The Exloo Infrasound Array (EXL) was constructed during the year 2005 following a collaboration between LOFAR (see: www.lofar.nl and www.lofar.org) and the Seismology Division of KNMI. Infrasound are air pressure fluctuations with frequencies below the human hearing threshold of 20 Hz. The lower frequency cut-off of infrasound is limited by the thickness of the atmosphere. Infrasound is inaudible sound traveling with the sound speed of approximately 330 m/s. In general, infrasound is generated when a large volume of air is displaced. Examples of sources are: volcanoes, explosions, meteors, sea waves, thunder storms, nuclear tests, supersonic flights, ..... Also infrasound from unknown origin is occasionally observed. Monitoring infrasound enables one to distinguish between sources in the solid earth, earthquakes, and atmospheric sources both causing vibrations. Furhermore, the measurement of infrasound is one of the verification techniques for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Within LOFAR several research objectives have been defined for the study of infrasound.

Array configuration and instrumentation

Infrasound is measured with an array of microbarometers. Using an array enables one to retrieve the direction of arrival of the pressure wave and increase the signal power by summing the different records. The infrasound array is located at LOFAR's Initial Test Station (ITS) near Exloo. The array consists of 6 KNMI-microbarometers configured for an optimal array response, see Figure 1. The microbarometer is a highly sensitive and high frequent barometer capable of measuring infrasound in the range of 500 seconds to 20 Hz with amplitudes of less than 0.01 Pa. Refer to the station table for the exact coordinates of the microbarometer locations given in meters in the Rijksdriehoek coordinate system.

To avoid temperature fluctuations, the instrument is installed in a pvc tube below the earth's surface, see Figure 2. An analog noise reducer is attached to the microbarometer to sample the atmosphere and reduce the negative effect of wind. More pictures of the array installation can be found here.

The data acquisition system consists of a pc running a Linux kernel patched for realtime processing (see: RTAI). Within the pc an analog-digital convertor is mounted being a 16 bits National Instruments PCI-card running at 40 Hz. The acquisition is controlled by drivers and programs from the Control and Measurement Device Interface (Comedi) project. Accurate timing is achieved through the usage of GPS time stamps provided by a Meinberg PCI-card and antenna.

Measurements and data processing

Figure 3 shows one of first coherent infrasound waves that traveled over EXL. The event took place around 22h00m13s UTC on September 07, 2005. Note the travel time differences over the array within the time slice of 12 seconds. Two clear arrivals of infrasound are visible on all traces with 4 seconds seperation.

The continuous recordings of the six microbarometers are automatically processed to detect coherent signals. The derived, so-called, Fisher ratio gives the statistical likehood of a coherent signal having traveled over the array and is used as trigger. From a triggered event the travel time differences over the array are used to characterize the source. Sources are characterized in the time and frequency domain in terms of back azimuth (bearing with respect to the North) and apparent sound speed. The apparent sound speed is the propagation speed of the infrasonic wave as measured by the microbarometers, this is the horizontal fraction of the true sound speed. Therefore, the apparent sound speed is a measure of the angle of incidence of the wave. The best beam is constructed by summing the time aligned the signals and shows an increase in signal-to-noise ratio. Figure 4 shows the proccesing results of the event.

The lower frame of Figure 4 gives the Fisher ratio as function of time also its linear relation to the signal-to-noise power ratio is shown on the right hand side of the lower frame. The middle two frames give the resolved apparent sound velocity and back azimuth, being 384.4 m/s and 154.4 degrees (see: Fig 5) at maximum coherency. The top frame shows the best beam for this events.

A clear infrasonic signature was detected by EXL, although, this event is of unknown origin. More events of known and unknown sources will be detected during the operation of the array. It is one of the goals of the project to identify sources of infrasound.

Data availability

Infrasound data of EXL are available through a web interface in near realtime. Here, the continuous recordings of EXL can be viewed and charateristics of the detected events are shown as explained in uitleg (in Dutch). The data pages are updated every half hour. The most recent recordings can be viewed here. Upon request, raw data can be made available.

Läslo Evers September 2005