Lowering the vertical component seismic noise-floor in the normal-mode band by frequency-dependent air-pressure corrections

Abstract

SUMMARY Contrary to popular belief in seismology, the vertical component seismic background level of Earth’s body in the normal-mode band is lower than current low-noise models would indicate. With a correction of atmospherically induced disturbances the low-noise level of the vertical component of the STS-1 at Black Forest Observatory (BFO) is reduced below the Peterson low noise model (NLNM). We demonstrate this with a moving window analysis (MWA) of 10 yr of data in the normal-mode band between 0.2 and 2 mHz. The 5th percentile low-noise level for the data initially is at the level of the NLNM. We lower this by 8 dB at the frequency of 0S2 (0.31 mHz) by partially correcting the seismometer recording in the time domain for disturbances caused by mass fluctuations in the atmosphere. The disturbances are estimated by the frequency dependent ‘improved Bouguer plate model’ (IBPM, defined by asymptotic admittance αDC and notch-frequency fn) from a recording of local air-pressure. In more than 90 per cent of the time windows of the MWA the signal level is lowered when using a priori set IBPM-parameters ($\alpha _{\text{DC}}= -3.5\, {\rm nm\, s}^{-2}\, {\rm hPa}^{-1}$, $f_{\text{n}}=2.25\, {\rm mHz}$). In 50 per cent of the time windows the background level at the frequency of 0S2 is reduced by at least 6 dB. With a priori set IBPM-parameters the level of the 1st percentile is lowered by up to 16 dB for windows of $1\, {\rm d}$ length and by 13 dB for windows of $7\, {\rm d}$ length. Overall, there is little additional benefit from optimizing the IBPM-parameters per time window. If optimized per time window, the parameters show a slight annual variation which apparently compensates for systematic deviations of the dynamics of the atmosphere from the simple considerations of the IBPM-model. By application of the correction procedure and with seismic sensors of sufficiently low self-noise, the range of observations for signals from Earth’s body can be extended to below current low-noise models at frequencies below 2 mHz.