Accuracy assessment of ocean tide loading computations for precise geodetic observations

The increasing resolution of ground based gravity measurements (e.g. by superconducting gravimeters) as well as satellite based gravity field studies allows to study very small signals, globally as well as local. On the other hand, this requires the correction of such signals to uncover others. To study the Earth’s deep interior and the on-going dynamic processes requires the correction of disturbing signals, and one of these signals is related to ocean tidal loading. Although new ocean tide models are being derived from current satellite missions, there are still uncertainties.In this paper we present an intercomparison ocean tide models to test their fit to world-wide observations. Therefore, three TOPEX/POSEIDON (T/P) satellite derived models (CSR3.0, FES95.2 and TPXO.2) beside the classical SCHW80 model were selected for an accuracy assessment study. The selected models have been subjected to an intercomparison test, tide gauge validation test and comparison to 59 tidal gravity stations.The intercomparison test shows a good agreement between the T/P-based models for the open ocean and remarkable disagreement between the selected models in the coastal regions indicating that such models are still problematic in these regions. The tide gauge validation shows that the T/P derived models fit tide gauges better than SCHW80, with a better fit for the semidiurnal constituents than for the diurnal constituents. Comparing the gravimetric ocean-tide loading computed from the selected models with the residuals from a set of 59 tidal gravity stations shows that there is an improvement of the T/P derived models with respect to the Schwiderski model, especially in M2. However, this improvement is not as significant as the result of the comparison with the pelagic data. The procedure developed for the comparison of T/P derived models with SCHW80 is presented. The results provide not only information and improvement with regard to SCHW80, but also information about the properties of the new models. It is intended to continue this work applying the very recent models to see how they perform compared to this study.With this study we provide boundary conditions for the improvement of new ocean-tide models in order to benefit from the gravity measurements now possible regarding the evaluation of Earth structures and dynamic processes.     

Determination of vertical displacements over the coastal area of Korea due to the ocean tide loading using GPS observations

This paper describes the GPS applicability for detecting the vertical displacements of ground stations caused by ocean tide loading effects. An experiment was carried out using 12 permanent GPS stations located in the coastal area of Korea using data in the period 1 July until 26 August 2003. The relative height differences were calculated from hourly DGPS data processing based on the carrier-phase observation. The power spectra of the M₂ and N₂ constituents of ocean tide loading were derived using the CLEAN algorithm. The differential vertical displacements generated by the ocean tide loading effect are typically 3–25 mm in coastal area of the Korea. We compared the results from GPS with those of the ocean tide models, NAO.99Jb regional model and GOT00.2, FES99 global models. The M₂ (N₂) amplitude differences of vertical displacements between GPS and GOT00.2 is 1.22 ± 3.61 mm (1.01 ± 1.48 mm), and that of the M₂ (N₂) amplitude difference between GPS and FES99 is 0.04 ± 4.64 mm (0.64 ± 1.75 mm), whereas the M₂ (N₂) amplitude difference between GPS and NAO.99Jb are 0.05 ± 1.03 mm (0.86 ± 1.18 mm). The highest vertical displacements at the PALM station are found for 24.5 ± 0.7 mm from GPS observation, and 22.9 mm from the regional model NAO.99Jb and 13.17 and 10.00 mm from the global models GOT00.2 and FES99, respectively. These values show that the vertical displacements derived from GPS are in good agreement with those of the regional model NAO.99Jb around Korea, more than with the global models. This result indicated that GPS is an effective tool to measure the vertical displacement caused by the ocean tide loading effect in the coastal area, and we need to use the NAO.99Jb ocean tide model rather than the global ocean tide models in and around the Korean peninsula for position determination with permanent GPS installations. This work demonstrates that vertical displacement caused by the M₂ and N₂ constituents of ocean tide loading can be measured by carrier-phase DGPS.     

