Nature of Low-Frequency, Atmosphere-Generated Seismic Noise

Characteristics of atmosphere-generated seismic noise below 0.05 Hz are investigated when surface pressure is large. In this paper, large pressure means pressure power spectral density exceeding 100 Pa-2/Hz (at 0.01 Hz). We discuss three main points. The first point is existence of two frequency ranges that show high coherence between co-located pressure and vertical seismic data. The lower frequency (LF) range is broad and its upper bound is about 0.002 Hz. The higher frequency (HF) range is bounded between about 0.01 and 0.05 Hz. Phase difference between pressure and vertical displacement is different for the two ranges. The LF range shows phase difference of zero, and the HF range shows phase difference of 180 degrees. The second point is on the excitation mechanism in the HF range. Using theory and data, we show that seismic noise in the HF range is primarily excited by wind-related pressure. When pressure is high, wind speeds become high, and wind directions become unidirectional. In such a case, a deterministic, moving pressure-source by Sorrells (1971, ) captures the characteristics of data better than stochastic source models. The third point is on the cause of phase differences between the LF range and the HF range. The root cause is that, even after removing the instrument response, vertical seismic data contain effects from gravity and Earth rotation. Gravity effects become significant for frequencies below 0.005 Hz and create discrepancies between deconvolved vertical displacements and true vertical ground displacements. Phase-difference results are naturally explained by it. Plain Language Summary When surface pressure becomes large, ground deformation can be detected in seismograms as seismic noise. If we had pressure and seismic sensors at a close distance, the data show high coherence, especially when surface pressure is large. Two broad frequency ranges are seen typically in the coherence. The lower frequency (LF) range is below 0.002 Hz but its lower bound is too low to determine by our data. This is from the limitation of seismic sensors. The higher frequency (HF) range is between about 0.01 and 0.05 Hz. Phase differences between pressure and seismic displacement are basically opposite between the two frequency ranges. The LF range shows phase difference of zero while the HF range shows 180 degrees. This can be explained by the fact that seismic sensors below 0.005 Hz is influenced by gravity signals. We illustrate it by using a simple model. We also show that seismic signals in the HF range is excited by wind-related surface pressure changes. This pressure deforms the Earth from which we can estimate elastic constants at shallow depths.