Gravity Gradient Anomalies Research Articles

Mapping the structural configuration of the northern part of the Central Indian Ridge from satellite gravity data using derivatives of the horizontal gradient

This study presents a novel approach to map the structural configuration of the northern part of the Central Indian Ridge from satellite-derived gravity data. The proposed approach is based on a combination of derivatives of the horizontal gradient of gravity anomalies. The robustness of the approach is tested on synthetic gravity examples before applying it to satellite gravity data of the northern part of the Central Indian Ridge. The results obtained from the synthetic examples show that the proposed method can map the structural boundaries more precisely and clearly compared to other techniques, and can avoid producing additional structures in output maps. For real application, the structural features determined from the proposed approach are consistent with the fracture zones that have been identified in the area. The obtained result reveals the prominent lineament trend is in the northeast-southwest direction, which is possibly related to plate movement during the Cenozoic. The result demonstrates a high lineament density that is in agreement with the slow spreading rate of the Central Indian Ridge. My findings also demonstrate that the proposed approach can be considered as a valuable tool in mapping structural boundaries.

3D density imaging using gravity and gravity gradient in the wavenumber domain and its application in the Decorah

Density imaging is a method that uses the inversion of the gravity and gravity gradient spectra in the wavenumber domain to create accurate 3D reconstructions of subsurface density distributions. This approach offers computational efficiency and rapid calculations. This research used preliminary inversions to examine the spectral characteristics of gravity and gravity gradient anomalies, as well as the resulting models, were scrutinized through preliminary inversions. 3D density imaging of gravity and gravity gradient was performed in the wavenumber domain using depth weighting on both noise-added and theoretical data, producing a density model that was consistent with the theoretical one. The technique was then used in the Decorah region of the United States, where 3D density imaging was performed and an examination of the properties of gravity and gravity gradient anomalies was conducted. The results showed where high-density Decorah complexes, low-density siliceous intrusive rocks, and high-density intrusive rock masses, were the distributed within the surrounding rock. Each of these provided comprehensive insights into the intrusive pathways to the rock mass. Thus, the appropriateness and effectiveness of the density imaging method were confirmed, supporting a deeper understanding of the structural division and geological evolution in the region.

Comparative Study on Predicting Topography from Gravity Anomaly and Gravity Gradient Anomaly

Owing to the dependence of algorithms on the measurement of ship soundings and geophysical parameters, the accuracy and coverage of topography still need to be improved. Previous studies have mostly predicted topography using gravity or gravity gradient, However, there is a relative lack of integrated research combining or comparing gravity and gravity gradient. In this study, we develop observation equations to predict topography based on vertical gravity anomalies (VG; also called gravity anomalies) and vertical gravity gradient (VGG) anomalies generated by a rectangular prism. The sources of interference are divided into medium- to high-frequency errors and low-frequency errors, and these new methods reduce these errors through regularization and error equations. We also use numerical simulations to test the efficiency of the algorithm and error-reduction method. Statistics show that VGG anomalies are more sensitive to topographic fluctuations; however, the linear correlation between VG anomalies and topography is stronger. Additionally, we use the EIGEN-6C4 model of VG and VGG anomalies to predict topography in shallow and deep-sea areas, with maximum depths of 2 km and 5 km, respectively. In the shallow and deep-sea areas, the root mean square (RMS) errors of VGG anomalies prediction are 93.8 m and 233.8 m, and the corresponding accuracies improved by 7.3% and 2.3% compared with those of VG anomaly prediction, respectively. Furthermore, we use cubic spline interpolation to fuse ship soundings and improve the accuracy of the final topography results. We develop a novel analytical algorithm by constructing an observation equation system applicable to VG and VGG anomalies. This will provide new insights and directions to refine topography prediction based on VG and VGG anomalies.

