基于CT成像和数字体图像相关法的岩石内部变形场量测方法的研究进展

Progress of internal deformation measurement of rock by using CT and digital volume correlation

在采矿工程、边坡工程、隧道工程、水利水电工程及新兴的岩体工程如深埋油气储库、地下核废料处置库、地热开发等生产开发过程中,岩石是主要工程对象.直观观测与定量表征应力场、渗流场和温度场等多物理场耦合作用下岩石内部非连续结构演化始终是岩石力学重要和具有挑战性的研究内容.高精度微CT能够在微细观甚至纳米尺度上观测岩石内部结构,通过与数字体图像相关法结合,可实现岩石内部变形场的透明可视化与定量解析,为岩石的非连续结构与多物理场效应的透明解析和推演提供了新的有效途径.本文回顾了近年来微焦点CT在岩石内部结构检测、数字岩心和内部变形场量测方面的应用,详细阐述了CT原位扫描实验与数字体图像相关法的原理及主要进展,分析了岩石微细观结构对应变场测量精度的影响及数字体图像相关法在岩石内部变形测量中的典型应用,探讨了数字体图像相关法测量岩石内部变形场面临的挑战.

Rock mechanics plays a critical role in mining engineering, slope engineering, tunnel engineering, water conservancy,hydropower engineering, and some newly developing rock mass engineering, such as deeply buried oil and gas storage,underground nuclear waste repository, geothermal development, etc. The evolution of the internal discontinuous structure of rock under the coupling of multiple physical fields such as stress field, seepage field, and temperature field is an important research topic in the field of rock mechanics. Scholars have investigated the internal structure characteristics and their evolution process at different scales with various equipment. High-resolution microtomography(Micro CT) with micron or even nanoscale resolution has been widely used in internal structure detection, digital core modeling, and numerical modeling of rock. The digital volume correlation(DVC) method can quantitatively analyze and visualize the internal 3D deformation field of rock, and its combination with Micro CT provides a new method for transparent analysis and deduction of discontinuous structures and multi-physical fields in rocks. This paper systematically reviews the research on internal deformation measurement of rocks by using Micro CT and DVC in terms of in-situ loading device with Micro CT, the theory of DVC, the influence of micro/meso-structures in rocks, and experimental measurements.To view the internal structures of rocks under different physical fields, various in-situ loading devices combined with Micro CT were developed, e.g., uniaxial loading device, triaxial loading coupled with seepage device, gas adsorption device, and heating device. In these devices, factors such as the strength of rock, physical field, material strength of chamber, X-ray energy and penetration, real-time performance of image acquisition, and quality of reconstructed images should be comprehensively considered.By using in-situ loading devices and Micro CT, a series of volume images of rocks are obtained before and after deformation, and then DVC is adopted to calculate the three-dimensional deformation field by analyzing the correlation between these volume images. According to the registration algorithms, DVC can be categorized into local DVC(L-DVC)and global DVC(G-DVC). The basic principle and development of DVC are reviewed. Due to the limitation of the heterogeneity of rocks and the accuracy of the image acquisition equipment, natural micro structures in rocks do not have ideal speckle patterns, which affects the accuracy of DVC. It is indicated that the larger the average gray gradient and Shannon entropy and the smaller the structural variation coefficient of a rock image, the higher the accuracy of DVC. The accuracy of G-DVC is slightly better than that of L-DVC, but its calculation efficiency is lower, especially when the mesh size is small. The applications of DVC for investigating strain localization in rocks, measuring carbon dioxide-induced strain in coal, and oil shale pyrolysis are presented.Despite some impressive achievements, DVC is still faced with some core challenges, e.g., the low accuracy of the DVC algorithm and artifact error correction, the influence of the microstructures in rocks, parameter identification and numerical model verification of rocks, and deformation field measurement under the coupling of multiple physical fields. With the popularization of Micro-CT in laboratories, DVC will be further applied to measuring the three-dimensional deformation field in rocks.