On Barrier-Lagoon Development and GPR Application in BO''AO Area
博鳌地区沙坝-泻湖沉积及探地雷达的应用

A suite of sedimentary sequences and a three-dimensional (3D) observation ofcoastal bar and shoal deposits from sandy gravel coast zone in Boao, eastetn Hainan Islandwere made using cores, outcrops and ground -penetrating -radar (GPR) profiles. According tolitho -facies distribution, three sedimentary sequences are distinguished in sedimentarycolumns, which constitutes an upward coarse-fine-coarse cycle.GPR reflectors show that water table in the area is located about 6 m below the surfaceand the reflective signals decrease rapidly under the water surface due to absorption ofsaturated sand to the electromagnetic (EM) waves. The GPR electromagnetic wave penetratesless than 5 m on modern beach because of the intrusion of salted sea water. The samePhenomenon is encountered on the shoal near the lagoon where the ground water invadessediments. Several GPR facies can be identified from the profiles. As to Yudai Tan coastalbar, two types of reflector configurations of regional significance were verified up and downthe water surface or unconformity in some places. Parallel to slightly wavy reflectors abovewater surface contrast with hummocky, chaotic, oblique and hyperbola reflectors below thewater surface. The former reflectors may indicate homogeneous and fine to medium sandwhi1e the latter indicates an increase in the content of coarse sand and gravel. Discontinues,hummocky and chaotic reflectors identified on Nangang Cun shoal may indicate analternation between sand and clay during deposition.The results show that the Nangang Cun shoal may develop in late Holocene period andbefore it joined the coastal bar, there was a lagoonal region separating them. It is indicatedthat GPR is a reliable, rapid, and economical method for high-resolution profiling ofsubsurface sediments in sandy grave1 coastal zone. In order to increase the reliability,continuous geological logs from test holes, wells and outcrops are needed to interpret theradar data.