Of interest to the Underground Nuclear Explosion Signatures Experiment are patterns and timing of explosion-generated noble gases that reach the land surface. The impact of potentially simultaneous flow of water and gas on noble gas transport in heterogeneous fractured rock is a current scientific knowledge gap. This article presents field and laboratory data to constrain and justify a triple continua conceptual model with multimodal multiphase fluid flow constitutive equations that represents host rock matrix, natural fractures, and induced fractures from past underground nuclear explosions (UNEs) at Aqueduct and Pahute Mesas, Nevada National Security Site, Nevada, USA. Capillary pressure from mercury intrusion and direct air–water measurements on volcanic tuff core samples exhibit extreme spatial heterogeneity (i.e., variation over multiple orders of magnitude). Petrographic observations indicate that heterogeneity derives from multimodal pore structures in ash-flow tuff components and post-depositional alteration processes. Comparisons of pre- and post-UNE samples reveal different pore size distributions that are due in part to microfractures. Capillary pressure relationships require a multimodal van Genuchten (VG) constitutive model to best fit the data. Relative permeability estimations based on unimodal VG fits to capillary pressure can be different from those based on bimodal VG fits, implying the choice of unimodal vs. bimodal fits may greatly affect flow and transport predictions of noble gas signatures. The range in measured capillary pressure and predicted relative permeability curves for a given lithology and between lithologies highlights the need for future modeling to consider spatially distributed properties.

中文翻译：

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火山岩的非均质多相流特性及其对地下核爆炸惰性气体传输的影响

地下核爆炸特征实验感兴趣的是爆炸产生的惰性气体到达地表的模式和时间。潜在的水和气同时流动对非均质裂隙岩中惰性气体传输的影响是当前的科学知识空白。本文提供了现场和实验室数据，以使用多模态多相流体流动本构方程来约束和证明三重连续体概念模型的合理性，该方程代表内华达州渡槽和帕胡特梅萨斯过去地下核爆炸 (UNE) 的母岩基质、天然裂缝和诱发裂缝美国内华达州国家安全站点。汞侵入产生的毛细管压力和火山凝灰岩岩心样品的直接空气-水测量表现出极端的空间异质性（即 多个数量级的变化）。岩相学观察表明，非均质性源于灰流凝灰岩组分中的多峰孔隙结构和沉积后蚀变过程。UNE 前后样品的比较揭示了不同的孔径分布，部分原因是微裂缝。毛细管压力关系需要多模态 van Genuchten (VG) 本构模型才能最好地拟合数据。基于单峰 VG 拟合毛细管压力的相对渗透率估计可能不同于基于双峰 VG 拟合的那些，这意味着单峰与双峰拟合的选择可能会极大地影响惰性气体特征的流动和传输预测。