Research Of Sub-bottom Profiler In Detecting Underwater Cultural Relics

China has nearly 18 thousand kilometers of coastline,and has a long history of marine development and utilization,which has created a rich and splendid underwater cultural heritage.Pingtan is located in the northern part of the Taiwan Straits,in the southeastern coast of China.It is an important maritime passage connecting the north and the south of China.There are many submerged reefs in the vicinity of Pingtan,and the water flow is swift and the hydrological elements are complex.This makes the sea area rich in underwater cultural heritage resources.As an important kind of underwater cultural heritage,the exploration and study of the sunked ship will help us to understand the economic and trade of the society and the level of the development of culture and technology.With the development of the economy,the marine development has further accelerated,and the possibility of the destruction of underwater cultural heritage has increased dramatically in the process of ocean development.How to effectively find the underwater cultural heritage and protect it properly has become an urgent problem.The marine geophysical method is an efficient and safe means of investigation.It is an important trend for the investigation of underwater relics.The investigation of sunken ships mainly includes side sonar,multi-beam,sub-bottom profile,underwater robot and so on.Sub-bottom profiler is a kind of high resolution geophysical exploration instrument,which is widely used in marine regional geological survey.It has the characteristics of high frequency,wide frequency band and convenient.For the identification of shallow abnormal bodies,sub-bottom profile is an important and effective means to investigate the buried and exposed underwater relics and obtain the size and shape of the target and the environment of the undersea surface.Since 2013,the third Institute of Oceanography,State Oceanic Administration has conducted a systematic investigation and study on underwater cultural relics in the Pingtan sea area.Based on the valuable data accumulated during this period,the sub-bottom section is used to study the underwater wreck.In this paper,the reflected wave characteristics of surface sediments,wood and steel are studied by forward modeling,and the effects of thin layers of different thickness on the amplitude and polarity of reflected waves are discussed.Based on the actual survey data in Pingtan sea area,the primary and second reflection of the seabed are picked up by the energy ratio method,then the amplitude of the signal at the main frequency is calculated by Fourier transform,and the polarity of reflection coefficient is judged by the single reflected wave and the multi channel mixed waveform.In the above calculation process,median filtering,background noise calculation and moving average processing methods are also involved.The following conclusions can be reached:1)The reflection coefficient of the iron shipwreck is greater than the reflection coefficient of the reef,and is obviously higher than the surrounding stratum,and it can be distinguished from the absolute value of the reflection coefficient.There are many disturbances in the calculation of the reflection coefficient polarity of the iron shipwreck,which can not effectively identify the iron sunken ship from the polarity.2)There is obvious negative reflection coefficient anomaly in the wood wreck site,and the precision of reflection coefficient polority from multichannel waveform covered is better than that from single waveform.3)sub-bottom profile is an effective means to detect underwater cultural relics,but its detection is linear,and other geophysical methods should be used to detect underwater cultural relics while used sub-bottom profiler.