Vertical Gravity Gradient Anomalies Research Articles

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.

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.

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.

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.

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.

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.

Three-dimensional estimate of the lithospheric effective elastic thickness of the Line ridge

Using a new bathymetry grid formed with vertical gravity gradient anomalies and ship soundings (BAT_VGG), a 1°×1° lithospheric effective elastic thickness (Te) grid of the Line ridge was calculated with the moving window admittance technique. As a comparison, both the GEBCO_08 and SIO V15.1 bathymetry datasets were used to calculate Te as well. The results show that BAT_VGG is suitable for the calculation of lithospheric effective elastic thickness. The lithospheric effective elastic thickness of the Line ridge is shown to be low, in the range of 5.5–13km, with an average of 8km and a standard deviation of 1.3km. Using the plate cooling model as a reference, most of the effective elastic thicknesses are controlled by the 150–300°C isotherm. Seamounts are primarily present in two zones, with lithospheric ages of 20–35Ma and 40–60Ma, at the time of loading. Unlike the Hawaiian-Emperor chain, the lithospheric effective elastic thickness of the Line ridge does not change monotonously. The tectonic setting of the Line ridge is discussed in detail based on our Te results and the seamount ages collected from the literature. The results show that thermal and fracture activities must have played an important role in the origin and evolution of the ridge.

Predicting Global Seafloor Topography Using Multi-Source Data

A new one-minute global seafloor topography model was derived from vertical gravity gradient anomalies (VGG), altimetric gravity anomalies, and ship soundings. Ship soundings are used to constrain seafloor topography at wavelengths longer than 200 km and to calibrate the topography to VGG (or gravity) ratios at short wavelengths area by area. VGG ratios are used to predict seafloor topography for wavelength bands of 100–200 km and to suppress the effect of crust isostasy. Gravity anomalies are used to recover seafloor topography at wavelengths shorter than 100 km. The data processing procedure is described in detail in this paper. The accuracy of the model is evaluated using ship soundings and existing models, including General Bathymetric Charts of the Oceans (GEBCO), DTU10, ETOPO1, and SIO V15.1. The results show that, in the discussed regions, the accuracy of the model is better than ETOPO1, GEBCO, and DTU10. Additionally, the model is comparable with V15.1, which is generally believed to have the highest accuracy. In the north-central Pacific Ocean, the accuracy of the model increased by approximately 29.5% compared with the V15.1 model. This indicates that a more accurate seafloor topography model can be formed by combining gravity anomalies, VGG, and ship soundings.

A program for bathymetry prediction from vertical gravity gradient anomalies and ship soundings

We design a program to calculate bathymetry model by combining vertical gravity gradients anomalies (VGG) and ship soundings. A bathymetry model in the experimental area (0 ∼ 30° N, 150 ∼ 210° E) from the North Pacific is calculated with the program. The bathymetry prediction procedure is validated by comparing the result to in situ ship depth soundings and existing models. The accuracy of the result is about 132.9 m, better than most of the existing models, such as GEBCO, DTU10, ETOPO1, and SIO V15.1. If VGG and gravity anomaly data are combined, the accuracy of recovered bathymetry model will increase by about 29.5 % to about 111.7 m, comparing to SIO V15.1. The bathymetry model constructed using VGG will be independent of gravity anomalies, and can be used to analyze the isostatic mechanism of ocean lithosphere with 3D admittance analysis technology. The accuracy of bathymetry prediction can be improved further by combining VGG and gravity anomaly data.

Bathymetry predicted from vertical gravity gradient anomalies and ship soundings

Abstract: In this paper, the admittance function between seafloor undulations and vertical gravity gradient anomalies was derived. Based on this admittance function, the bathymetry model of 1 minute resolution was predicted from vertical gravity gradient anomalies and ship soundings in the experimental area from the northwest Pacific. The accuracy of the model is evaluated using ship soundings and existing models, including ETOPOl, GEBCO, DTU10 and V15. 1 from SIO. The model's STD is 69.481m, comparable with V15. 1 which is generally believed to have the highest accuracy.

A study of regional gravity field recovery from GOCE vertical gravity gradient data in the Auvergne test area using collocation

Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the first satellite mission that observes gravity gradients from the space, to be primarily used for the determination of high precision global gravity field models. However, the GOCE gradients, having a dense data distribution, may potentially provide better predictions of the regional gravity field than those obtained using a spherical harmonic Earth Geopotential Model (EGM). This is investigated in Auvergne test area using Least Squares Collocation (LSC) with GOCE vertical gravity gradient anomalies (Tzz), removing the long wavelength part from EGM2008 and the short wavelength part by residual terrain modelling (RTM). The results show that terrain effects on the vertical gravity gradient are significant at satellite altitude, reaching a level of 0.11 Eotvos unit (E.U.) in the mountainous areas. Removing the RTM effects from GOCE Tzz leads to significant improvements on the LSC predictions of surface gravity anomalies and quasigeoid heights. Comparison with ground truth data shows that using LSC surface free air gravity anomalies and quasi-geoid heights are recovered from GOCE Tzz with standard deviations of 11 mGal and 18 cm, which is better than those obtained by using GOCE EGMs, demonstrating that information beyond the maximal degree of the GOCE EGMs is present. Investigation of using covariance functions created separately from GOCE Tzz and terrestrial free air gravity anomalies, suggests that both covariance functions give almost identical predictions. However, using covariance function obtained from GOCE Tzz has the effect that the predicted formal average error estimates are considerably larger than the standard deviations of predicted minus observed gravity anomalies. Therefore, GOCE Tzz should be used with caution to determine the covariance functions in areas where surface gravity anomalies are not available, if error estimates are needed.

Lithological Mapping by Correlating Magnetic and Gravity Gradient Airborne Measurements

We report here two new applications of the Poisson relation useful when a joint magnetic and gravity gradient survey has been made. In the first, an apparent susceptibility contrast to density contrast ratio map (which we call the map) can be estimated. In the second, selected values of the above ratio are used in a filter to subtract from the magnetic field measurements the effect of a particular rock type. This would be especially useful as a filter of geological due perhaps to weathered magnetite products, particularly if the latter had an effect over a large area. As a test of these techniques we have applied them to a simulated joint airborne survey over a model which we designate Mini-Elura, in which the magnetic magnitude and vertical gravity gradient anomalies are not visible above the due to surficial variations. The computed pseudo-lithology map clearly distinguishes the regions of the map dominated by the alluvium source from those due to the Mini-Elura orebody, while the pseudo-magnetic filter does very effectively remove the noise due to the alluvium from the magnetic map.