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1.
Summary. A new set of 1×1° mean free-air anomalies in the Indian Ocean is determined on the basis of previously published free-air anomaly maps (Talwani & Kahle) and the most recent Lamont surface ship gravity measurements. The data are then used to compute a (total) 1×1° gravimetric Indian Ocean geoid. The computation is carried out by combining the Goddard Space Flight Center (GSFC) GEM-6 geoid and a difference geoid that corresponds to the differences between the set of 1×1° surface gravity values and the GEM-6 gravity anomalies. The difference geoid is highest over the Madagascar Ridge (+ 20 m) and lowest over the Timor Trough (-30 m). The total geoid is compared with GEOS-3 radar altimeter derived geoid profiles and geophysical implications are discussed. 相似文献
2.
A. B. Watts D. P. McKenzie B. E. Parsons M. Roufosse 《Geophysical Journal International》1985,83(1):263-298
Summary. Surface-ship and satellite derived data have been compiled in new free-air gravity anomaly, bathymetry and geoid anomaly maps of the Pacific Ocean basin and its margin. The maps are based on smoothed values of the gravity anomaly, bathymetry and geoid interpolated on to a 90 × 90 km grid. Each smoothed value was obtained by Gaussian filtering measurements along individual ship and subsatellite tracks. The resulting maps resolve features in the gravity, bathymetry and geoid with wavelengths that range from a few hundred to a few thousand kilometres. The smoothed values of bathymetry and geoid anomaly have been corrected for age. The resulting maps show the Pacific ocean basin is associated with a number of ENE–WSW-trending geoid anomaly highs with amplitudes of about ± 5 m and wavelengths of about 3000 km. The most prominent of these highs correlate with the Magellan seamounts–Marshall Gilbert Islands–Magellan rise and the Hess rise–Hawaiian ridge regions. The correlation between geoid anomaly and bathymetry cannot be explained by models of static compensation, but is consistent with a model in which the geoid anomaly and bathymetry are supported by some form of dynamic compensation. We suggest that the dynamic compensation, which characterizes oceanic lithosphere older than 80 Myr, is the result of mantle convection on scales that are smaller than the lithospheric plates themselves. 相似文献
3.
Summary. A total of 3708 1 × 1° free-air gravity anomaly averages have been used to construct a new 1 × 1° gravimetric geoid of the Northwest Pacific Ocean. The 1 × 1° averages are based on a compilation of 147000 surface ship and pendulum gravity measurements. The gravimetric geoid reveals information in the geoid of the Northwest Pacific not present in currently used satellite derived models. The RMS difference between the 1 × 1° geoid and satellite derived models is about ±6 m. Difference geoid undulations range from a maximum of +19 m over the Hawaiian ridge to a minimum of −31 m over the junction of the Kuril and Aleutian trenches. The Hawaiian swell is associated with a geoidal high of up to +15 m with wavelengths of about 2200 km and the topographic rises seaward of deep-sea trenches are associated with geoidal highs of up to 4m with wavelengths of about 220–900 km. The main difference between the gravimetric geoid and the satellite derived models occurs over the Pacific basin where discrepancies reach +10 m with wavelengths of 4000 km. The agreement between the gravi-metric geoid and Skylab-4 and Geos-3 altimeter data is close for wavelengths greater than about 300 km but poor for shorter wavelengths. 相似文献
4.
The reduction of aliasing in gravity anomalies and geoid heights using digital terrain data 总被引：8，自引：0，他引：8
Observations of gravity can be aliased by virtue of the logistics involved in collecting these data in the field. For instance, gravity measurements are often made in more accessible lowland areas where there are roads and tracks, thus omitting areas of higher relief in between. The gravimetric determination of the geoid requires mean terrain-corrected free-air anomalies; however, anomalies based only on the observations in lowland regions are not necessarily representative of the true mean value over the topography. A five-stage approach is taken that uses a digital elevation model, which provides a more accurate representation of the topography than the gravity observation elevations, to reduce the unrepresentative sampling in the gravity observations. When using this approach with the Australian digital elevation model, the terrain-corrected free-air anomalies generated from the Australian gravity data base change by between 77.075 and −84.335 mgal (−0.193 mgal mean and 2.687 mgal standard deviation). Subsequent gravimetric geoid computations are used to illustrate the effect of aliasing in the Australian gravity data upon the geoid. The difference between 'aliased' and 'non-aliased' gravimetric geoid solutions varies by between 0.732 and −1.816 m (−0.058 m mean and 0.122 m standard deviation). Based on these conceptual arguments and numerical results, it is recommended that supplementary digital elevation information be included during the estimation of mean gravity anomalies prior to the computation of a gravimetric geoid model. 相似文献
5.
