The geoid over the north Indian Ocean has a significantly large negative amplitude, even if the excess flattening of the Earth beyond its equilibrium shape is considered. The various mechanisms ...proposed for this geoid anomaly vary and acceptable geoid predictions are obtained for specific tomographic models only. In this study, we identified model-independent features in the mantle beneath the Indian Ocean and Ross Sea region by analyzing eight recent global tomographic models. We standardized each of the models and applied cluster analysis to regionalize geophysically significant features in various depth ranges. These regionalizations are compared grid-by-grid to construct vote-maps. They highlight the anomalous features consistent across models and their approximate dimensions. Low velocity anomalies of dVS ∼-1.1% in the ∼400–680 km depth range are consistent in almost all the models beneath the Indian Ocean and Ross Sea. High velocity anomalies of dVS≥1% at depths below 1600 km, incoherent in dimension and orientation, are also observed. High velocity anomalies are most likely subducted slabs while low velocity anomalies could be partial melts generated by hydration of mantle. Additionally, a consistent low velocity structure is seen throughout the mantle beneath the southwestern Indian Ocean and east Africa. It is mostly likely a plume rising from the African LLSVP. It connects to the probable partial melts beneath the Indian Ocean via a remnant trail. Forward modelling of the geoid using vote-maps reveals that the E-W extent of the Indian Ocean Geoid Low is precisely reproduced by the consistent low velocity anomalies in the upper mantle. However, the N-S extent, most likely dependent on lower mantle anomalies, is not expressed since the dimension, orientation and dVS characteristics of high velocity structures are inconsistent in different models. The inter-model agreement is insufficient to identify structure(s) seen across models that can explain the geoid low over the Ross Sea.
•Cluster and rank-based analysis have identified coherent features within the mantle.•Upper mantle low velocity anomalies are consistent in almost all models.•Dimension and orientation of lower mantle high velocity anomalies inconsistent.•Upper mantle low velocity anomalies precisely predict the E-W extent of the IOGL.•The effect of lower mantle high velocity anomalies on geoid impaired by incoherency.•Inaccurate quantification of relevant mantle anomalies results in ambiguous prediction
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The primary objective of the 1-cm geoid experiment in Colorado (USA) is to compare the numerous geoid computation methods used by different groups around the world. This is intended to lay the ...foundations for tuning computation methods to achieve the sought after 1-cm accuracy, and also evaluate how this accuracy may be robustly assessed. In this experiment, (quasi)geoid models were computed using the same input data provided by the US National Geodetic Survey (NGS), but using different methodologies. The rugged mountainous study area (730 km
×
560 km) in Colorado was chosen so as to accentuate any differences between the methodologies, and to take advantage of newly collected GPS/leveling data of the Geoid Slope Validation Survey 2017 (GSVS17) which are now available to be used as an accurate and independent test dataset. Fourteen groups from fourteen countries submitted a gravimetric geoid and a quasigeoid model in a 1′
×
1′ grid for the study area, as well as geoid heights, height anomalies, and geopotential values at the 223 GSVS17 marks. This paper concentrates on the quasigeoid model comparison and evaluation, while the geopotential value investigations are presented as a separate paper (Sánchez et al. in J Geodesy 95(3):1.
https://doi.org/10.1007/s00190-021-01481-0
, 2021). Three comparisons are performed: the area comparison to show the model precision, the comparison with the GSVS17 data to estimate the relative accuracy of the models, and the differential quasigeoid (slope) comparison with GSVS17 to assess the relative accuracy of the height anomalies at different baseline lengths. The results show that the precision of the 1′ × 1′ models over the complete area is about 2 cm, while the accuracy estimates along the GSVS17 profile range from 1.2 cm to 3.4 cm. Considering that the GSVS17 does not pass the roughest terrain, we estimate that the quasigeoid can be computed with an accuracy of ~ 2 cm in Colorado. The slope comparisons show that RMS values of the differences vary from 2 to 8 cm in all baseline lengths. Although the 2-cm precision and 2-cm relative accuracy have been estimated in such a rugged region, the experiment has not reached the 1-cm accuracy goal. At this point, the different accuracy estimates are not a proof of the superiority of one methodology over another because the model precision and accuracy of the GSVS17-derived height anomalies are at a similar level. It appears that the differences are not primarily caused by differences in theory, but that they originate mostly from numerical computations and/or data processing techniques. Consequently, recommendations to improve the model precision toward the 1-cm accuracy are also given in this paper.
