南海北部陆架海底沙波基本特征及属性

Characteristics of Sand Waves on the Northern South China Sea Shelf and Its Formation

南海北部陆架海底沙波是否为残留沙波长期存在争议。为配合陆丰油田的开发,在LF13-2和LF13-1站位之间进行了详细的地形、地貌和海底底流测量。观测数据显示,本研究区发育两组海底沙波。两组沙波的分布和当今的海底地形密切相关,沙波的波宽、波高,沙脊的长度及延伸的方向也都和现今的水深密切相关。此外,沙波区的底质、沉积物颜色等也随现今水深的变化而变化。跨年度的测量结果显示,从2003年夏季到2005年冬季,海底沙波发生了明显的变化,除有明显的沙波迁移,并有新的沙波生成外,沙波区的水深也整体变深,研究区东南端平均加深了0.29m。由于东沙海底高原的存在,东沙区底流有了复杂的变化。实际的观测结果显示,在9MKII站位,向南的退潮流明显大于向北的涨潮流,但在AEM-HR站位,WNW向的涨潮流则稍稍大于ESE向的退潮流。9MKII站位测得的最大底流速为48cm/s,有22%的底流速大于20cm/s,这是启动海底沉积物并能形成海底沙波的最小底流速。本文指出,东沙隆起后,由于强烈的底流侵蚀,在东沙区的中陆架和外陆架区几乎没有现代沉积。构成海底沙波的沉积物主要来自海底地层的剥蚀。测深结果显示的海底加深可能和海底剥蚀有关。很明显,海底沙波和现今的海底底流体系有很好的吻合关系,从而是在现今底流体系条件下形成的,而非晚更新世末次冰期的残留沙波。

Sand waves on the northern South China Sea Shelf had been considered as stable relict bed form. For the industry use of sea bed between LF13-2 and LF13-1, a new round exploration was carried out. The newly obtained data show that both spacings and amplitudes of sand waves are all systematically changing with water depth. Repeated observations since 2003 to 2004 showed that the sea bed is currently active. Due to strong erosion of surface sediment since Dongsha uplifting, there are almost no modern sediments on the shelf of Dongsha area. Sand materials in the study area mainly originate from the erosion of the bed sediment formation. Water depth increment revealed by repeated echo sounder data is mainly due to erosion. Bottom currents are quite complex in the area of Dongsha under water plateaus. At site 9MKII, the south ward ebb current is stronger than the north ward flood current, while at site AEM-HR, the WNW ward flood current is slightly stronger than the ESE ward ebb current. At site 9MKII, the maximum bottom current speed is 48 cm/s, and 22% of the observed bottom current speeds are larger than 20 cm/s, that meet the minimum bottom current speed required for the creation of sand wave. This paper points out that present-day oceanographic condition couples well with the sand wave morphologies, and the sand waves are to a great extent in equilibrium with the ongoing present-day oceanographic bottom current condition, and active.