1,721,250 research outputs found
Exploration of tectonic structures with GOCE in Africa and across-continents
No full textThe gravity anomaly field over the whole Earth obtained by the GOCE satellite is a revolutionary tool to reveal geologic information on a continental scale for the large areas where conventional gravity measurements have yet to be made. It is, however, necessary to isolate the nearsurface geologic signal from the contributions of thickness variations in the crust and lithosphere and the isostatic compensation of surface relief. Here Africa is studied with particular emphasis on selected geological features which are expected to appear as density inhomogeneities. These include cratons and fold belts in the Precambrian basement, the overlying sedimentary basins and magmatism, as well as the continental margins. Regression analysis between gravity and topography shows coefficients that are consistently positive for the free air gravity anomaly and negative for the Bouguer gravity anomaly. The error and scatter on the regression is smallest in oceanic areas, where it is a possible tool for identifying changes in crustal type. The regression analysis allows the large gradient in the Bouguer anomaly signal across continental margins to be removed. After subtracting the predicted effect of known topography from the original Bouguer anomaly field, the residual field shows a continent-wide pattern of anomalies that could be attributed to regional geological structures. A few of these are highlighted, such as those representing Karoo magmatism, the Kibalian foldbelt, the Zimbabwe Craton, the Cameroon and Tibesti volcanic deposits, the Benue Trough and the Luangwa Rift. A reconstruction of the pre-break up position of Africa and South America (the plates forming West Gondwana) is made for the residual GOCE gravity field. The reconstruction allows the positive and negative anomalies to be compared across the continental fragments, and so helps identify common geologic units that extend across both the now-separate continents
Estimating the hydrologic induced signal in geodetic measurements with predicitive filtering methods
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The European Alps sensed by GOCE
No full textThe observations of the GOCE satellite in western Europe give a major contribution over mountain ranges and in the transition zone between ocean and continent. The area centered on the Alpine arc highlights the improvement, three mountain ranges meeting here, the Alps, the Dinarides, and the Appennines, and there being the continent-ocean transition to the Tyrrhenian Sea. The gravity observations of GOCE produce an unparalleled
global field that allows to detect geological features and classify types of continental crust. GOCE is superior to existing global fields based on terrestrial data, as is EGM2008, as is seen from the regional variability of the root mean square difference between the two fields. The difference is governed by which is biased by varying quality of the terrestrial observations. The gravity anomaly and Bouguer fields of GOCE and residual fields based on a regression between topography and gravity are studied. The root mean square amplitude variation of the
residual Bouguer field is reduced by 56%, demonstrating that the regression efficiently eliminates the isostatic field. As expected, for the gravity anomaly the reduction is less and amounts to 20%. The residual fields highlight geological units, as deep sedimentary basins (Po-basin sediments, Alpine foreland basin), the Ivrea body and the Periadriatic intrusions, and the gravity high centered on the Tuscan Arcipelagus connecting Western Corsica and the Tuscan geothermal fields. The transition of the Alpine Arc towards the Pannonian basin is marked by two subparallel NS striking positive anomalies separated by a negative linear anomaly. The observation of these anomalies is new and is important in modeling the Alps eastern transition. I demonstrates the strength of GOCE
in mapping the field homogeneously crossing geologic, orographic and national boundaries
A grip on geology with GOCE
No full textThe development of today's emerging countries of Africa requires the retrieval of the sufficient funds to build the necessary governmental structures and institutions. An important source of wealth is given by mineral and underground fossil energy resources, but must yet be explored and discovered. More developed countries have invested in geological surveys, which have had the time of decades to geologically map the country and commission terrestrial geophysical soundings. The greater geological context is therefore known in general terms, and it is clear in which areas natural resources can be expected and where further detailed investigations are necessary. In emerging countries the geological surveys are at the start of the complex work of investigating the entire territory. The discovery of deposits is greatly accelerated if there is the possibility to guide detailed investigations to the right place, based on the regional knowledge of the geologic history, the recognition of main crustal structures, and the identification of potentially productive geological units. Density is a fundamental rock parameter that distinguishes different rock types due to their compactness and chemical or mineralogical composition.
The link between the presence of natural resources and the density variation of the rock is not necessarily that of direct cause and effect, but is based on the correlation of processes that can produce the subsurface deposit. Diamonds are found in the extremely dense kymberlite pipes, because they are transported by dense mantle rocks that quickly move upwards through the less dense crust to the surface. It is an example of a situation in which the presence of the high-density rocks guides the exploration to the limited area in which there is a higher probability to find the mineral or hydrocarbon resource. The new satellite GOCE gives the first time opportunity to map such structures from space, under the condition that their extent is sufficiently big. This is due to the highly precise observation of the gravity gradient tensor in space at low altitude.