利用我国最新地球重力场模型 WDM94,给出了南极 (纬度范围为 - 60°～ - 90°)大地水准面高和平均空间重力异常。为了全面总结分析南极大地水准面特征 ,收集了国外最新地球重力场模型 OSU91 ( 360阶次 )和 JGMOSU( 360阶次 ) ,计算了相应的大地水准面高和平均重力异常。其结果分别与 WDM94的结果作了比较 ,WDM94与 OSU91和 JGMOSU的大地水准面高标准差分别为± 1 .90 m和± 2 .0 9m,平均空间重力异常标准差分别为± 8.97mgal和± 9.32 mgal 相似文献
6.
利用NCEP/NCAR位势高度再分析资料分析了1979~2005年青藏高原及其邻近地区30hPa、100hPa、300hPa和500hPa位势高度场变化趋势的时空特征。研究表明:(1)1979~2005年青藏高原区域平均位势高度场在对流层中层存在上升趋势,且主要发生在冷半年;随着等压面的升高,高度场上升趋势减弱;到平流层高度场呈显著降低趋势,且主要发生在暖半年。(2)从空间上看,平流层位势高度场年平均变化整体呈显著降低趋势,大致呈纬向分布,南部降低趋势强于北部,特别是高原东南、孟加拉湾北部降低趋势最强;对流层中层位势高度场变化趋势以上升为主,高原东北部上升趋势尤其显著。(3)尽管与高原各层位势高度值本身相比,变化趋势是一个小量,但国内外不同区域及不同空间尺度的研究都发现有类似现象,故对此应给予重视。今后应进一步加强青藏高原位势高度场时空变化趋势的驱动机制、影响及其对全球变暖的区域响应研究。(4)1979年以来NCEP/NCAR位势高度再分析资料用于青藏高原及其邻近地区的气候变化研究是有效的。 相似文献
7.
MSA在冰芯深层的“迁移”现象的存在 ,关系到能否应用大气中 MSA季节特征进行极地冰芯断代 ,并以冰芯分析结果确定大气中生物硫化物含量的方法基础。本文展示各类最有代表性的冰雪 MSA剖面 ,对提出冰芯深层 MSA“迁移假说”的理论依据进行探讨 ,并根据最新资料 ,说明冰川中 MSA的“迁移”和重新分布 ,在表层粒雪和渗浸 -冻结冰层中同样可能发生。对“迁移假说”的延伸补充在于 ,粒雪中的 MSA迁移是在积雪中空气与外界贯通的“开放”条件下进行的 ,渗入雪层中的融水将 MSA溶出后 ,在晶粒间宏观向下输送 ,在阳离子集中的层位发生反应 ,生成盐类冰点的改变使之发生“冻结”,重新分布。而渗浸 -冻结冰层中的迁移机制可能和深层冰川冰中的情况接近 ,即在气体与外界隔绝条件下 ,主要在“封闭”的晶间脉状纹理中以“微观”形式进行。南极半岛 MSA迁移过程需要很长时间 ,而表层渗浸 -冻结冰层中 MSA迁移过程很快出现 ,说明冰中脉状纹理的加快形成、较高冰川温度和冰层中的较大含水量等因素会对 MSA“迁移”进程起促进作用。最后 ,对 MSA“迁移”情况下 ,如何进行有关冰芯 MSA计算进行了讨论 ,对设计实验验证并完善迁移理论提出了设想。 相似文献
8.
联合基于GRACE重力卫星观测资料恢复的重力场模型(EIGEN-GL04S1)和卫星测高推求的平均海面高模型(KMSS04)来构造南极绕极流区域的平均海面动力地形,并利用小波滤波方法去掉短波及噪声信号,进而推算大、中尺度的绕极流。与非卫星重力场模型、同化资料及海洋水文资料确定相应结果的验证分析表明:基于新的卫星重力场模型推算的南极绕极流区域的海面动力地形、PF、SAF和表层流场等都与海洋学结果相吻合,且局部特征更加清晰。表明卫-卫跟踪重力卫星计划确定的地球重力场模型较之以前存在的重力场模型在中长波部分精度有较大提高,从大地测量(从空间)角度来探测南极绕极流已达到较高的精度。 相似文献
9.