Computation of gravimetric geoid model using free air vertical gravity gradient anomaly in geoid-quasigeoid formula

Geoid-quasigeoid separation term is a crucial parameter that allows the conversion between different national height systems based on geoid and quasigeoid surfaces. The consideration of free-air Vertical Gravity Gradient Anomalies (VGGA) in the separation term formula is generally ignored and is not sufficiently investigated. However, this may be important, particularly for the mountainous areas. In this study, the impact of considering modelled VGGA in the special formulation of separation term is investigated. The study area is in mountainous western Turkey where modelled VGGA are previously validated with the homogenous and dense VGGA measurements (in total: 159). The separation term is calculated with and without considering the VGGA. Thus, gravimetric geoid models have been obtained with two different strategies. These gravimetric geoid models are compared with geometric geoid model calculated from recently measured GPS/levelling measurements. The results demonstrate that the consideration of modelled VGGA in the special formula of separation term changes the gravimetric geoid model with the statistics of min: −10.6 cm, max: 17 cm, mean: 0.2 cm, std.: 1.5 cm. This is quite significant for a goal of cm accurate geoid modelling. Especially at elevations higher than 1000 m, the use of modelled VGGA is found to be more effective on the gravimetric geoid.

Depth distribution of Moho model and tectonic patterns in South China Sea and adjacent areas

This study aims to investigate the depth distribution of Mohorovicic discontinuity (Moho) and its relationship with the tectonic pattern of the South China Sea and adjacent areas. To achieve this, the spatial characteristics of the full tensor gravity gradient data are analyzed to identify 17 large and deep faults and to divide the study area into 9 tectonic units with distinct geological structures. Using a three-dimensional (3D) interface inversion method, the Moho depth is determined, constrained by the Moho depth information obtained from sonar-buoy detection and submarine seismograph detection profiles. By analyzing the relationship between the distribution characteristics of Moho and tectonic units, the study summarizes the trend, relief, gradient of Moho, and crustal properties in the study area. Additionally, the seismically constrained Moho undulation combined with the gravity data, gravity gradient anomalies and unconstrained 3D correlation imaging are employed to study the crustal structure of the South China Sea, investigate the vertical and horizontal changes of the crustal structure, and reveal the large-scale crustal and regional structure of the South China Sea. Through the coupling analysis of shallow and deep structures, the study reveals that the gravity gradient anomalies and 3D correlation imaging are consistent with the variations of the Moho depth, indicating the presence of a trench-island arc-back arc basin system and the distribution of continental crust, oceanic crust, and transitional crust in the South China Sea.

On performance of vertical gravity gradient determined from CryoSat-2 altimeter data over Arabian Sea

SUMMARYHigh-precision and high-resolution satellite altimetry data from CryoSat-2 are widely utilized for marine gravity inversion. The vertical gravity gradient is a crucial parameter of the Earth's gravity field. To evaluate the performance of vertical gravity gradient determined from CryoSat-2 altimeter data, the pre-processed along-track sea surface heights (SSHs) are obtained through error correction. The study area focused on the Arabian Sea and its surrounding region, where the along-track geoid was derived by subtracting the mean dynamic topography of the ocean from the along-track SSH of CryoSat-2. The residual along-track geoidal gradients were obtained by adjusting the along-track geoid gradients calculated from CryoSat-2 altimeter data using the remove-restore method. This was done by subtracting the geoid gradients calculated by the gravity field model XGM2019e_2159. After obtaining the residual along-track geoidal gradients, the residual gridded deflections of the vertical (DOV) are calculated using the least-squares collocation (LSC) method. The residual gridded DOV are then used to compute the residual gridded gravity anomaly gradients in the study area using the finite-difference method. After restoring the gravity anomaly gradients computed by the XGM2019e_2159 model, a high-resolution gravity anomaly gradient model with a resolution of 1′ ×1′ is obtained for the Arabian Sea and its surrounding area. To evaluate the accuracy of the gravity anomaly gradient model derived from CryoSat-2, it was compared with the SIO V32.1 gravity anomaly gradient model released by the Scripps Institution of Oceanography. The comparison showed that the root mean square (RMS) of the differences between the two models is 7.69E, demonstrating the high accuracy and precision of the vertical gravity gradient determined from CryoSat-2 altimeter data.

