探地雷达(GPR)在海南岛东北部海岸带调查中的应用 *

对海南岛东北部海岸带调查采用了探地雷达(GPR)研究海岸带沙体的结构、展布和沉积序列,取得了很好的效果。文章通过和有限的露头资料对比以及对反射波形态、结构的研究,确定了海滩脊、海岸沙丘的反射特征以及潜水面的位置。雷达图像显示五龙港古海滩脊由亚水平、不连续、高振幅和透镜状反射波组成;木兰头海岸沙丘已受到人类活动的扰动,短的、陡倾斜反射可能代表未受扰动的海岸风成沙的前积层。研究表明探地雷达是一种可靠、快速和经济的地球物理方法,在砂砾质海岸可产生高质量、高分辨率和连续的反射剖面,值得加以推广。

APPLICATION OF GROUND-PENETRATING RADAR IN COASTAL SURVEY OF NORTHEASTERN HAINAN ISLAND

Ground-penetrating Radar (GPR) system was used to study unconsolidated deposits on their distribution, internal structure and sedimentary succession along northeastern Hainan Island, South China. Ground-penetrating Radar is a surface-geophysical method based on the emission, transmission, reflection, and reception of an electromagnetic pulse and can produce continuous high-resolution subsurface profiles rapidly and efficiently. The study area is located geologically within North Hainan graben and the distribution of capes, embayments and small-scale estuaries along coastal zone are greatly controlled by NNW-SSE trend and ENE-WSW trend faults. The Paleozoic rocks and the Mesozoic granites, which expose along the faults form the substrate of study area. From the Mulantou Cape to the Jingxin Cape, the coast has a quasi-half-moon shape. The northern beach from Mulantou Cape to Chaotanbi is composed primarily of siliciclastic, medium to coarse-grained sand. The area is characterized by coastal systems up to approximately 10m high, composed of Pleistocene sands covered by eolian dunes. Along the southern part of the NE coast between Chaotanbi and Huxin the beach is formed primarily of carbonate material derived from the shallow shoreface area where extensive reefs have developed in the past. The inland part is characterized by a beach ridge capped by dunes. GPR survey was conducted on eolian deposits in a quarry at Mulantuo Cape, and on beach ridge deposits at Wulonggang harbor. The radar facies of beach ridge and coastal dune as well as watertable were delineated after correlated with limited outcrop data available. The radar image shows a series of discontinuous, seaward, slightly oblique reflectors of beach deposits at most left of Wulonggang profile. Toward inland a beach ridge up to 30m wide and 3~4m high possesses a quasi-horizontal, discontinuous, high-amplitude and lens-shaped reflectors. Toward flat back ridge area, below watertable, the quasi-horizontal, discontinuous short reflectors are explained as beach deposits as well. The radar reflectors indicate that the aeolian deposits are partly interrupted by human activities at the Mulantou Cape. The short, high oblique reflectors bounded by erosion surfaces between 160TWT and 280TWT can be interpreted as foresets of coastal dune, which are resemble to what van Overmeeren has described in his paper. A cut-and-fill reflector between 70m and 100m is suggested to be a man made structure. The quasi-continuous strong reflectors between 280TWT and 350TWT probably represent the transition between overlying soft deposits and underlying hard deposits. Below 350TWT, the parabolic reflectors may represent hard substrate. It is suggested that GPR is a very potential geophysical method to study internal structure of coastal deposits in sandy gravel coastal areas. It can produce high quality, high resolution and continuous profiles which are very useful for coastal study and management both for geomorphologists and administrates.