Application Research On Side Scan Sonar And Multi-beam Sonar In Underwater Cultural Relics In Pingtan Sea Area

China is a major country of underwater cultural heritage,and has abundant underwater cultural relics resources.The protection of underwater cultural heritage is of special meaning to the development,utilization and management of the ocean.Underwater archaeology is a subject of research on underwater cultural heritage.This article collects and summarizes the historical information of underwater cultural relics,analyzes the distribution and preservation status of underwater cultural relics along the southeast coast of China,and selects Fujian Pingtan sea area as the target area,analyze the sea area characteristics and sedimentary environment around Pingtan,as well as the buried characteristics of underwater cultural relics in the sea area.Based on modern marine geophysical survey methods,combined with the topography of Pingtan sea area,used side scan sonar and multi-beam sonar to investigate comprehensive marine geological and geophysical survey systematically,and carry out underwater cultural relic detection and research.Because side scan sonar can intuitively determination the condition of the seabed,analyze the seabed micro-topography,identify seabed obstacles,divide the bottom quality characteristics of the target,and judge the obvious irregular topography of the seabed;multi-beam sonar can accurately measure the depth,size and position of the target,and can perform three-dimensional visualization analysis.On the other hand,in the side scan sonar detection,the middle position is the blind area,and the graph shows the images on both sides of the ship’s route.However,for multi-beam sonar detection,the image resolution below the hull is the highest,and the resolution decreases gradually from both sides.In conclusion,when the two sonars are used in combination with the ship,they can complement each other and give consideration to the intermediate and marginal areas of underwater exploration.As a result,the detection accuracy is improved,the high definition image of the underwater cultural relics and the damage to the underwater cultural relics are reduced,the investigation efficiency of the underwater cultural relics is improved,and the detection error is reduced.By studying the reflection image characteristics of underwater artifacts under side scan sonar and multi-beam sonar acoustic instruments,we systematically investigated several shipwreck relics characterized by revealing the characteristics of its underwater landforms and cultural relics,and got the following conclusions:(1)Through the analysis of the characteristics of Pingtan sea area,the comprehensive marine geophysical detecting methods combining side scan sonar and multi-beam sonar is selected to detect the underwater cultural relics in Pingtan sea area.Through comparative analysis of the characteristics of the suspected sunken ship,there were 17 suspicious underwater relics identified by side scan sonar,and 9 relics identified by multi-beam sonar;(2)Through the application of side scan sonar to carry out the survey of different azimuth,different spacing and different scan width,it is found that the anomalous shape of the target object can be better identified by the parallel direction of the target object.Under the same scan width,the closer the distance to the target,the clearer the image.And the narrower scan widths are,the higher the resolution is at approximately the same distance.(3)During the detection of multi-beam sonar,it is suggested that the resolution is better in the equiangular mode than in the equidistance mode within ±60°.In addition,multi-beam sonar adopts single-sided focusing mode,which can focus all beams on single-sided target imaging,and can obtain better beam density and higher resolution.(4)The analysis of reflection coefficient of sediment shows that there is a linear positive correlation between the particle size and the reflection coefficient of sediment.The reflection coefficient of metal hull is high,iron shipwrecks can be better distinguished from reefs and sediment from the value of reflection coefficient.Because the tangential and radial reflection coefficients of the wood are negative,so it is more convenient to distinguish wooden shipwreck relics from other disturbances.Most bricks and tiles are magnetic(thermal remanence).It can also be detected by magnetic geophysical detecting method;(5)The magnetometer is mainly suitable for detecting ferromagnetic targets,and the sub-bottom profilers is suitable for fully or semi-shallowly buried underwater relics.The combination of magnetometer and sub-bottom profilers can make up for the deficiency of side scan sonar and multi-beam sonar detection.By means of comprehensive marine geophysical detecting methods,several underwater shipwreck relics were systematically investigated to reveal their underwater landforms and relic features,which can provide reference and technical support for the subsequent underwater archaeology.(6)Because the acoustic and magnetic geophysical detecting methods belong to the telemetry method,they cannot directly verify the properties of the resolved objects.Therefore,it is necessary to investigate by means of underwater exploration and underwater robot.However,the disadvantages of this method are that the open angle of the underwater viewing angle is small,and the underwater turbidity will have a certain influence on the working efficiency and the recognition of the final resolution ratio,etc.,and it is not suitable for the sea area with rapid current.