The Seismology Division of the Koninklijk Nederlands Meteorologisch Instituut (KNMI) operates a network of seismic and acoustic sensors in the Netherlands. Information on vibrations is provided to the Dutch society on the basis of signals detected by this network. The main causes of vibrations are earthquakes and low frequency acoustic waves, i.e., infrasound, in the atmosphere. The latter can be generated by shock waves associated with for example explosions and supersonic flying. By monitoring both the solid earth and the atmosphere, a distinction can be made between the causes of vibrations which enhances the capability to identify the sources.

Vibrations were reported by citizens of Friesland on 2010, February 03. This province has recently gained seismological attention because of its citizen who felt vibrations and associated those with subsurface salt explorations. A network of borehole seismometers is currently being realized to monitor the possible occur- rence of earthquakes. In this report, measurements from one of these borehole seismometers near Firdgum will be used (see Figure 1.1) in analyzing the possible cause of the reported vibrations on February 03. Such a borehole setup consists of four levels, separated by 30 meters, equipped with three-component geophones. Infrasound measurements near Friesland are conducted at the island of Texel with a six element microbarom- eter array (see Figure 1.1). A microbarometer is sensitive to infrasound within the frequency range of 0.002 to 20 Hz. An analog noise reducer is connected to each microbarometer to reduce the effect of wind noise on the measurements [Evers, 2008].

The aim of this report is to identify the source of the reported vibrations based on observations of atmo- spheric pressure variations. Furthermore, attention will be paid to wave propagation through a dynamic atmosphere.

Reference:
L.G. Evers, Seismo-acoustic analysis of the infrasound events on 2010, February 03, KNMI Technical Report (TR-314), 31 pages, 2010.