The Study Of Integrated Marine Geophysical Method In Detecting Underwater Cultural Heritage

Pingtan is situated at the southeast coast of China,and guards the throat of Taiwan Strait,which has been closely related to the two ports of Quanzhou and Fuzhou since ancient times,Consequently it has become the transportation of China's famous maritime Silk Road.Nearby the waters in Pingtan has many shallow reefs and complex hydraulic flow pattern.Due to various factors,underwater cultural heritage resources are abundant,there are many wrecks have been found in Wanjiao No.1,Wanjiao No.2,Five Dynasties Fenliuwei island,Song Dynasty Southwest island of Dalian island,Yuan Dynasty Dongjiao village of Xiaolian island,etc.There may still be many underwater cultural heritage(UCH)resources that lay on the sea floor,waiting for refreshing glory.With the accelerated development of marine development and utilization,submerged heritages nearby the waters in Pingtan are suffered from the growing risk of damage.It is imperative to protect UCH,further,the exploration and identification of UCH is the first step to take protective measures.Marine geophysical technology is a kind of harmless detection method,which is the main development direction for detecting UCH.The physical properties of submerged heritages are often different with the surrounding medium of sea floor,such as acoustic backscatter intensity,acoustic impedance,magnetic property,electrical property,radioactivity,etc,which lay the foundation for applying side-scan sonar(SSS),multi-beam echo sounding(MBES),sub-bottom profiler(SBP),magnetometer(MAG),electrical technology and radioactive technology and other marine geophysical methods to detect UCH.Depending on a wide range of factors,such as nature(e.g.metallic/non metallic),size,shape,exposed and buried of the targets,water depth,water quality,hydrodynamic,lithology,sediment particle size,topographic and geomorphic conditions,geological setting,different geophysical methods have their peculiarity and detection capability.The application of integrated marine geophysical methods enhances the accuracy and reliability of UCH detection.Marine underwater cultural heritage is mainly distributed in the coastal area,channel area,sea war area,fishing area and sea peril area.Based on the modern shipwreck,ancient shipwreck and other underwater cultural sites identified in the investigation of the waters near Pingtan,the imaging quality of SSS,MBES,SBP and MAG is studied in turn.The following conclusions can be reached:(1)For UCH that is all or partially exposed to the seabed,SSS,MBES,SBP and MAG play a role.SSS and MBES have advantages in the recognition of the shape,size and external structure of UCH,and MBES provides higher accuracy and 3-D imaging,SBP can determine the buried conditions of UCH,MAG can determine the material(metal)and quality of the UCH.(2)For UCH that is all buried in the seabed,SSS and MBES are powerless,therefore,which relys on SBP and MAG and other methods.(3)The application of integrated marine geophysical methods will significantly enhance the recognition of UCH,but the geophysical method is a remote sensing means.Confirming whether targets belong to UCH by diving or underwater robots that are able to perform underwater photography and seabed sampling.Based on the application of marine geophysical methods nearby the waters in Pingtan,the marine geophysical survey pattern of UCH is established as follows: Firstly,SSS and MBES are used to rapidly achieve full coverage scanning for the target sea area in order to determine the detailed investigation programme,SSS should be set to the larger range,MBES should be set to the larger swath angle,in addition,anomaly targets that are identified by equipment could be delineated.Second,combined with the distribution rule of UCH,marine geophysical methods are used to refinedly achieve full coverage scanning for main surveying unit within the scope of the target sea area,such as the channel area.The measuring lines should be densely laid in main surveying unit,meanwhile the equipment resolution is set to the highest level as far as possible to achieve the highest level.Thirdly,the anomaly targets of UCH were delineated for the first two stages,which need to be confirmed by diving or underwater robot.