Satellite-derived Gravity Anomalies Research Articles

Определение гравитационных аномалий в акватории Вьетнама и на прилегающей территории по данным спутниковой альтиметрии CryoSat-2

The purpose of this study is to determine marine gravity anomalies with high accuracy using the CryoSat-2 satellite altimetry data in the Middle Vietnamese Sea and the adjacent territory. For this, the mentioned data and the process of determining gravity anomalies from the CryoSat-2 data were analyzed. The record from 818 GNSS-leveling points in Vietnam were used to evaluate and select the best global gravity field model for use in the calculation process. The findings from 31 gauge tidal were also used to assess and select the best mean dynamic topography model for the study area. Marine gravity anomalies in the study area were calculated using data from the CryoSat-2 satellite altimeter at 72 483 measurement points. The calculated results are compared with the ship-derived gravity anomalies at 1025 measurement points for assessment. The research results show that of the Earth Geopotential Model EIGEN6C4 and the Mean Dynamic Topography Model DTU15MDT are the most suitable for the research area. The accuracy of satellite-derived gravity anomalies, calculated from the CryoSat-2 satellite altimetry data in the study area, reaches ±1,18 mGal. Compared to the available results in the study area, this accuracy is the highest.

A Script-Driven Approach to Mapping Satellite-Derived Topography and Gravity Data Over the Zagros Fold-and-Thrust Belt, Iran

Abstract Integrated geophysical mapping benefits from visualizing multi-source datasets including gravity and satellite altimetry data using 2D and 3D techniques. Applying scripting cartographic approach by R language and GMT supported by traditional mapping in QGIS is presented in this paper with a case study of Iranian geomorphology and a special focus on Zagros Fold-and-Thrust Belt, a unique landform of the country affected by complex geodynamic structure. Several modules of GMT and ’tmap’ and ’raster’ packages of R language were shown to illustrate the efficiency of the console-based mapping by scripts. Data sources included high-resolution raster grids of GEBCO/SRTM, EGM-2008, SRTM DEM and vector geologic layers of USGS. The cartographic objective was to visualize thematic maps of Iran: topography, geology, satellite-derived gravity anomalies, geoid undulations and geomorphology. Various cartographic techniques were applied to plot the geophysical and topographic field gradients and categorical variations in geological structures and relief along the Zagros Fold-and-Thrust Belt. The structures of Elburz, Zagros, Kopet Dag and Makran slopes, Dasht-e Kavir, Dasht-e Lut and Great Salt Desert were visualized using 3D-and 2D techniques. The geomorphometric properties (slope, aspect, hillshade, elevations) were modelled by R. The study presented a series of 11 new maps made using a combination of scripting techniques and GIS for comparative geological-geophysical analysis. Listings of R and GMT scripting are provided for repeatability.

Marine gravity anomaly mapping for the Gulf of Tonkin area (Vietnam) using Cryosat-2 and Saral/AltiKa satellite altimetry data

Marine gravity anomalies are essential data for determining coastal geoid, investigating tectonics and crustal structures, and offshore explorations. The objective of this study is to present a methodology for estimating marine gravity anomalies from CryoSat-2 and Saral/AltiKa satellite altimeter data for the Gulf of Tonkin of Vietnam with a high-resolution 2′ × 2′ grid. A total of 15,665 sea surface height (SSH) grid points, including derived from the Cryosat-2 (6842 grid points) and Saral/AltiKa (8823 grid points) satellite altimeter data were used. Then, the remove-restore technique and the crossover adjustment algorithm were used to remove the long-wavelength geoid height, the mean dynamic topography hMDT, and time-varying sea-surface topography ht. The residual geoid heights ΔN were used to determine the residual gravity anomalies δg using the Least-Squares Collocation method, whereas the Earth Geopotential Model was employed to restore the long-wavelength gravity anomalies ΔgEIGEN. GPS/leveling and tidal gauge of 31 tidal stations were used for assessing and choosing the best Earth Geopotential Model and Mean Dynamic Topography models for the study area (EIGEN6C4 and DTU15MDT models). The accuracy of the final marine gravity anomaly result was assessed using 56,978 marine gravity points, which were distributed in the study area. The result showed that the standard deviation between the satellite-derived gravity anomalies and checked points is ± 3.36 mGal, indicating good accuracy. After improving with ship-measured gravity anomalies, the accuracy of satellite-derived marine gravity anomaly improves to ± 2.63 mGal. The results of this research are useful for geodetic and geophysical applications in the region.

Inversion of marine gravity anomalies over southeastern China seas from multi-satellite altimeter vertical deflections

The accuracy and resolution of marine gravity field derived from satellite altimetry mainly depends on the range precision and dense spatial distribution. This paper aims at modeling a regional marine gravity field with improved accuracy and higher resolution (1′×1′) over Southeastern China Seas using additional data from CryoSat-2 as well as new data from AltiKa. Three approaches are used to enhance the precision level of satellite-derived gravity anomalies. Firstly we evaluate a suite of published retracking algorithms and find the two-step retracker is optimal for open ocean waveforms. Secondly, we evaluate the filtering and resampling procedure used to reduce the full 20 or 40Hz data to a lower rate having lower noise. We adopt a uniform low-pass filter for all altimeter missions and resample at 5Hz and then perform a second editing based on sea surface slope estimates from previous models. Thirdly, we selected WHU12 model to update the corrections provided in geophysical data record. We finally calculated the 1′×1′ marine gravity field model by using EGM2008 model as reference field during the remove/restore procedure. The root mean squares of the discrepancies between the new result and DTU10, DTU13, V23.1, EGM2008 are within the range of 1.8–3.9mGal, while the verification with respect to shipboard gravity data shows that the accuracy of the new result reached a comparable level with DTU13 and was slightly superior to V23.1, DTU10 and EGM2008 models. Moreover, the new result has a 2mGal better accuracy over open seas than coastal areas with shallow water depth.