Bathymetric Prediction Using Multisource Gravity Data Derived From a Parallel Linked BP Neural Network

AbstractGravity anomalies (GAs) and vertical gravity gradient anomalies (VGGs) are regularly used to predict bathymetry. However, few studies have explored the combination of the GAs and VGGs to predict bathymetry. We introduce the back propagation (BP) neural network into bathymetric prediction field and propose a method to predict depth from GAs and VGGs. The method was tested in the Mariana Trench region, and a neural network bathymetry model was constructed using feature data obtained from GAs and VGGs as the input of the BP neural network. Additionally, single‐beam sounding data were used as label data. By comparing the neural network and the gravity‐geologic method (GGM), the neural network was found to provide better performance with an accuracy improvement of 19%. The root‐mean‐square of the absolute difference between the neural network bathymetry model and the single‐beam sounding data was 72.40 m, with a relative accuracy of 1.71%. Approximately 50% of the differences were distributed within ±20 m, and 90% were distributed within ±100 m. The neural network bathymetry model was also compared with the GGM bathymetry model for different depths and topographies, and the results verified the feasibility and effectiveness of the BP neural network method.

Revised depth of the Challenger Deep from submersible transects; including a general method for precise, pressure-derived depths in the ocean

We present a revised estimate of the maximum depth of the Challenger Deep, generally considered the deepest area of the world's oceans, based on a series of submersible dives conducted in June 2020. These depth estimates are derived from acoustic altimeter profiles referenced to in-situ pressure and corrected for observed oceanographic properties of the water-column, atmospheric pressure, gravity and gravity-gradient anomalies, and water-level effects. We also present comprehensive methods to determine depth from observed pressure using modern standards and estimate the associated uncertainty. For the Challenger Deep, the deepest observed seafloor depth was 10,935 m (±6 m at 95% C.I.) below mean sea level. For this work, the error term is dominated by the uncertainty of the pressure sensor used but we show that the gravity correction is substantial. We compare to these new results other recent acoustic and pressure-based measurements for the Challenger Deep.

Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India

Abstract The Southern Granulite Terrain (SGT) is a large tract of exposed Archean continental crust, divided into the Madurai Block (MB), Trivandrum Block (TB), and Nagercoil Block (NB). These crustal domains are linked with the NW-SE trending Achankovil Shear Zone (AKSZ). We combine gravity and magnetic data with previously published ground observations and geochronological data to re-evaluate the crustal architecture, evolution of the AKSZ, and possible extension of AKSZ into Madagascar. Analyses indicate that the long wavelength trends of the magnetic anomalies originate at ~20 km depth of different SGT blocks. These observations are corroborated with the gravity as well as computed gravity gradient anomalies. The presence of khondalite outcrops in Trivandrum Block implies that high magnetization crust is the main source of positive magnetic anomalies. Such magnetic anomalies advocate that SGT preserves the remanent of Archean crustal blocks in South India, a part due to variation in thermal and geochemical processes. The AKSZ, TB, and MB exhibit contrasting magnetic crustal signatures. The joint modeling results reveal a three-layered crustal configuration with varying Moho ranging from 41 to 34 km in NE to SW, respectively. It is also noted that AKSZ is a narrow and deep structure near to the Western Ghats Escarpment while it is wide and shallow in the far-east, which implies that the evolution of the Western Ghats is a late geological event.

Study on gravity anomaly changes and crustal structure characteristics in Weihe Basin

Based on the Earth Gravitational Model (EGM2008) and the global terrain model (ETOPO1), we calculate and obtain the free air gravity anomaly and Bouguer gravity anomaly in the Weihe Basin. The wavelet multi-scale analysis method is used to decompose the Bouguer gravity anomaly with 1st to 5th order wavelet detail and wavelet approximation. At the same time, based on the logarithmic power spectrum method, the average depth of the field source corresponding to the 1st to 5th order wavelet detail and approximation is calculated. The research results show that: (1) the wavelet approximation of Bouguer gravity anomaly of order 1st to 5th in the Weihe Basin corresponds to a field source depth of 12 to 84 km. From the wavelet approximation images, the basic pattern of different tectonic units in the study area can be clearly seen. (2) The field source depth corresponding to the wavelet detail of the 1st to 5th order Bouguer gravity anomalies in the Weihe Basin is 3 to 49 km. From the wavelet detail images, the characteristics of different tectonic divisions and fault distribution in the study area can be clearly seen. (3) Gravity anomaly gradient zones often correspond to relatively large-scale faults, such as the Longxian-Baoji fault, the northern Qinling fault, the Weihe fault, and the Huashan piedmont fault. In addition, from the wavelet detail images, the main structural units such as the Baoji uplift, the Xi'an sag, the Lishan uplift and the Gushi sag can be identified.