Verifying the body tide at the Canary Islands using tidal gravimetry observations

Gravity tide records from El Hierro, Tenerife and Lanzarote Islands (Canarian Archipelago) have been analyzed and compared to the theoretical body tide model (DDW) of Dehant el al. (1999). The use of more stringent criterion of tidal analysis using VAV program allowed us to reduce the error bars by a factor of two of the gravimetric factors at Tenerife and Lanzarote compared with previous published values. Also, the calibration values have been revisited at those sites. Precise ocean tide loading (OTL) corrections based on up-to-date global ocean models and improved regional ocean model have been obtained for the main tidal harmonics O₁, K₁, M₂, S₂. We also point out the importance of using the most accurate coastline definition for OTL calculations in the Canaries. The remaining observational errors depend on the accuracy of the calibration of the gravimeters and/or on the length of the observed data series. Finally, the comparison of the tidal observations with the theoretical body tide models has been done with an accuracy level of 0.1% at El Hierro, 0.4% at Tenerife and 0.5% at Lanzarote.     

海潮负荷对沿海地区宽幅InSAR形变监测的影响

海岸带地区是全球自然生态环境最为复杂和脆弱的地域之一,合成孔径雷达干涉测量（InSAR）技术可以为全球人类活动、气候变暖和俯冲带剧烈构造运动等背景下的大范围海岸带地理环境变化研究提供重要观测资料.海洋潮汐导致固体地球长周期形变,波长尺度为10²~10³ km的海潮负荷引入mm级至cm级的形变梯度,此类非构造信号对海岸带InSAR精密形变分析（如：大范围、微小、缓慢且非稳态构造过程等）造成显著影响.本文以宽幅模式SAR数据为例,基于多种海潮模型研究了全球典型海岸带地区（福建、智利和阿拉斯加湾）海潮负荷效应对宽幅InSAR形变监测的影响,给出了宽幅InSAR海潮负荷三维分量估计与差分相位提取方法,并进一步讨论了基于不同海潮模型估计海潮负荷位移的差异.海潮负荷影响不仅与研究范围大小有关,其形变梯度变化与研究区域地形特征存在强相关,对于长波长形变分析而言,传统平面或者曲面拟合方法难以有效分离海潮负荷位移.

     

南极中山与昭和站重力海潮负荷效应及背景噪声研究

利用南极中山站LCR-ET21重力仪器与昭和站GWR058仪器获得的重力潮汐观测资料,采用最新的三个全球海潮模型（Dtu10,Eot11A和HAM11A）研究了南极地区的海潮负荷效应和背景噪声.结果表明,由三个海潮模型计算的重力负荷均值改正后,中山站O1和M2振幅观测残差分别由13.83%和20.55%下降到5.32%和5.95%,昭和站O1和M2振幅观测残差分别由10.84%和21.52%下降到1.91%和3.40%,说明海潮负荷改正的有效性.利用加汉宁窗的FFT变换,获得了地震频段的地震噪声等级（SNM）,其值分别为1.574（中山站）和1.289（昭和站）.而在潮汐频段,中山站的背景噪声比昭和站高一个数量级,主要由不同观测仪器和台站局部环境所致.本文结果可为进一步利用南极重力资料研究局部环境和全球动力学问题提供有效参考.     

海潮对卫星重力场恢复的影响

本文讨论了海潮对卫星重力测量的影响问题. 首先介绍了海潮对卫星重力测量影响的基本理论;采用FES02和TPXO6海潮模型计算了海潮负荷对卫星重力结果前60阶的影响;并用两个模型之间的差异作为海潮模型精度的估计量,据此计算了海潮模型误差对卫星重力结果的影响. 与GRACE恢复的重力场精度的比较说明：海潮对重力场40阶以下的影响都超过了目前重力场恢复精度;尽管由于卫星测高技术的发展,海潮模型的精度有了很大的提高,但目前的全球海潮模型用于GRACE重力场恢复的前12阶的改正还是不够精确. 另外,我们也利用中国东海和南海潮汐资料以及FES02海潮模型讨论了中国近海潮汐效应对GRACE观测的影响. 结果说明该影响与海潮模型的误差相当. 这反映了当前海潮模型的不确定度,因此通过结合全球验潮站资料有望提高海潮对卫星重力测量的改正精度.     

