Reducing magnetic field induced noise in broad-band seismic recordings

Abstract

SUMMARY Seismic broad‐band sensors are known to be sensitive to the magnetic field. Magnetic storms and man‐made disturbances of the magnetic field can produce significant noise in seismic recordings. I show that variations in the magnetic field translate directly into apparent acceleration of the seismic sensor within the period range from 60 to 1200 s for all leaf‐spring sensors under investigation. For a Streckeisen STS‐1V this is shown even for periods down to 1 s. The sensitivity is quantified in magnitude and direction. Both are quite stable over many time windows and signal periods. The sensitivities obtained by linear regression of the acceleration signal on magnetic field recordings during a magnetic storm can effectively be applied to reduce noise in seismic signals. The sensitivity varies in magnitude from sensor to sensor but all are in the range from 0.05 to 1.2 m s−2 T−1. Seismograms from sensors at Black Forest Observatory (BFO) and stations of the German Regional Seismic Network were investigated. Although these are mainly equipped with leaf‐spring sensors, the problem is not limited to this type of instrument. The effect is not observable on the horizontal component STS‐1s at BFO while it is significant in the recordings of the vertical STS‐1. The main difference between these instruments is the leaf‐spring suspension in the vertical component that appears to be the source of the trouble. The suspension springs are made of temperature compensated Elinvar alloys that inherently are ferromagnetic and may respond to the magnetic field in various ways. However, the LaCoste Romberg ET‐19 gravimeter at BFO, which uses this material too, does not respond to magnetic storms at a similar magnitude neither do the Invar‐wire strainmeters. An active shielding, composed of three Helmholtz coils and a feedback system, is installed at station Stuttgart and provides an improvement of signal‐to‐noise ratio by almost a factor of 20 at this particular station. The passive Permalloy shielding commonly installed with STS‐1V sensors performs similarly well.

Clear evidence for the sign-reversal of the pressure admittance to gravity near 3mHz

“…Of course, because seismic waves and noise of instrumental and other origin (e. g. Forbriger, 2007) will provide some energy at these frequencies thus the best one can expect is to see a minimum. Peterson (1993) and Berger et al (2004) show in their studies of broad-band seismic noise at many stations of the Global Seismic Network for many time windows that the lower envelope of vertical acceleration noise power spectral densities (PSD) has a minimum near 3 mHz not present in the horizontal components.…”

Influence of high-latitude geomagnetic pulsations on recordings of broadband force-balanced seismic sensors

“…Although direct influence of the magnetic field on the electromagnetic force transducers is a universal mechanism explaining sensitivity of force balanced seismometers to variations of geomagnetic field, this influence was not considered in the previous studies by Forbriger (2007) and Forbriger et al (2010) who suggested the needle effect as the most effective. Our study shows that, indeed, the needle effect might be the major factor responsible for sensitivity of the STS-2 seismometer in Kilpisjärvi (Fig.…”

“…Forbriger (2007) also noticed that different types of seismic sensors have different sensitivity to the variations of the magnetic field. These differences in sensitivity of different sensors to magnetic field can hardly be due to the variations in magnetic properties of suspensions only.…”

“…Another argument in favour of different mechanisms is the fact that gravimeters and strainmeters, having suspensions with similar properties, are not so sensitive to the variations of magnetic field as broadband seismometers (Pálinkás et al, 2003;Forbriger, 2007). Forbriger (2007) also noticed that different types of seismic sensors have different sensitivity to the variations of the magnetic field.…”

“…For instance, Forbriger (2007) reported about a signature of the magnetic storm on 31 March 2001, which was clearly observed in the recordings of the Streckeisen STS-2 seismometers at the German Regional Seismic Network (GRSN).…”