中国台湾浅滩海砂砂体的地球物理特征及有利赋存标志*

探测查明海砂分布及其资源量, 对海洋矿产资源规划和社会经济发展有重要意义。利用在中国台湾浅滩采集的单波束测深、侧扫声呐、浅地层剖面、单道地震等多源地球物理数据, 识别出海底沙脊、沙波、埋藏古河道等有利于海砂赋存的地貌标志, 以及厚泥盖层等不利标志, 它们在平面上渐变交错分布。地球物理数据与表层沉积物、钻孔岩心结果对比, 证实浅地层剖面能有效识别表层砂体上部和排除厚泥盖层(厚度>10m), 表层砂体上部呈断续中弱振幅杂乱反射; 单道地震剖面能有效识别砂-泥地层界面, 垂向上刻画表层及埋藏砂体形态, 砂体最主要的特征为连续性差的弱振幅反射中夹杂空白、杂乱的“白雾”反射。砂体响应特征反映了砂体声学较难穿透的特性, 同时也可能是砂体含气所致。分析表明在砂体形态多变或钻孔分布密度低时, 利用地球物理方法, 结合一定数量的钻孔岩心约束, 相比仅依靠大量钻孔岩心, 可更为经济、高效地对砂体的垂向形态和平面展布进行精细刻画, 从而有利于更准确地估算海砂的资源量。

Geophysical characteristics and favorable occurrence signs of marine sand-gravel body in Taiwan banks*

An efficient and economical exploration of the distribution and resources of marine sand is of great significance to social and economic development and marine mineral resources planning. Multi-source geophysical data acquired in Taiwan banks, such as single beam sounding, side-scan sonar, sub-bottom profile, and single-channel seismic, were used to identify the geomorphic signs favorable to the occurrence of marine sand and gravel, such as submarine sand ridges, sand waves, and buried ancient channels, and the signs unfavorable to the occurrence of marine sand, such as thick argillaceous overlying strata. These geomorphic signs alternate progressively on the plane. Comparison of geophysical data with surface sediments and borehole cores shows that the sub-bottom profile can effectively distinguish the upper surface sand-gravel body and thick argillaceous overlying strata (thickness > 10 m). The upper surface sand-gravel body on the sub-bottom profile shows discontinuous reflection with medium or weak amplitude. The single-channel seismic profile can effectively identify the sand-mud interface, and depict the shape of the surface and buried sand-gravel body vertically; and the main characteristic of the sand-gravel body on the single-channel seismic profile is weak amplitude reflection with poor continuity mixed with blank and disordered “white cloud” reflection. The seismic response characteristics of the sand-gravel body reflect that the sand-gravel body is difficult to penetrate, and may also be due to the gas contained in the sand-gravel body. Our analysis shows that when the shape of the sand-gravel body is changeable or the distribution density of boreholes is low, the use of geophysical method combined with a certain number of borehole core constraints, compared with relying on a large number of borehole cores only, can more economically and efficiently make a fine depiction of the vertical shape and plane distribution of sand-gravel body, which is conducive to more accurate estimates of sea sand resources.