ALBERT

Beschreibung: We show a set of forward model equations in the Fourier domain for calculating the 3-D gravity and magnetic anomalies of a given 3-D distribution of density or magnetization. One property of the potential field equations is that they are given by convolution products, providing a very simple analytic expression in the Fourier domain. Under this assumption, the domain of the density or magnetization parameters is connected by a biunivoc relationship with the data space, and potential field anomalies can be seen as filtered versions of the corresponding density or magnetization distributions. A very fine spatial discretization can be obtained by using a large number of points within a unique 3-D grid, where both the source distributions and field data are defined. The main advantage of this formulation is that it dramatically reduces execution times, providing a very fast forward model tool useful for modeling anomalies at different altitudes. We use this method to evaluate an average magnetization of 8 A/m for the Palinuro Seamount in the Tyrrhenian Sea (southern Italy), thus performing a joint interpretation of morphological and newly acquired magnetic data.

Beschreibung: Published

Beschreibung: B02103

Beschreibung: 2.6. TTC - Laboratorio di gravimetria, magnetismo ed elettromagnetismo in aree attive

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: A scientific debate has developed in the last few years as to whether a 130 m diameter sag pond surrounded by a saddle-shaped rim and neighboring smaller sags from the Sirente Plain (Abruzzi, Italy) represent the only known Italian meteoritic crater field, a mud volcano, or an anthropogenic feature. To decipher the nature of the Sirente landforms, we carried out geophysical and geochemical investigations. Geoelectric profiles document two karstified shelf carbonate ridges lying at 10–40 m depth below calcareous lacustrine silts (and deeper more conductive sediments, likely soils/tephra) filling the plain. The smaller sags lie just above the ridges, implying a karstic origin, whereas the main sag (also resting above a carbonate ridge) shows no roots in excess of 10–20 m depth, in contrast to the "crater" interpretation. High-resolution magnetic surveys reveal negative/positive anomaly stripes in correspondence with the buried ridges/ valleys, respectively. The smaller sags, as well as the main crater are located in the domain of negative residuals. The positive long-wavelength magnetic signature is likely due to the strongly susceptive soils/tephra filling the buried valleys. Magnetic modeling shows that the field observed over the crater is incompatible with the field generated by a buried meteorite with realistic characteristics. The smaller sags are characterized by small magnetic anomaly couplets, perfectly reproducible considering the susceptibility contrast between the fill-in soil and the surrounding silts. Our data show that the Sirente crater and the minor depressions are simply the results of human activity and karstic processes, respectively.

Beschreibung: Published

Beschreibung: B03103

Beschreibung: 1.6. Osservazioni di geomagnetismo

Beschreibung: 2.2. Laboratorio di paleomagnetismo

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: We discuss the tectonic implications of a new residual magnetic map of the Apennine belt/Adriatic-Apulian foreland obtained by integrating ground and offshore data sets [Chiappini et al., 2000a]. Negative anomalies are documented over the Adriatic-Apulian foreland areas, whereas the external Apennine belt is characterized by a ubiquitous lowamplitude (〈30 nT), long-wavelength positive anomaly. In the central northern Apennines, three 100 km wide more intense (100–200 nT) round-shaped anomalies are superimposed to the long-wavelength feature. Finally, in the Tyrrhenian Sea and margins, high-intensity, short-wavelength positive-negative couplets coincide with magmatic outcrops or bodies at shallow depth. The low-amplitude anomaly pattern over Italy suggests that the magnetic basement beneath the Triassic evaporites is ubiquitously incorporated in the external belt compressive fronts, implying a thick-skinned tectonic style for the external Apennines. The new residual magnetic map resolves the inconsistency between previous aeromagnetic data [AGIP SpA. Italia, 1981], which suggested a lack of basement involvement in the Apennine belt, and recent seismic data, which imaged deep reflectors penetrating the basement. Two magnetic models along NE-SW transects in the northern and southern Apennines suggest consistent structural styles. In the northern Apennines, positive anomalies roughly coincide with the external compressive fronts, although there are local second-order differences between the belt front and the edges of the anomaly. Here the magnetic data show that the basement rises southwestward along the thrust fronts from 6–7 km depth in the Adriatic foreland to 2–3 km depth in the axial belt, where some exploration wells have penetrated basement. Within the belt front, basement exhumation is inferred to occur along high-angle, low-displacement thrust faults inverting preexisting normal faults. In the southern Apennines, a remarkable positive magnetic anomaly is parallel with and tens of kilometers southwest of the belt front. Seismic data and oil wells show that the basement surface cannot be shallower in the belt than in the foreland. Therefore the observed magnetic anomaly is produced by strongly magnetic basement beneath the belt, likely an internal crustal wedge tectonically interposed between the Apulian carbonate sequences and basement.

