Study of Seismo-Electromagnetic signals in an area characterized by an intense hydrothermal activity: a case study from the Solfatara area (Campi Flegrei, Italy)

In 2014 t In the framework of the MEDSUV project, the Solfatara area, part of the Campi Flegrei caldera, was chosen as test site for the RICEN (Repeated InduCed Earthquakes and Noise) experiment mainly oriented to the use of seismic waves (both in passive and active mode) as a diagnostic tool to investigate the changes in the properties of the medium at small scales. Besides the study of seismic waves (both in passive and active mode), part of the RICEN activities was focused on the detection and characterization of the Seismo-Electromagnetic signals (SES) associated with their propagation.

Considering the abundance of fluids that characterize the shallow hydrothermal system of Solfatara area, SES arewere expected to be detectable and informative of the subsoil structure. On May 2014, Hence for their detection during RICEN experimthree magnetotelluric (MT) stations were installed outside the seismic grid and close to the main volcanic fumaroles in the Solfatara area. Thusing electrical and magnetic components concurrently with seismic and geochemical measurements were recorded. As a result, SES related to Vibrosesis seismic source energization in 100 sites, distributed on an almost regular grid on an area of about 115m x 90m, were analyzed. The m.

Although Unfortunately the electrical part of the SES could not be extracted by the recorded time series due to the severe effects of the Solfatara volcanic environment on the unmpolarizable electrodes. ,Converselyagnetic components, instead of the electrical ones, were generally better appreciable, in terms of amplitude, with respect to the natural electromagnetic fluctuations(s?). This circumstance allowed to verify the strict causality relationship between of the SES with and seismic signals for interstation distances (Seismic source -MT stations) ranging between 100 m and 200 m..

Focusing on the magnetic part, a comparative Wavelet analysis on SES and on seismic source permitted to evaluate that in the time domain that SES signals are mainly associated with Rayleigh wave, due to relatively large average distance between shot and MT sites, ranging between 60 m and 200 m.

Once defined SES characteristics, in terms of frequency content and amplitude, possible information on the subsurface status and new inferences on fluids characterizing the subsoil of the studied area were obtained. This was possible by investigating the spatial distribution of SES amplitude and by comparing it with 3D model of Vp, Vs and resistivity as well as with temperature and CO₂ flux maps.