三沙永乐龙洞的三维形态

Three-Dimensional Morphology of Sansha Yongle Blue Hole

三沙永乐龙洞是世界已知最深的海洋蓝洞,其三维几何参数和精细形态与蓝洞的形成和演化密切相关。2017年,采用ROV(Remote Operated Vehicle)和多波束系统对龙洞测量,获取了龙洞精细的水深和三维形态数据。本文利用获取的点云数据建立了龙洞的精细三维模型,计算了龙洞的几何参数,展示和分析了龙洞的三维形态特征。龙洞底部基于当地10 a平均海面的水深为301.19 m,体积1 417 149 m3,表面积98 151 m2,其形态复杂曲折,洞体平面总体向东北方向展布,洞口中心点与洞底部尖角水平错位距离约118 m,整体上酷似一只“运动芭蕾舞鞋”。洞体在垂向上分为5层:洞体Ⅰ为洞口部分,呈现浅盘口状,坡度平缓;洞体Ⅱ和洞体Ⅲ呈现近似圆柱体,垂直下降,宽度均匀;洞体Ⅳ形态极为复杂,其体积最大,形态在90 m和158 m处出现2次北向转折;洞体Ⅴ向东北延伸,洞径逐渐收窄变小,形成碗形洞底,洞底平均宽度约36 m。洞体内壁凹凸不平,存在窝穴、突起、波纹、侧洞、横向沟槽、台阶、纵向断裂等小形态特征,展现了龙洞内部形态的复杂性。龙洞的整体复杂大形态和内部姿态各异的小形态特征为龙洞的演化和成因研究提供可靠证据。

The Sansha Yongle Blue Hole is the deepest known marine blue hole in the world,and its 3D geometric parameters and fine morphology are closely related to the formation and evolution of the Blue Hole.In 2017,ROV and a multibeam echosounding system were used to survey the Blue Hole,and fine water depth and 3D morphological data of the hole were obtained.In this study,the point cloud data obtained during the surveying period are used to establish a detailed 3D model for the Blue Hole,calculate the geometric parameters of the Blue Hole,and analyze the 3D morphological characteristics of the Blue Hole.The depth of the bottom of the Blue Hole is calculated to be 301.19 m based on the local ten-year average sea level,the volume is 1417149 m3,and the surface area is 98151 m2.Its morphology is complex and twisted,and the hole plane is generally spreading northeastwards,and the sharp corner of the hole bottom deviates from its entrance by 118 m,and the overall appearance resembles a sport ballet dancer’s shoe.The hole body can be subdivided into five layers in the vertical direction as follows.CaveⅠis the cave entrance part,showing a shallow disk mouth shape with a gentle slope;caveⅡandⅢshow a nearly cylindrical body with vertical descent and uniform width;caveⅣhas an extremely complex form,and its volume is the largest,and the form turns northward twice at 90 m and 158 m;caveⅤextends to the northeast,and the diameter of the cave gradually becomes smaller,forming a bowl-shaped cave bottom with an average width of about 36 m.The inner wall of the cave is uneven,and there are small morphological features such as nests,protrusions,corrugations,side holes,lateral grooves,steps and longitudinal fractures,which show the complexity of the internal morphology of Blue Hole.The overall complex morphology and the small morphological features with different internal morphological characteristics of Blue Hole provide some reliable evidences for its evolution and genesis.