Small Displacement Faults in Sand: What Control Do They Exert on Saturated and Unsaturated Flow and Transport?

我们建议研究小位移断层对弱固结砂质沉积物中饱和和非饱和水流和化学输运的影响。以前的工作主要集中在断层对岩石中饱和流体流动的影响，而不是沉积物。这些研究不能直接应用于沉积物中的流动，因为沉积物的变形力学和由此产生的结构不同于岩石，也不能应用于非饱和流动，因为给定材料在非饱和和饱和条件下的渗透率分布不同。一组复杂的变量控制着断层对流体流动的影响。由于缺乏关于松散或松散沉积物中断层行为的信息，解决所有相关变量超出了本提案的范围。我们将重点关注问题的一个小但非常重要的方面：变形如何影响美国西南部最常见的含水层材料砂中小位移断层的饱和和不饱和渗透率。小位移断层远比地质图上通常包含的大位移断层更常见。我们的初步工作表明，这种材料中的断层会导致饱和渗透率降低，而对于该地区典型的干旱水分条件，则会导致非饱和渗透率增加，这两者都是由于变形引起的孔隙尺寸减小。因此，在饱和的地下水位区，断层起着屏障的作用，而在瓦杜兹区，断层可能是将化学物质直接带到地下水位的快速通道。在研究过程中，我们将检验以下假设：在构造活跃的伸展区，如里约热内卢大裂谷，砂中的断层，特别是小位移断层是相对常见的特征。在孔隙介质尺度上，断裂带饱和和不饱和渗透率通过变形和成岩作用发生改变，主要表现为粒度减小、晶粒重定向和胶结作用。我们将集中注意前两个过程。（第三个过程将是未来研究的主题。）断裂带渗透率的改变影响了饱和地下水流动和非饱和Vaduz带流动，以及与之相关的化学物质运动。其影响程度取决于断层带与主砂体的渗透率对比、断层的空间频率。在饱和砂质含水层中，由于饱和渗透率降低，这些断层带成为流动的障碍。在渗流带中，断裂带可以成为美国西南部典型干旱条件下的首选流动路径；在潮湿的条件下，它们变成了屏障。我们将结合地质和水文分析来检验这些假设。几个研究地点的地质情况将以不同的比例尺绘制。然后，他们将确定在变形和未变形材料中绘制的水文地质单元的渗透率和岩石学特征。这些基于现场的数据将用于构建断层系统的数值模型，在该模型中可以评估断层特性对流动和输运的影响。我们还将对胶结进行地球化学测量，以诊断过去的流动状态。通过跨学科的方法，在不同的尺度上使用各种技术来解决这个问题，我们希望确定这些断层是否是重要的水文地质特征。

9614385 Wilson We propose to study the influence of small-displacement faults on both saturated and unsaturated water flow and chemical transport in poorly consolidated sandy sediments. Previous work has focused on the influence of faults on saturated fluid flow in rock, rather than sediments. These studies cannot be directly applied to flow in sediments because the mechanics of deformation and resultant structures are different for sediments than for rocks, and cannot be applied to unsaturated flow because the distribution of permeability in a given material is different under unsaturated versus saturated conditions. A complex set of variables controls the influence of faulting on fluid flow. Given the lack of information about the behavior of faults in unconsolidated or poorly consolidated sediments, addressing all of the pertinent variables is beyond the scope of this proposal. We will focus on a small, but very important, aspect of the problems: how deformation influence saturated and unsaturated permeability of small-displacement faults in sand, the most common aquifer material in the southwestern United States. Small-displacement faults are far more common than the large-displacement faults typically included on geologic maps. Our preliminary work suggests that faulting in such material will result in decreased saturated permeability and, for the arid moisture conditions typical of the region, increased unsaturated permeability, both resulting from pore-size reduction caused by deformation. Thus in the saturated groundwater zone faults act as barrier, while in the Vaduz zone they may act as fast paths that lead chemicals directly to the water table. We will test the following hypotheses the course of the study: Fault in sand, especially small-displacement faults, are relatively common features in tectonically active extensional regions like the Rio Grande Rift. At the porous-media scale, fault-zone saturated and unsaturated permeability is altered by deformation and d iagenesis, through grain-size reduction, grain reorientation, and cementation. We will focus our attention on the first two processes. (The third process will be the subject of future research.) The altered fault-zone permeability influences saturated groundwater flow and unsaturated Vaduz zone flow, and the associated movement of chemicals. The degree influence depends on the permeability contrast between fault zones and host sands, spatial frequency of faults. In saturated sandy aquifers these fault zones become barriers to flow because of reductions in saturated permeability. In the vadose zone fault zones can become preferred flow paths under the arid conditions typical of the southwestern US; in wetter conditions they become barriers. We will test these hypotheses using a combination of geologic and hydrologic analyses. The geology of several study sites will be mapped at different scales. They will then determine the permeability and petrographic characteristics of the mapped hydrogeologic units in both deformed and undeformed materials. This field-based data will be used to construct numerical models of fault systems in which the influence of fault properties on flow and transport can be evaluated. We will also make geochemical measurement of cementation that is diagnostic of past flow regimes. By addressing the problem with a interdisciplinary approach, using a variety of techniques at different scales, we hope to determine whether these faults are significant hydrogeologic features.