Study on pore structure characteristics of sea ice based on CT observation
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摘要: 孔隙结构是影响海冰力学性质的重要细观物理特征。为探究融化海冰的细观结构，在渤海盛冰期采集平整冰，并置于低温恒温环境（−1.0℃）内恒温48 h，进行CT扫描观测。确定气体−冰体−卤水的CT值阈值为−310 HU和−30 HU，对冰样CT图像进行三值化分割，识别气体和卤水孔隙，分析孔隙的二维形态特征；在此基础上进行冰样孔隙的三维重构，分析孔隙的三维形态特征。观测发现沿冰厚方向，冰样扫描断面气体面积分数为5.00%～35.93%，卤水分布不连续，面积分数最大为0.06%。气体和卤水孔隙平行冰面方向的截面形状近似圆形，圆度大于0.60。扫描断面内气体孔隙等效圆直径为1.1～3.2 mm，卤水孔隙等效圆直径为0.2～2.0 mm，且发现孔隙等效圆直径与面积分数呈正相关，与圆度呈负相关。三维结构上，根据孔隙球度（Rsph）将气体孔隙分为4种典型形态：冠状孔隙（Rsph ≤ 0.25）、不规则孔隙（0.25 < Rsph ≤ 0.45）、条状孔隙（0.45 < Rsph ≤ 0.60）和球形孔隙（0.60 < Rsph ≤ 1.00）。其中，冠状孔隙单体体积最大[平均体积（
11522.8 ±5610.2 mm3）]且数量最少，球形孔隙尺寸最小（平均直径2.0 ± 1.1 mm）且数量最多。卤水孔隙分为卤水通道（0.45 < Rsph ≤ 0.60）和卤水胞（0.60 < Rsph ≤ 1.00）。卤水通道平均长度17.1 ± 12.1 mm，卤水胞平均直径1.5 ± 0.9 mm；卤水通道数量较少，但体积占比与卤水胞相当。Abstract: Pore structure is an important mesoscopic feature of sea ice affecting its mechanical properties. In order to investigate the mesoscopic structure of melting sea ice, a sea ice block was collected during severe ice period in the Bohai Sea. The ice block was put in a low temperature environment (−1.0℃) for 48 h, which was then observed using a CT scanner. The thresholds of CT values among gas, ice, and brine were set to −310 HU and −30 HU for segmentation of the CT image, respectively. The gas and brine inclusions were able to be identified in the CT image, and the two-dimensional morphological characteristics of the pores in ice were analyzed. On the basis of image segmentation, the three-dimensional reconstruction of the ice pores was carried out, and the three-dimensional morphological characteristics of the pores were analyzed. It was found that along the ice thickness, the gas area fraction was 5.00%～35.93%, and the brine distribution was discontinuous with maximum area fraction of 0.06%. The cross-sectional shape of the gas and brine pores parallel to the ice surface was approximately circular, with a roundness more than 0.60. The equivalent circle diameter of gas pore was 1.1～3.2 mm, and that of brine was 0.2～2.0 mm. The equivalent circle diameter of pores was positively correlated with the area fraction and negatively correlated with the roundness. In terms of three-dimensional structure, 4 types of gas pores were divided according to sphericity (Rsph) into coronary pores (Rsph ≤ 0.25), irregular pores (0.25 < Rsph ≤ 0.45), strip pores (0.45 < Rsph ≤ 0.60), and spherical bubbles (0.60 < Rsph ≤ 1.00). The coronary pore was the largest (average volume (11522.8 ±5610.2 ) mm3) with smallest amount, and the spherical bubble was the smallest (average diameter (2.0 ± 1.1) mm) with the largest amount. The brine pores were divided into brine channels (0.45 < Rsph ≤ 0.60) and brine cells (0.60 < Rsph ≤ 1.00). The average length of the brine channel was (17.1 ± 12.1) mm, and the average diameter of the brine cell was (1.5 ± 0.9) mm. The amount of brine channels was less, but the volume proportion was comparable to that of brine cells.-
Key words:
- sea ice /
- pore /
- structure characteristics /
- CT scanning /
- 3D reconstruction
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图 5 不同深度位置冰样断面CT图像（左）、阈值分割图像（中）和CT值分布（右）
Fig. 5 CT images of ice sections at different depth (left), corresponding images after segmentation (middle) and the distributions of CT values (right)
图 6 海冰扫描断面各组分CT值沿冰厚的变化，误差线为标准差
Fig. 6 Profile of CT values of air, ice, and brine in the scanning sections along ice thickness. The error bar represents the standard deviation.