Mapping the geoid for Iberia and the Macaronesian Islands using multi-sensor gravity data and the GRACE geopotential model

A new gravimetric geoid model (ICAGM07) has been determined for the North-East Atlantic Ocean, Iberia, and the Macaronesian Islands using multi-sensor gravity data and a GRACE derived Earth geopotential model. A high resolution gravity model, determined using least squares optimal interpolation of marine, land, and satellite derived gravity anomalies, was used to resolve the medium and short wavelengths of the geoid. Long wavelengths of the geoid were provided by the GRACE derived Earth geopotential model. The topographic effects were computed in the spectral domain using a high resolution (100 m) digital terrain model derived from SRTM mission data and cartographic charts. The remove–restore technique was used to compute the geoid model on a 1.5′ grid, and the residual geoid height was computed using spherical FFT and a modified Stokes’ kernel. The effects of different Earth tide models on the geoid were computed and analysed. Comparison over the sea with an oceanographic geoid determined from CLS01 MSSH and Rio05 MDT yield a relative accuracy of 8 cm, with larger differences close to the shoreline. Over land, comparisons with 1646 GPS/levelling marks indicate an overall precision of 8–10 cm and relative vertical datum offsets of up to 2 m. This new geoid model represents a significant improvement over existing geoid models, with a homogeneous relative accuracy of 8–10 cm over both marine and land areas, and exposes the inaccuracies of local vertical datums as references for studies of vertical deformation.

Iberia-Azores Gravity Model (IAGRM) using multi-source gravity data

Abstract A consistent high precision and high resolution gravity model in the north-east Atlantic off Iberia peninsula using multi-source gravity data, ship-borne and satellite derived gravity anomalies, is presented. A solution strategy based on least squares optimal interpolation was used to assimilate into a coherent gravity model, gravity data with different spectral and spatial resolutions. Satellite derived gravity anomalies from KMS02 model, with an error covariance of 25 mGal2, and marine data carefully edited and validated by bias crossover error adjustment, were used in this study. The observation error variance was determined from ship-borne track adjustment and assigned an independent value for each track determined from error variance propagation. Unbiased ship-borne gravity observations were assimilated into the satellite derived gravity KMS02 model by the least squares optimal interpolation algorithm (OI) with bias removed by applying a regional bias to all ship tracks (OI-b) and alternatively by constraining all ship tracks to KMS02 using bias and tilt (OI-t). External error of the model was determined by comparing with recent surveys and it was verified that OI-t approach improved the final gravity model to an accuracy of about 3 mGal. The effect of different merging approaches on geoid solution was also evaluated and it was verified that the merging process can contribute to improve the geoid accuracy up to 4 cm with the OI-t approach.

Satellite Gravity Anomalies and Their Correlation with the Major Tectonic Features in the South China Sea

The South China Sea (SCS) is a region of interaction among three major plates: the Pacific, Indo-Australian and Eurasian. The collision of the Indian subcontinent with the Eurasian plate in the northwest, back-arc spreading at the center, and subduction beneath the Philippine plate along Manila trench in the east and the collision along Palawan trough in the south have produced complex tectonic features within and along the SCS. This investigation examines the satellite-derived gravity anomalies of the SCS and compares them with major tectonic features of the area. A map of Bouguer gravity anomaly is derived in conjunction with available seafloor topography to investigate the crustal structure. The residual isostatic gravity anomaly is calculated assuming that the Cenozoic sedimentary load is isostatically compensated. The features in the gravity anomalies in general correlate remarkably well with the major geological features, including offsets in the seafloor spreading segments, major faults, basins, seamounts and other manifestations of magmatism and volcanism on the seafloor. They also correlate with the presumed location of continental-oceanic crust boundary. The region underlain by oceanic crust in the central part of the SCS is characterized by a large positive Bouguer gravity anomaly (220–330 mgal) as well as large free-air and residual isostatic anomalies. There are, however, important differences among spreading segments. For example, in terms of free-air gravity anomaly, the southwest section of mid-ocean has an approximately 50 km wide belt of gravity low superimposed on a broad high of 45 mgal running NW–SE, whereas there are no similar features in other spreading segments. There are indications that gravity anomalies may represent lateral variation in upper crustal density structure. For instance, free air and isostatic anomalies show large positive anomalies in the east of the Namconson basin, which coincide with areas of dense volcanic material known from seismic surveys. The Red River Fault system are clearly identified in the satellite gravity anomalies, including three major faults, Songchay Fault in the southwest, Songlo Fault in the Northeast and Central Fault in the center of the basin. They are elongated in NW–SE direction between 20±30'N and 17°N and reach to Vietnam Scarp Fault around 16°30'N. It is also defined that the crustal density in the south side of the Central Basin is denser than that in the north side of the Central Basin.