Geomagnetic anomaly in opening basin structures: the Tyrrhenian Sea example

Abstract

The aim of this work is to improve the informative content of regional survey aeromagnetic data to allow high frequency anomaly studies in regional analysis too. The analysis has been applied to study Tyrrhenian seamounts and to link them in structural configuration and age. To cancel spectral difference due to distance and position of magmatic sources (seamounts and volcanic islands) with respect to altitude survey, the South Tyrrhenian geomagnetic anomaly field has been reduced to bottom topographic surface by the metrologic technique BTM (Bottom Reduction Method, Faggioni et al., 2001). The data set comes from the aeromagnetic surveys performed by AGIP over the South Tyrrhenian Sea in 1977. The flight pattern covers a surface of 69173 Km² and is made up of 230850 measurements; its location is approximately between 38° and 41°N longitude and 10° and 15°E latitude. Data have been organized in a grid with latitude and longitude as channels, over which the matrix computation of maps are built. The anomaly map is the result of subtracting the IGRF (International Geomagnetic Reference Field) model from the total anomaly field map. Anomaly signal, as residual of IGRF procedure, is produced by more superficial sources (i.e. seamounts), therefore it is directly informative about their physical characteristics. It shows values of little coherence, because the reduction of the reference field was derived from a subjective surface composed by the envelope of local planes at the time of the survey. This distortion has been removed resorting to DSP (Digital Signal Processing). After obtaining the “cleaned” magnetic anomaly map, the following step was the matrix transport in frequency to carry out the spectral analysis of data and to identify the cut-frequency to separate low (L) and high (H) frequency components. From the first, the S.R.F. (Spectral Reference Field) is obtained i.e. the cotransformation of low frequency and gives information about deep magnetic structures. The high frequency component, computed as direct reduction (difference between correct magnetic anomaly field and low frequency one), is the S.A.F. (Spectral Anomaly Field). The bottom reduced magnetic anomaly field comes from merging the frequency bands (H e L) after projection of vertical gradient through increasing parameters of geomagnetic field and the depth of all grid points. The modelling of BTM field in some interesting areas is shown.