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

Abstract

Abstract. Seismic broadband sensors with electromagnetic feedback are sensitive to variations of surrounding magnetic field, including variations of geomagnetic field. Usually, the influence of the geomagnetic field on recordings of such seismometers is ignored. It might be justified for seismic observations at middle and low latitudes. The problem is of high importance, however, for observations in Polar Regions (above 60 • geomagnetic latitude), where magnitudes of natural magnetic disturbances may be two or even three orders larger. In our study we investigate the effect of ultra-low frequency (ULF) magnetic disturbances, known as geomagnetic pulsations, on the STS-2 seismic broadband sensors. The pulsations have their sources and, respectively, maximal amplitudes in the region of the auroral ovals, which surround the magnetic poles in both hemispheres at geomagnetic latitude (GMLAT) between 60 • and 80 • . To investigate sensitivity of the STS-2 seismometer to geomagnetic pulsations, we compared the recordings of permanent seismic stations in northern Finland to the data of the magnetometers of the IMAGE network located in the same area. Our results show that temporary variations of magnetic field with periods of 40-150 s corresponding to regular Pc4 and irregular Pi2 pulsations are seen very well in recordings of the STS-2 seismometers. Therefore, these pulsations may create a serious problem for interpretation of seismic observations in the vicinity of the auroral oval. Moreover, the shape of Pi2 magnetic disturbances and their periods resemble the waveforms of glacial seismic events reported originally by Ekström (2003). The problem may be treated, however, if combined analysis of recordings of co-located seismic and magnetic instruments is used.

On the observation of magnetic events on broad-band seismometers

“…Forbriger (2007) presented seismograms showing clear signals corresponding to a couple of SSC events recorded at stations of the German Regional Seismic Network and proposed that the magnetic field disturbances affected the suspension springs of the inertial mass, resulting in apparent accelerations proportional to the variations of the magnetic field which, at low frequencies, can be larger than the accelerations due to mechanical accelerations generated by soil vibration. Alternatively, Kozlovskaya and Kozlovsky (2012) proposed that the origin of the magnetic signals generated by geomagnetic pulsations in seismic records has their origin in the feedback system of the sensor. Under this hypothesis, the geomagnetic field variations would result in an induced current modifying the electrical current flowing through the large capacitor and the feedback coil and resulting in apparent accelerations not related to ground motion.…”