1,721,194 research outputs found
Application of the DEXP method to the streaming potential data
No full textWe interpret the self-potential data related to groundwater flow by the depth from extreme points (DEXP) method; a multiscale method in which the data are upward-continued and scaled by a scaling law depend on the structural index. The depth to the water table is estimated from extreme points of the DEXP image without a priori estimate of the hydraulic coupling coefficient. The method is tested with a synthetic model of the water table and applied to a real self-potential dataset near a pumping well. The obtained results agree well with the known information
Analysis of direct current resistivity data using continuous wavelet transform
No full textWe have developed a new method for the direct current resistivity interpretation, based on the continuous wavelet transform
(CWT) of electric potential-difference data. It exploits the main
properties of the CWT, such as stability versus noise, and does
not require a starting model or other a priori information such as a
model weighting function or constraints. Because the approximate integral equation of the resistivity problem has the same
form as the forward problem for potential fields, the authors analyze geoelectric data (with dipole-dipole configuration) using the
wavelets belonging to the Poisson kernel semigroup. They find
that the CWT analysis of the measured electric potential difference is able to identify buried bodies, defining their depth, position, and extent. Such parameters are estimated with no prior
knowledge of the resistivity contrast between the bodies and
the background. We consider several synthetic models, such as
dikes, compact bodies, and contacts. In general, the depth and
the lateral thickness of the buried bodies are estimated with good
accuracy, using a diagram relating the singular point estimations
to the different values of the dipole separation factor n. Thanks
to the good results obtained from synthetic data, we test the
method with data generated during laboratory experiments. In
two laboratory-scale models, our method displays a better precision compared with smoothness-constrained least-squares inversion in identifying the exact position of the edges of a buried
body. Finally, we find that combining CWT and inversion is
advantageous: after constraining the inverse problem with a priori
information from the CWT analysis, we obtain an improved
inverse model
Imaging Methods Versus Inverse Methods: An Option or An Alternative?
No full textBoth imaging and inversion of potential fields allow the estimation of the source-property distribution. Here, we compare these methods in order to assess their relative advantages and performances. Specifically, we use an iterative imaging algorithm, which is based on the compact depth from extreme points (CDEXP), and the data-space inverse algorithm. This choice was determined because both the methods use a depth weighting function and a compacting function, i.e., they yield a compact source solution. Inverted and imaged solutions are compared with each other, for two sets of noise-corrupted synthetic data, one relative to a simple prism and the other to two oppositely dipping dikes. In both cases, the two models show a noticeable similarity. However, the execution times are substantially different, with the inversion times being an order of magnitude greater. Finally, we interpret two real gravity data sets by using both the approaches: gravity data sets acquired over 1) the Galinge iron-ore deposit of Northwest China and 2) Jiaodong gold deposit of East China. We found that the source models obtained by imaging and inversion methods are once again similar
Una pastorale europea
No full textIl saggio propone un'ampia lettura critica dell'opera pastorale in dialetto siciliano di Giovanni Meli, medico e poeta, figura di spicco nella stagione del riformismo illuminista, letterato noto in Europa e apprezzato anche da Herder e da Goethe. La “Buccolica” (pubblicata in una prima redazione nel 1787 e poi – accresciuta – nel 1814) è forse la sua opera più celebre, e si configura come una raccolta poetica che comprende 18 testi, distribuiti (a parte due sonetti introduttivi) nelle quattro parti intitolate canonicamente alle Stagioni. Adottando questo pattern Meli aveva scelto di guardare alla tradizione classica, certamente, per rifarsi con orgoglio al retaggio del siciliano Teocrito; ma dimostrò di avere assimilato bene anche la più recente lezione di Alexander Pope e di James Thomson.