Error covariance estimates are necessary information for the combination of solutions resulting from different kinds of data or methods, or for the assimilation of new results in already existing solutions. Such a combination or assimilation process demands proper weighting of the data, in order for the combination to be optimal and the error estimates of the results realistic. One flexible method for the gravity field approximation is least-squares collocation leading to optimal solutions for the predicted quantities and their error covariance estimates. The drawback of this method is related to the current ability of computers in handling very large systems of linear equations produced by an equally large amount of available input data. This problem becomes more serious when error covariance estimates have to be simultaneously computed. Using numerical experiments aiming at revealing dependencies between error covariance estimates and given features of the input data we investigate the possibility of a straightforward estimation of error covariance functions exploiting known characteristics of the observations. The experiments using gravity anomalies for the computation of geoid heights and the associated error covariance functions were conducted in the Arctic region north of 64° latitude. The correlation between the known features of the data and the parameters variance and correlation length of the computed error covariance functions was estimated using multiple regression analysis. The results showed that a satisfactory a priori estimation of these parameters was not possible, at least in the area considered. 相似文献
10.
Summary. The geopotential is usually expressed as an infinite series of spherical harmonics, and the odd zonal harmonics are the terms independent of longitude and antisymmetric about the equator: they define the 'pear-shape' effect. The coefficients J 3 , J 5 , J 7 , … of these harmonics have been evaluated by analysing the variations in eccentricity of 28 satellite orbits from near-equatorial to polar. Most of the orbits from our previous determination in 1974 are used again, but three new orbits are added, including two at inclinations between 62° and 63°, which have been specially observed for more than five years by the Hewitt cameras. With the help of the new orbits and revised theory, we have obtained sets of J -coefficients with standard deviations about 40 per cent lower than before. A 9-coefficient set is chosen as representative, and is as follows (all × 109 ): J 3 =– 2530 ± 4, J 5 =–245 ± 5, J 7 =–336 ± 6, J 9 =–90 ± 7, J 11 = 159 ± 9, J 13 =–158 ± 15, J 15 =– 20 ± 15, J 17 =– 236 ± 14, J 19 =– 27 ± 19. With this set of values, the pear-shape asymmetry of the geoid (north polar minus south polar radius) amounts to 45.1 m instead of the previous 44.7 m. The accuracy of the longitude-averaged geoid profile is estimated as 50 cm, except at latitudes above 86°. The geoid profile and predicted amplitude of the oscillation in eccentricity are compared with those from other sources. 相似文献
11.
The search for the optimal spatial scale for observing landforms to understand physical processes is a fundamental issue in geomorphology. Topographic attributes derived from Digital Terrain Models (DTMs) such as slope, curvature and drainage area provide a basis for topographic analyses. The slope–area relationship has been used to distinguish diffusive (hillslope) from linear (valley) processes, and to infer dominant sediment transport processes. In addition, curvature is also useful in distinguishing the dominant landform process. Recent topographic survey techniques such as LiDAR have permitted detailed topographic analysis by providing high-quality DTMs. This study uses LiDAR-derived DTMs with a spatial scale between 1 and 30 m in order to find the optimal scale for observation of dominant landform processes in a headwater basin in the eastern Italian Alps where shallow landsliding and debris flows are dominant. The analysis considered the scaling regimes of local slope versus drainage area, the spatial distribution of curvature, and field observations of channel head locations. The results indicate that: i) hillslope-to-valley transitions in slope–area diagrams become clearer as the DTM grid size decreases due to the better representation of hillslope morphology, and the topographic signature of valley incision by debris flows and landslides is also best displayed with finer DTMs; ii) regarding the channel head distribution in the slope–area diagrams, the scaling regimes of local slope versus drainage area obtained with grid sizes of 1, 3, and 5 m are more consistent with field data; and iii) the use of thresholds of standard deviation of curvature, particularly at the finest grid size, were proven as a useful and objective methodology for recognizing hollows and related channel heads. 相似文献
12.
Gravity measurement is of great importance to the height datum in Antarctica.The absolute gravity measurement was carried out at Great Wall Station,Antarctica,using FG5 absolute gravity instrument.The gravity data was processed with corrections of earth tide,ocean tide,polar motion and the atmospher,and the RMS is within ±3×10-8 ms-2.The vertical and horizontal gravity gradients were measured using 2 LaCoaste & Romberg(LCR) gravimeters.The absolute gravity measurement provides the fundamental data for the validation and calibration of the satellite gravity projects such as CHAMP,GRACE and GOCE,and for the high accuracy geoid model. 相似文献
13.
