Publication Date:
2017-04-04
Description:
We have investigated the mechanical response of a magnetically shielded
room that hosts a magnetoencephalography system that is subject to
external vibrations. This is a superconducting quantum interference device,
which are the most sensitive sensors for magnetic flux variations. When
the magnetoencephalography operates with people inside the room, the
spectrum of the flux of the magnetic field shows anomalous peaks at several
frequencies between 1 Hz and 20 Hz, independent of the experiment that is
being run. As the variations in the flux of the magnetic field through the
sensors might not only be related to the electrical currents circulating
inside the brain, but also to non-damped mechanical oscillations of the
room, we installed seismic instrumentation to measure the effective motion
inside the room and to compare it to the external motion. For this analysis,
we recorded the ambient seismic noise at two very close stations, one inside
the magnetically shielded room, the other one outside in the room in which
the magnetically shielded room is itself located. Data were collected over
four days, including a week-end, to study the response of the magnetically
shielded room subjected to different energy levels of external vibrations.
The root mean square, Fourier spectra and power spectral density show
significant differences between the signal recorded inside and outside the
magnetically shielded room, with several anomalous peaks in the
frequency band of 1 Hz to 20 Hz. The normalized spectral quantities
(horizontal to vertical spectral ratio, and ratio between the internal and
external spectra) show large amplification at several frequencies, reaching
in some cases one order of magnitude. We concluded that the magnetically
shielded room does not dampen the external vibrations, but it instead
appears to amplify these across a broad frequency range.
Description:
Published
Description:
223-233
Description:
4.1. Metodologie sismologiche per l'ingegneria sismica
Description:
JCR Journal
Description:
open
Keywords:
Seismic noise
;
Spectral ratio analyses
;
Magnetoencephalography
;
04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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