Resolution and variance operators of gravity and gravity gradiometry

Geophysics ◽  
1989 ◽  
Vol 54 (7) ◽  
pp. 889-899 ◽  
Author(s):  
D. W. Vasco
Keyword(s):  
Inverse Problem ◽  
Vertical Component ◽  
Maximum Error ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Density Structure ◽  
Gravity Gradiometry ◽  
Data Set ◽  
Model Parameter ◽  
Vertical Gravity Gradient

Gravity gradiometry represents a new potential field data set which may better constrain the density structure of the earth. Using singular value (spectral) decomposition of the gravity and gravity gradient kernels, the model parameter resolution and model parameter variance of the two data types are compared using data from the Defense Mapping Agency and a recently acquired collection of airborne gradient measurements from Bell Aerospace Textron’s Gravity Gradient Survey System (GGSS). The GGSS was flown over a portion of southwestern Oklahoma, where the gravitational anomaly from the buried Wichita basement rocks is over 60 mGal. The corresponding maximum vertical gravity gradient was found to be 46.2 Eötvös. The determination of the subsurface density structure is cast as a linear inverse problem and, for comparison, a nonlinear inverse problem. For both the linear and nonlinear inversions, the gravity gradients improve the resolution and result in smaller variances than the vertical component of gravity. The density resolution and variance were computed for a subset of tracks from an airborne gravity gradient survey made in the summer of 1987. For the linear inversion, the resolution of the density is not adequate below the second layer (20 km). Furthermore, the estimated error of the actual gradient observations for a resolution of 0.9 km is 10E, for which the maximum error of the density values is [Formula: see text]. The linearized resolution of the boundary perturbations is better, with most parameters being well resolved. The standard errors for the layer perturbations are less than 1 km for the shallower layer (5.0 km) when using the gradiometer data. For the deeper layer (25.0 km), the maximum error is larger, 4.3 km.

scholarly journals AIRBORNE GRAVITY GRADIOMETRY – DATA PROCESSING AND INTERPRETATION

2013 ◽  
Vol 31 (3) ◽  
pp. 427 ◽  
Author(s):  
Dionisio Uendro Carlos ◽  
Marco Antonio Braga ◽  
Henry F. Galbiatti ◽  
Wanderson Roberto Pereira
Keyword(s):  
Iron Ore ◽  
A Priori ◽  
Synthetic Data ◽  
Real Data ◽  
Geological Mapping ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Gravity Gradiometry ◽  
Quadrilátero Ferrífero ◽  
Airborne Gravity Gradiometry

ABSTRACT. This paper discusses some processing techniques (all codes were implemented with open source software) developed for airborne gravity gradient systems, aiming at outlining geological features by applying mathematical formulations based on the potential field properties and its breakdown into gradiometric tensors. These techniques were applied to both synthetic and real data. These techniques applied to synthetic data allow working in a controlled environment, under- standing the different processing results and establishing a comparative parameter. These methodologies were applied to a survey area of the Quadrilátero Ferrífero to map iron ore targets, resulting in a set of very helpful and important information for geological mapping activities and a priori information for inversion geophysical models.Keywords: processing, airborne gravity gradiometry, iron ore exploration, FTG system, FALCON system. RESUMO. Neste trabalho apresentamos algumas técnicas de processamento (todos os códigos foram implementados em softwares livres) desenvolvidas para aplicação aos dados de aerogradiometria gravimétrica. Os processamentos foram aplicados tanto a dados sintéticos como a dados reais. A aplicação a dados sintéticos permite atuar em um ambiente controlado e entender o padrão resultante de cada processamento. Esses mesmos processamentos foram aplicados em uma área do Quadrilátero Ferrífero para o mapeamento de minério de ferro. Todos os resultados desses processamentos são muito úteis e importantes para o mapeamento geológicoe como informação a priori para modelos de inversão geofísica.Palavras-chave: processamento, dados de aerogradiometria gravimétrica, exploração de minério de ferro, sistema FTG, sistema FALCON.