Bathymetry Model in the Northwestern Pacific Ocean Predicted from Satellite Altimetric Vertical Gravity Gradient Anomalies and Ship-Board Depths

New bathymetry models in the northwestern Pacific Ocean are presented at 1 arc-minute and 15 arc-second resolution. The latest version of the altimetric vertical gravity gradient (VGG) anomalies from Scripps Institute of Oceanography, ∼7 million single-beam depths from the National Centers for Environmental Information, and ∼80 GB of multibeam grids from the Japan Agency for Marine-Earth Science and Technology are used. The ship-board depths are used to constrain bathymetry at wavelengths longer than 200 km, and calibrate the local topography to VGG ratio at 15–200 km wavelength bands. The VGG is used to predict bathymetry at 15 ∼ 200 km wavelength bands. The spectrum analysis results show that the 1 arc-minute model has more power than models predicted from gravity anomalies at wavelengths shorter than 100 km. The standard deviation of differences between the 1 arc-minute model and ship-board depths is 44.76 m, and it is 102.842 m comparing to the SIO topo_20.1.nc model. The accuracy of the new 1 arc-minute model has been improved significantly from our last bathymetry model, BAT_VGG, and has a better accuracy than that of the DTU18, GEBCO_08, and ETOPO1 models. The accuracy of the 15 arc-second model is consistent with that of SRTM + V2.1 and GEBCO_2020.

Environmental Influence Analysis of Global Gravity Anomaly Gradient Basing on Neighbourhood Satistics in GIS

A series of abnormal environmental phenomena were found related with local earth gravity field anomaly through statistics and calculation analysis. Gravity anomaly gradient was used to describe gravity anomaly change in local extent. Total of 79% in earth surface gravity anomaly gradient express stable (<30 mGal/Degree), 7.37% area express major gravity anomaly gradient (> 50 mGal/Degree), of 3.43% area gravity anomaly gradient is very high (> 70 mGal/Degree). While gravity anomaly is over 30 mGal/Degree, there will may more easily appear sandstorm, tornadoes and haze weather, if thermodynamic conditions and material conditions are proper. Many natural disasters start from these areas, where gravity anomaly gradient is very high.

Sequential inversion of GOCE satellite gravity gradient data and terrestrial gravity data for the lithospheric density structure in the North China Craton

Abstract. The North China Craton (NCC) is one of the oldest cratons in the world. Currently, the destruction mechanism and geodynamics of the NCC remain controversial. All of the proposed views regarding the issues involve studying the internal density structure of the NCC lithosphere. Gravity field data are among the most important data in regard to investigating the lithospheric density structure, and gravity gradient data and gravity data each possess their own advantages. Given the different observational plane heights between the on-orbit GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) satellite gravity gradient and terrestrial gravity and the effects of the initial density model on the inversion results, sequential inversion of the gravity gradient and gravity are divided into two integrated processes. By using the preconditioned conjugate gradient (PCG) inversion algorithm, the density data are calculated using the preprocessed corrected gravity anomaly data. Then, the newly obtained high-resolution density data are used as the initial density model, which can serve as constraints for the subsequent gravity gradient inversion. Several essential corrections are applied to the four gravity gradient tensors (Txx, Txz, Tyy, Tzz) of the GOCE satellite, after which the corrected gravity gradient anomalies (T′xx, T′xz, T′yy, T′zz) are used as observations. The lithospheric density distribution result within the depth range of 0–180 km in the NCC is obtained. This study clearly illustrates that GOCE data are helpful in understanding the geological settings and tectonic structures in the NCC with regional scale. The inversion results show that in the crust the eastern NCC is affected by lithospheric thinning with obvious local features. In the mantle, the presented obvious negative-density areas are mainly affected by the high-heat-flux environment. In the eastern NCC, the density anomaly in the Bohai Bay area is mostly attributed to the extension of the Tancheng–Lujiang major fault at the eastern boundary. In the western NCC, the crustal density anomaly distribution of the Qilian block is consistent with the northwest–southeast strike of the surface fault belt, whereas such an anomaly distribution experiences a clockwise rotation to a nearly north–south direction upon entering the mantle.