Nella “Buccolica” infatti trovano spazio molti temi filosoficamente cruciali e anche questioni di forte ‘attualità’: in primo luogo un’idea di natura che interpretava Newton – per così dire – dal lato di Epicuro, e non di Platone, cioè rilanciando il concetto lucreziano dell’amore come forza attrattiva, capace di riprodurre in scala, agendo sugli esseri viventi dal più grande al più insignificante, il meccanismo fondamentale dell’universo. Inoltre l’involucro poetico della pastorale si piegava bene a ‘velare’ e a rendere fruibili anche altre argomentazioni di peso: per esempio un’istanza ‘pacifista’ in nome della quale perfino l’autorità di Omero, poeta sovrano, veniva messa in discussione: e questo proprio nel secolo e in un contesto (quello della cultura meridionale) in cui le «favole antiche» del ciclo troiano erano oggetto di una interpretazione allegorico-naturalistica e rivendicate come patrimonio dell’antica sapienza italica. L’obiettivo di Meli, però, è soprattutto quello di scalzare il primato dell’epica (poesia della guerra e della morte) nella scala dei generi illustri, opponendo alla tromba omerica l’umile zampogna di Teocrito, e al paradigma della forza quello della saggezza (Idillio IV, Teocritu: a chiusura de La stati).
Estremamente interessante nell’opera poetica di Meli, è infine la prospettiva economica, nella misura in cui l’insistenza sulla necessità di assecondare l’ordine naturale sottende anche un’apologia diffusa delle teorie fisiocratiche. Nell’isola prediletta da Cerere, dove Vulcano aveva forgiato il primo aratro, il riscatto dopo secoli d’inerzia e povertà non poteva venire se non dall’agricoltura, fonte di ogni ricchezza e presupposto di tutte le altre attività produttive, come si legge chiaramente nell’egloga intitolata I monti Erei. Andava infatti in questa direzione anche il tentativo riformista promosso dal viceré Caracciolo, amico e mentore di Meli, sostenitore convinto della necessità di incardinare comunque sull’agricoltura il piano complessivo di interventi di cui la Sicilia aveva disperatamente bisogno
Non stationary Analysis of Geomagnetic Time Sequences from Mt. Etna and North Palm Springs Earthquake
No full textJoint Inversion of DC Resistivity and Magnetic Data, Constrained by Cross Gradients, Compactness and Depth Weighting
No full textIn this paper we perform a 2-D joint inversion of DC resistivity and magnetic data, constrained by cross-gradients. Inspired by methods developed for potential fields, we introduce into both the separate and joint inversion algorithms also compactness and depth weighting functions, under the form of a model weighting-function. These constraints, usually not considered for DC resistivity inversion, reveal to be decisive for its joint inversion with magnetic data. A linear approximated forward problem of the resistivity is used for the joint inversion so that both the resistivity and magnetic problems are expressed as a linear integral equation under the form of a Fredholm integral of 1st kind. To examine the feasibility of the joint inversion algorithm, we first test the method with two synthetic cases: a thick dyke in a two-layered medium and a cavity located above a conductor. A third synthetic case involves a multisource model. The results are encouraging, revealing that the cross-gradient constraint is an effective tool to improve the separate inversions of DC resistivity and magnetic data. The joint inversion algorithm is also applied to data in the archeological area of the old Pompeii city, nearby Naples. Comparing the results of joint and separate inversions, we obtain a significant improvement in the interpretation of both kind of data in terms of buried walls of an ancient roman villa. In all the studied cases, the cross-gradient constraint appears to be a key-diagnostic tool to assess whether actual coherence is gained among DC resistivity and magnetic susceptibility models
Inhomogeneous Magnetization of Tyrrhenian Seamounts Revealed From Gravity and Magnetic Correlation Analysis
Full text linkWe perform a joint analysis of gravity and magnetic data sets in the Tyrrhenian Sea region to infer the rock physical properties of several volcanic seamounts. We propose a moving-window application using Poisson's theorem, which relates the total gradient of the magnetic field to the total gradient of the first-order vertical derivative of the gravity field data. In volcanic environments, where strong intensity of remanent magnetization is expected, the total gradient of the magnetic field is particularly useful since it is almost independent on the direction of the total-magnetization. The moving-window approach resulted necessary due to the heterogeneous magnetization distribution of the volcanoes. First, we perform synthetic tests based on realistic seamount models which exhibit inhomogeneous magnetization intensity and orientation. Using the total gradients, we demonstrate that our approach can provide an appropriate magnetization-to-density ratio in different subareas of seamounts. The results of the correlation analysis for the Palinuro, Marsili, Vavilov, and Magnaghi seamounts provide interesting information on the variability of magnetization associated with different epochs of formation and demagnetization effects due to hydrothermal alteration processes
