Atmospheric Processes Modulating Noise in Fairfield Nodal 5 Hz Geophones

Atmospheric processes are documented to modulate seismic noise in Fairfield Nodal three-component geophones. Spectral analysis has shown high-amplitude signals between 40 and 50 Hz in all waveforms inspected. The changes in spectral amplitudes and frequency are found to be modified by daily variations in wind velocity and temperature, which are temporally correlated for much of the study. The wind velocity is shown to affect a wide spectral band with peak amplitudes that depend on the distance from in situ structures coupling wind energy into the shallow crust. The wind velocity increases the spectral amplitudes, most noticeably in the 40-50 Hz band; it produces a 15 Hz frequency modulation in the conditions of highest wind, with resonance frequencies up to 150 Hz. These signals likely reflect a superposition of multiple local and regional sources producing wind-generated ground motions and nonlinear wave propagation in the shallow subsurface. During periods of temperatures below 0 degrees C, a similar frequency modulation is observed, but the amplitudes are not as pronounced without the elevated wind velocity. A possible source of the continuous noise signal and the temperature-dependent frequency modulation is the spike mount that is attached to the nodal housing. The noise signals modulated by the wind and temperature variations require installation procedures in order to mitigate the effects of the contaminating noise on the geophysical processes of interest.