Analysis Of Ocean Tide Loading Displacements And Gravity In The Coastal Areas Of China

With the advancement of science and technology,the accuracy of instruments such as GPS,VLBI and Superconducting Gravimeter has been greatly improved,and the accuracy of observations required by scientific research and applications is becoming higher and higher,and because of ocean tide loading(OTL)is the second signal only to the solid Earth tides in geodetic.In the coastal areas of China,the displacements and gravity changes caused by the OTL have reached several centimeters and several μGal,respectively.So the OTL effect is the important influencing factors that must be considered in the geodetic observation data processing.For example,the impact of OTL on the station displacements must be considered in high-precision GPS measurement,and the influence of gravity OTL must be considered in high-precision Superconducting Gravimeter measurement.In addition,because the density and elastic structure characteristics inside the Earth control the spatial and temporal characteristics of the load response,the response of the OTL can be observed by high-precision geodetic methods to provide constraints on the crustal and upper mantle structure of Earth.In the OTL effect,gravity and displacements are more sensitive to the Earth’s internal structure than tilt and strain.In this paper,displacements and gravity are selected to analyze their sensitivity to the Earth model and provide a reference for inversion of the Earth’s interior.Since the gravity OTL effect includes two parts: elastic effect and Newtonian effect,and only the elastic part is controlled by the internal structure of the Earth,the premise of invert the internal structure of the Earth by gravity is to separate the two parts first.OTL displacements are all contributed by the elastic part,so this step is not required.This paper separates the elastic and Newtonian effects of gravity OTL based on the SPOTL OTL calculation program.First,based on the HAMTIDE11 a global ocean tide model and the Gutenberg-Bullen A Earth model,the integral Green function method was used to calculate and analyze the gravity elastic effect and Newtonian effect of M2,S2,K1,and O1 constituents,as well as the displacements at grid site.The geographical distribution of the OTL effects in the coastal areas of China and their influence on the gravity field and displacements of coastal stations.Then analyze the influence of global ocean tide model,Earth model,local ocean tide model and station elevation on the calculation of the three.Finally,the sensitivity of OTL displacements and gravity to the Earth model is calculated and compared,which provides a useful reference for the study of OTL displacements and gravity in the surrounding areas of China,and the theoretical feasibility of gravity inversion.The results of study are as following:1.The Newtonian and elastic effects of the gravity OTL of the four main constituents M2,S2,K1 and O1 have reached the magnitude of μGal in eastern China and the surrounding coastal areas,with horizontal and vertical displacements up to several millimeters,of which the maximum vertical displacements of M2 is greater than 20 mm.The spatial distribution shows a decreasing trend from the coast to the inland,and this trend is more obvious on diurnal constituents(O1,K1).The spatial distribution of M2 and S2 constituents,K1 and O1 loading effects are consistent with each other.In the southeast coastal area,diurnal constituents(K1,O1)have a more impact on inland than semi-diurnal constituent(M2,S2).2.Different ocean tide models have a greater influence on the calculation results and the spatial distribution of vector differences between the different models is highly consistent.The differences occur in the eastern coastal area.The biggest difference of the four constituents appears between the NAO99 b and FES04 models.For the K1 consistent,the difference between the FES04 and NAO99 b models is the largest,The horizontal and vertical directions of displacements and the elastic effects of gravity are about 0.4mm,2.3mm and 0.51μGal,respectively.For the M2 consistent,the largest difference occurs in the coastal area in southeastern Jiangsu of the FES04 and NAO99 b models.The horizontal and vertical directions of displacements and the elastic effects of gravity are about 1.3mm,7.5mm,and 1.75μGal,respectively.It shows that the ocean tide model is still the main factor restricting the calculation accuracy.3.The local tidal effect has a greater impact on the eastern coastal area.As far as the M2 consistent is concerned,the maximum difference about Newtonian effect and elastic effect of the gravity OTL occurs at the coastal stations,which are 1.87μGal(Rongcheng Station)and 2.32μGal(Xiamen Station);The displacements has the greatest impact,the east,south and vertical directions are 3.16,3.90 and 8.28 mm respectively.This indicates that the accuracy of the global ocean tide model in the offshore area is relatively poor,so it is necessary to consider the high-precision local tide model when calculate the deformations due to OTL at offshore stations.4.The elevation of the station only affects the Newtonian effect.The Newtonian effect is more sensitive to the elevation of coastal stations,and the influence of elevation on the Newtonian effect(M2)is up to 1.4μGal at Xiamen Station.Therefore,the station elevation affect on the gravity effect should be taken into account in the high-precision geodesy.5.The differences caused by different Earth models occur in coastal areas and the distribution rules are relatively consistent.The Newtonian effect is little affected by the differences between the Earth models;the distribution of elastic effect and gravity OTL are more consistent,and the differences are mainly distribute in the eastern coastal area.There are large differences between PREM and other Earth models,and the difference between PREM and AK135 models is the largest.The largest difference between the east direction of the M2 consistent occurs in the coastal area of northwestern Taiwan,and the vector difference is about 1.2mm;Jiangsu,Fujian,and Zhejiang coastal areas follow,and the vector difference is about 1.0mm.The difference in the north direction mainly occurs in the south of Liaoning and the north of the Yellow ocean,the southwest of Taiwan and the adjacent ocean,and the vector difference is about 0.9mm.The maximum vector difference of the vertical direction appears in the southern coast of Liaoning and the western coast of the Taiwan Strait is about 1.8mm;the elastic effect reaches the maximum in the coastal area of Fujian and surrounding area,about 0.52μGal.6.The elastic effect of gravity OTL in some areas is more sensitive to the Earth model than the vertical direction of OTL displacements,indicating that the elastic effect may be a better choice.However,the current research on gravity OTL mainly focuses on the correction and deduction of solid Earth tides and other studies,the elastic part and Newtonian part of the gravity OTL in the instrument measurement results cannot be separated.In addition,Superconducting Gravimeters are expensive,have high requirements on conditions,and have few distributions worldwide.Relatively speaking,GPS has the advantages of low price,full time period,global high-resolution coverage,low requirements for observation conditions,and easy access to data.So far,the OTL displacements still has an absolute advantage in constrain the crustal and upper mantle structure.