Abstract

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.