These infrasonic waves were detected world-wide on traditional barographs as small air pressure fluctuations. Due to its low-frequency content, infrasound hardly experiences attenuation in the atmosphere and travels to thermospheric altitudes of over 100 km. In addition to seismology, infrasound was used to measure the occurrence of atmospheric nuclear tests and to estimate their yield (2). It was also in this period that the possibility to use infrasound as passive atmospheric probe started to be recognized( 3). This interest diminished after nuclear tests were confined to the subsurface under the Limited Test Ban Treaty (1963).
Recently, the study of infrasound is experiencing a renaissance as it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), that opened for signing in 1996 (4). Infrasound science currently concentrates on source identification and passive remote sensing of the upper atmosphere.
The construction of the largest radio-telescope in the world in the northern part of the Netherlands and neighbouring countries, the Low Frequency Array (LOFAR), opened the possibility to co-locate geophysical sensors and realize an efficient multi-sensor network. The KNMI, Delft University of Technology and TNO, all partners in LOFAR, make use of the advanced LOFAR infrastructure to build-up an infrasound and seismological research network. This network consists of a temporary 80 element high density array, a permanent 30 element microbarometer array with an aperture of 100 km and, at the same locations, a 20 to 30 element seismological component. Here, we present the scientific background, goals and first results.
Sound waves below 20 Hz are inaudible for the human ear. These sound waves are called infrasound. The lower limit of infrasound is controlled by the thickness of the atmosphere or of an atmospheric layer. For the troposphere, the acoustic cut-off period is roughly 5 minutes. For longer period waves gravity acts as restoring force, instead of the molecular relaxation for sound waves, and hence these are called gravity waves. These gravity waves propagate with typical wind speed velocities of 5 to 10 m/s. Infrasonic waves travel with the sound speed which is 340 m/s for air of 20°C. The amplitudes of infrasonic waves are small with respect to the ambient pressure and vary between milli-pascals (Pa) to tens of Pa.
The propagation of infrasound is controlled by the effective sound speed, which is a function of the temperature and wind along the source-receiver trajectory (5). Infrasonic waves will be refracted if vertical gradients in the effective sound speed exist. Waves will be bended back towards the earth's surface in case these gradients are strong enough. There are three regions in the atmosphere where strong wind and/or temperature gradients (may) exist that lead to turning infrasonic waves. (1) In the troposphere, in case of a temperature inversion near the surface or a strong jet stream around the tropopause at 10 km altitude. (2) In the stratosphere, due to the combined effect of a temperature increase with increasing altitude due to presence of ozone and strong seasonal winds around the stratopause at 50 km altitude. (3) In the thermosphere, where from 100 km and upwards the temperature strongly increases with increasing altitude due to direct influence of solar radiation on the molecules. As an example, the temperature and wind for a winter and summer atmosphere in De Bilt (the Netherlands) are shown in Figure 1.
A large amount of infrasound is continuously being recorded from a variety of man-made and natural sources. Anthropogenic sources include: explosions, nuclear tests, mining, military activities and supersonic flights. The latter is the cause of frequent reports in the Netherlands of felt tremors in buildings, similar to the sensation of an earthquake. Natural sources comprise: avalanches, oceanic waves, severe weather, sprites (lightning from cloud top to ionosphere), earthquakes, meteors, lightning, volcanoes and aurora. The measurement of infrasound is affected by noise due to wind and turbulence in the boundary layer. Therefore, infrasound is measured with arrays to increase the signal-to-noise ratio (SNR) through signal summation. Arrays are also used to estimate the direction of arrival of a wave and its propagation velocity. Typical sizes, i.e., apertures, are in the order of 100 to 1000 meters. Additional noise reduction at each array element is achieved by a wind barrier, a porous hose or pipe array with discrete inlets6). The recorded signals are thus a function of the state of the boundary layer and the upper atmosphere, which changes with time and geographical location. To unravel this complex picture, is the major challenge in source identification (7).
The surface based microbarometers can also be used as a passive probe for the upper atmosphere (higher than 30 km) with the large amount of sources continuously present. Actual recordings of the basic properties, like wind and temperature, of the upper atmosphere are sparse. Meteorological balloons reach an altitude of roughly 35 km but lack spatial and temporal coverage. Rocket sondes can reach the upper atmosphere but also lack coverage. Satellites have global coverage but have a limited vertical resolution and are difficult to validate for stratospheric altitudes. Therefore, most information currently depends on numerical weather prediction model characteristics. Infrasound can validate such models, and even information on a finer temporal and spatial scale is expected to be retrieved. Such information is very welcome for future atmospheric research. The troposphere and stratosphere have long been considered two isolated layers, split by an impermeable tropopause. The influence of the stratosphere on our daily weather and climate has recently been firmly established, showing that processes in the upper atmosphere do couple to the troposphere8).
The geophysical application within LOFAR, i.e., GEO-LOFAR, consists of seismological and infrasound equipment. The astronomical application will be realized on antenna fields where infrastructure, like power and high-speed Internet, is being established. GEO-LOFAR will make use of this infrastructure. The acquired data will be gathered at Groningen University from where they are distributed to the GEO-LOFAR partners, which are TU Delft, TNO and KNMI. The infrasound contribution consists of a High Density Infrasound Array (HDIA) and a Large Aperture Infrasound Array (LAIA).
A six element infrasound array (EXL) was realized at LOFAR's Initial Test Station near Exloo. EXL has an aperture of 250 meter and contributed to the discovery of exceptional fast infrasonic phases observed after the explosion of an oil-depot in the UK (10,11) (see Figure 4). The array also showed its value in the detection of lightning, by combining infrasound and electro-magnetic measurements within LOFAR (12,13).
In order to also sense gravity waves, the KNMI-microbarometer (KNMI-mb) has been adapted to periods of 1000 seconds, as lower cut-off. The earlier version of the KNMI-mb had a cut-off at 500 seconds and was specially designed to measure acoustic waves. The influence of temperature on the differential KNMI-mb is of main concern when lowering the frequency response. Temperature stability is ensured by properly insulating the instrument's fault and by its subsurface mounting. A detailed study of the amplitude and frequency response has been carried out (16).