Beschreibung: Published

Beschreibung: 2290

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: Spherical Cap Harmonic Analysis has been applied to obtain a reference model of geomagnetic secular change for Antarctica valid for the last forty years. In this paper, we use the latest available observatory data to update this model and to compare it with the 8th generation IGRF. In addition, the selected set of total field values used for the generation of the Oersted Initial Field Model have been employed together with observatory data to develop the first complete Antarctic Reference Model (ARM). This model improves the fit to the secular variation deduced from observatory data by about 60% relative to IGRF, and the fit to observatory and satellite field data by 8%. The model allows merging data sets taken at different altitudes and epochs in Antarctica, where significant temporal geomagnetic variations occur.

Beschreibung: Published

Beschreibung: 1192

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: We show a set of forward model equations in the Fourier domain for calculating the 3D gravity and magnetic anomalies of a given 3D distribution of density or magnetization. One property of the potential-field equations is that they are given by convolution products, providing a very simple analytic expression in the Fourier domain. Under this assumption, the domain of the density or magnetization parameters is connected by a biunivoc relationship with the data space, and potential-field anomalies can be seen as filtered versions of the corresponding density or magnetization distributions. A very fine spatial discretization can be obtained by using a large number of points within a unique 3D grid, where both the source distributions and field data are defined. The main advantage of this formulation is that it dramatically reduces execution times, providing a very fast forward model tool useful for modeling anomalies at different altitudes. We use this method to evaluate an average magnetization of 8 A/m for the Palinuro Seamount in the Tyrrhenian Sea (Southern Italy), thus performing a joint interpretation of morphological and newly acquired magnetic data.

Beschreibung: In press

Beschreibung: 2.6. TTC - Laboratorio di gravimetria, magnetismo ed elettromagnetismo in aree attive

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: In this paper we show and discuss the results of gravity and magnetic surveys of Panarea Island and its archipelago. The most recent volcanic manifestation occurred in November 2002 with a shallow submarine gas eruption between the islets of Dattilo, Panarelli, Lisca Bianca, Bottaro and Lisca Nera. Currently, the activity of Panarea is monitored through a multidisciplinary study under the umbrella of the Italian Department of Civil Protection with the goal of defining the hazard of this area. With this aim, in May 2006 the first gravity and magnetic surveys of Panarea Island and its archipelago were performed. The offshore magnetic data were obtained using a marine magnetometer, a Geometrics G880, from the Istituto Idrografico dell Marina (IIM). Onshore and offshore magnetic data were integrated into an unique dataset for complete magnetic coverage of the study area. By using two micro-gravimeters (LaCoste & Romberg), gravity data were collected along tracks every 250 meters. The gravity dataset was processed using the standard method. A Bouguer reduction was applied to the free-air gravity dataset using a detailed digital elevation model of the island and the neighbouring sea after evaluation of the optimal Bouguer density to reduce the topographic effect. The result is a Bouguer anomaly map that shows lateral variations in density distribution and the relationships between the shallow volcanic/crustal features and tectonic lineaments. This evidence is also highlighted by the magnetic pattern, which suggests the importance of the youngest volcanic deposits with respect to the magnetic features of the island.

Beschreibung: Published

Beschreibung: 25-38

Beschreibung: 2.6. TTC - Laboratorio di gravimetria, magnetismo ed elettromagnetismo in aree attive

Beschreibung: 3.4. Geomagnetismo

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal variations from the recorded total magnetic field intensity data. At mid latitudes, one or a few base stations are typically located within or near the survey area and are used to monitor and remove time dependent variations. These are usually assumed to be of external origin and uniform throughout the survey area. Here we investigate the influence on the magnetic base station correction of the time varying magnetic field variations generated by internal telluric currents flowing in anomalous regional 2D/3D conductivity structures. The study is based on the statistical analysis of a data set collected by four magnetovariational stations installed in northwestern Italy. The variometer stations were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. They recorded the geomagnetic field from the beginning to the end of April 2005, with a sampling rate of 0.33 Hz. Cross-correlation and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different stations. This indicates spatial uniformity of the source field and of the induced currents in the 1D Earth. Additionally, the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic signature of the Sestri-Voltaggio line, interpreted as a major regional tectonic boundary. Induced telluric currents flowing through this 2D/3D electrical conductivity discontinuity affect mainly the vertical magnetic component at the closer locations. By comparing this component at near (32 km) and far (70 km) stations, we have found that the mean value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band ranging from 2.5×10−3 to 5.5×10−5 Hz. This energy, folded in the spatial domain of an hypothetical survey in this region produces unwanted noise in the dataset. Considering a fifth of nyquist frequency the optimal tie-line spacing to assure complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and 100 knots (aeromagnetic survey) respectively. Similar power spectra analysis can be applied elsewhere to optimise tie-line spacing for levelling and filtering parameters utlilised for microlevelling.