After making some reductions to the data for the deeper lying density sources and the obvious mass effect of topography, the geological signal is enhanced. The residual gradient map from GOCE defines the large scale geologic structures over their entire extent, revealing a relatively orderly alignment of subparallel lineaments that flank the African hardcore, the Congo craton. The Congo craton carries on the surface the Congo basin, and extends down to 400km with the cratonic root, signalized by an increased seismic velocity. It's age is estimated to be 3300 Ma, that is two third of Earth's history. The geometry of this rigid and deep reaching core which has moved since 200Ma from the South Pole to the present equatorial position has determined the directionalities of the orogenic belts, the rifts and the shear zones, that were formed by the forces acting on the crust being pushed against the rigid core. An important fact for mineral exploration is, that existing planes of weakness along which the mineral rich fluids can percolate and accumulate in mineral veins, are preserved during Earth history, increasing the deposit with time. Such a situation is recorded along the borders of the craton, as the same lineaments that were formed in an orogen at some time in earth history are reused as planes for rifting or shearing, as they present stable zones of weakness. The GOCE observations show the alternation of high and low density rock- alignments of different ages, demonstrating that the directionalities are long lasting and span over 1000Ma. Partial correlation of these density lineaments with the surface geology is demonstrated by geospatial analysis.
The geologic units are identified unequivocally by the gravity and gradient signals, directly linked to the different characteristic rock densities. Relevant for the geodynamical context of Gondwana is the western series of high and low density belts that run parallel to the African coast, presumably defining the weakness line along which the South American continent was separated through rifting from the African plate
Interpretation of Continental Scale Gravity Signatures from GOCE at Smaller Scale Mineral Hosting outcrops
No full textThe GOCE gravity field is globally homogeneous at the resolution of about 50km or better allowing for the first time to analyze tectonic structures on the continental scale. Geologic correlation studies propose to continue the tectonic lineaments across continents to the pre-breakup position. Tectonic events that induce density changes, as metamorphic events and magmatic events, should then show up in the gravity field. Applying geodynamic plate reconstructions to the GOCE gravity field places today’s observed field at the pre-breakup position (Braitenberg, 2014). The same reconstruction can be applied to the seismic velocity models, to allow a joint gravity-velocity analysis. The geophysical fields bear information to control the likeliness of the hypothesized continuation of lineations. Total absence of a signal, makes the cross-continental continuation of the lineament unprobable, as continental-wide lineaments are controlled by rheologic and compositional differences of crust and upper mantle. Special attention is given to Greenstone belts, which are associated to a class of important mineralizations. The outcrops are limited in extent, but are associated with a much broader gravity signature, which cannot be explained by the outcropping masses alone. The gravity requires a mass source residing at lower crustal level, giving evidence of the mantle-crust melting processes influencing the tectonic characteristic at surface. The study is carried out over the African and South American continents
An assessment of the impact of Next Generation Gravity Missions on earthquake signal retrieval. Constructing a database of time-varying co-seismic and post-seismic gravity change and a detectability assessment strategy
No full textAdvancements in mission concepts and instrumentation, such as those investigated under the Mass change And Geosciences International Constellation (MAGIC), could enable significant enhancements in the spatial and temporal resolution of gravity field products. Closed-loop simulations of these new constellations of instruments estimate radical improvements in the error budget in the retrieval of geophysical signals, including those arising from mass movement in the solid Earth – earthquakes included. The displacements caused by co-seismic dislocation and post-seismic relaxation are sensed by a broad array of seismological and geodetic techniques. Gravity has the potential of providing additional information, especially when the mass movement is a-seismic and when its surface expression occurs mostly in areas that are difficult or impossible to sense with other remote-sensing techniques (GNSS, dInSAR), such as off shore.
In order to assess by how much the improvements in MAGIC would lower the detectability threshold, we modelled a database of synthetic earthquake gravity signal, including the effect of post-seismic viscoelastic relaxation. We computed the gravity change in time using the QSSPSTATIC [1] code, set up in a way to obtain the spherical harmonics (SH) coefficients of the geopotential change through time. This data, which we then used in a detectability assessment, also allow comparing different modelling strategies and signal retrieval methods. We devised its data structure, by design, to be easily included as part of time-varying signals used in simulations, enhancing the solid-Earth component of models such as AOHIS [2].
We test detectability in terms of the SH-domain SNR between the earthquake signal and the gravity model errors. The SH coefficients of both quantities undergo a spatio-spectral localization procedure [3] and are compared in terms of their localized degree variances. We show how a spatial-scale dependent analysis, such as the one that a spectral-domain method allows, is needed to fully exploit the signal in the optimal range of spatial wavelengths owing to the coloured spectra of signal and noise. We perform a parametric study of the effect on detectability of moment magnitude, source parameters (focal mechanism, depth), and rheological profiles – with magnitude being the first-order predictor of detectability in co- and post-seismic signals. As a methodological test, we also present an experiment on the signal omission arising from approximating an earthquake dislocation as a point-source, comparing its signal to the one we can obtain using a finite fault solution of a real event instead. We assess and discuss the impact of a simpler model on the trade-space between the precision of a detectability assessment and the added computational effort.
References
[1] Wang et al., 2017 DOI:10.1093/gji/ggx259
[3] Dobslaw et al., 2015 DOI:10.1007/s00190-014-0787-8
[2] Wieczorek and Simons, 2005 DOI:10.1111/j.1365-246X.2005.02687.
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