Alexandre Ganachaud Carl Wunsch Myung-Chan Kim Byron Tapley 《Geophysical Journal International》1997,128(3):708-722
A global estimate of the absolute oceanic general circulation from a geostrophic inversion of in situ hydrographic data is tested against and then combined with an estimate obtained from TOPEX/POSEIDON altimetric data and a geoid model computed using the JGM-3 gravity-field solution. Within the quantitative uncertainties of both the hydrographic inversion and the geoid estimate, the two estimates derived by very different methods are consistent. When the in situ inversion is combined with the altimetry/geoid scheme using a recursive inverse procedure, a new solution, fully consistent with both hydrography and altimetry, is found. There is, however, little reduction in the uncertainties of the calculated ocean circulation and its mass and heat fluxes because the best available geoid estimate remains noisy relative to the purely oceano-graphic inferences. The conclusion drawn from this is that the comparatively large errors present in the existing geoid models now limit the ability of satellite altimeter data to improve directly the general ocean circulation models derived from in situ measurements. Because improvements in the geoid could be realized through a dedicated spaceborne gravity recovery mission, the impact of hypothetical much better, future geoid estimates on the circulation uncertainty is also quantified, showing significant hypothetical reductions in the uncertainties of oceanic transport calculations, Full ocean general circulation models could better exploit both existing oceanographic data and future gravity-mission data, but their present use is severely limited by the inability to quantify their error budgets. 相似文献
14.
利用FG5绝对重力仪进行南极长城站绝对重力测定 总被引：1，自引：0，他引：1
在南极地区进行重力测量是建立高程基准的基础,2005年,在南极长城站进行了绝对重力测量,观测仪器采用FG5绝对重力仪,经固体潮改正、海潮改正、极移改正及气压改正等,精度达±3×10-8m s-2,并同时利用2台LCR相对重力仪进行了重力垂直梯度测量和水平梯度测量。长城站绝对重力测量的实施,对于新一代卫星重力计划如CHAMP、GRACE和GOCE的地面校准及建立南极地区的高精度、高分辨率的大地水准面模型提供了基础数据。 相似文献
15.
16.
High-resolution mean sea surface computed with altimeter data of Ers-1 (geodetic mission) and topex-poseidon 总被引：1，自引：0，他引：1
A. Cazenave P. Schaeffer M. Berge C. Brossier K. Dominh M. C. Gennero 《Geophysical Journal International》1996,125(3):696-704
The remote-sensing satellite ERS-1, launched in 1991 to study the Earth's environment, was placed on a geodetic (168-day repeat) orbit between 1994 April and 1995 March to map, through altimetric measurements, the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along-track and cross-track directions. We have analysed the precise altimeter data of the geodetic mission, and, by also using one year of Topex-Poseidon altimeter data, we have computed a global high-resolution mean sea surface. The various steps involved in pre-processing the ERS-1 data consisted of correcting the data for environmental factors, editing, and reducing, through crossover analyses, the radial orbit error, which directly affects sea-surface height measurements. For this purpose, we adjusted sinusoids at 1 and 2 cycle rev−1 along the ERS-1 profiles in order to minimize crossover differences between ERS-1 and yearly averaged Topex-Poseidon profiles. In effect, the orbit of Topex-Poseidon is very accurately determined (within 2–3 cm for the radial component), so Topex-Poseidon altimeter profiles can serve as a reference to reduce the ERS-1 radial orbit error. The ERS-1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission. The along-track ERS-1 and Topex-Poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1/16°× 1/16° size. The mapping of the gridded sea-surface heights reveals the very fine structure of the marine geoid, up until now unknown at a global scale. This new data set will be most useful for marine geophysical and tectonic investigations. 相似文献
17.
Georg Kaufmann 《Geophysical Journal International》2000,143(1):142-156
During the last glacial cycles, global sea level dropped several times by about 120 m and large ice sheets covered North America, northern Europe and Antarctica during the glacial stages. The changes in the iceocean mass balance have displaced mantle material mainly via viscous flow, and the perturbation of the equilibrium figure of the Earth by glacial isostatic adjustment is still observable today in timedependent changes of gravitational and rotational observations. Contemporary iceocean mass balance from volume changes of polar ice caps also contributes to secular variations of the Earth's gravitational field.
