应用探地雷达方法对海南岛博鳌海岸沙坝的研究

APPLICATION OF GROUND-PENETRATING-RADAR TO COASTAL BAR STUDIES IN BO'AO IN EASTERN HAINAN ISLAND OF CHINA

了解海岸沙坝的内部结构对于动态地恢复海岸沙坝与海平面变化的相互关系具有重要的意义 ,探地雷达 (GPR)为海岸沙坝的研究提供了一种快速、经济和高分辨率的地球物理方法 ;探地雷达利用高频电磁波在介电特性不均匀的沉积体中产生反射获得图像 ,通过解译可确定地下沉积体的性质和结构 ;在海南岛东部博鳌玉带滩海岸沙坝的近口门处、决口堤处和南岗村东侧玉带滩海岸沙坝顶部进行了探地雷达反射剖面的测量 ;研究结果表明自然堆积的沙体反射波连续 ,受风暴浪改造的沙体同相轴错断 ,反射波杂乱不连续 ,无法进行对比 ;运用探地雷达相分析方法并结合钻孔资料 ,认为 5～ 6 m以上的平行反射波形可能代表中细砂沉积 ,以下的丘状或杂乱反射波形可能代表中粗砂沉积 。

Yudaitan coastal bar, located at Bo'ao, eastern Hainan Island, is a large structure about 8 km long (Qionghai segment) and 6 m high and it has a relatively narrow beachface backed by a steep slope of the bar. It is composed of coarse, light gray yellowish feldspathic quartz sand with occasional comminuted shell fragment. Ground penetrating radar survey was carried out at northernmost (barrier spit), middle (scoured bar) and south segments. The result shows that the continuous, parallel and horizontal reflectors occur at northernmost barrier spit, which indicates homogenous fine to medium grained sand as described by Beres & Haenu. The continuous but strong reflector at about 80 TWT mostly results from water table, which impedes the penetrating of electromagnetic wave far to the underground. An unconformity appears on the farthest right on profile YUDT between 40 and 50 m, which shows the beach sand on the left overlies the estuarine deposits on the right. The radar profiles across the scoured bar show irregular reflectors. Uncontinuous, chaotic and broken reflectors characterizes the scoured bar, which indicates the reworking by storm wave and surge. The longitudinal profile at the south segment of Yudaitan coastal bar shows several radar facies. The quasi parallel reflectors above 100 TWT may probably represent fine to medium grained beach sands and the sigmoid typed reflectors between 55 and 60 m and 70 and 80 m indicate the prograding of beach sands. The continuous and strong reflector at 100 TWT shows the position of water table. From below water table to 250 TWT, hummocky reflectors appear, indicating the increase of coarse grains, but below 250 TWT, no signal is reflected. The GPR method is reliable when used for unconsolidated coastal sediments with less content of clay particle, but continuous geologic logs from test holes and wells are needed to interpret the radar data if the geology is complex. GPR is a reliable, high resolution and intrusive method for studying the internal structure of coastal bars.