Mathematical properties and physical meaning of the gravity gradient tensor eigenvalues

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. G115-G124
Author(s):  
Carlos Cevallos
Keyword(s):  
Physical Meaning ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Model Data ◽  
Gradient Tensor ◽  
Gravity Gradient Tensor ◽  
Mathematical Properties ◽  
Vertical Gravity Gradient ◽  
Vertical Gravity ◽  
Two Parameters

The eigenvalues of the gravity gradient tensor can be expressed as functions of two parameters: a magnitude and a phase. The decomposition gives physical meaning to the eigenvalues: The magnitude measures the amount of curvature, and the phase is related to the type of source. Their understanding allows one to propose new quantities to interpret. As an example, a modified phase eigenvalue offers the interpreter a subtle enhanced version of the vertical gravity gradient, which is evaluated with model data and applied to FALCON airborne gravity gradiometer data from the Perth Basin, Australia.

Consistency investigation, vertical gravity estimation, and inversion of airborne gravity gradient data — A case study from northern Sweden

Geophysics ◽  
2016 ◽  
Vol 81 (3) ◽  
pp. B65-B76 ◽  
Author(s):  
Sayyed Mohammad Abtahi ◽  
Laust Börsting Pedersen ◽  
Jochen Kamm ◽  
Thomas Kalscheuer
Keyword(s):  
Scale Effects ◽  
Gravity Data ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Small Scale ◽  
Northern Sweden ◽  
Data Set ◽  
Vertical Gravity ◽  
The Individual ◽  
Topographic Relief

For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.

Geophysical expression of a buried niobium and rare earth element deposit: The Elk Creek carbonatite, Nebraska, USA

2014 ◽  
Vol 2 (4) ◽  
pp. SJ23-SJ33 ◽  
Author(s):  
Benjamin J. Drenth
Keyword(s):  
Rare Earth ◽  
Rare Earth Element ◽  
Earth Element ◽  
Primary Source ◽  
Physical Property ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Magnetic Survey ◽  
Lower Paleozoic ◽  
Vertical Gravity Gradient

The lower Paleozoic Elk Creek carbonatite is a 6–8-km-diameter intrusive complex buried under 200 m of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S. and a rare earth element (REE) deposit. The carbonatite is composed of several lithologies, the relations of which are poorly understood. Niobium mineralization is most enriched within a magnetite beforsite (MB) unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes Proterozoic country rocks. Efforts to explore the carbonatite have used geophysical data and drilling. A high-resolution airborne gravity gradient and magnetic survey was flown over the carbonatite in 2012. The carbonatite is associated with a roughly annular vertical gravity gradient high and a subdued central low and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. Geophysical, borehole, and physical property data are combined for an interpretation of these signatures. The carbonatite is denser than the country rocks, explaining the gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than those of the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are present in large volumes. MB is very dense (mean density [Formula: see text]) and strongly magnetized (median 0.073 magnetic susceptibility), producing a gravity gradient high and contributing to the aeromagnetic high. Barite beforsite has physical properties similar to most of the carbonatite volume, making it a poor geophysical target. Geophysical anomalies indicate the presence of dense and strongly magnetized rocks at depths below existing boreholes, either a large volume of MB or another unknown lithology.

3D inversion of airborne gravity-gradiometry data using cokriging

Geophysics ◽  
2014 ◽  
Vol 79 (4) ◽  
pp. G37-G47 ◽  
Author(s):  
Meixia Geng ◽  
Danian Huang ◽  
Qingjie Yang ◽  
Yinping Liu
Keyword(s):  
Estimation Error ◽  
A Priori ◽  
Error Variance ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Gravity Gradiometry ◽  
Multiple Components ◽  
Smoothing Effects ◽  
Priori Information ◽  
Airborne Gravity Gradiometry

We developed a new method for interpretation of airborne gravity gradiometry data, based on cokriging inversion. The cokriging method that we evaluated minimized the theoretical estimation error variance by using auto- and crosscorrelations of several variables. It does not require iterations and can easily include complex a priori information. Moreover, the smoothing effects in the inverted density structure model can be reduced to a certain extent due to the anisotropy constrain in the covariance model. We compared the recovered models obtained by inverting the different combinations of gravity-gradient components to understand how different component combinations contributed to the resolution of the recovered model. The results indicated that including multiple components for inversion increased the resolution of the recovered density model and improved the structure delineation. Moreover, in the case in which the parameters of the variogram model are not well chosen, cokriging with multicomponent combinations can still correctly recover the geometry of the targeted sources. The survey data of the Vinton dome were considered as a case study. The results of the inversion were in good agreement with the known formation in the region. This supports the validity of our method.

