GPR Detection Technology Study Of The Internal Structure And Hidden Defects Of The Dengfeng Guanxingtai Observatory

As a world cultural heritage,the Dengfeng Observatory is the earliest observatory in China.It was built in the Yuan Dynasty,with a history of more than 700 years.Based on the data measured in the Observatory,Guo Shoujing issued the Shoushi calendar,and the one-year period he calculated is exactly the same as the Gregorian calendar which is now universal all over the world,while three hundred years earlier,which reflects the importance of the Observatory in the field of astronomical sciences building.The Observatory is of a brick hybrid structure,which consists of two parts,the stage body and Shi Gui.The height of the stage is about 9.36 meters,and about 12.95 meters together with the upper small room.The length of each side is 8.28 meters on the top,while 16.52 meters on the base.How to maintain the structure of the Observatory,such a large-scale construction,stable at that time? What materials and special construction methods are used in the process of construction? What about the internal structure of the Observatory? Whether there are hidden chambers in the building's body? What about the distribution of the internal and external wall,after over 700 years,especially the destruction of the artillery bombardment of World War II? In the absence of literature,all of these problems need to be detected and studied.Ground Penetrating Radar(GPR)have the advantages of no damage to the target,high resolution,fast imaging and so on,which meets the needs of fine exploration of the structure of cultural relics well.In this paper,the author explores the surface of the Observatory using GPR,and studies the Observatory through a variety of methods,which contain profile method,wide-angle reflection method and transmission method.The profile method is adopted to get fine near-surface structure information around the observatory.The wide-angle reflection method is used to get the electromagnetic velocity of the brick layer,and then get the dielectric permittivity.And transmission tomography method is good at detecting the velocity distribution inside the target body,which provides insight into the internal structure.The results show that the electromagnetic velocity of the Observatory's external brick layer is 0.14m/ns,and the dielectric constant is 4.59.The overall structure of the Observatory is composed of the exterior high-speed brick layer and the internal low-velocity rammed earth.The fine structure of each outer wall of the Observatory isn't same on the transverse and longitudinal,but the structure of the area is mainly composed of external three-layer brick and internal rammed earth.But there are changes in the structure of some regions,not only the overall thickness of the brick is different,but also the number of brick layers are four,not three.There is no symmetry between the east wall and the west wall.Whether the reason for this construction is on purpose,or to make up for the inaccurate measurement due to the limitation of the constructing conditions,further analysis is needed.The author also adopts three attributes(three instantaneous attributes,related attributes and coherent attributes)to analysis the wall cracks in the Observatory.The results show that coherent attributes is most sensitive to the wall cracks,which can not only indicate the crack positions,but also reflect the depth of the cracks.