武汉超导重力仪观测最新结果和海潮模型研究

利用武汉台站GWR_C032超导重力仪观测资料,在对原始数据进行有效预处理的基础上作调和分析,获得反映地球内部介质特征的重力潮汐参数.基于卫星测高技术和有限元方法同时考虑验潮站数据作约束条件获得的多个全球海潮模型,利用负荷理论和数值褶积积分技术计算了重力负荷,对周日和半日频段内的重力潮汐参数实施负荷改正,提出了“负荷改正有效性”概念,研究了全球海潮模型适应性.数值结果说明,海潮改正的有效性高达91%（O1,NAO99）和92%（M2,ORI96）.基于11个海潮模型对主波（O1,K1,M2和S2）的负荷改正说明平均有效性为（86%,70%,73%和84%）,振幅因子与理论模型间的差异分别从（212%,155%,116%和080%）降到（031%,039%,034%和008%）,同时还说明利用NAO99和ORI96全球海潮模型能获得比其他模型更佳的负荷改正效果.文章还利用国际地球动力学计划网络其他7个台站的超导重力仪观测研究了全球海潮模型的适定性问题,结果说明不同模型中不同潮波具有明显的区域特点,早期构制的SCW80全球海潮模型仍可作为大地测量研究中的重要参考模型.     

不同海潮模型对中国大陆构造环境监测网络GNSS数据解算的影响

选取2018年中国大陆构造环境监测网络GNSS基准站及中国周边IGS站数据,基于CSR4、FES2004、GOT00、NAO99b等海潮负荷改正模型,采用单一变量的解算方式,分析不同海潮模型对我国GNSS基准站基线、三维坐标、时间序列等造成的影响。结果表明：利用不同海潮模型解算基线精度,水平方向相差不大于1 mm,垂向相差不大于3 mm;各测站NEU坐标精度均在毫米级以下,以耿马站时间序列为例,与其他模型相比,基于FES2004模型解算的各向偏差趋势波动较弱,其年序列图的精度及连续性、稳定性更佳。简言之,利用FES2004海潮模型解算GNSS站点位置,效果较好。     

印尼地区地震断层面上的海潮负荷应力特征

对印尼地区3574次M≥5.0地震震源断层面上的海潮负荷应力进行计算,研究了地震断层上的负荷应力随深度变化以及随断层倾角和走向的变化特征.结果表明,地震断层上的负荷正应力大于负荷切应力.逆断层上的负荷应力最大,其它依次为斜滑断层、正断层和走滑断层.倾角范围在0°—50°的断层上的负荷正应力随倾角增大有减小趋势,负荷切应力在30°—50°倾角范围的断层上较大.负荷应力随断层走向的趋势性变化不明显.不同类型断层上的负荷正应力和切应力在震源深度15km处均达到最大,之后随深度增加而减小.当震源深度小于40km时,断层面上的海潮负荷应力的量级与固体潮应力相当.因此指出,在潮汐应力对地震影响的研究中,对震源深度小于40km的海洋和沿海地震,除固体潮汐应力外还需要考虑海潮负荷应力的影响.     

香港地区重力固体潮和海潮负荷特征研究

介绍了在香港地区重力固体潮合作观测成果, 获得了该地区完整的重力固体潮实测模型. 利用全球和近海海潮模型以及岛屿验潮站数据较系统地研究了海潮负荷特征, 反演了全球海潮模型的适定性. 数值结果说明周日频段内的海潮模型要比半日频段内的模型更加稳定, 实施验潮站潮位高变化改正对精密确定重力固体潮相位滞后起重要作用. 文章还研究了重力观测残差和台站背景噪声水平. 本项研究填补了中国地壳运动观测网络在该地区重力固体潮观测空白, 为地表和空间大地测量提供有效参考和服务.     

