645 research outputs found

    Maximum-likelihood estimation of lithospheric flexural rigidity, initial-loading fraction, and load correlation, under isotropy

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    Topography and gravity are geophysical fields whose joint statistical structure derives from interface-loading processes modulated by the underlying mechanics of isostatic and flexural compensation in the shallow lithosphere. Under this dual statistical-mechanistic viewpoint an estimation problem can be formulated where the knowns are topography and gravity and the principal unknown the elastic flexural rigidity of the lithosphere. In the guise of an equivalent "effective elastic thickness", this important, geographically varying, structural parameter has been the subject of many interpretative studies, but precisely how well it is known or how best it can be found from the data, abundant nonetheless, has remained contentious and unresolved throughout the last few decades of dedicated study. The popular methods whereby admittance or coherence, both spectral measures of the relation between gravity and topography, are inverted for the flexural rigidity, have revealed themselves to have insufficient power to independently constrain both it and the additional unknown initial-loading fraction and load-correlation fac- tors, respectively. Solving this extremely ill-posed inversion problem leads to non-uniqueness and is further complicated by practical considerations such as the choice of regularizing data tapers to render the analysis sufficiently selective both in the spatial and spectral domains. Here, we rewrite the problem in a form amenable to maximum-likelihood estimation theory, which we show yields unbiased, minimum-variance estimates of flexural rigidity, initial-loading frac- tion and load correlation, each of those separably resolved with little a posteriori correlation between their estimates. We are also able to separately characterize the isotropic spectral shape of the initial loading processes.Comment: 41 pages, 13 figures, accepted for publication by Geophysical Journal Internationa

    Characterization of geological boundaries using 1‐D wavelet transform on gravity data: Theory and application to the Himalayas

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    International audienceWe investigate the use of the continuous wavelet transform for gravity inversion. The wavelet transform operator has recently been introduced in the domain of potential fields both as a filtering and a source-analysis tool. Here we develop an inverse scheme in the wavelet domain , designed to recover the geometric characteristics of density heterogeneities described by simple-shaped sources. The 1-D analyzing wavelet we use associates the upward continuation operator and linear combinations of derivatives of any order. In the gravity case, we first demonstrate how to localize causative sources using simple geometric constructions. Both the upper part of the source and the whole source can be studied when considering low or high altitudes, respectively. The ho-mogeneity degree of the source is deduced without prior information and allows us to infer its shape. Introducing complex wavelets, we derive analytically the scaling behavior of the wavelet coefficients for the dyke and the step sources. The modulus term is used in an inversion procedure to recover the thickness of the source. The phase term provides its dip. This analysis is performed on gravity data we measured along a profile across the Himalayas in Nepal. Good agreement of our results with well-documented thrusting structures demonstrates the applicability of the method to real data. Also, deeper, less constrained structures are characterized

    Mechanisms of isostatic compensation in areas of lithospheric extension : examples from the Aegean.

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    The results of previous studies in the Aegean have greatly influenced our understanding of continental extensional tectonics. However, although lithospheric rheology is known to exert a significant control on tectonic style, it had not been assessed across the province. The effective elastic thickness, Te, provides a good measure of lithospheric rheology. Through analyses of the isostatic mechanism, the spatial variability in rheology across the Aegean is examined. A crucial step in the process is to obtain a good gravity dataset. This study compiles new and existing gravity data across both onshore and offshore regions to produce the best gravity dataset currently available for the Aegean region. Short wavelength components of the Bouguer gravity (less than ~80 km) are seen to correlate with surface geological features. Longer wavelength components are coherent with topographic features, which may be interpreted in terms of the lithosphere's isostatic response to loading. The amplitudes of Bouguer anomaly minima across the sedimentary basins of central Greece are used to estimate the depth to basement. A model is derived for the gravitational signature of buried sedimentary strata whose porosity decreases exponentially with burial depth. The model results are presented in graphical form which may have general application. The resulting depth estimates for the eastern Gulf of Corinth (2.7 to 3.7 km) are used along with a seismic depth conversion and erosional marine terrace geometries to model the generation of topography across an active rift segment. These boundary element elastic models estimate the local rift-flank Te as ~6 km. The primary objective of this thesis - to assess the spatial variability in lithospheric rheology - is achieved through the analysis of gravity coherence. Windowed (short time) Fourier transforms are used, and the observed coherence is modelled with a thin elastic plate to estimate Te. Across the Aegean extensional province, Te is between 10 and 20 km, and values of between 8 and 10 km are observed across the extending Rhodope metamorphic core complex. The broad pattern is similar to that in measured surface heat flow, indicating that geothermal gradient is an important factor in controlling lithospheric rheology. This thesis presents the first comparison between the conventional Fourier transform method and the wavelet transform method in a-coherence analysis. Due to the localisation properties of the continuous wavelet transform, an improved spatial resolution of the variability in coherence is observed. The weakest lithosphere across the Rhodope complex is observed to occur in a narrow zone (~ 100 km wide) trending parallel to normal faulting, where a Te of less than 5 km is predicted. These features are not resolved with the Fourier method. The results of this thesis are then compared to observations across the Basin and Range and the Tibetan Plateau. It is argued that rheological similarities between these collapsing orogens account for similarities in the tectonic style. A scheme is subsequently suggested for the evolution of the Aegean province since approximately 20 Ma

