Inversion Of Gravity Anomalies Research Articles

The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling

The lithospheric structure of the Falkland Plateau region has been modelled by making an initial estimate on the basis of local isostasy and then refining the geometries by inversion of gravity anomalies. The model predicts a crustal thickness beneath the Falkland Plateau Basin that is more than twice that previously inferred from seismic evidence. The preferred explanation is that the seismic survey detected high-velocity (?underplated) lower crust rather than upper mantle. This is compatible with the results of deep seismic experiments over the conjugate Filchner Block of Antarctica and also appears likely on the basis of the position of the Falkland Plateau Basin in relation to the Karoo–Ferrar magmatic province at the time of extension. Continental crust is inferred to be continuous beneath the northern part of the Falkland Plateau, as there are consistent magnetic and flexural anomalies associated with the continent–ocean boundary. Flexural modelling of the southern margin of the plateau indicates lateral variations in strength, with the strongest lithosphere beneath the Falkland Plateau Basin. This may indicate thick oceanic crust beneath the southern part of the basin or the effect of thinning relatively weak continental crust and replacing it with relatively strong underplating.

An Inversion of Gravity Anomalies by Using a 2.5 Dimensional Rectangle Gridded Model and the Simulated Annealing Algorithm

AbstractIn accordance with the present state of gravity anomaly inversion, we use the 2.5 dimensional model construction method to divide the underground density structure into rectangle gridded units to simulate density models with arbitrary interfaces or bodies, then perform gravity forward modeling. This model construction is flexible and adaptable especially for the complicated geological models with horizontal varying densities usually appearing in practical situations. We employ the improved very fast simulated annealing algorithm (VFSA), which has obvious advantage of finding the global optimal solution in inverse problems to perform gravity anomaly inversion. Only the value of each rectangle unit needs to be inversed, the interfaces or shapes and the density distributions can be simultaneously determined during gravity anomaly inversion. Through the complicated model test, this approach is proved effective and practicable. The application in the lower Yangtze area of Jiangsu shows that this inversion approach could increase the accuracy of inversion under the constrained conditions of prior seismic data, the objective strata of Devonian‐Silurian and related geological problems could also be solved efficiently.

Inversion of gravity anomalies over spreading oceanic ridges

Abstract Models of spreading ocean ridges are derived by Bayesian gravity inversion with geophysical and geodynamic a priori information. The aim is to investigate the influence of spreading rate, plate dynamics and tectonic framework on crust and upper mantle structure by comparing the Mid Atlantic Ridge (MAR), the Indian Ocean Ridge (IND) and the East Pacific Rise (PAC). They differ in mean spreading rate, dynamic settings, as attached slabs, and plume interaction. Topography or bathymetry, gravity, isostasy, seismology and geology, etc. are averaged along the ridges and guide the construction of initial 2D models, including features as mean plumes, i.e. averaged along the ridge. This is a gross simplification, and the results are considered preliminary. Three model types are tested: (a) the temperature anomaly; (b) asthenospheric rise into thickening lithosphere; (c) a crustal root as had been anticipated before seafloor spreading was discovered. Additional model components are a mean plume, a non-compensated ridge uplift, an under-compensated asthenospheric rise, e.g. of partially molten material, and seismic velocity models for P and S waves. Model type (c), tends to permute to model type (b) from thick crust to thin axial lithosphere. Model type (a) renders ‘realistic’ values of the thermal expansivity, but is insufficient to fit the gravity data; partial melt may disturb the simple temperature effect. A combination of (a) and (b) is most adequate. Exclusive seismic velocity models of S or P waves do not lead to acceptable densities nor to adequate gravity fitting. The different ridges exhibit significant differences in the best models: ATL and IND show an axial mass excess fostering enhanced ridge push, and ATL, in addition, suggests a mean plume input, while PAC shows an axial mass deficit reducing ridge push, most probably due to dominance of slab pull in the force balance. Goodness of the gravity fit alone is no justifiable criterion for goodness of model, indeed minor modifications to each model within the uncertainties of the assumptions can make the fit arbitrarily good. Goodness of model is quantified exclusively by a priori information.

