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Repeat-station surveys: implications from chaos and ergodicity of the recent geomagnetic field

Wed, 17 Apr 2013 22:00:00 GMT

Title: Repeat-station surveys: implications from chaos and ergodicity of the recent geomagnetic field Authors: De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Qamili, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Cianchini, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: The present geomagnetic field is chaotic and ergodic: chaotic because it can no longer be predicted beyond around 6 years; and ergodic in the sense that time averages correspond to phase-space averages. These properties have already been deduced from complex analyses of observatory time series in a reconstructed phase space [Barraclough and De Santis 1997] and from global predicted and definitive models of differences in the time domain [De Santis et al. 2011]. These results imply that there is a strong necessity to make repeat-station magnetic surveys more frequently than every 5 years. This, in turn, will also improve the geomagnetic field secular variation models. This report provides practical examples and case studies.

Curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy)

Tue, 19 Mar 2013 23:00:00 GMT

Title: Curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy) Authors: De Ritis, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Ravat, D.; Department of Earth and Environmental Sciences, University of Kentucky; Ventura, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Chiappini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: The Salina, Lipari, and Vulcano volcanic ridge and the surrounding sea sectors (Aeolian Archipelago, Southern Tyrrhenian Sea, Italy) are characterized by vents responsible for a recent (<40 ka—1889/1890 AD) effusive and explosive subareal activity and repeated, 56 to 7 ka in age, submarine explosive eruptions from source areas located between Lipari and Vulcano. A spectral depth estimation of the magnetic bottom using a fractal method on aeromagnetic data from Vulcano, Lipari, and Salina volcanic ridge allows us to constrain the Curie isotherm depth. The elevated portion of the isotherm is between 2 and 3 km below Salina and Vulcano and about 1 km below Lipari. The Curie depth results in the context of other geological and geophysical evidence suggest that the rise of the Curie isotherm is mainly due to the occurrence of shallow heat sources such as magma ponds and associated hydrothermal systems. The short-wavelength magnetic anomaly field reflects magnetic contrasts from highly magnetized volcanic bodies, low-magnetization sediments, and hydrothermally altered rocks. Borehole temperature data verify the Curie temperature derived from the magnetic methods on the island of Vulcano.We conclude that the whole Vulcano, Lipari, and Salina volcanic ridge is active and should be monitored.

Antarctic geomagnetic reference model updated to 2010 and provisionally to 2012

Mon, 31 Dec 2012 23:00:00 GMT

Title: Antarctic geomagnetic reference model updated to 2010 and provisionally to 2012 Authors: Tozzi, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Gaya-Piqué, L.-R.; On-Site Inspection Division, Comprehensive Nuclear-Test Ban Treaty Organization PrepCom, Vienna, Austria. Abstract: The Antarctic Reference Model (ARM) has been here updated using recent geomagnetic data measured over the Antarctic continent from both ground observatories and satellites. This regional geomagnetic model is based on a Spherical Cap Harmonic Analysis (SCHA) of geomagnetic field measurements over a polar cap of 30° half-angle centred at the geographic South Pole, fixing the maximum spatial expansion index kmax=8 and the maximum temporal order of polynomials qmax=4. The importance of updating ARM model lies, for instance, in its usefulness for the reduction of magnetic surveys, performed during the period of model validity over the Antarctica, or for geomagnetic anomaly field estimations. Moreover, so far, ARM still remains the only regional reference magnetic model specifically constructed for the Antarctic continent. The present updated version can be considered valid from 1955.5 to 2010.0 with predictive coefficients up to 2012.0. The model includes the most recent available data but, in contrast to previous versions not only does it take advantage of a stricter selection of satellite data in order to consider even quieter periods of external magnetic activity, but it also includes ground observatory data previous to 1960 going back to 1955.8. Like the previous versions, the new updated model has been tested and compared with major global models to show its reliability over the region under investigation.