Mapping of basement structure beneath the Kohima Synclinorium, north-east India via Bouguer gravity data modelling

Kohima Synclinorium is one of the most tectonically active corridors of Indian subcontinent and displays complex tectonics of the region. Mapping the basement structure beneath the Kohima Synform is, therefore, vital to provide deep insight into the understanding of the crucial thrust geometry of the region. The vertical gravity gradient anomalies and available geological evidences suggest that the underlying area is occupied by thrust geometry embedded with prominently known tectonic trends of Schuppen Belt (SB), Kohima–Patkai Synclinal (KS–PS) and adjoining Inner Fold Belts (IFB). By keeping in view the massive complex tectonic upheaval in the region, we carried out 2D Bouguer gravity data analysis using the radially averaged power spectral techniques and GMSYS modelling to map the basement depth more precisely. Our results suggest that there is a wide range of heterogeneity in the underlying undulating basement indicating an average sedimentary thickness of the order of 2.2–5.5 km. The gravity PDEPTH modelling results show that source depth varies from 2.5 to 6.5 km. There is an uplifted basement tending towards the southwestern part while gradual deepening of basement was observed towards the eastern part of the study area. The profile modelling results show the presence of basement in a depth range of 2.5–3.8, 3.8–4.0, and 3.8–4.2 km beneath Foreland Basin (FB), Kohima–Patkai Synclinal structures (KS–PS), and Inner Fold Belts (IFB), respectively. The underlying results of integrated profiles, PDEPTH and GMSYS modelling would be useful to understand the detailed basement structure and tectonic trends of Belt of Schuppen (BS), Kohima–Patkai Synclinal structures (KS–PS) and adjacent Inner Fold Belts (IFB) of north-eastern region of India.

Structure and tectonic setting of the Chingale Igneous Ring Complex, Malawi from aeromagnetic and satellite gravity data: Implication for Precambrian terranes collision and Neogene - Quaternary rifting

This work examines the structure and tectonic setting of the possibly late Neoproterozoic - early Cambrian Pan-African (henceforth Pan-African) Chingale Igneous Ring Complex (CIRC) in southern Malawi to document its internal emplacement structure and solicit implications for Precambrian terranes collision and Neogene – Quaternary rifting. The CIRC is located along the boundary between the Mesoproterozoic – Neoproterozoic Southern Irumide orogenic belt in the northwest (dominantly amphibolite facies metamorphic terrane) and the Unango complex to the southeast (dominantly granulite facies metamorphic terrane). Portion of the Southern Irumide orogenic belt has been recently recognized as the Niassa craton. The CIRC is also dissected by the southeastern border fault of the Malawi rift, which represents a segment of the Neogene – Quaternary Western Branch of the East African Rift System. This work uses reduced to the pole (RTP) aeromagnetic data enhanced through tilt derivative and three-dimensional (3D) magnetic susceptibility modeling to reveal that the CIRC comprises three overlapping 7 km–12 km wide circular to elliptical ring structures that are aligned in a NE-SW direction along the boundary between the Southern Irumide orogenic belt and the Unango complex. This work also uses Bouguer gravity anomalies of the World Gravity model 2012 (WGM 2012) subjected to upward continuation of 2 km and 12 km to show that the CIRC is located along a relatively steep gravity anomaly gradient coinciding with the boundary between the Southern Irumide orogenic belt and the Unango complex. Together with structural, lithological, metamorphic, and lithospheric structure observations, this work proposes that the CIRC mark the location of a Pan-African suture zone resulting from the collision of the Southern Irumide orogenic belt (Niassa craton) and the Unango complex and this is named here the Chingale suture zone. The presence of this Pan-African suture zone facilitated localization of extensional strain during the onset of the Malawi rift allowing it to change its orientation from NNW-trending in its northern part to NE-trending in its southern part. Igneous ring complexes are hypabyssal intrusions. Hence, the exposure of the CIRC within the southern Malawi rift suggests the lack of rift-related subsidence. This makes much of the ~300 m–~500 m high relief of the topographic escarpment of the southeastern border fault of the rift the result of rift flank uplift.

