ON NOISE-REDUCTION IN VERTICAL SEISMIC RECORDS BELOW 2 MHZ USING LOCAL BAROMETRIC-PRESSURE

We propose a simple method for the reduction of noise in vertical seis mic recordings at frequencies below 2 mHz. The method consists in subt racting: the locally recorded, scaled, atmospheric pressure signal fro m the gravity record. The scale factor is frequency independent and ca n be linearly estimated. from the data. While such a pressure correcti on was previously only used in the analysis of tidal records, it is eq ually efficient at the low-frequency end of the normal mode band and w e use gravity recordings of the big 1994 Shikotan and Bolivian quakes to demonstrate this point. Our examples also show that this correction permits to achieve noise levels well below the New Low Noise Model (N LNM) [Peterson, 1993]. The success of the correction also sheds some l ight on the physical processes responsible for the generation of low-f requency seismic noise at quiet observatories.