Paleomagnetic secular variation at the Azores during the last 3 ka

Wed, 11 Jul 2012 22:00:00 GMT

Title: Paleomagnetic secular variation at the Azores during the last 3 ka Authors: Di Chiara, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Speranza, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Porreca, M.; Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, Ponta Delgada, Portugal. Abstract: We report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on São Miguel, the largest island of the Azores. The data provide 27 well-dated directions from historical or 14C dated flows which, together with 6 directions previously gathered from the same flows by Johnson et al. (1998), yield the first paleomagnetic directional record of the last 3 ka from the Atlantic Ocean. Within-flow directions are consistent, suggesting that inclination swings from 60 to 25 and declination changes between 10 to 20 reflect variations in the geomagnetic field over the last 3 ka. To a first approximation, the declination record is consistent with predictions from CALS3k.4 and gufm1 global field models. Conversely, inclination values are lower than model predictions at two different ages: 1) four sites from the 1652 AD flow yield I = 48 instead of I = 63 predicted by gufm1; 2) data from several flows nicely mimic the inclination minimum of 800–1400 AD, but inclination values are lower by 10 than CALS3k.4 model predictions. By interpolating a cubic spline fit on declination / inclination versus age data, we tentatively infer the directional evolution of the geomagnetic field at the Azores from 1000 BC to 1600 AD. The obtained curve shows three tracks in virtual overlap during the 1000–800 BC, 800–500 BC, and 400–700 AD time spans.

Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism

Wed, 29 Feb 2012 23:00:00 GMT

Title: Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism Authors: Speranza, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Di Chiara, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Rotolo, S. G.; Dipartimento di Scienze della Terra e del Mare (DISTeM), Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy Abstract: Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.

Repeat Station Activities

Sun, 31 Jul 2011 22:00:00 GMT

Title: Repeat Station Activities Authors: Barraclough, D. R.; British Geological Survey, Edinburgh, U. K.; De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Editors: Mandea, M.; Institut de Physique du Globe, Paris, France; Korte, M.; GFZ German Research Centre for Geosciences Abstract: A repeat station is a site whose position is accurately known and where accurate measurements of the geomagnetic field vector are made at regular intervals in order to provide information about the secular variation of the geomagnetic field. In this chapter we begin by giving a brief history of the development of repeat station networks. We then describe the instruments used to make measurements at a repeat station. These include fixing the position of the station, finding the direction of true north and measuring the components of the geomagnetic field. Emphasis is given to techniques and instruments that are in current use. We next discuss the procedures that are used to reduce the measurements to a usable form and consider the uses to which the reduced data are put. Finally, we discuss the continued importance of such data in the present era of satellite geomagnetic surveys.

Ergodicity of the recent geomagnetic field

Tue, 31 May 2011 22:00:00 GMT

Title: Ergodicity of the recent geomagnetic field Authors: De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Qamili, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Cianchini, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: The geomagnetic field is a fundamental property of our planet: its study would allow us to understand those processes of Earth’s interior, which act in its outer core and produce the main field. Knowledge of whether the field is ergodic, i.e. whether time averages correspond to phase space averages, is an important question since, if this were true, it would point out a strong spatio-temporal coupling amongst the components of the dynamical system behind the present geomagnetic field generation. Another consequence would be that many computations, usually undertaken with many difficulties in the phase space, can be made in the conventional time domain. We analyse the temporal behaviour of the deviation between predictive and definitive geomagnetic global models for successive intervals from 1965 to 2010, finding a similar exponential growth with time. Also going back in time (at around 1600 and 1900 by using the GUFM1 model) confirms the same findings. This result corroborates previous chaotic analyses made in a reconstructed phase space from geomagnetic observatory time series, confirming the chaotic character of the recent geomagnetic field with no reliable prediction after around 6 years from definitive values, and disclosing the potentiality of estimating important entropic quantities of the field by time averages. Although more tests will be necessary, some of our analyses confirm the efforts to improve the representation of the geomagnetic field with more detailed secular variation and acceleration.

