Extracting quantitative dynamics in Earth's apparent resistivity time series by using the detrended fluctuation analysis

The magnetotelluric method (MT) consists in the observation of the surface natural variations of the electromagnetic field in a broad range of periods. The apparent resistivity is proportional to the modulus of the surface impedance, which is the tensorial relation between the horizontal components of the electric with the magnetic field. The subsurface resistivity distribution is obtained by inverting the apparent resistivity. The method of the detrended fluctuation analysis (DFA) has been applied to investigate the dynamics of apparent resistivity, which. represents one of the most important scientific challenges in the studies devoted to the geophysical monitoring. Our results show that during July 2003 the time series representing variations of the apparent resistivity in the seismic area of Val d'Agri (southern Italy) are persistent at all the periods considered, tending to pure randomness with the increase of the period. Due to the relationship between the period and the depth, the observed variations are compatible with resistivity changes in the shallow layers. (c) 2006 Elsevier B.V. All rights reserved.