Beschreibung: Published

Beschreibung: 397-406

Beschreibung: 3.4. Geomagnetismo

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: The present essay is the result of a cooperative work between geophysicists and archaeologists in which the authors carried out an integrated geophysical prospecting in an archaeological site near Rome. This paper describes the methodology and the results of a geophysical survey carried out on Villa ai Cavallacci, an ancient roman building in Albano Laziale (Rome) discovered in the late seventies. It is often possible to obtain very important results planning a fast geophysical survey opportunely; within this framework (due to the fact that an archaeological excavation was planned in a short time), an integrated geophysical techniques survey (GPR, magnetic, and geoelectric tomography) has been carried out on the areas indicated by the archaeologists. Even if the described geophysical survey should be considered only a first step analysis, the data pointed out some very interesting features confirmed by the excavation.

Beschreibung: Published

Beschreibung: 291-299

Beschreibung: 1.8. Osservazioni di geofisica ambientale

Beschreibung: 3.8. Geofisica per l'ambiente

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Salina Island (Italy) is located in the central sector of the Aeolian Islands and represents the northernmost volcanic structure of an elongated ridge emplaced on a regional shear zone characterized by NNW-SSE strike-slip faults and by second order N-S and NE-SW faults. High-resolution, low-altitude aeromagnetic data collected in 2003 and 2005 allow us to study the subsurface structure of Salina Island. The magnetic data show a pattern with a wide range of wavelengths and intensities. Magnetic modeling constrained by volcanological data allow us to reconstruct the inner structure of the Salina volcanoes and surrounding marine regions. Long wavelength negative anomalies overlap E-W elongated sedimentary basin related to the Early Pliocene-Pleistocene opening of the Southern Tyrrhenian Sea back-arc basin. The shorter wavelength positive magnetic anomalies are related to the Late Pleistocene-Holocene volcanic (conduits, dikes) and tectonic (faults) structures. The magnetic and volcanological data indicate that the early (168- about 100 ka), basaltic to basaltic andesitic Salina volcanism developed along N-S and NE-SW tectonic structure, whereas the more recent basaltic andesitic to rhyolitic products (about 100-13ka) were emitted by vents related to the main NW-SE fault tectonic structures. The tectonic structures also control the location of the seamounts around the island and the geometry of the volcano-tectonic collapses. The Salina volcanism emplaced on a NNW-SSE regional discontinuity that represents a tear fault of the present-day roll-backing slab in the Southern Tyrrhenian Sea.

Beschreibung: Published

Beschreibung: 1.6. Osservazioni di geomagnetismo

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2007

Beschreibung: Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor. Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation, grain boundary cracks form the interconnected network required for pervasive alteration of the oceanic lithosphere. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers.

Beschreibung: Research was funded by a MIT Presidential Fellowship and NSF grants OCE-0095936, OCE-9907224, OCE-0137329, OCE-6892222, and OCE-6897400.