Two geomagnetic regional models for Albania and south-east Italy from 1990 to 2010 with prediction to 2012 and comparison with IGRF-11

Thu, 31 Dec 2009 23:00:00 GMT

Title: Two geomagnetic regional models for Albania and south-east Italy from 1990 to 2010 with prediction to 2012 and comparison with IGRF-11 Authors: Qamili, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Cianchini, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Duka, B.; Department of Physics, Faculty of Natural Sciences, University of Tirana, Albania; Gaya-Piqué, L. R.; On-Site Inspection Division, Comprehensive Nuclear-Test Ban Treaty Organization PrepCom, Vienna, Austria; Dominici, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Hyka, Niko; Department of Physics, Faculty of Natural Sciences, University of Tirana, Albania Abstract: Here we present a revised geomagnetic reference model for the region comprising Albanian territory, south-east part of Italian Peninsula and Ionian Sea from 1990 to 2010 with prediction to 2012. This study is based on the datasets of magnetic measurements taken during different campaigns in Albania and Italy in the time of concern, together with a total intensity data set from the Ørsted and CHAMP satellite missions. The model is designed to represent the Cartesian components, X, Y, Z and the total intensity F of the main geomagnetic field (and its secular variation) for the period of interest. To develop the model, we applied a Spherical Cap Harmonic Analysis (SCHA) of the geomagnetic potential over a 16° cap with most of the observations concentrated in the central 4° half-angle. The use of a larger cap than that containing the data was made to reduce the typical problems in SV modelling over small regions. Also a new technique, called ``Radially Simplified Spherical Cap Harmonic Analysis" (RS-SCHA), was developed to improve the model especially in the radial variation of the geomagnetic field components. Both these models provide an optimal representation of the geomagnetic field in the considered region compared with the International Geomagnetic Reference Field model (IGRF-11) and can be used as reference models to reduce magnetic surveys undertaken in the area during the time of validity of the model, or to extrapolate the field till 2012.

Shannon information of the geomagnetic field for the past 7000 years

Thu, 11 Feb 2010 23:00:00 GMT

Title: Shannon information of the geomagnetic field for the past 7000 years Authors: De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Qamili, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia Abstract: The present behaviour of the geomagnetic field as expressed by the International Geomagnetic Reference Field (IGRF) deserves special attention when compared with that shown over the past few thousands of years by two paleomagnetic/archeomagnetic models, CALS3K and CALS7K. The application of the Information theory in terms of Shannon Information and K-entropy to these models shows characteristics of an instable geomagnetic field. Although the result is mitigated when we correct the CALS7K model for its typical spectral damping, the present geomagnetic field as represented by IGRF is still rather distinct, at least for the past 4000 years, a result that is further confirmed by the CALS3K model. This is consistent with a significant global critical state started at around 1750, and still present, characterised by significant decays of the geomagnetic dipole, energy and Shannon information and high K-entropy. The details of how these characteristics may develop are not clear, since the present state could move toward an excursion or a geomagnetic polarity reversal, but we cannot exclude the possibility that the “critical” behaviour will become again more “normal”, stopping the apparent trend of the recent geomagnetic field decay.

Three years continuous record of the Earth's magnetic field at Concordia Station (DomeC, Antarctica)

Sat, 31 Jan 2009 23:00:00 GMT

Title: Three years continuous record of the Earth's magnetic field at Concordia Station (DomeC, Antarctica) Authors: Chambodut, A.; EOST, France; Di Mauro, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Schott, J.-J.; EOST, France; Bordais, P.; IPEV, France; Agnoletto, L.; ENEA, Italy; Di Felice, P.; ENEA, Italy Abstract: The magnetic observatory deployed at DomeC, Antarctica, in the French-Italian base known as Concordia has now been permanently running for more than three years. This paper focuses on these long-term results which are more relevant for an observatory intended to provide absolute values of the field. The problems which emerged in this fairly long record are discussed and solutions suggested to upgrade the observatory to the standards of an absolute one (i.e. Intermagnet standards).