than 1 microfracture per millimeter (mf/mm) for every sample. The pre-ex sample containing the most fractures was collected near a previously identified minor fault at 26.5 m depth (Townsend et al., 2012). The post-ex cores U15n#10, U15n#12, and U15n#13 showed higher and more variable TABLE 1. CHRONOLOGY OF CORE DRILLING AND SOURCE PHYSICS EXPERIMENT (SPE) EXPLOSIONS Core name Drill completion date Explosion name and size Explosion depth (m) Explosion date U15n 28 July 2010 SPE-1, 88 kg 54.9 3 May 2011 SPE-2, 997 kg 45.7 25 October 2011 U15n#10 9 December 2011 SPE-3, 905 kg 45.8 24 July 2012 U15n#12 26 November 2012 U15n#13 17 December 2012 Note: Earliest events are at the top. Explosion sizes are in kilograms of trinitrotoluene (TNT) equivalent. Figure 1. Sample locations. (A) Simplified geologic map of Climax Stock, modified from Townsend et al. (2012). (B) Location within Nevada National Security Site (green outline); CA—California; SPE—Source Physics Experiment. (C) Cross-section X-X′ showing sample locations (colored ellipses). Ellipses are colored by core, with size scaled to total (open + filled) transgranular microfractures. Damage extents from SPE-2 and SPE-3 explosions (from Swanson et al., 2018) are indicated by orange-shaded circle and box. (D) Planview image at same scale as C, showing borehole collar locations. White circles indicate distance from U15n. U-15n#10

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爆炸诱导裂缝中的快速粘土沉淀

地球内部的裂缝控制着岩石强度和流体流动，但它们的动态性质尚不清楚。作为内华达州花岗岩地下一系列化学爆炸的一部分，我们收集并分析了个别爆炸前后的微裂缝密度数据集。我们的工作表明，爆炸后开放和填充的微裂缝均增加了约 4 倍。根据取芯的时间，在浅层（<100 m）地壳条件下，裂缝张开后短短 6 周内就会出现一些新裂缝的充填。这些结果表明，近地表裂缝可能会非常迅速地填充，可能会在与石油、天然气和地热能生产相关的时间尺度上改变渗透率；碳汇; 地震周期；以及来自核废料储存和地下核爆炸的放射性核素迁移。引言 岩石裂缝填充和密封所需的时间受到严格限制，特别是在浅地壳环境中。通过充填和密封减少的裂缝孔径对石油、天然气和地热能生产很重要（Hippler，1993 年；Boles 等人，2004 年；Dempsey 等人，2013 年）、碳封存（Shukla 等人， 2010)、核废料储存中的放射性核素迁移（Ticknor 等人，1989 年；Moreno 和 Neretnieks，1993 年）和地下核爆炸检测、监测和表征（Jordan 等人，2015 年）。虽然通常假设裂缝填充处于“地质时间尺度”（例如，Lander 和 Laubach，2015 年），一些研究表明，101-104 年的封闭可以更好地解释构造加载断层中地震周期的时间（例如，Blanpied 等，1992；Byerlee，1993；Chester 等，1993）。根据地震速度的瞬时变化（Tadokoro 和 Ando，2002；Hiramatsu 等，2005；Li，2006）和渗透率（Rojstaczer 和 Wolf，1992；Claesson 等， al., 2007; Xue et al., 2013; Wästeby et al., 2014) 在地震之后，但这些变化发生的机制（即裂缝可能如何闭合）没有用这些技术观察到。实验室研究揭示了裂缝可以密封的潜在过程，包括由于矿物在 150–1000 °C 的温度下沉淀引起的裂缝填充（Morrow 等，1981，2001；Moore 等，1994；Brantley 等，1990；Tenthorey 等，2003），去除凹凸不平通过溶解/再沉淀（Beeler 和 Hickman，2004 年；Gratier 等人，2013 年；Aben 等人，2017 年），或没有流体的粗糙接触屈服（Dieterich 和 Kilgore，1994 年），这会导致渗透率降低（例如，Morrow等人，1981 年；摩尔等人，1994 年）。