图 8 冰样扫描断面CT均值与面孔隙率的关系（a）；粒状冰层（b）和柱状冰层扫描断面CT均值与面孔隙率的关系（c）
Fig. 8 The relationships between mean CT values and porosities of scanning sections in the whole ice sample (a), granular ice (b), and columnar ice (c)
图 9 气体与卤水孔隙的圆度（a）和等效圆直径（b）沿冰厚的变化。误差线为标准差
Fig. 9 Profiles of roundness (a) and equivalent diameter (b) of air and brie inclusions along ice thickness. The error bar represents the standard deviation
图 10 气体孔隙等效圆直径随面积分数（a）和圆度（b）的变化；卤水孔隙等效圆直径随面积分数（c）和圆度（d）的变化
Fig. 10 Variations of equivalent diameters of air inclusions with area fraction (a) and roundness (b), and of brine inclusions with area fraction (c) and roundness (d)
图 11 冰样三维孔隙结构（a）、典型气体孔隙（b−e）和典型卤水孔隙（f−g）
Fig. 11 Three-dimensional pore structure of ice sample (a), typical gas inclusion structure (b−e) and typical gas inclusion structure (f−g).
图 12 球形气泡等效球直径频率密度分布
Fig. 12 The probability density distribution of equivalent diameter of spherical gas bubbles
图 13 球形气泡（a）和卤水胞（b）等效球直径与球度的关系（红色虚线为包络线）
Fig. 13 The relationships between equivalent diameters and sphericities of spherical gas bubbles (a) and brine cells (b), where the red dashed lines are envelopes
表 1 粒状冰和柱状冰层二维孔隙特征平均值
Tab. 1 Mean two-dimensional characteristics of pores in the granular ice and columnar ice
气体面积分数/% 卤水面积分数/% 气体孔隙圆度 卤水孔隙圆度 气体孔隙等效圆直径/mm 卤水孔隙等效圆直径/mm 粒状冰层 7.43 ± 4.24 0.02 ± 0.02 0.81 ± 0.05 0.82 ± 0.09 1.9 ± 0.4 1.0 ± 0.5 柱状冰层 6.59 ± 8.59 0.02 ± 0.01 0.83 ± 0.05 0.72 ± 0.18 1.8 ± 0.4 1.2 ± 0.6 
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表 2 冰样孔隙分类和特征
Tab. 2 Classification and characteristics of pores in ice
气体孔隙 特征 卤水孔隙 特征 冠状孔隙 结构复杂，单体体积最大 卤水通道 呈细长形，长度为6.1～30.0 mm 不规则孔隙 形态各异，单体体积小于冠状孔隙，但比其余两种气体孔隙大 条状孔隙 呈细长形，长度为0.7～41.6 mm，数量较多 卤水胞 近似球体，直径为0.4～4.1 mm 球形气泡 近似球体，直径为0.4～9.3 mm，数量最多
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表 3 粒状冰层与柱状冰层孔隙参数比较
Tab. 3 Comparison of pore characteristics between granular ice and columnar ice
参数 粒状冰层 柱状冰层 体积占比/% 平均体积/mm3 平均长度/mm 平均直径/mm 体积占比/% 平均体积/mm3 平均长度/mm 平均直径/mm 卤水通道 23.3 7.0 10.6 2.3 76.7 46.4 30.0 4.5 卤水胞 100 3.8 2.6 1.5 / / / / 冠状孔隙 / / / / 16.5 11420.8 74.8 27.9 不规则孔隙 39.5 553.5 34.9 9.7 27.0 521.0 41.9 8.8 条状孔隙 52.2 71.4 13.5 4.0 42.7 62.6 13.3 3.7 球形气泡 52.4 8.9 3.3 2.0 43.3 9.0 3.5 1.9 注：“/”为无资料。
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