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The Indian Ocean geoid low (IOGL) is one of the lowest geoid anomalies on Earth. Several theories have been proposed to explain this geoid low, including past subduction (Nerlich et al., 2016; Rao ...and Kumar, 2014), subduction coupled with low velocity anomalies in the upper mantle (Spasojevic et al., 2010) and hot, low density anomaly in the upper to mid mantle depths (Reiss et al., 2017; Ghosh et al., 2017). It was also argued by Ghosh et al. (2017) that subducted slabs in the lower mantle have minimal role to play in contributing to this geoid signal. We further investigate that claim here by looking at the geoid contributed from various processes (density vs dynamic topography) as well as by inspecting the contribution from different spherical harmonic degrees. Our current findings substantiate the earlier claim that lower mantle slabs indeed play a minimal role in creating this anomalous geoid low.
•Individual contribution from density anomaly and surface dynamic topography show that low density anomaly causes the Indian Oceean geoid low.•Lower mantle slabs have negligible effect in creating this anomaly.•Amongst the recent tomography models, GyPSuM-P and S yield the highest fit to the IOGL.
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One of the most pronounced geoid lows on Earth lies in the Indian Ocean just south of the Indian peninsula. Several theories have been proposed to explain this geoid low, most of which invoke past ...subduction. Some recent studies have also argued that high‐velocity anomalies in the lower mantle coupled with low‐velocity anomalies in the upper mantle are responsible for these negative geoid anomalies. However, there is no general consensus regarding the source of this particular anomaly. We investigate the source of this geoid low by using models of density‐driven mantle convection. Our study is the first to successfully explain the occurrence of this anomaly using a global convection model driven by present‐day density anomalies derived from tomography. We test various tomography models in our flow calculations with different radial and lateral viscosity variations. Some of them produce a fairly high correlation to the global geoid, but only a few (SMEAN2, GyPSuM, SEMUCB, and LLNL‐JPS) could match the precise location and pattern of the geoid low in the Indian Ocean. The source of this low stems from a low‐density anomaly stretching from a depth of 300 km down to ∼900 km in the northern Indian Ocean region. This density anomaly potentially originates from material rising along the edge of the African Large Low Shear Velocity Province and moving toward the northeast, facilitated by the movement of the Indian plate in the same direction.
Plain Language Summary
One of the lowest geoid anomalies on Earth lies in the Indian Ocean just south of the Indian peninsula. Several theories have been proposed to explain this gravity low. Most of these theories try to explain the existence of this anomaly with the help of cold, dense oceanic plate that sank into the mantle in the past and which could potentially be present below the Indian Ocean at depths greater than 1,000 km. However, there is no general consensus regarding the source of this particular negative geoid anomaly. We investigate the source of this low by using models of density‐driven mantle convection. Our study finds that the source of this low stems from a low‐density anomaly stretching from a depth of 300 km down to ∼900 km in the northern Indian Ocean region. This density anomaly potentially originates from hot buoyant material rising from deep mantle beneath Africa and moving toward the northeast, facilitated by the movement of the Indian plate in the same direction. Our study is the first to successfully explain the occurrence of this geoid low using present‐day density anomalies.
