The purpose of the International Monitoring System (IMS) Infrasound network is to detect and provide information on low frequency sound waves generated by a nuclear explosion detonated in the atmosphere or at shallow depths in the ocean. An explosion will produce sound waves (atmospheric pressure variations) over a wide range of frequencies. Higher frequencies are rapidly attenuated or absorbed with distance; however very low frequency energy is not so readily absorbed and can be detectable at ranges of thousands of kilometres. The frequency range of signals IMS infrasound stations are designed to detect is 0.02 - 4 Hz (cycles per second).
An infrasound station consists of a number of very sensitive microbarometers. To increase the sensitivity of the station each microbarometer is connected to several radiating pipes with each pipe having small holes (ports) along its length. This arrangement of pipes and inlet ports, commonly called a space filter, averages and largely cancels out pressure variations localised over an area smaller than the filter - such as wind generated turbulence. The maximum dimension of the space filter is limited by the frequency range of the signals of interest and the speed of sound in air. For a normal IMS station the space filter will be limited to a maximum diameter of about 18 metres. If the diameter is larger than this the higher frequency signals of interest would start to cancel out because there will be a significant portion of a signal wavelength being sampled by the space filter at any one instant.
A typical IMS infrasound station will comprise eight microbarometer sensors and associated space filters, four located at the extremities of a lopsided quadrangle with sides of one to two kilometres and an additional four elements clustered at one of the corners. The exact layout of the station is not critical and individual elements are sited to take advantage of local conditions - e.g. topography, isolated patches of forest etc. The reason for having multiple sensor elements is first to further increase the signal/noise ratio by using each as an element of a larger array, second to record signals over a wide frequency band to cover the signals of interest and thirdly, to be able to determine the direction from which an incoming infrasound signal arrives. The signal direction can be determined from the difference in time at which the signal arrives at each of the elements.
Topic contact: firstname.lastname@example.org Last updated: July 19, 2011