Next Generation High Resolution Airborne Gravity Reconnaissance in Oil Field Exploration

1993 ◽  
Vol 11 (3-4) ◽  
pp. 198-234 ◽  
Author(s):  
J.M. Bodard ◽  
J.G. Creer ◽  
M.W. Asten
Keyword(s):  
Gravity Field ◽  
Oil Field ◽  
Petroleum Exploration ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Practical Implementation ◽  
Gravity Gradients ◽  
Gravity Gradiometry ◽  
Near Surface ◽  
Migration Pathways

Simple modelling studies of gravity fields using elementary structural forms, oilfield-type structures and geological reconnaissance situations, show that gravity gradiometry technology offers significant petroleum exploration potential. In geological environments of interest, gravity gradients are primarily due to density displacement along (near) vertical boundaries. Gradient images therefore reveal the edges and corners of intrusions, faults, fault intersections, and other such structures often associated with hydrocarbon migration pathways and traps, and/or significant basinal trends. Recent technological advances may make gravity gradiometry an airborne reconnaissance tool capable of providing sensitivity and resolution superior to the best gravimetry available today. This capacity, and the array of gradient components that may be measured, will embellish aspects of the gravity field important to developing regional geologic interpretations. While the potential advantage of gravity gradiometry is greater lateral resolution and sensitivity from a moving platform, the disadvantage is the high sensitivity to topographic and shallow buried irregularities unrelated to the deeper geological structures of interest. A further difficulty is the complex gravity field representations produced for density structures of certain geometries. Buried features that have near surface expressions will be easiest to map. However, full use of gravity gradient technology will require application-focused data processing techniques and new interpretation skills. When the technology becomes commercially available it could find application in preseismic reconnaissance, structural (and possibly stratigraphic) mapping, acreage management and assessment, and in the evolution and mapping of controls on oilfield distribution. The technology could help develop exploration in remote and inaccessible areas, and provide a new look at well-explored regions. An immediate practical implementation appears to be in offshore exploration applications, possibly linked to deepwater exploitation strategies.

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

2019 ◽  
Vol 56 (5) ◽  
pp. 452-470
Author(s):  
Mike D. Thomas ◽  
Mark Pilkington ◽  
Mike McCuaig
Keyword(s):  
Iron Oxide ◽  
Regional Scale ◽  
Vertical Component ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Canadian Cordillera ◽  
Iron Range ◽  
Vertical Gravity Gradient ◽  
Magnetic Modelling

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.

scholarly journals On the Feasibility of Seafloor Topography Estimation from Airborne Gravity Gradients: Performance Analysis Using Real Data

2020 ◽  
Vol 12 (24) ◽  
pp. 4092
Author(s):  
Junjun Yang ◽  
Zhicai Luo ◽  
Liangcheng Tu ◽  
Shanshan Li ◽  
Jingxue Guo ◽  
...  
Keyword(s):  
Short Wavelength ◽  
Estimation Accuracy ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Airborne Gravimetry ◽  
Gravity Gradients ◽  
Gravity Gradiometry ◽  
Seafloor Topography ◽  
Topographic Features ◽  
Airborne Gravity Gradiometry

Compared with airborne gravimetry, a technique frequently used to infer the seafloor topography at places inaccessible to ship soundings due to the presence of ice shelf or ice mélange, airborne gravity gradiometry inherently could achieve higher spatial resolution, thus it is promising for improved inference of seafloor topography. However, its estimation capability has not been demonstrated by real projects. Theoretical analysis through admittance shows that compared with gravity disturbance, gravity gradient is more sensitive to the short-wavelength seafloor topography but diminishes faster with the increase of the distance between the seafloor and airplane, indicating its superiority is recovering short-wavelength topographic features over shallow waters. We present the first numerical experiment that estimates seafloor topography from a 0.4-km resolution, real airborne gravity gradients. It is shown that airborne gravity gradiometry can recover smaller topographic features than typical airborne gravimetry, but the estimation accuracy is only ±17 m due to the presence of subsurface density variations. The long-wavelength effect of the subsurface density variations can be removed with the aid of constraining bathymetry inside the study area, whereas the short wavelengths cannot. This study expands the applications of airborne gravity gradiometry, and helps glaciologists understand its performance in seafloor topography estimation.

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