Magnetic field background variations can limit the resolution of seismic broad-band sensors

Analysing magnetic field recordings from a quiet period in
January 2007 we find that seismometers with
a sensitivity to horizontal component of magnetic field
larger than 0.2 m/s/s/T would not be able to resolve the
NLNM (New Low Noise Model, Peterson 1993) between 0.3 mHz and 3 mHz (i.e. in
the low-frequency normal-mode band).
In a previous study the sensitivity to magnetic field of
stations in the GRSN (German Regional Seismic Network) was analysed.
Only recently we became aware that this unwanted sensitivity can limit the
resolution of some of these stations even during magnetically quiet periods.
The situation will be even worse during less quiet periods and
at other sites of the global network of
broad-band seismometers since amplitudes of
natural variations of Earth's magnetic field are likely
to be larger at higher geomagnetic latitude.
Vertical or oblique components of broad-band sensors which require a
suspension spring can suffer from this since the springs are made from
elinvar alloys.
Elinvar alloys necessarily are ferromagnetic and magnetostrictive.
It is thus crucial to use appropriate means (e.g. a permalloy shield)
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to ensure a low
sensitivity to magnetic fields when designing and installing high-resolution
broad-band seismometers for the observation of normal modes.
Artificial sources of magnetic fields should be avoided in the vicinity of the
seismometers.
We demonstrate that even the small magnetic fields caused by supply currents
in data acquisition systems can generate disturbances in seismic broad-band
recordings.
Although making use of invar and elinvar alloys too, the LaCoste Romberg earth
tide gravimeter ET-19 and the invar wire strainmeters at BFO (Black Forest
Observatory) are not limited
due to magnetic field variations in their resolution for seismic normal modes.