    Localization of gravity and topography: constraints on the tectonics and mantle dynamics of Venus

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    We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, T_e, was less than 20 km. Estimates of T_e at highland rises are consistently less than 30 km. Our inability to find regions with T_e > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

    UFGM - 2006 Annual Report

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    INGV, SEZIONE DI CATANIAPublished2.6. TTC - Laboratorio di gravimetria, magnetismo ed elettromagnetismo in aree attiveope

    An aeromagnetic denoising-decomposition-3D inversion approach for mineral exploration

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    Reduction of aeromagnetic noise and extraction of mineralization-related residual anomalies are critical for aeromagnetic data processing in mineral exploration. This study introduced a multifractal singular value decomposition (MSVD) method to remove the noise and improved the bi-dimensional empirical mode decomposition (BEMD) algorithm to extract residual magnetic anomalies. It is shown that MSVD and improved BEMD could effectively reduce the noise and extract residual magnetic anomalies. Then, a wavenumber–domain iterative approach is applied in 3D imaging of magnetic anomalies and gradients with depth constraints, which is a rapid tool for qualitative and quantitative interpretation of magnetic data and is suitable for rapidly imaging large-scale data. The 3D inversion result is verified by four geological sections along the regional tectonic directions and some drilling holes on the deposits. It is revealed that this proposed approach is practical and effective in dealing with aeromagnetic data interpretation and inversion for mineral exploration

    Localization of gravity and topography : constraints on the tectonics and mantle dynamics of Earth and Venus

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996.Includes bibliographical references (p. 197-209).by Mark Simons.Ph.D

    Learning-Based Modeling of Weather and Climate Events Related To El Niño Phenomenon via Differentiable Programming and Empirical Decompositions

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    This dissertation is the accumulation of the application of adaptive, empirical learning-based methods in the study and characterization of the El Niño Southern Oscillation. In specific, it focuses on ENSO’s effects on rainfall and drought conditions in two major regions shown to be linked through the strength of the dependence of their climate on ENSO: 1) the southern Pacific Coast of the United States and 2) the Nile River Basin. In these cases, drought and rainfall are tied to deep economic and social factors within the region. The principal aim of this dissertation is to establish, with scientific rigor, an epistemological and foundational justification of adaptive learning models and their utility in the both the modeling and understanding of a wide-reaching climate phenomenon such as ENSO. This dissertation explores a scientific justification for their proven accuracy in prediction and utility as an aide in deriving a deeper understanding of climate phenomenon. In the application of drought forecasting for Southern California, adaptive learning methods were able to forecast the drought severity of the 2015-2016 winter with greater accuracy than established models. Expanding this analysis yields novel ways to analyze and understand the underlying processes driving California drought. The pursuit of adaptive learning as a guiding tool would also lead to the discovery of a significant extractable components of ENSO strength variation, which are used with in the analysis of Nile River Basin precipitation and flow of the Nile River, and in the prediction of Nile River yield to p=0.038. In this dissertation, the duality of modeling and understanding is explored, as well as a discussion on why adaptive learning methods are uniquely suited to the study of climate phenomenon like ENSO in the way that traditional methods lack. The main methods explored are 1) differentiable Programming, as a means of construction of novel self-learning models through which the meaningfulness of parameters arises from emergent phenomenon and 2) empirical decompositions, which are driven by an adaptive rather than rigid component extraction principle, are explored further as both a predictive tool and as a tool for gaining insight and the construction of models

