Evaluation of Infrasonic Spatial Filters

The infrasound element of the global International Monitoring System (IMS) will consist of 60 stations. Each station in the global network will consist of an array of sensors. As the local surface wind strongly influences ambient infrasonic noise levels in the band of interest to the nuclear treaty monitoring community (between 0.05 and 4 Hz), each of these sensors will include a spatial filter designed to reduce the effects of wind noise. There are, however, many different design suggestions for spatial filtering system. In order to maximize the signal-to-noise ratio of signals of interest, it is essential that the spatial noise reduction systems be optimized to provide the most significant noise power reduction over the broadest band possible. We believe that substantial progress in these areas requires the comparative testing of the major noise reducing systems exposed to the same meteorological conditions. Under our new DTRA contract we will construct and test several spatial filtering systems by simultaneous operation at the Pinon Flat Observatory (PFO) in southern California. These systems include the wind fence, microporous pipes, and a recent design in which a large number of open ports are connected to a summing manifold, and then to the microbarometer, by solid pipes. We will also compare these mechanical noise suppression schemes with a new design based on light-speed pressure averaging via optical fibres. The observatory is an ideal test site as it is subjected to winds that come from all azimuths at speeds ranging from 0 to over10 m/s.