## 3 Test Results
### Effect of Temperature on Sandstone Anisotropy  
To quantitatively judge the degree of anisotropy of rock strength, [PERSON] et al. (1989) proposed the concept of the \"anisotropy degree\", that is, the ratio of the maximum compressive strength when the bedding angle is 0\({}^{\circ}\) or 90\({}^{\circ}\) to the minimum compressive strength when the bedding angle is between 0\({}^{\circ}\) and 90\({}^{\circ}\). Using a similar idea, we can define and calculate a series of anisotropy degrees of sandstone's physical and mechanical parameters:  
\[R_{\rm s}=\frac{S_{max}}{S_{min}} \tag{6}\]  
where \(R_{\rm s}\) is the anisotropy of mechanical parameters such as the elastic modulus, peak stress, internal friction angle, and cohesion of rock samples. These are obtained by dividing the maximum and minimum values of mechanical indices at various temperatures. Figure 12a shows the variation law of \(R_{p}\) and \(R_{\rm s}\) of sandstone with temperature. It can be seen from the figure that Rp increases gradually with temperature. When \(T=600\)\({}^{\circ}\)C and \(T=800\)\({}^{\circ}\)C, the anisotropy is relatively close. It can be speculated that the temperature increase has an amplification effect on the anisotropy of the sandstone's P-wave velocity. The S-wave anisotropy degree \(R_{\rm s}\) of sandstone also shows an overall increasing trend with increasing temperature, but the trend is gentler than that of the P wave. When \(T<400\)\({}^{\circ}\)C, the temperature has no obvious effect on the S-wave anisotropy of sandstone. When the temperature rises from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, \(R_{\rm s}\) rapidly increases, but as the temperature continues to rise, \(R_{\rm s}\) decreases when \(T=800\)\({}^{\circ}\)C compared with 600 \({}^{\circ}\)C, and the \(R_{\rm s}\) of sandstone reaches the maximum value when \(T=1000\)\({}^{\circ}\)C. This shows that the temperature can amplify the anisotropy of the S-wave velocity of sandstone as a whole, but when \(T=800\)\({}^{\circ}\)C, \(R_{\rm s}\) decreases more than when \(T=600\)\({}^{\circ}\)C and 1000 \({}^{\circ}\)C.  
Figure 11: (a) \(\vartheta_{t}/\varphi_{0}\) variation trend with temperature for rock samples with different bedding angles; (b) \(\vartheta\) change trend with dip angle for rock samples treated at different temperatures. Figure 12b shows the change rule of the degree of anisotropy of the sandstone's mechanical index with increasing temperature. It can be seen from the figure that with increasing temperature, \(R_{e}\) increases initially, then decreases, and then increases again. Temperatures below 400 \({}^{\circ}\)C have no obvious effect on the anisotropy of sandstone, and the sandstone undergoes almost no change. When the temperature rises to 400 \({}^{\circ}\)C, \(R_{e}\) rapidly increases, which indicates that the temperature range of 400-600 \({}^{\circ}\)C has a relatively obvious enhancement effect on sandstone anisotropy. As the temperature continues to rise, reaching 800 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, \(R_{e}\) is significantly lower than that at room temperature, and it is speculated that temperatures above 800 \({}^{\circ}\)C will weaken the peak stress anisotropy of sandstone. The influence of temperature on \(R_{E}\) fluctuates greatly. It can be seen from the figure that the \(R_{E}\) of sandstone fluctuates once every 200 \({}^{\circ}\)C temperature interval, but the \(R_{E}\) of sandstone samples after high temperature treatment is basically greater than the value under room temperature. Most high-temperature conditions can improve the anisotropy of the sandstone elastic modulus. Although there is slight fluctuation of \(R_{\varphi}\) in the whole temperature range, there is no significant change overall. \(R_{c}\) generally decreases with increasing temperature, but it suddenly changes at 600 \({}^{\circ}\)C and reaches a maximum.