    Geophysical studies of the Limpopo Belt

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    A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the Degree of Master of Science Johannesburg, 1991The multiply deformed, metamtlJphic Limpopo belt separates the granite- greenstone terranes of the Kaapvaal and Zimbabwe Cratons. The belt is divided into three zones, the Southern Marginal Zone (SMZ), the Central Zone (CZ) and the Northern Marainal Zone (NMZ), all consisting of high- grade metamorphic rocks, Previous geophysical investigations, which included gravity (Coward and Fairhead, 1980) and electrical metbods (Van Zijt, 1978) presented models in which the Kaapvaal Craton (KC) had been thrust over the Zimbabwe Craton (ZC). Recent geophysical studies (De Beer and Stettler. 1988) indicate a south dipping. contact between. the ZC and NMZ and a north dipping contact between the KC and SMZ. which suggests a pop-up structure (Van Reenen et al., 1990). This study includes seismic reflection and refractton work as well as gravity modelling. The seismic reflection section is typical of Archean sections, i.e. a reflective uppet crust overlying a seismically transparent lower crust. The SMZ and a block thought to be the NMZ are eharacterlsed by numerous short, dipping reflectors. The CZ is largely seismically transparent except for sedimentary rocks close to surface. The KC is characterised by sub-horizontal reflectors indicating thin skinned tectonics. The contact between the SMZ and KC is seen as a zone of discontinuous north dipping reflectors. No continuous reflector at mid-crustal levels is seen connecting the SMZ and NMZ which could represent the decollement as postulated by McCourt and Veamcornbe (1987), free-air gravity modelling results indicate that the positive anomaly over the SMZ is " result of dense rocks within the upper crust haying a maximum thickness of 7 km, The lower regional value over the CZ can (l)eaccommodated by increasing the crustal thickness beneath the CZ by between 2 and 5 km. Thls is in agreement with the refraction data. The 0.6 s slowdown seer. . the reduced travel-time refraction data near the boundory between the CZ and SMZ is modelled using a 3 to 4 km thick low velocity layer at a depth of 42 km rather than an increase in crusta) thickness

    Structure and evolution of the Australian continent : insights from seismic and mechanical heterogeneity and anisotropy

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.Includes bibliographical references (p. 235-261).In this thesis, I explore the geophysical structure and evolution of the Australian continental lithosphere. I combine insights from isotropic and anisotropic seismic surface-wave tomography with an analysis of the anisotropy in the mechanical properties of the lithosphere, inferred from the coherence between gravity anomalies and topography. With a new high-resolution waveform tomographic model of Australia, I demonstrate that the depth of continental high wave speed anomalies does not universally increase with age, but is dependent on the scale and the tectonic history of the region under consideration. I construct an azimuthally anisotropic three-dimensional model of the Australian upper mantle from Rayleigh-wave waveforms. I compare Bayesian inverse methods with discretely parameterized regularization methods, and explore the use of regular, tectonic and resolution-dependent tomographic grids. I advocate the use of multitaper spectral estimation techniques for coherence analysis of gravity and topography, applied to Australian isostasy. I investigate the importance of internal loading, the directional anisotropy of the gravitational response to loading, and the estimation bias affecting the long wavelengths of the coherence function. I develop a method for non-stationary coherence analysis which enables a complete characterization of continental strength by the dependency of gravity-topography coherence on wavelength, direction and geologic age. Combining high-resolution, depth-dependent anisotropy measurements from surface-wave tomography with the mechanical anisotropy from gravity/topography coherence, I assess the validity of two competing theories regarding the cause of continental anisotropy (vertically coherent deformation or simple asthenospheric flow) quantitatively for the very first time.by Frederik Jozef Maurits Simons.Ph.D
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