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Department of Earth and Environmental Sciences, University of Kentucky
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- PublicationRestrictedA study of spectral methods of estimating the depth to the bottom of magnetic sources from near-surface magnetic anomaly data(2007)
; ; ; ; ;Ravat, D.; Southern Illinois University C'dale ;Pignatelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Nicolosi, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Chiappini, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; ; ; Based on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models, we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations: (1) using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows (>300-500 km); (2) avoiding filtering to remove arbitrary regional fields, accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 Earth's main field models e.g. recent International Geomagnetic Reference Field models (IGRF) or Comprehensive models (CM); (3) ascertaining the near-circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends; and (4) avoid determining the slopes from the exponential, low wavenumber part of the spectra in the cases of layered magnetization. We also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results. We found that, despite all these precautions, in some cases, the results can still be erroneous and, therefore, we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible. In the southcentral US, east of the Rockies, where the surface heat flow ranges between 40 and 65 mW m?2, we obtained the magnetic bottom depth of 40 ± 10 km using the approach of the forward modelling of the spectral peak. This range is similar to the seismically derived crustal thickness of 45-50 km, suggesting, therefore, that the entire crust may be magnetic in this region. Because of the uncertainties in the various heat flow contributing parameters, such as the variations in thermal conductivity, radiogenic heat and hydraulic regime, we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km.355 51 - PublicationRestrictedNew model alternatives for improving the representation of the core magnetic field of Antarctica(2006)
; ; ; ; ;Gaya-Piqué, L. R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Ravat, D.; Department of Geology 4324, Southern Illinois University Carbondale, Carbondale, IL 62901-4324, USA ;De Santis, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Torta, J. M.; Observatori de l’Ebre, CSIC - URL, Horta Alta 38, 43520 Roquetes, Spain; ; ; Use of the International Geomagnetic Reference Field Model (IGRF) to construct magnetic anomaly maps can lead to problems with the accurate determination of magnetic anomalies that are readily apparent at the edges of local or regional magnetic surveys carried out at different epochs. The situation is severe in areas like Antarctica, where ionospheric activity is intense and only a few ground magnetic observatories exist. This makes it difficult to properly separate from ionospheric variations the secular variation of the core magnetic field. We examine two alternatives to the piecewise-continuous IGRF core magnetic field in Antarctica for the last 45 years: the present global Comprehensive Model (CM4) and the new version of the Antarctic Reference Model (ARM). Both these continuous models are better at representing the secular variation in Antarctica than the IGRF. Therefore, their use is recommended for defining the crustal magnetic field of Antarctica (e.g. the next generation of the Antarctic Digital Magnetic Anomaly Map).215 29 - PublicationRestrictedCurie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central Aeolian Ridge (Southern Tyrrhenian Sea, Italy)(2013-03-20)
; ; ; ; ;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; ; ; 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.766 110