5. Array configuration and response
While measuring infrasound one has to deal with poor
signal-to-noise ratios. Arrays are deployed to enable the use of beamforming techniques
which increase the signal-to-noise ratio and can resolve source characteristics.
Array configuration on the basis of array response calculations are explained in this
chapter.
Figure 5.1 shows the layout of 16 KNMI microbarometer, creating The Deelen Infrasound Array
(DIA), installed at Airfore base Deelen.
Figure 5.1: The layout of 16 microbarometers of DIA in RD coordinates
The aperture of DIA is approximately 1.5 km with an inter station distance of 200 to 300
meters. Array response calculations form the basis for this irregular and non symmetric
layout. The solution of the wave equation is described by the elementary plane wave, being:
By letting the elementary vertical incident on the array elements, differential travel times
become zero, i.e. all instruments record the wave at the same time. The response of a set of
N instruments is given by:
The response in the infrasonic domain can be modeled by adequately chosen the slowness (p)
space. In figure 5.2 the normalized response of the 16 microbarometers of DIA is given for 0.15 Hz
(vertical incident) plane wave.
Figure 5.2: The response of 16 microbarometer layout of DIA to a 0.15 Hz plane wave
The shown response has several important characteristics:
- The circular shape of the main lob. This means the atmosphere above the array is
uniformly sampled; the array shows no directionality. The array will be equally sensitive
to all infrasonic energy independent of its incoming angle.
- Side lobs are of low amplitude and located at considerable distance of the main lob.
This implies a unique identification of infrasonic sources; the energy is mainly concentrated
in the main lob.
- The main lob has a sharp and peaked form. This will result in a high resolution array.
The resolved source characteristics, like apparent velocity and back azimuth, will have small
errors.
Bringing together seismology and atmospheric sciences - Läslo Evers
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