Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL 61801, USA.
J Contam Hydrol. 2010 Apr 1;113(1-4):1-24. doi: 10.1016/j.jconhyd.2010.01.001. Epub 2010 Jan 28.
Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods' advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.
在多孔介质中发生的污染物水文地质过程通常无法直接观察。因此，通常使用间接测量（例如，固定位置的污染物突破）来推断在不同尺度、位置或时间发生的过程。为了克服这一限制，非侵入性成像方法越来越多地应用于污染物水文地质学研究。本文综述了四种最常见的方法，即使用紫外线或可见光的光学成像、双能伽马辐射、X 射线微断层扫描和磁共振成像（MRI）。非侵入性成像技术为各种复杂系统和过程提供了有价值的见解，包括多孔介质特性、多相流体分布、流体流动、溶质传输和混合、胶体传输和沉积以及反应。本文综述了这些方法的基本原理、这些方法在污染物水文地质学研究中的应用以及这些方法的优缺点。不出所料，对于非侵入性成像，没有一种完美的方法或工具。然而，光学方法通常是在人工二维（2D）多孔介质中成像流体分布、溶质和流体流动、胶体传输和反应最经济、最简单的选择。伽马辐射方法为在达西尺度上对二维流体分布进行特征描述提供了最佳机会。X 射线方法为三维（3D）天然多孔介质特征描述以及天然多孔介质中流体分布的 3D 特征描述提供了最高的分辨率和灵活性。MRI 为人工多孔介质中流体分布、流体流动、胶体传输和反应的 3D 特征描述提供了最佳选择。显然，需要开发新的方法来解决一些明显的缺陷，如在三维空间中对天然多孔介质中流体流动和胶体传输等瞬态过程进行成像的能力、在人工和天然多孔介质中对许多环境感兴趣的反应进行成像的能力以及在人工和天然多孔介质中对选定过程在一系列尺度上进行成像的能力。