Key Points
The current study successfully explains the occurrence of the Indian Ocean Geoid low using a global convection model driven by present‐day density anomalies derived from tomography
Upper to middle mantle low‐density anomalies are mainly responsible for the formation of this low
The recent tomography models have the necessary resolution to reproduce this geoid low
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We provide an unprecedented ultrahigh resolution picture of Earth's gravity over all continents and numerous islands within ±60° latitude. This is achieved through augmentation of new satellite and ...terrestrial gravity with topography data and use of massive parallel computation techniques, delivering local detail at ~200 m spatial resolution. As such, our work is the first‐of‐its‐kind to model gravity at unprecedented fine scales yet with near‐global coverage. The new picture of Earth's gravity encompasses a suite of gridded estimates of gravity accelerations, radial and horizontal field components, and quasi‐geoid heights at over 3 billion points covering 80% of Earth's land masses. We identify new candidate locations of extreme gravity signals, suggesting that the Committee on Data for Science and Technology standard for peak‐to‐peak variations in free‐fall gravity is too low by about 40%. The new models are beneficial for a wide range of scientific and engineering applications and freely available to the public.
Key Points
Satellite, terrestrial and topographic gravity combined at ~200 m resolution
Model covers all continents within SRTM data, and developing countries
Gravity accelerations on Earth estimated to vary within ~7000 mGal, or 0.7 %.
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This study presents a solution of the ‘1 cm Geoid Experiment’ (Colorado Experiment) using spherical radial basis functions (SRBFs). As the only group using SRBFs among the fourteen participated ...institutions from all over the world, we highlight the methodology of SRBFs in this paper. Detailed explanations are given regarding the settings of the four most important factors that influence the performance of SRBFs in gravity field modeling, namely (1) the choosing bandwidth, (2) the locations of the SRBFs, (3) the type of the SRBFs as well as (4) the extensions of the data zone for reducing the edge effect. Two types of basis functions covering the same spectral range are used for the terrestrial and the airborne measurements, respectively. The non-smoothing Shannon function is applied to the terrestrial data to avoid the loss of spectral information. The cubic polynomial (CuP) function which has smoothing features is applied to the airborne data as a low-pass filter for filtering the high-frequency noise. Although the idea of combining different SRBFs for different observations was proven in theory to be possible, it is applied to real data for the first time, in this study. The RMS error of our height anomaly result along the GSVS17 benchmarks w.r.t the validation data (which is the mean results of the other contributions in the ‘Colorado Experiment’) drops by 5% when combining the Shannon function for the terrestrial data and the CuP function for the airborne data, compared to those obtained by using the Shannon function for both the two data sets. This improvement indicates the validity and benefits of using different SRBFs for different observation types. Global gravity model (GGM), topographic model, the terrestrial gravity data, as well as the airborne gravity data are combined, and the contribution of each data set to the final solution is discussed. By adding the terrestrial data to the GGM and the topographic model, the RMS error of the height anomaly result w.r.t the validation data drops from 4 to 1.8 cm, and it is further reduced to 1 cm by including the airborne data. Comparisons with the mean results of all the contributions show that our height anomaly and geoid height solutions at the GSVS17 benchmarks have an RMS error of 1.0 cm and 1.3 cm, respectively; and our height anomaly results give an RMS value of 1.6 cm in the whole study area, which are all the smallest among the participants.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Within the context of the remove-restore technique, this research seeks to determine the best combination of gravity field wavelengths for Egypt’s geoid computation. There are various methods for ...such a wavelength combination. It has been proposed to merge the regional data signals and the global geopotential earth models, potentially using a modified Stokes’ kernel with various methods. Firstly, from our computations, it can be concluded that the conventional remove-restore technique should not be applied for the determination of gravity anomalies and geoid determination. Also, the outcomes demonstrated that the estimated gravity anomalies utilizing the window approach are independent, the finest, and unbiased and have a minimal difference between the maximum and minimum values. The geoid produced from the GPS levelling has fewer differences between it and the geoids computed using the modified Stokes’ kernels as well as the geoid determined using the window technique than in the case of utilizing the unmodified Stokes’ classical kernel. Finally, the window approach gives, however, completely better outcomes compared to the Stokes unmodified kernel method.
