###### Abstract  
The micropore structure of porous media (such as natural rocks and man-made materials) is very complex and has strong micro heterogeneity, and pore structure is a critical parameter to estimate the rock quality. However, the pore structure characterization of rocks under load is not studied well. In this paper, sandstone specimens were preloaded to six different stress levels, and then the pore structure of rock was characterized by SEM and NMR, respectively. The results show the following: (1) The damage in sandstone increases with predefined stress, and the rate significantly increases over 0.8 uniaxial compressive strength (UCS). (2) There is a critical value in the process of rock damage (0.8 UCS), and when it is less than this critical value, the microstructure in the rock is mainly composed of pores and micro-cracks, and the length is generally less than 5 \(\upmu\)m; when it exceeds the critical value, there are obvious cracks or even groups of cracks inside the rock. (3) The changes in porosity can be divided into three stages, showing a \"\(\surd\)\" shape tendency. (4) The pore structure can be visually presented using NMR and SEM, and the distribution mode of the pores changes from separated points to concentrated patches to finally interconnected networks of pores with an increase in the predefined stress. Overall, NMR provides a new method for characterizing rock damage and studying rock microstructure. s +
Footnote †: journal: Applied  
1
Footnote 1: School of Physics, University of California, Berkeley, CA 94720  
**Citation:** [PERSON]; [PERSON]; [PERSON]; [PERSON] An Investigation on the Pore Structure Characterization of Sandstone Using a Scanning Electron Microscope and an Online Nuclear Magnetic Resonance System. _Appl. Sci._**2024**, _14_, 7063.  
ease of analysis and calculations, these studies often use various assumptions, decreasing the accuracy of actual changes in the rock. These simulation results and theoretical analysis do have a certain guiding purpose on the actual geotechnical engineering construction, yet a quantitative analysis of microscopic rock damage and its stability are needed to improve the accuracy of field applications. In recent years, the development of and improvement in experimental techniques have provided the possibility of observing the internal microscopic damage of rocks. In particular, nuclear magnetic resonance (NMR) has been successfully applied in many fields due to its advantages of continuous, rapid, non-destructive testing, low cost and simple operation. NMR can clearly and accurately visualize the internal structure, composition, material and defects of an object and can quantitatively study the internal pore size based on NMR relaxation. Currently, NMR has gradually developed into a mature tool in the field of geotechnical engineering to detect the internal microscopic damage of rocks and evaluate rock stability, covering, for instance, CO\({}_{2}\) storage [2; 3], shale gas [4; 5; 6; 7], exploration studies of hydrocarbon reservoirs [8; 9], and the ability of geotechnical structures [10; 11]. [PERSON] et al. [12] used a low-field NMR to study fluid percolation and distribution during imbibition and self-drainage. The distribution of pore size was obtained using NMR from the linear relationship between the pore volume and signal strength and the relationship between the melting point and pore size [13]. [PERSON] et al. [11] studied the structure and seepage characteristics of coal samples under different stresses and pore water pressures using NMR and found that the internal fissure-pore structures of coal samples were interwoven after water injection, and the distribution of the pore radius was diverse. Four main pore types can be identified and classified according to low-field NMR results [4]. [PERSON] et al. [14; 15] used the NMR technique to study the damage evolution law of sandstone under loading and seepage and established the discriminant equation of the damage degree of the rock body.