然而，在地壳条件下，尤其是在低温、近地表条件下，这些过程中的每一个的时间尺度和相对重要性都没有得到很好的量化。提高我们对裂缝填充和封闭率的理解需要隔离单个变形事件。地震预测非常困难，以至于无法将震前断裂特性与震后断裂特性的时间序列进行直接比较。因此，源物理实验 (SPE)，内华达州花岗岩中的一系列地下化学爆炸，提供了在（非构造）损坏事件前后收集岩心的独特机会。方法 作为了解损伤对地下化学爆炸地震特征的作用的一部分（Snelson 等人，2013 年），在任何化学爆炸之前（“pre-ex”，U15n）和每次爆炸之后收集岩心。较大的两次爆炸（“post-ex”，U15n#10、U15n#12 和 U15n#13；表 1；图 1 中的位置）。这些爆炸是在位于干旱的内华达州国家安全站点的 Climax Stock 花岗岩的石英二长岩成员中进行的。空气泡沫，而不是钻井泥浆，是在钻井过程中使用的。先收集 U15n 岩心，然后扩大钻孔，以便安置炸药。SPE-2爆炸后6周收集U15n#10核；U15n#12 和 U15n#13 核分别在 SPE-3 爆炸后 18 周和 21 周和 SPE-2 后一年多时收集。岩心 U15n#10 和 U15n#12 与垂直方向成约 30° 的角度钻孔（以确保与爆炸源区域相交），而 U15n 和 U15n#13 则垂直钻孔（图 1）。所有岩心都交付并存储在内华达州水星的美国地质调查局 (USGS) 数据中心和岩心图书馆。用于微观结构分析的样品沿每个岩心从大约每 6 m 的间隔中选择，并且从每个样品制成三个相互垂直的薄片。这些与 Swanson 等人用于定义损坏程度的薄切片相同。(2018)，有关薄切片制备和方向的更多信息可以在那里和 GSA 数据存储库中找到。对于每个薄片，穿晶微裂缝的特征是开放或填充（参见数据存储表 DR1、图 DR1 和分类标准的文本），以及它们的 *电子邮件：emswanson@lanl.gov 1GSA 数据存储库项目 2019399，详细方法，包括一张显示开放与填充微裂缝的图；包含此处提供的数据的表格；以及显示填充微裂缝比例的图，可在 http://www.geosociety.org/datarepository/2019/ 在线获取，或应editing@geosociety.org 的要求提供。†当前地址：Sandia National Laboratories, Albuquerque, New Mexico 87185, USA。来自 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46957.1/4848234/g46957.pdf 由访客于 2019 年 11 月 2 日下载 2 www.gsapubs.org | 卷XX | 编号 XX | 地质 | 美国地质学会各自的线性密度是使用 Swanson 等人使用的相同技术测量的。(2018) 并在数据存储库中进行了详细描述。对于每个样品（即深度），比较薄片三个方向中每一个的微裂缝密度值，并选择最大的微裂缝密度进行比较，以促进潜在定向损伤的识别。岩心没有定向，但可以在 Swanson 等人的文章中找到中尺度裂缝定向的讨论。(2018)。为了确定裂缝填充物是粉碎的主岩还是新沉淀的材料，我们使用 FEI Inspect 扫描电子显微镜 (SEM) 和 15 kV 光束收集了一些选定样品的图像。制备包括用金作为导电层溅射涂覆样品。使用能量色散谱 (EDS) 进行元素分析。元素使用 EDAX EDS 软件进行识别，并使用无标准 ZAF 量化进行量化，该量化解释了原子序数 (Z)、吸收 (A) 和荧光 (F) 的影响。我们使用术语“填充”来描述其中含有可见量材料的裂缝，与之前研究中使用的术语“密封”形成对比（例如，Wilson 等人，2003 年；Mitchell 和 Faulkner，2009 年），以避免暗示这些裂缝是不透水的或已经恢复强度。然而，裂缝填充是朝着密封迈出的一步，并且可能发生了一些渗透率损失和/或强度恢复（相对于开放裂缝），随着时间的推移，这些裂缝可能会完全封闭。结果 我们将所有样品的开放微裂缝密度与深度进行了比较（图 2A）。在 pre-ex 岩心 U15n 中，开放式微裂缝密度都相对较低，每个样品每毫米 (mf/mm) 不到 1 个微裂缝。包含最多裂缝的 pre-ex 样本是在先前确定的 26.5 m 深度的小断层附近收集的（Townsend 等，2012）。防爆后的岩心 U15n#10、U15n#12 和 U15n#13 表现出更高和更多的变化 表 1. 岩心钻孔和源物理实验 (SPE) 爆炸的年代 岩心名称 钻孔完成日期 爆炸名称和尺寸 爆炸深度 (m ) 爆炸日期 U15n 2010 年 7 月 28 日 SPE-1，88 公斤 54.9 2011 年 5 月 3 日 SPE-2，997 公斤 45。7 2011 年 10 月 25 日 U15n#10 2011 年 12 月 9 日 SPE-3，905 kg 45.8 2012 年 7 月 24 日 U15n#12 2012 年 11 月 26 日 U15n#13 2012 年 12 月 17 日 注意：最早的事件在顶部。爆炸大小以千克三硝基甲苯 (TNT) 为单位。图 1. 示例位置。(A) Climax Stock 的简化地质图，由 Townsend 等人修改。(2012)。(B) 内华达州国家安全站点内的位置（绿色轮廓）；CA——加利福尼亚；SPE——源物理实验。(C) 横截面 XX' 显示样本位置（彩色椭圆）。椭圆按岩心着色，大小按总（开放 + 填充）穿晶微裂缝缩放。SPE-2 和 SPE-3 爆炸（来自 Swanson 等人，2018 年）的损坏程度由橙色阴影圆圈和框表示。(D) 与 C 相同比例的平面图图像，显示钻孔接箍位置。白色圆圈表示与 U15n 的距离。U-15n#10