One of the challenges in the unification of the global vertical datum is to determine the vertical offset between the local vertical datum and the global vertical datum. Since 2015, the scientific ...community has been working on the unification of the global vertical datum, following the publication of the resolution for the definition and realization of an International Height Reference System. This paper aims to estimate the vertical offset in Brazil (Imbituba and Santana Data) by combining a regional gravity field model, using the Geodetic Boundary Value Problem approach applying the Remove-Compute-Restore procedure, and GNSS/leveling data. In the Imbituba vertical datum 1271 stations were used and 66 stations were connected to the Santana tide gauge. The results showed that the geopotential value of Imbituba was 62,636,849.87 ± 0.224 m2s-2, and the vertical offset concerning the reference surface (W0) was 0.358 ± 0.023 m. In Santana, the geopotential value was 62,636,836.88 ± 4.108 m2s-2, and the vertical offset was 1.689 ± 0.420 m. The offset between Imbituba and Santana was computed once the physical connection between Brazil's two local vertical data is a challenge. The estimated value was 1.331 ± 0.421 m. This means the Santana vertical datum is 1.331 m higher than the Imbituba vertical datum.
•Improving the Brazilian height system.•Using GNSS/leveling data to estimate the vertical offset in Brazil.•Connecting the Brazilian local vertical data to the International Height Reference Frame.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Indian Ocean Geoid Low (IOGL) is the most prominent geoid anomaly (−106 m) on the globe, whose origin remains elusive. In the present study, we investigate the mantle transition zone (MTZ) ...structure beneath the region using P receiver functions (PRFs), to examine its role in the genesis of this feature. Results from 3‐D time to depth migration of PRFs reveal a thin MTZ primarily due to an elevation of the 660 km discontinuity. This is suggestive of anomalously hot temperatures in the mid mantle beneath the IOGL region, possibly sourced from the African Large Low Shear Velocity Province (LLSVP). The combined effect of the hot (low‐density) material in the MTZ proposed in this study and the (high‐density) cold slab graves atop the core‐mantle boundary inferred from previous studies can possibly explain this geoid low.
Key Points
The mantle transition zone (MTZ) beneath the Indian Ocean Geoid Low is investigated using receiver functions
A thin MTZ primarily due to an elevated 660 km discontinuity provides evidence for a hot anomaly in the mid mantle
A hot low‐density material in the mid mantle is one of the sources to explain the world's largest geoid low
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Em 2018, o IBGE alinha-se às recomendações internacionais determinando as chamadas altitudes normais. Em razão disso, criou-se a necessidade de modelar a Separação entre Geoide e Quase Geoide (SGQG) ...para permitir a conversão entre altitudes normais e ortométricas para os países que optarem por utilizar-se desta última. Esse estudo se propõe avaliar o valor da SGQG a partir das diferenças entre as altitudes ortométrica e normal. Para o desenvolvimento deste estudo foram utilizados: dados altimétricos e dados gravimétricos; mapa de densidades das massas topográficas derivadas a partir do mapa geológico; bem como os coeficientes do Modelo Global do Geopotencial EGM2008. A metodologia consiste em calcular e avaliar a altitude ortométrica obtida a partir do número geopotencial, com o valor da densidade topográfica lateral constante e variável; determinar a SGQG a partir da diferença entre as altitudes ortométrica e normal, e verificar as discrepâncias em relação a SGQG obtida a partir da diferença entre a anomalia de altitude e da ondulação geoidal do modelo EGM2008. Os resultados mostram que a SGQG apresenta uma variação entre cm a 10 cm, sendo que as discrepâncias maiores são observadas no município de São José dos Ausentes e adjacências. A negligência da omissão dos efeitos da densidade topográfica lateral na determinação da altitude ortométrica foi estimada em 2 cm. Os resultados mostraram a necessidade da efetiva incorporação das variações das densidades das massas topográficas aos cálculos da SGQG, quando se almeja a determinação das altitudes ortométricas.
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