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1997

Beschreibung: A new tomographic technique is employed to investigate the structure and dynamics of the Pacific upper mantle. We invert band-center travel times of ScS reverberations and frequency-dependent travel times of direct S phases, upper-mantle guided waves such as SS and SSS, and the R1 and G1 surface waves for the 2D composite structure in the plane of two Pacific corridors. The frequency-dependent travel times of the turning and surface waves are measured from all three components of ground motion as phase delays relative to a radially-anisotropic, spherically-symmetric oceanic mantle model, and their 2D Fréchet kernels are constructed by a coupled-mode algorithm. The travel times of the primary ScSn and sScSn phases and their first-order reverberations from the 410 and 660 discontinuities are measured as individual phases and the 2D Fréchet kernels for these band-limited signals are calculated using the paraxial ray approximation. The model parameters include shear-speed variations throughout the mantle, perturbations to radial shear-wave anisotropy in the uppermost mantle, and the topography of the 410 and 660 discontinuities. We construct vertical tomograms through two mantle corridors: one between the Tonga subduction zone and Oahu, Hawaii, which traverses the central Pacific Ocean; and the other between the Ryukyu subduction zone and Oahu, which samples the northern Philippine Sea, the western Pacific, and the entire Hawaiian swell. Tests demonstrate that the data sets for the two corridors resolve the lateral structure in the upper mantle with a scale length of a few hundreds kilometers and greater but that the resolving power decreases rapidly in the lower mantle. The model for the Tonga-Hawaii corridor reveals several interesting features, the most significant being a regular pattern of high and low shear velocities in the upper mantle between Tonga and Hawaii. These variations, which are well resolved by the data set, have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid-height variations along this corridor. The geoid highs correspond to a series of northwest-trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topography is supported by thermal buoyancy. This result is substantiated by the model from the Ryukyu-Hawaii corridor, which exhibits a prominent, fast region that extends beneath the entire Hawaiian swell. This anomaly, which resides in the uppermost 200-300 km of the mantle, is also positively correlated with the undulations of the Hawaiian-swell height. The other dominant features in the Ryukyu-Hawaii model include the high-velocity subducting slabs beneath the Ryukyu and Izu-Bonin seismic zones, which extend throughout the entire upper mantle; a very low-velocity in the uppermost 160 km of the mantle beneath the northern Philippine Sea, which is ascribed to the presence of extra water in this region; and a pronounced minimum in the amount of radial anisotropy near Hawaii, which is also seen along the Tonga-Hawaii corridor. A joint inversion of the data from the two corridors reveals the same anomaly pattern and clearly demonstrates that the swells in the Central Pacific are underlain by fast velocities. It is therefore implied that the topography of the swells in the central Pacific is supported by a chemical buoyancy mechanism which is generated by basaltic volcanism and the formation of its low-density peridotitic residuum. While the basaltic depletion mechanism can produce high shear velocities in the uppermost 200 km, it cannot explain the depth extent of the fast anomalies beneath the swells which, along Tonga-Hawaii corridor, extend well into the transition zone. It is therefore hypothesized that the central Pacific is underlain by a system of convective rolls that are confined above the 660-km discontinuity. It is likely that these rolls are predominantly oriented in the direction of plate motion (like "Richter rolls ") but the limited depth of the fast anomaly beneath the Hawaiian swell (200-300 km) suggests that their pattern is probably more complicated. Nevertheless, this convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep-mantle plumes.

Beschreibung: This research was supported by the National Science Foundation under grant EAR- 9628351 and by the Defense Special Weapons Agency under grant DSW A-F49620-95-1- 0051.

Beschreibung: Submitted in partial fulfillment of the requirements of the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2004

Beschreibung: Laterally extensive, well-developed clinoforms have been mapped in Early Cretaceous deposits located in the northeastern 27,000 km2 of the Colvile Basin, North Slope of Alaska. Using public domain 2-D seismic data, well logs, core photographs, and grain size data, depositional geometries within the Nanushuk and Torok formations were interpreted in order to constrain the transport conditions associated with progradation of the shoreline and construction of the continental margin out of detritus shed from the ancestral Brooks Range. Using STRATA, a synthetic stratigraphic modeling package, constructional clinoform geometries similar to those preserved in the North Slope clinoform volume (32,400 km3) were simulated. Sediment flux, marine and nonmarine diffusivities, and basin subsidence were systematically varied until a match was found for the foreset and topset slopes, as well as progradation rates over a 6 milion year period. The ability of STRATA to match the seismically interpreted geometries allows us to constrain measures of possible water and sediment discharges consistent with the observed development of the Early Cretaceous c1inoform suite. Simulations indicate that, in order to reproduce observed geometries and trends using constant input parameters, the subsidence rate must be very small, only a fraction of the most likely rate calculated from the seismic data. Constant sediment transport parameters can successfully describe the evolution of the prograding margin only in the absence of tectonic subsidence. However, further work is needed to constrain the absolute magnitude of these values and determine a unique solution for the NPR-A clinoforms.

Beschreibung: A correlation has been recently found between large earthquakes and the succeeding largest 9 explosive eruptions of the last century, which has been interpreted as a product of co- and post-seismic stress diffusion. Here, we check the statistical significance of the proposed coupling by using a larger dataset, and investigate the reliability of the causality hypothesis. We find that the volcanoes with VEI ≥ 4 eruptions underwent, in the few decades before the volcanic event, higher seismic stress perturbations due to large earthquakes compared to other volcanic areas. The correlation is statistically significant and it is not explained by a spatio-temporal clustering of eruptions and earthquakes due to tectonic pulses. This implies that the large earthquakes indeed triggered the eruptions.

Beschreibung: Gruppo Nazionale di Vulcanologia and e-Ruption projects

Beschreibung: Published

Beschreibung: reserved