The study of infrasound: a geophysical application in the framework of LOFAR

Infrasound was first discovered after the destructive eruption of the Krakatoa in Indonesia (1883). Low frequency acoustic waves appeared to have traveled around the globe up to four times. These infrasonic waves were detected world-wide on traditional barographs as small air pressure fluctuations, with respect to the ambient pressure. Due to its low frequency content, infrasound hardly experiences attenuation in the atmosphere. Therefore, it can travel over large distances and reach thermopsheric altitudes of over 100 km. Infrasound was used in the nuclear testing era to measure the occurrence of atmospheric nuclear tests and to estimate the yield. It was also in this period that the capacity of infrasound as passive atmospheric probe started to be recognized. This interest diminished as nuclear tests were confined to the subsurface under the Limited Test Ban Treaty (1963). Recently, the study of infrasound is experiencing a renaissance since it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty, that opened for signing in 1996. Science currently concentrates on source identification and passive remote sensing of the upper atmosphere.
The Low Frequency Array (LOFAR) initiative aims at developing an infrastructure for radio-astronomy, geophysics and high-precision agriculture, i.e., a multi sensor network. Most deployments will be in the northern part of the Netherlands where the geophysical network will consist of seismological and infrasound sensors. Both the astrophysical and geophysical application aim at the detection of a wavefront using a spatially distributed network. The wavefront contains valuable information of the propagation path of the wave, which can be used to derive properties from (unknown) sources and physical processes. Astrophysics aims at the detection of electromagnetic waves, while for the geophysical applications the waves consist of mechanical vibrations in the earth and atmosphere. In astrophysics, the propagation path is through a three-dimensional universe, which is inflating with time but otherwise uniform. In infrasound, the propagation is through a layered medium, which can often be approximated to be two-dimensional, but the layered structure itself is time dependent. Due to the layered structure, refraction plays a significant role in the propagation of waves. Refraction of waves allows for a convenient determination of the structure of the medium, and is particular helpful for source identification.
Here, we present the scientific background and goals regarding the infrasonic network which consists of a 80 element high density array and a 30 element microbarometer array with an aperture of 100 km.