GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.3 619 formulation, is the one proposed by Pride (1994) who describes the governing equations coupling Electromagnetic and seismic fields in a porous media. Pride’s results can be summarized by using the author’s word in the following way: «The derived equations have the form of Maxwell’s equations coupled to Biot’s equations with coupling occurring in the flux-force (or transport) relations.». What basically emerges from Pride’s work (containing more than 350 formulas) is that two different kinds of ElectroMagnetic (EM) signals are associated with mechanical perturbation propagating within the subsoil: a pure EM signal (traveling at the speed of EM signals in a medium) generated when the mechanical perturbation crosses a discontinuity with strong permeability contrast and a SES signal which travels with the seismic wave velocity. Pride’s work, along with Gao and Hu (2010), is fundamental because it allows, theoretically speaking, to predict the SES produced by a seismic wave knowing the characteristics of the medium in which the seismic propagation takes place. In addition, reversing the point of view, it makes plausible and possible the definition of a new geophysical method based on the simultaneous recording of seismic and SES signals, “integrated dataset”, to define the characteristics of the subsoil. The outline of this potential new method is now being defined (i.e Warden et al., 2012), but there is still a lot to do in order to make really feasible the use of the SES. New experimental evidences are needed and, considering the unpredictability (in terms of occurrence time and location) of the earthquakes, a systematic study on the SES needs dedicated experiments and data analysis techniques tuned on the characteristic of the problem. To this aim, ElectroMagnetic signal were recorded during a active seismic experiment (performed in the framework of the RICEN project). By using MagnetoTelluric (MT) instruments, the SES generated by the controlled seismic source have been detected for a their future characterization and to define a reliable and objective analysis technique. In this work we present some of the results obtained by analysing the collected data. Our purpose is to show the strengths and weaknesses of using a SES based integrated dataset. The first challenge that will be presented is related to the electromagnetic part of the dataset. Contrary to what happens for the seismic data, also in a favourable condition (small source-receiver separation), most of the recorded EM signal has an origin not related to the SES presence. Even knowing the activation time of the seismic source, the SES, in terms of amplitude, represents only a fraction of the whole recorded signal (natural+cultural+SES). Thus, the EM signals have to be firstly cleaned from the “noise” (natural+cultural). Once shown that it is possible to extract SES information, the main issue that we are trying to address is to understand how the medium, in which the seismic waves propagate, affects the SES characteristics and how and if changing the distribution of the observation points (MT station) respect to the seismic source may allow to infer information on the subsoil. References Balasco M., Lapenna V., Romano G, Siniscalchi A., Stabile T. A. and Telesca L.; 2014: Electric and magnetic field changes observed during a seismic swarm in Pollino area (Southern Italy). Bulletin of the Seismological Society of America, 104 , ISSN: 0037-1106, doi: 10.1785/0120130183 Gao, Y., and Hu H.; 2010: Seismoelectromagnetic waves radiated by a double couple source in a saturated porous medium . Geophys. J. Int., 181 , 873–896. Honkura, Y., Satoh H. and Ujihara N.; 2004: Seismo dynamo effects associated with the M 7.1 earthquake of 26 May 2003 off Miyagi Prefecture and the M 6.4 earthquake of 26 July 2003 in northern Miyagi Prefecture, NE Japan . Earth Planets Space, 56 , 109–114. Honkura, Y., Ogawa Y., Matsushima M. , Nagaoka S., Ujihara N. and Yamawaki T.; 2009: A model for observed circular polarized electric fields coincident with the passage of large seismic waves . J. Geophys. Res. 114, no. B10103, doi: 10.1029/2008JB006117. Hu, H., and Gao Y.; 2011: Electromagnetic field generated by a finite fault due to electrokinetic effect. J. Geophys. Res., 116 , no. B08302, doi: 10.1029/2010JB007958. Pride, S. R.; 1994: Governing equations for the coupled electromagnetics and acoustics of porous media . Phys. Rev. B, 50 , 15,678–15,696 Warden S., Garambois S., Sailhac P., Jouniaux L. and Bano M.; 2012: Curvelet-based seismoelectric data processing. Geophys. J. Int., 190 , 1533–1550