Influence of the modified global ocean tide model with local tides of East and South China Seas on load gravity in China and its neighbor area

Introduction With the development of science and technology, the accuracy of gravity measurement is im-proved. The gravity observation with FG5 gravimeter has reached accuracy of μ magnitude. And the high accurate superconducting gravimeter can detect the tiny signal of 10?2 μ magnitude in frequency domain. With the high-accuracy gravity observation on Earth′s surface, the Earth′s tidal parameters can be determined precisely. And the observations can also be used to invert the struc-ture…     

The computation of the free barotropic oscillations of a global ocean model including friction and loading effects

A set of 61 normal modes with periods between 7.8 and 133.1 h has been calculated, using a 1° model of the global ocean, including the Arctic Ocean. The model explicitly considers frictional forces and ocean self-attraction and loading effects. The latter effects have generally been taken into account by parameterization, but for some modes the effects have also been considered fully. Due to friction, the computed eigenfrequencies are complex, exhibiting also the varying dissipative properties of the modes and their dependence on the distribution of potential and kinetic energies over the oceanic regions. In detail, gravity modes having periods less then 80 h and dominating the semi-diurnal and the diurnal tides, topographically controlled vorticity modes with periods longer than diurnal, and two planetary vorticity modes with periods of 96.8 and 119.4 h have been identified. These planetary vorticity modes have their energies distributed over Pacific, Atlantic, and Indian Oceans, while the other modes with periods longer than 80 h, as vorticity modes, have their energies concentrated on topographic structures of restricted extension. The modes are discussed with respect to their wave properties, e.g., concerning quasi-standing-wave resonances and to the appearance of Kelvin waves of different orders and trapped by different coastlines. In particular, the relevance of specific modes for the development of the fields of the most important semi-diurnal and diurnal tidal constituents is investigated.     

大连台重力固体潮受海潮负荷影响特征研究

大连台重力固体潮汐观测受海潮负荷影响较大,选取2015年1月1日～2019年8月14日的重力数据进行预处理,获得重力潮汐参数,基于选取的8个海潮模型对O₁、K₁和M₂波进行海潮负荷改正及评价分析。数值分析结果表明,O₁、K₁和M₂波的重力海潮负荷振幅分布频段在2.4～2.8μGal之间,3个主潮波的残差负荷改正有效性为43％～53％,8个海潮模型对大连台主潮波的海潮负荷改正差别较小。     

中国大陆精密重力潮汐改正模型

利用理论和实验重力固体潮模型,充分考虑全球海潮和中国近海潮汐的负荷效应,建立了中国大陆的精密重力潮汐改正模型.结果表明,采用不同的固体潮模型会对重力潮汐结果产生相对变化幅度小于0.06％的差异;在沿海地区海潮负荷的影响约为整个潮汐的4％,而中部地区约为1％,其中中国近海潮汐模型的影响约占整个海潮负荷的10%,内插或外推潮波的负荷约占海潮负荷的3％.通过比较实测的重力数据表明,本文给出的重力潮汐改正模型的精度远远优于0.5×10-8 m·s-2,说明了本文构建的模型的实用性,可为中国大陆高精度重力测量提供有效参考和精密的改正模型.     

Sea level time series and ocean tide analysis from multipath signals at five GPS sites in different parts of the world

We present sea level observations derived from the analysis of signal-to-noise ratio (SNR) data recorded by five coastal GPS stations. These stations are located in different regions around the world, both in the northern and in the southern hemisphere, in different multipath environments, from rural coastal areas to busy harbors, and experience different tidal ranges.The recorded SNR data show periodic variations that originate from multipath, i.e. the interference of direct and reflected signals. The general assumption is that for satellite arcs facing the open sea, the rapid SNR variations are due to reflections off the sea surface. The SNR data recorded from these azimuth intervals were analyzed by spectral analysis with two methods: a standard analysis method assuming a static sea level during a satellite arc and an extended analysis method assuming a time dependent sea level during a satellite arc.The GPS-derived sea level results are compared to sea level records from co-located traditional tide gauges, both in the time and in the frequency domain. The sea level time series are highly correlated with correlation coefficients to the order of 0.89–0.99. The root-mean-square (RMS) difference is 6.2 cm for the station with the lowest tidal range of 165 cm and 43 cm for the station with the highest tidal range of 772 cm. The relative accuracy, defined as the ratio of RMS and tidal range, is between 2.4% and 10.0% for all stations.Comparing the standard analysis method and the extended analysis method, the results based on the extended analysis method agree better with the independent tide gauge records for the stations with a high tidal range. For the station with the highest tidal range (772 cm), the RMS is reduced by 47% when using the extended analysis method. Furthermore, the results also indicate that the standard analysis method, assuming a static sea level, can be used for stations with a tidal range of up to about 270 cm, without performing significantly worse than the extended analysis method.Tidal amplitudes and phases are derived by harmonic analysis of the sea level records. Again, a high level of agreement is observed between the tide gauge and the GPS-derived results. Comparing the GPS-derived results, the results based on the extended analysis method show a higher degree of agreement with the traditional tide gauge results for stations with larger tidal ranges. Spectral analysis of the residuals after the harmonic analysis reveals remaining signal power at multiples of the draconitic day. This indicates that the observed SNR data are to some level disturbed by additional multipath signals, in particular for GPS stations that are located in harbors.     

