Geophysical characteristics and migration mechanism of active submarine sand waves off the coast of Dongfang, Hainan
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摘要:
海南东方近岸海底发育有大量沙波，利用多波束测深、侧扫声呐、浅地层剖面、单道地震资料综合分析了活动沙波的地球物理特征，探讨了沙波的分布特征、迁移机制、活动性及形态演变特征。结果表明，研究区海底沙波分布和规模具有显著空间差异性，大中型沙波主要发育于沙脊上，小型沙波主要发育于沙脊两侧，坑槽区发育近对称沙波，研究区西南部沙波不发育。受潮流和科氏力约束，在海底沙脊西侧沙波迁移方向主要为向北（略偏东），在沙脊东侧主要为向南（略偏西）；受地形制约，坑槽区近对称沙波迁移可能停止或方向发生改变。沙波活动性强的标志主要包括：① 形态呈不对称的“脊尖槽缓”，② 叠置小沙波与沙纹发育，③ 浅部有透明层，④ 陡坡面反射模糊，⑤ 内部斜交前积结构。分析认为，沙波活动性与其形态密切相关，包括弱运动、强运动、不运动3个演变阶段。
Abstract:A large numbers of sand waves are developed on the seafloor off the coast of Dongfang, Hainan. This paper is devoted to the study of the geophysical characteristics of active submarine sand waves by means of multi-beam echo-sounding, side-scan sonar, sub-bottom profile, and single-channel seismic, and to the discussion on the distribution pattern, migration mechanism, activity, and morphologic evolution of the sand waves. The results show that there are significant spatial differences in the distribution and scale of submarine sand waves, for examples, large and medium-sized sand waves are mainly developed on the top of sand ridges, small sand waves developed mainly on the two sides of sand ridges, and near-symmetrical sand waves developed in the areas of pits. Submarine sand waves are not developed in the southwest of the study area. Constrained by tidal current and Coriolis force, the migration direction of the sand wave on the west side of the submarine sand ridge is mainly northwards with a few towards east, while those on the east side is mainly southwards with a few slightly towards west. Restricted by topography, the migration of near-symmetric sand waves may stop or change direction. Strongly active submarine sand waves are characterized by the features as follows. (a) Asymmetric shape with "sharp ridge and gentle trough"; (b) Small sand waves and sand ripples superimposed on sand waves; (c)Sub-bottom profile shows transparent layers; (d) Blank reflection on the steep slope; (e) Internal oblique progradation configuration. The analysis shows that the movement of the sand waves is closely related to its morphology, and three stages of evolution from inactive to highly active to motionless are recognized.
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表 1 主要调查设备及数据采集参数
Table 1. Main parameters of the surveying systems
参数 多波束测深 侧扫声呐 浅地层剖面 单道地震 设备型号 EM710S EdgeTech4200 SES2000Medium SIG 2Mille震源Geo-sense 48电缆 中心频率/kHz 120 110 6 0.6～1 量程/m 通常为水深的3～4倍 单侧100 60 海底以下100 声源发射率/Hz 大于5 约为7 约为12 1 观测系统 船底安装 拖曳于船尾后约120 m，近底观测 船底安装 沉放约0.5 m，拖曳于右船尾后约45 m 
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表 2 海南岛西南近岸海域海底沙波形态参数统计
Table 2. Morphological parameters of submarine sand waves in the southwestern offshore area of Hainan
形态参数 研究区 海南岛西部VI区[7] 东方岸外东区[12] 海南岛西南海域[25] 海南岛东方海域[26] 水深/m 9～46，平均25 19.3～21.3 20～50 30～40 平均15 波长/m 7.1～329，平均93.3 41～148 − 5.8～91.8 41.5～719.7 波高/m 0.3～13.2，平均4.7 1.7～5.9 2～10 0.1～4.3 0.84～9.9 垂直形态参数 5.5～54，平均22 16～29 18～44 − 14.5～196.2 缓坡倾角/（°） 3.2～13，平均5 2.7～5.1 − − 0.34～8.05 陡坡倾角/（°） 3.2～25，平均10.7 10.4～16.8 10～20 − 2.0～13.3 对称指数 1.0～12.8，平均4.5 3.1～4.6 7～12.6 0.32～6.52 0.67～9.63 注：缓坡水平距离（a），陡坡水平距离（b），波长（L=a+b），波高（H），垂直形态参数（L/H），对称指数（a/b）。
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表 3 研究区海底活动沙波地球物理特征
Table 3. Geophysical characteristics of active submarine sand waves in the study area
观测方法 地球物理特征 多波束测深 平面上呈韵律的新月形条带状；剖面上呈“脊尖槽缓”的波状排列，成群出现。 侧扫声呐 海底线呈“竹节状”变化，波脊线两侧回波强度变化明显；发育叠置小沙波与沙纹。 浅地层剖面 波状排列，通常波形不对称；浅部有一层透明层；缓坡面反射连续清晰，陡坡面反射模糊；内部可见迁移活动底界面。 单道地震 波状排列，波形陡缓分明；缓坡面表层为连续强振幅反射，陡坡面表层反射模糊。沙波内部为斜交前积反射结构，下伏为亚平行的层状反射。
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