In the near future, several satellite gravity missions will significantly improve the accuracy of the observed timedependent gravitational field. In view of the expected improvements in the observations, we predict glacially induced perturbations of the gravitational field, induced by Late Pleistocene and contemporary ice volume changes, for a variety of radial mantle viscosity profiles. We assess the degree of uncertainty for the glacially induced contributions to gravitational and rotational parameters, both in the spectral and the spatial domain.
Predictions of power spectra for the glacially induced freeair gravity and geoid anomalies are about one order of magnitude lower than the observed values, and uncertainties arising from different plausible viscosity profiles are around 0.150.4 mGal and 0.21.5 m, respectively. Uncertainties from different ice models are of secondary importance for the predicted power spectra. Predicted secular changes in geoid anomalies in formerly glaciated areas are mainly controlled by the viscosity profile and contemporary ice volume changes. We also show that the simple threelayer viscosity profiles currently employed for the majority of postglacial rebound studies represent a limited subset for model predictions of the timedependent gravitational field. 相似文献
In the near future, several satellite gravity missions will significantly improve the accuracy of the observed timedependent gravitational field. In view of the expected improvements in the observations, we predict glacially induced perturbations of the gravitational field, induced by Late Pleistocene and contemporary ice volume changes, for a variety of radial mantle viscosity profiles. We assess the degree of uncertainty for the glacially induced contributions to gravitational and rotational parameters, both in the spectral and the spatial domain.
Predictions of power spectra for the glacially induced freeair gravity and geoid anomalies are about one order of magnitude lower than the observed values, and uncertainties arising from different plausible viscosity profiles are around 0.150.4 mGal and 0.21.5 m, respectively. Uncertainties from different ice models are of secondary importance for the predicted power spectra. Predicted secular changes in geoid anomalies in formerly glaciated areas are mainly controlled by the viscosity profile and contemporary ice volume changes. We also show that the simple threelayer viscosity profiles currently employed for the majority of postglacial rebound studies represent a limited subset for model predictions of the timedependent gravitational field. 相似文献
18.
F. Bocchio 《Geophysical Journal International》1978,55(2):503-506
Summary. A new computation of the relevant geometrical and mechanical quantities pertaining to the three-dimensional normal ellipsoidal field (Bocchio) has been worked out with reference to a spatial grid ranging from 0 to 9000 m in dynamic height and from 0 to 90° in latitude, with a mesh size of 500 m and 1° respectively giving a better accuracy. An estimate of the latter at the upper end of the vertical interval gives 10−2 m for the radii of curvature of the equipotential surfaces and 10−2 mgal for normal gravity. Twenty-six functions of interest have been numerically computed including the metric tensor, Christoffel's symbols and the tensor of gravity gradients giving about 45 000 tabulated data. 相似文献
19.
Precise location of unsurveyed seamounts in the Austral archipelago area using SEASAT data 总被引：1，自引：0，他引：1
Summary. SEASAT altimetric geoid data are used to detect uncharted sea-mounts in the Austral archipelago area. The various physical parameters which affect the geoid signature of a seamount are inspected to analyse their influence on the precision of the location. These parameters are the shape of the seamount, its density, the crustal model, the effective elastic thickness of the lithosphere and the distance between the seamount and sub-satellite tracks, respectively. The results of the tests performed on synthetic data as well as on charted structures show that when a seamount is detected on at least two neighbouring tracks, it is possible to locate it with good confidence and to give an estimation of its height. If the correct elastic thickness is assumed, the precision on the located is order 15 km. Ten previously unsurveyed seamounts have been located in the Austral archipelago. It appears that they are emplaced along two well-defined azimuths (N 110°E and N 150°E) and that two parallel distinct volcanic chains form the Austral archipelago. 相似文献
20.
Jean Goslin Jean-Louis Thirot Olivier Noël & Jean Francheteau 《Geophysical Journal International》1998,135(2):700-710
Among the mantle hotspots present under oceanic areas, a large number are located on—or close to—active oceanic ridges. This is especially true in the slow-spreading Atlantic and Indian oceans. The recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active slow-spreading ridges. The observed correlations allow us to draw preliminary conclusions on the general links between surficial processes, which shape the detailed morphology of the ridge axes, and deeper processes, active in the upper mantle below the ridge axial domains as a whole. The interactions are first studied at the scale of the Atlantic (the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correlation between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axis basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and upper-mantle tomography models. The interactions appear complex, ranging from the effect of large and very productive plumes, almost totally overprinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the ridge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stress the need for additional observations in various plume/ridge configurations. 相似文献