Sediment thicknesses of the western Anatolia graben structures determined by 2D and 3D analysis using gravity data

Western Turkey is tectonically one of the most active and rapidly deforming regions of continental crust in the world. The most pronounced structural and morphological features of this region are defined by normal faulting in an E–W direction, which creates the boundaries of the Büyük Menderes, Küçük Menderes and Gediz grabens. The graben systems may have become active between 11–7 Ma and are now deeper than the surrounding topography. The Gediz and Büyük Menderes grabens exhibit negative Bouguer gravity anomalies. The Bouguer gravity anomalies increase towards the center of the Aegean Sea in a regional sense. The grabens are filled with recent sediments giving rise to relative negative gravity anomalies. These anomalies were interpreted using 2D and 3D gravity inversion techniques in order to determine the depth to the metamorphic basement. Inversion of geophysical anomalies implies fitting the observed data to theoretical anomalies possibly produced by subsurface bodies. Inversion of gravity anomalies is a non-linear problem in the sense that body parameters are not linearly related to the anomalies. Gravity anomalies caused by structures are mostly determined by a 2D model, which has an n-sided polygonal cross-section. For non-linear problems, theoretically it is more suitable to determine the parameters of the structure by a series of juxtaposing prisms or vertical dykes extending to infinity, rather than using a single n-side polygonal model. The depths to the top surfaces of a rectangular prisms with infinite depth represent the relief of the basement. The basement relief can be computed on the basis of density contrast if the reference depth is assessed. As a result of 3D analysis of the Bouguer gravity map, the maximum thickness of sedimentary cover appears to be more than 2.0 and 1.5 km for the Gediz and Büyük Menderes grabens, respectively. Towards the Sarayköy-Kızıldere area, the sedimentary thickness cover thickens to more than 2.0 km. 2D analysis of the Bouguer gravity profiles indicate that the maximum thickness of the sedimentary cover is between 2.5 and 3.5 km in the Büyük Menderes graben, and between 0.5 and 2.0 km in the Gediz Graben.

INVGRAFALT–A Fortran code for Marquardt inversion of gravity anomalies of faulted beds with varying density

INVGRAFALT–A Fortran code for Marquardt inversion of gravity anomalies of faulted beds with varying density

Best Student Papers for 1987 Spring Meeting

As they did at previous national meetings, several AGU sections selected Best Student Papers at the 1987 Spring Meeting in Baltimore, Md., during May 1987 as a means of encouraging student participation.Pal Wessel was selected by the Geodesy Section to receive their Best Student Paper Award for his paper entitled “Global Gravity Crossover Corrections: Implications and Applications,” which was coauthored by A. B. Watts (Lamont‐Doherty Geological Observatory (LDGO), Palisades, N.Y.). Wessel received his B.Sc. (1982) and M.S. (1984) from the University of Oslo, Norway, in applied geophysics, while working on inversion of gravity anomalies over a continental rift (Oslo Graben). He is currently a Ph.D. candidate at LDGO. Wessel's research in gravity includes statistical analysis of shipboard gravity data, gridding algorithms, and computation of gravimetric geoids. He is also working on flexre of young oceanic lithosphere caused by thermal stresses.

Linear inversion of gravity data using the spectral expansion method

Inversion of gravity anomalies in terms of an anomalous mass distribution with irregular outline but uniform density leads to a nonlinear inverse problem. An alternative approach based on the thin‐sheet approximation can, however, be formulated as a linear inverse problem provided the structures are two‐dimensional. The anomalous mass is represented by a thin sheet, which is located at a depth [Formula: see text] and is divided into M strips with N < M data points. Thus, an underdetermined system of linear equations is obtained which is solved by the spectral expansion method for the surface density distribution of each segment. This set of parameters is then transformed into a prismatic structure with variable depth but uniform density. The modeling procedure involves a noniterative method. A gravity problem is investigated, and the solution obtained compares well with previous interpretations.