Characteristics and spatial variability of wind noise on near-surface broadband seismometers

Abstract

By coupling with the ground, wind causes ground motion that appears on seismic records as noise across a wide bandwidth. This wind-generated noise can drown out important features such as small earthquakes and prevent observation of normal modes from large earthquakes. Because the wind field is heterogeneous at local scales due to structures, diurnal heating, and topography, wind-induced seismic noise may be different on seismometers installed just meters apart. We investigated the spatial variability of wind-induced noise using two weather sensors separated by ∼100 m and collocated with one deep borehole and four near-surface broadband seismometers. We found that at longer periods (>5 s), increasing wind speed causes noise increases on the horizontal components of seismometers. Although this has been previously observed, we also measured γ2 coherences of less than 0.2 between the wind speed, wind direction, and the pressure recorded by our weather stations. We further observed a loss of coherence between the vertical components of our seismometers from an 8–20 s period. The amplitude of the drop in coherence appears to depend on the substrate surrounding the seismometer. Based on two previously developed theoretical models, we found that the local material surrounding the sensor could be amplifying the wind-generated noise. We also investigated the frequency dependence of wind-induced noise and found that the dominant source of high-frequency seismic noise at some sites could be anthropogenic rather than induced by wind. In addition, we estimated the linear relationships between the root mean square (rms) of wind speed and rms seismic velocity for each sensor, finding substantial variability between different installments. A more detailed understanding of the complex processes by which wind-induced noise is generated can inform the installation of sensors and the development of methods for mitigation of these effects, thus improving the overall quality of seismic data.