A review of spectral analysis of magnetic data for depth estimation
No full textSpectral analysis has been used for studying a variety of geologic structures and processes, such as estimation of the depth to the crystalline basement or estimation of the Curie temperature isotherm from magnetic anomalies. However, the analysis is not standard because it refers to different theoretical frameworks, such as statistical ensembles of homogeneous sources and uncorrelated or fractal randomly distributed sources. We have aimed to unify the approaches by reformulating all of the common spectral expressions in the form of a product between a depth-dependent exponential factor and a factor, which we call the spectral correction factor, that incorporates all of the a priori assumptions for each method. This type of organization might be useful for practitioners to quickly select the most appropriate method for a given study area. We also establish a new formula for extending the Spector and Grant method to the centroid depth estimation. Practical constraints on the depth estimation and intrinsic assumptions/limitations of the different approaches are examined by generating synthetic data of homogeneous ensemble sources as well as random and fractal models. We address the statistical uncertainty of depth estimates using ordinary error propagation on the spectral slope. Critical parameters, such as the window size, are also analyzed in terms of the type of method used and the geologic complexity. We find that the window size is smaller for the centroid/modified centroid methods and larger for the spectral peak, defractal, and nonlinear parameter depth estimation methods. In any case, the window size can be large in tectonically stable regions and relatively small over volcanically, tectonically, and geothermally active areas. Finally, we estimate and discuss the depth to the magnetic top and bottom in the Adriatic Sea region (eastern Italy) in the context of the heat flow, Moho depth, and gravity data of the region
Deep Learning to estimate the basement depth by gravity data using Feedforward neural network
No full textWe have developed a Deep Learning method based on the neural network of the Feed Forward type to estimate the depth to the carbonate basement from potential fields. The data used to train and test the network are related to the Bishop synthetic model. The training was organized associating the depth values of the basement to the data through a moving window, running along profiles in the N-S and E-W directions. In this way we generated a set of about 300000 examples. We verified the robustness of the trained net by carrying out a test related to another synthetic model, extracted from the Himalaya Digital Elevation Model. The inherent ambiguity of the problem led us to test two hypotheses for the estimation of the basement depth, the first related to a priori information on the density contrast and the shallowest depth, the second assuming the knowledge of the depth at least at two points, but not that of the density contrast. In both cases, our data-driven approach yielded interesting results leading to estimate the maximum depth in the first case and the density contrast in the second one. We finally applied the method to the isostatic anomaly of the Yucca Flat sedimentary basin, Nevada. The results are consistent with previous interpretation of the area, that were based on gravity inversion methods
Fractional-order Local Wavenumber - An Improved Source-parameter Estimator
No full textWe present an improved local wavenumber method, based on the fractional-order differentiation of potential fields. Such kind of differentiation allows a fractional-order local wavenumber to be defined, whose usefulness is two-fold: a) the positions of the peaks of the two different-order local wavenumber are essentially the same and also holds in their difference; b) the noise-enhancement may be kept to a minimum, if compared to standard local wavenumber, based on integer-order field differentiation.The fractional-order local wavenumber is applied to synthetic and real examples and it provided a good estimation of both depth to sources and structural index
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