## 4 Conclusions  
Basic physicochemical information on red sandstone specimens was obtained by using ultrasonic detection, microscopic electron microscopy (SEM), X-ray diffraction (XRD) and X-ray computed tomography (X-CT). Based on using the ultrasonic wave velocity and CT grayscale distribution map to analyze the specificity of the degradation degree of red sandstone in the horizontal direction, a method is proposed to calculate the degradation degree of the overall plane. The degradation degree of each level is calculated, and the degradation degree of \(L\)S is predicted using the optimal gray prediction GM (1, 1) model. The relationship between the degradation degree and the characteristic parameters of the CT grayscale image is further investigated using the gray correlation method. The conclusions obtained are as follows:  
(1) Microstructural analysis shows that the degradation of red sandstone specimens is accompanied by the development of defects such as cracks, fissures, and holes, as well as transformation and generation of new substances. The internal structure of red sandstone has white dolomite generated under weak weathering, while the surface sandstone structure under strong weathering all year round has dissolved flocculent sand grains with authigenic chlorite, which is able to fill the structural pores and protect the internal sandstone from further weathering and corrosion to a certain extent. The degree of degradation decreases with increasing depth. (2) When analyzing the ultrasonic test results of the red sandstone specimens, it can be found that the ultrasonic velocity value is decreasing in the longitudinal direction from \(L1\) to \(L5\), and the erosion degree is different in the Y-axis and X-axis directions in the horizontal plane due to the spatial location. Combined with the CT grayscale images, it can be found that in the \(Z\) direction, there is a certain connection between the characteristic parameters  
Figure 12: Degradation degree versus third-order central moment and Pg > 200.  
of CT grayscale images and ultrasonic waves, and the root cause of the change in ultrasonic wave velocity values is the change in the degree of degradation, so the correlation between the characteristic parameters of CT grayscale images and the degree of degradation can be derived. It is further verified that the degradation degree is different in the X and Y directions when in the same layer, and thus a method to characterize the overall planar degradation degree of red sandstone specimens is required, as proposed in this paper. (3) A method based on the gray GM (1, 1) model to predict the degree of degradation of layer \(L5\) of a red sandstone specimen was proposed. Combined with the actual situation of this red sandstone specimen, in order to investigate the degradation of its surface layer relative to the un-weathered layer, four gray predictive GM (1, 1) models were used to predict the degradation of \(L5\), and it was found that the mean relative error of the EDGM model was the smallest. The predicted degradation of the \(L5\) layer was 32.39%. (4) The correlation between the characteristic parameters of the CT grayscale images and the degree of degradation was analyzed using gray correlation theory. It was found that the correlation between the third-order central moments and Pg \(>\) 200 of the CT grayscale images of the red sandstone specimens and the true degree of degradation of the red sandstone specimens were 0.7934 and 0.8581, respectively, which further confirmed the good prediction effect of the EDGM model. (5) The results of this paper show that there is a good correlation between the red sandstone CT grayscale image and the degree of red sandstone degradation, so it is practicable to characterize the degree of red sandstone degradation by using the features of a red sandstone CT grayscale distribution map.