Modelling and validation of defects on infrasound wind-noise-reduction pipe systems.

Infrasound signals are detectable from many different sources, such as earthquakes and man-made explosions. Wind-generated turbulent noise can mask incoming infrasound signals; however, pipe-array wind-noise-reduction systems (WNRSs) have been designed to reduce the level of noise in the observed pressure time series. Given that the arrival times of the signals need to be well-known to calculate the source back azimuth and trace velocity, the response of the WNRS must be known in magnitude and phase. Previous work has been performed to optimize these systems and effectively model them. The goal of this research is to determine the effects of different defects which may occur during normal operation in typical field-experiment conditions. The models were extended to include the effects of defective systems, such as blockages or leaks. It was found that these models could effectively recreate the responses observed in an experimental setting, and several different defects were tested and are summarized in this paper.

Wind imaging using simultaneous fringe sampling with field-widened Michelson interferometers.

The first, to our knowledge, successful laboratory implementation of an approach to image winds using simultaneous (as opposed to sequential) fringe imaging of suitable isolated spectral emission lines is described. Achieving this in practice has been a long-standing goal for wind imaging using airglow. It avoids the aliasing effects of source irradiance variations that are possible with sequential fringe sampling techniques. Simultaneous fringe imaging is accomplished using a field-widened Michelson interferometer by depositing phase steps on four quadrants of one of the mirrors and designing an optical system so that four images of the scene of interest, each at a different phase, are simultaneously produced. In this paper, the instrument characteristics, its characterization, and the analysis algorithms necessary for use of the technique for this type of interferometer are described for the first time, to the best of our knowledge. The large throughput associated with field-widened Michelson interferometers is sufficient for the spatial resolutions and temporal cadences necessary for ground based imaging of gravity waves in wind and irradiance to be achieved. The practical demonstration of this technique also validates its use for proposed monolithic satellite instruments for wind measurements using airglow on the Earth and Mars.