天津地区井水位固体潮观测的加卸载响应比变化与附近地震关系的研究

用加卸载响应比的方法,处理了天津地区2002年以来静海地震台井、宝坻地震台井、高村井和张道口地震台深井地下水位固体潮观测数据,并以加卸载响应比计算值的2倍方差作为异常判定指标,将天津及周边地区发生的M_L≥4.0地震参数与井水位固体潮响应比异常变化对比分析。结果表明:该区4口地下水位固体潮井的加卸载响应比异常变化与附近的地震活动有较好的对应关系。从而证明,利用井水位固体潮观测的加卸载响应比是一种能有效识别和提取地震前兆的方法。     

Applying local Green's functions to study the influence of the crustal structure on hydrological loading displacements

The influence of the elastic Earth properties on seasonal or shorter periodic surface deformations due to atmospheric surface pressure and terrestrial water storage variations is usually modeled by applying a local half-space model or an one dimensional spherical Earth model like PREM from which a unique set of elastic load Love numbers, or alternatively, elastic Green's functions are derived. The first model is valid only if load and observer almost coincide, the second model considers only the response of an average Earth structure. However, for surface loads with horizontal scales less than 2500 km², as for instance, for strong localized hydrological signals associated with heavy precipitation events and river floods, the Earth elastic response becomes very sensitive to inhomogeneities in the Earth crustal structure.We derive a set of local Green's functions defined globally on a 1° × 1° grid for the 3-layer crustal structure TEA12. Local Green's functions show standard deviations of ±12% in the vertical and ±21% in the horizontal directions for distances in the range from 0.1° to 0.5°. By means of Green's function scatter plots, we analyze the dependence of the load response to various crustal rocks and layer thicknesses. The application of local Green's functions instead of a mean global Green's function introduces a variability of 0.5–1.0 mm into the hydrological loading displacements, both in vertical and in horizontal directions. Maximum changes due to the local crustal structures are from −25% to +26% in the vertical and −91% to +55% in the horizontal displacements. In addition, the horizontal displacement can change its direction significantly. The lateral deviations in surface deformation due to local crustal elastic properties are found to be much larger than the differences between various commonly used one-dimensional Earth models.     

面向大气热力潮汐结构分析的Hough函数计算

作为一种刻画行星尺度大气波动基本结构的函数,Hough函数为开展有关大气潮汐全球结构的分析提供了具有物理基础的手段,它们也被作为一种数值工具应用于大气模式中以提高数值积分的效率.本文介绍了我们以拉普拉斯潮汐方程为基础,在发展求取Hough函数方面取得的进展,包括对有关计算方案和详细步骤的说明.针对39种分别具有三种主要频率,即周日、半日和1/3日,与此同时纬向波数落在[6,6]区间的潮汐成份,通过计算得到了与每一种成份对应的本征值、本征函数（Hough函数）集合.作为计算结果的示例,本文给出了针对迁移性周日潮和迁移性半日潮开展计算取得结果.这些结果说明这些函数具有正确的空间结构,与此同时还说明相关的本征值计算结果都达到了相当准确的水平.所有这些结果显示当前计算结果已经可以满足普通的应用.