## 2 Experimental Principles and Method
### Experimental Procedure and Device  
To simulate the drying and wetting process in a more realistic way, the water samples were taken from the hydro-fluctuation zone of the Three Gorges Reservoir. According to the chemical elements tests, the water consisted of Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\), HCO\({}_{3}^{-}\),Cl\({}^{-}\), SO\({}_{4}^{2-}\) and NO\({}_{3}^{-}\), with a PH = 7.2-8.3. Generally, the composition of water is stable perennially. All the samples for the experiments were taken from the same place. In nature, the water level of the Three Gorges Reservoir rises from June to August, while it drops from November to December. To accelerate the damage of sandstone under dry-wet alternation action, the vacuum saturation and oven drying methods were employed to simulate the effect of dry-wet alternation [41; 42]. In detail, rock samples were firstly put into water under tank pressure of \(-\)80 kPa for 6 h. Then, samples were soaked for 18 h until the negative pressure increased to 0 kPa. After that, rock samples were dried at 105 \({}^{\circ}\)C for 24 h. The test flow is shown in Figure 3, where \(n\) represents the dry-wet alternation number. After the tests (\(n\) = 0, 1, 5, 10, 15 and 20), the jointed sandstones were taken out, and the meso-damage tests by means of NMR and SAXS technology were conducted. Throughout the testing process, we performed parallel tests (i.e., three sets of tests under identical conditions for each protocol) to minimize the uncertainty of the results.  
Figure 2: Experiment samples: (**a**) part of jointed sandstone and (**b**) joint distribution.  
In this research, a vacuum saturation instrument and electric drying oven were selected to carry out testing, as illustrated in Figure 3a. The advent and application of NMR technology provides a new and effective method to study rock pore structures. To quantitatively analyze the variation of mesoscopic pore structure of jointed sandstone subjected to the dry-wet alternation action, MacroMR12-150H-I low-field nuclear magnetic resonance spectrometer systems were employed to observe the mesostructure of the saturated rock samples. The non-destructive NMR equipment is shown in Figure 3b, and the main technical parameters of the low-field NMR system are compiled in Table 1. The jointed sandstone samples with different dry-wet cycle numbers were processed into rock powder after drying and grinding. After that, the mineral composition and proportion of rock samples after the action of dry-wet cycles were identified using the small angle X-ray scattering instrument (Figure 3c).  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**System Parameters** & **Value** & **System Parameters** & **Value** \\ \hline Temperature of magnet \(T\) (\({}^{\circ}\)C) & 32 & RF 90\({}^{\circ}\) pulse width \(P_{1}\) (\(\mu\)s) & 3.80 \\ RF frequency SF (MHz) & 12.68 & RF 180\({}^{\circ}\) pulse width \(P_{2}\) (\(\mu\)s) & 10.20 \\ RF offset frequency O\({}_{1}\) (Hz) & 682,035.1 & Repeat sampling interval TW & 500 \\ RFD (ms) & 0.020 & Sampling number TD & 99,980 \\ Analog gain (RG1) & 20.0 & Data radius DR & 1 \\ Cumulative collecting times & 8 & Pulse echo times NECH & 5000 \\ (NS) & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: CPMG sequence parameters of low-field NMR system.  
Figure 3: Experimental flow and instruments: (**a**) water saturator instrument and drying oven; (**b**) non-destructive NMR system; and (**c**) small angle X-ray scattering equipment.