Ambient infrasound noise

Abstract

The ambient infrasound noise environment is characterized for 21 globally distributed infrasound arrays in the frequency band of 0.03 to 7 Hz. Power Spectral Density (PSD) is measured for one site of each array for 21 intervals at each of four times of day from January 2003 through January 2004. The ambient noise at infrasound stations is highly variable by season, time of day and station. Noise spectra for an individual station may vary by four orders of magnitude at any given frequency. Preliminary infrasound noise models are defined, which can be used as baselines for evaluating ambient noise at current and new infrasound stations. Median noise levels in the microbarom band centered on 0.2 Hz vary smoothly in an annual pattern, with most stations observing maximum noise during local winter. Noise amplitudes do not have a normal or log‐normal distribution, but rather are skewed to larger amplitudes.

The IDC Seismic, Hydroacoustic and Infrasound Global Low and High Noise Models

“…Except for a very slight deviation at the microseism peak, the seismic global low noise model returns identically the PETERSON (1993) NLNM low noise curve. The global infrasonic low noise model is found to agree with that of BOWMAN et al (2005BOWMAN et al ( , 2007 but disagrees with the revised results presented in BOWMAN et al (2009) by a factor of 2 in the calculation of the PSD. The global hydroacoustic low and high noise curves are found to be in quantitative agreement with Urick's oceanic ambient noise curves for light to heavy shipping.…”

“…9 are the global low and high infrasonic noise curves reported by BOWMAN et al (2009) Fig. 10 are consistently out in the value of the PSD's by a factor of 2, suggesting from this analysis that the original analysis of BOWMAN et al (2005BOWMAN et al ( , 2007) is correct and a systematic error was introduced in the more recent work.…”

SOGRO (Superconducting Omni-directional Gravitational Radiation Observatory)

“…6, we plot the expected strain sensitivities of SOGRO and aSOGRO, as well as those of aLIGO, AdV and eLISA. The NN due to the Rayleigh waves and infrasound waves before mitigation are also plotted [6,13]. The SOGRO detector noise is dominated by the Hz and fills the frequency gap between them.…”

“…With both waves present, the resulting mixed NN cannot be removed by using these methods. We propose to use an array of external seismometers to first remove the Rayleigh waves independently of the infrasound to one part in 10 3 , and then combine the cleaned-up h' 13 , h' 23 , h' 33 and microphone signals to remove the remaining NN due to the infrasound to one part in 10 3 . This will satisfy the rejection requirement for both types of NN for SOGRO as well as aSOGRO.…”

“…Due to the transverse nature of the GW, h' 13 In the general case where more than two waves are incident simultaneously, SOGRO must be supplemented with external seismometers and microphones and the NN must be removed by using a correlation method. Figure 7(a) shows the residual NN achieved for Rayleigh waves in the absence of infrasound waves by using h' 13 , h' 23 , h' 33 and a z (CM), plus seven seismometers with SNR of 10 3 at the radius of 5 km as the input of the Wiener filter. The NN has been removed to ~10 -3 with environmental sensors (seismometers) alone.…”