High-frequency analysis of seismic background noise as a function of wind speed and shallow depth

Abstract

We used a deep (1500 m) cased borehole near the town of Datil in west-central New Mexico to study high-frequency (>1 Hz) seismic noise characteristics. The remote site had very low levels of cultural noise, but strong winds (winter and spring) made the site an excellent candidate to study the effects of wind noise on seismograms. Along with a three-component set of surface sensors (Teledyne Geotech GS-13), a vertical borehole seismometer (GS-28) was deployed at a variety of depths (5, 43, and 85 m) to investigate signal and noise variations. Wind speed was measured with an anemometer. Event-triggered and time-triggered data streams were recorded on a RefTek 72-02 data acquisition system located at the site. Our data show little cultural noise and a strong correlation between wind speed and seismic background noise. The minimum wind speed at which the seismic background noise appears to be influenced varies with depth: 3 m/sec at the surface, 3.5 m/sec at 43 m in depth, and 4 m/sec at 85 m in depth. For wind speed below 3 to 4 m/sec, we observe omni-directional background noise that is coherent at frequencies below 15 Hz. This coherence is destroyed when wind speeds exceed 3 to 4 m/sec. We use a test event (Md ∼ 1.6) and superimposed noise to investigate signal-to-noise ratio (SNR) improvement with sensor depth. For the low Q valley fill of the Datil borehole (DBH) site, we have found that SNR can be improved by as much as 20 to 40 dB between 23 and 55 Hz and 10 to 20 dB between 10 and 20 Hz, by deploying at a 43-m depth rather than at the surface. At the surface, there is little signal above noise in the 23- to 55-Hz frequency band for wind speeds greater than 8 m/sec. Thus, high-frequency signal information that is lost at the surface can be recorded by deploying at the relatively shallow depth of 40 m. Because we observe only minor further reductions in seismic background noise (SBN) at deeper depths, 40 m is likely to be a reasonable deployment depth for other high-frequency-monitoring sites in similar environmental and geologic conditions.

Babbling brook to thunderous torrent: Using sound to monitor river stage

“…Wind can be the most intrusive sound on a recording and, if the wind is constantly louder than the river, blowing out the microphone, then it is not possible to extract a river component. In a remote location away from cultural noise sources, wind has also been recorded on seismic monitors at the surface, contaminating the recorded signal at speeds of only 6.5 mph (Withers et al., 1996). Ronan et al.…”

“…Wind can be the most intrusive sound on a recording and, if the wind is constantly louder than the river, blowing out the microphone, then it is not possible to extract a river component. In a remote location away from cultural noise sources, wind has also been recorded on seismic monitors at the surface, contaminating the recorded signal at speeds of only 6.5 mph (Withers et al, 1996). Ronan et al (2017) found wind to be the most probable source of high power, low spectral coherence noise on infrasound recordings, while Anthony et al (2018) suggested that without wind filters, and if deployed close to a river, infrasound acoustic signals were likely unsuitable for smaller river systems.…”

Urbanization effect on Hyderabad seismic station

“…Ambient seismic noise is broadly divided into two categories, microseisms (infra low frequency waves) (Longuet-Higgins 1950 ; Oliver and Ewing 1957 ; Stutzmann et al 2000 ) and earth’s hum (infra gravity waves) (Rhie and Romanowicz 2004 ; Nishida et al 2007 ; Webb 2007 ). Cultural noise, wind and infrasound pressure variations together are generally the dominant source of noise in the frequency band 1–20 Hz (e.g., Withers et al 1996 ; Le Pichon 2004 ; McNamara and Buland 2004 ; Marzorati and Bindi 2006 ; Burtin et al . 2008 ) which is also the band where most of microearthquakes at local and regional distance appear.…”

Low coherency of wind induced seismic noise: Implications for gravitational wave detection

“…Sensitivity of seismic measurements to wind has been extensively studied. References [11,12] demonstrated the coherent signature of wind induced seismic noise, while references [13,14] reported low coherency of this type of seismic noise though with limited and sparse spatial measurements. By comparing wind speed and direction from two anemometers, [14] showed that wind is incoherent on the 100 m length scale.…”

“…The turbulent interaction of wind with surface objects that results in the more challenging low coherent wind induced seismic noise is the subject of this paper. We use higher sensor density than [13,14] and measure coherence lengths to analyze the coherency of wind induced seismic noise. We use the ability to resolve background coherent seismic noise to highlight the effect.…”