Comparative Evaluation of Selected Infrasound Noise Reduction Methods

The objective of this project is to identify and characterize potential low-cost alternatives to the conventional pipe array wind-noise-reduction method. Initial efforts have focused upon the use of a porous medium as a wind noise filter. A simple theoretical model of wind-generated noise in a rigid, porous medium has been developed. This model predicts that the attenuation, A(w), of the noise in this type of medium is related to the observation depth, d, by an equation of the form; A(w)=exp(-alpha(w)d) where alpha(w)=Re(w(2)/c(2)+i(sigmaw/P(o))1/2 and, c, is the convection velocity of the wind-generated pressure field, sigma, is the effective flow resistance of the medium, and P(o) is the static atmospheric pressure at the elevation of the observation point. The results of a small-scale field trial are described in the main body of this report. They validate the predicted exponential attentuation of the wind noise. They also indicated that a finite, porous body would act as a half-space for wavelengths that are less than 3-4 times its horizontal dimensions. The theoretical model of Attenborough et al (1986) has been adopted to predict the attenuation of airborne sound in a porous medium. This model predicts that the attenuation, A(i), of the infrasound signal in this type of medium is related to the observation depth, d, by an equation of the form; A(i)=exp(-alpha(i)d). Sabatier et al (1993) have shown that in the bandwidth of interest to the infrasound community alpha(i) approx. Re(i(sigmaw/P(o)). The combined wind noise and infrasound signal models therefore predict that infrasound signal-to-noise ratios will exponentially increase as a function of the observation depth in a frequency range defined by the constraint that; f is less than sigmac(2)/4(pi)P(0) where f is the frequency. The preliminary results of a field experiment to test this constraint and a discussion of its practical implications are summarized in the main body of this report. Presented at the Seismic Research Review: Worldwide Monitoring of Nuclear Explosions (23rd) held in Jackson Hole, WY on 2-5 October 2001. Published in the Proceedings of the Seismic Research Review: Worldwide Monitoring of Nuclear Explosions (23rd), p131-139, 2001. The original document contains color images.