Challenges and perspectives for lowering the horizontal component long-period detection level
Thomas Forbriger & Walter Zürn
GPI / BFO
Mass fluctuations in the atmosphere produce a permanent background noise level in normal mode data (period band from about two minutes to one hour) and such limit the signal-to-noise ratio. The mechanisms coupling this atmospheric signal into the vertical component data are quite well understood and mitigation procedures exist. The situation is more difficult for horizontal component data, where horizontal gradients of pressure loading and local elastic structure near the sensor play a significant role. In the current presentation we present results obtained from the analysis of horizontal component data. Horizontal component and strain data is essential to observe toroidal free oscillations of the globe. Spectral analysis of these toroidal modes can put integral constraints on the viscoelastic properties of the Earth's mantel, such complementing the analysis of body wave amplitude decay. These properties, commonly expressed by the Q parameter of viscoelastic material, are desired to constrain models of mantle convection. Tilting of the ground due to loading by the variable atmosphere is known to corrupt very long-period horizontal seismic records (below 10 mHz) even at the quietest stations. At BFO (Black Forest Observatory, SW-Germany) the opportunity arose to study these disturbances on a variety of state-of-the-art broadband sensors operated simultaneously. A series of time windows with clear atmospherically caused effects was selected and attempts were made to model these 'signals' in a deterministic way. This was done by least squares fitting the locally recorded barometric pressure and its Hilbert transform simultaneously to the ground accelerations in a bandpass between 3600s and 100s period. Variance reductions of up to 97% were obtained. We show our results by combining the 'specific pressure induced accelerations' for the two horizontal components of the same sensor as vectors on a horizontal plane, one for direct pressure and one for its Hilbert transform. It turned out that at BFO the direct pressure effects are large, strongly position dependent, and largely independent of atmospheric events for instruments installed on piers, while three posthole sensors are only slightly affected. The infamous 'cavity effects' are invoked to be responsible for these large effects on the pier sensors. On the other hand, in the majority of cases all sensors showed very similar magnitudes and directions for the vectors obtained for the regression with the Hilbert transform, but highly variable especially in direction from event to event. Therefore this direction most certainly has to do with the gradient of the pressure field moving over the station which causes a larger scale deformation of the crust. The observations are very consistent with these two fundamental mechanisms of how fluctuations of atmospheric surface pressure causes tilt noise. The results provide a sound basis for further improvements of the models for these mechanisms. The methods used here can already help to reduce atmospherically induced noise in long period horizontal seismic records.