## 3 Experimental Results and Analysis
### \(T_{1}\)-\(T_{2}\) Spectrum Test Results
When the temperature ranged from \(-2\)\({}^{\circ}\)C to \(10\)\({}^{\circ}\)C, the growth slope changed abruptly, the ice in different pores of the coal sample began to melt, the unfrozen water was distributed in different areas of the coal sample and the distribution range was wide. Similar to the characteristic distribution pattern of the characteristic peaks presented by the 1D NMR T\({}_{2}\) curve, as shown in Figure 7, each different peak in the 3D peak diagram represents hydrogen-containing substances with the same relaxation characteristics, and different colors are used to distinguish different hydrogen-containing substances due to different amplitudes. Since this paper mainly studies the content of unfrozen water in pores, bound water and free water are highlighted here. When the pores of the coal body were  
Figure 5: Distribution range of T\({}_{1}\) and T\({}_{2}\) of coal samples at different temperatures. Figure 6: Relative distribution space of unfrozen water in coal samples at different temperatures. evenly distributed, the pore size or characteristics had large differences and the inter-pore connectivity was poor, the fluid relaxation signals in the pores showed large differences, which made the pores have strong sorting properties. Even if the same coal sample is used in the test, coal at different melting temperatures will show different peak shapes. As shown in Figure 7, at \(-\)196 \({}^{\circ}\)C, the coal sample only had a single-peak state, and the peak value of the highest peak was very low and the pore structure was relatively simple, mainly consisting of coal and ice pores. At \(-\)20 \({}^{\circ}\)C, the coal sample presented a bimodal state, and the difference between the two peaks was larger, indicating that the pore results at this time were more complex than those at \(-\)196 \({}^{\circ}\)C. However, mainly small pores were observed, and only a small number of medium pores could be measured. When the coal sample ranged from \(-\)2 \({}^{\circ}\)C to 10 \({}^{\circ}\)C, the pore structure was more complex, and the peak value of the highest peak gradually decrease, while the peak value of other peaks gradually increased and the unfrozen water in the middle and large pores gradually melted; the pore types that could be detected gradually increased. At extremely low temperatures, free water was frozen, and the signal at this time represented the distribution state of unfrozen water in the pore. Figure 8 shows the peak centroid positions of coal samples at different melting temperatures. When the coal samples gradually melted with room temperature, the peak centroid of coal samples at different melting temperatures changed: the centroid of coal samples at \(-\)196 \({}^{\circ}\)C and \(-\)20 \({}^{\circ}\)C moved in the direction of increasing T\({}_{2}\). In addition, from \(-\)20 \({}^{\circ}\)C to 10 \({}^{\circ}\)C, the position of coal sample centroid moved along the increasing direction of T\({}_{1}\)-T\({}_{2}\). The trend difference between coal samples at different temperatures is caused by the different development of unfrozen water content in the pores, which results in different lateral relaxation (spin-spin relaxation) and longitudinal relaxation (spin-lattice relaxation) of the fluid in different pores under the influence of magnetic field. By combining the two dimensions, T\({}_{1}\)-T\({}_{2}\) methods can more accurately identify changes in fluid relaxation characteristics in pores than simple T\({}_{1}\) or T\({}_{2}\) methods. The two-dimensional NMR method can identify the space of different fluid parts in different rock masses and reflect the specific fluid content in different pores. In the 3D peak diagram, the peak volume can directly represent the different fluid content in the coal. Figure 9 shows the unfrozen water content in coal samples at different melting temperatures.