Evaluation of mineral exploration targets defined by airborne gravity gradiometry through gravity and magnetic modelling: vicinity of the Iron Range Fault, Purcell anticlinorium, southern Canadian Cordillera

An airborne gravity gradiometer survey was recently flown over the Iron Range Fault in the Purcell anticlinorium, southern Canadian Cordillera. The fault is commonly associated with iron oxide mineralization having characteristics similar to those of iron oxide Au ± Cu deposits. Drilling near the fault has revealed Au ± Cu–Pb–Zn mineralization. Prominent positive vertical gravity gradient (VGG) anomalies defined by the survey were identified as targets for follow-up exploration. Possible sources of the target anomalies were investigated by modelling gravity, VGG, and magnetic data along several profiles. Modelling of regional-scale profiles of the vertical component of gravity crossing exploration targets provides a regional perspective on the regional geological setting, dominated by the broad Goat River anticline, whose axis closely follows the Iron Range Fault. Modelling indicates that several VGG anomalies are related to Moyie sills, although one anomaly is modelled as a narrow vertical body (120 m wide, 1000 m vertical extent, 40 m deep) just west of the Iron Range Fault. Its apparent high density of 3500 kg/m3 suggests metallic content, making it a choice candidate for follow-up investigation. Drilling at the southern end of this geophysical target intersected a Moyie intrusion, but untested geochemical anomalies in the vicinity encourage follow-up exploration. The densities of modelled units derived from VGG profiles across two other specific targets indicate that Moyie sills represent one target and iron oxide mineralization the other, as supported by magnetic modelling, which also delineated vertical zones of significantly magnetic material along the Iron Range Fault.

Density interface topography recovered by inversion of satellite gravity gradiometry observations

A radial integration of spherical mass elements (i.e. tesseroids) is presented for evaluating the six components of the second-order gravity gradient (i.e. second derivatives of the Newtonian mass integral for the gravitational potential) created by an uneven spherical topography consisting of juxtaposed vertical prisms. The method uses Legendre polynomial series and takes elastic compensation of the topography by the Earth’s surface into account. The speed of computation of the polynomial series increases logically with the observing altitude from the source of anomaly. Such a forward modelling can be easily applied for reduction of observed gravity gradient anomalies by the effects of any spherical interface of density. An iterative least-squares inversion of measured gravity gradient coefficients is also proposed to estimate a regional set of juxtaposed topographic heights. Several tests of recovery have been made by considering simulated gradients created by idealistic conical and irregular Great Meteor seamount topographies, and for varying satellite altitudes and testing different levels of uncertainty. In the case of gravity gradients measured at a GOCE-type altitude of \(\sim \)300 km, the search converges down to a stable but smooth topography after 10–15 iterations, while the final root-mean-square error is \(\sim \)100 m that represents only 2 % of the seamount amplitude. This recovery error decreases with the altitude of the gravity gradient observations by revealing more topographic details in the region of survey.

Geopotential field anomalies and regional tectonic features – two case studies: southern Africa and Germany

Abstract. Maps of magnetic and gravity field anomalies provide information about physical properties of the Earth's crust and upper mantle, helpful in understanding geological conditions and tectonic structures. Depending on data availability, whether from the ground, airborne, or from satellites, potential field anomaly maps contain information on different ranges of spatial wavelengths, roughly corresponding to sources at different depths. Focussing on magnetic data, we compare amplitudes and characteristics of anomalies from maps based on various available data and as measured at geomagnetic repeat stations. Two cases are investigated: southern Africa, characterized by geologically old cratons and strong magnetic anomalies, and the smaller region of Germany with much younger crust and weaker anomalies. Estimating lithospheric magnetic anomaly values from the ground stations' time series (repeat station crustal biases) reveals magnetospheric field contributions causing time-varying offsets of several nT in the results. Similar influences might be one source of discrepancy when merging anomaly maps from different epochs. Moreover, we take advantage of recently developed satellite potential field models and compare magnetic and gravity gradient anomalies of ∼ 200 km resolution. Density and magnetization represent independent rock properties and thus provide complementary information on compositional and structural changes. Comparing short- and long-wavelength anomalies and the correlation of rather large-scale magnetic and gravity anomalies, and relating them to known lithospheric structures, we generally find a better agreement in the southern African region than the German region. This probably indicates stronger concordance between near-surface (down to at most a few km) and deeper (several kilometres down to Curie depth) structures in the former area, which can be seen to agree with a thicker lithosphere and a lower heat flux reported in the literature for the southern African region.

Complete gravity field of an ellipsoidal prism by Gauss–Legendre quadrature

The increasing availability of geophysical models of the Earth’s lithosphere and mantle has generated renewed interest in computation of theoretical gravity effects at global and regional scales. At the same time, the increasing availability of gravity gradient anomalies derived from satellite measurements, such as those provided by GOCE satellite, requires mathematical methods that directly model the gravity gradient anomalies in the same reference frame as GOCE gravity gradients. Our main purpose is to interpret these anomalies in terms of source and density distribution. Numerical integrationmethods for calculating gravity gradient values are generally based on a mass discretization obtained by decomposing the Earth’s layers into a finite number of elementary solid bodies. In order to take into account the curvature of the Earth, spherical prisms or ‘tesseroids’ have been established unequivocally as accurate computation tools for determining the gravitational effects of large-scale structures. The question which then arises from, is whether gravity calculation methods using spherical prisms remain valid when factoring in the ellipticity of the Earth. In the paper, we outline a comprehensive method to numerically compute the complete gravity field with the help of the Gauss–Legendre quadrature involving ellipsoidal shaped prisms. The assessment of this new method is conducted by comparison between the gravity gradient values of simple sources obtained by means of numerical and analytical calculations, respectively. A comparison of the gravity gradients obtained from PREM and LITHO1.0 models using spherical- and ellipsoidalprism- based methods is also presented. Numerical results indicate that the error on gravity gradients, caused by the use of the spherical prism instead of its ellipsoidal counterpart to describe an ellipsoidally shaped Earth, is useful for a joint analysis with those deduced from GOCE satellite measurements. Provided that a suitable scaling of prism densities has been performed, the spherical approximation error at GOCE height hardly reaches 1mE for the entire Earth’s lithosphere. The error attains 6mE at a peak for a complete modeling of the Earth, from the crust down to the internal core.

Using ground gravity to improve ice mass change estimation from GOCE gravity gradients in mid-west Greenland

Vertical gravity gradient anomalies from the Gravity and steady-state Ocean Circulation Explorer (GOCE) DIR-3 model have been used to determine gravity anomalies in mid-west Greenland by using Least-Squares Collocation (LSC) and the Reduced Point Mass (RPM) method. The two methods give nearly identical results. However, compared to LSC, the RPM method needs less computational time as the number of equations to be solved in LSC equals the number of observations. The advantage of the LSC, however, is the acquired error estimates. The observation periods are winter 2009 and summer 2012. In order to enhance the accuracy of the calculated gravity anomalies, ground gravity data from West Greenland is used over locations where the gravity change resulting from ice mass changes is negligible, i.e. over solid rock. In the period considered, the gravity anomaly change due to changes in ice mass varies from −5 mGal to 4 mGal. It is negative over the outlet glacier Jacobshavn Isbrae, where the mass loss corresponds to a gravity change of approximately −4 mGal. When using only GOCE vertical gravity gradients, the error estimates range from 5 mGal at the coast to 17 mGal over the ice sheet. Introducing the ground gravity data from West Greenland in the prediction reduces the errors to range from 2 to 10 mGal.