Design of Infrasonic Arrays
Abstract
The Infrasound Experts Group of the Geneva Conference on Disarmament Ad Hoc Committee on a Nuclear Test Ban has recommended an infrasound array design consisting of four elements, with three elements forming an equilateral triangle and the fourth at the center of the triangle. The Experts recommended that the sides of the triangle be in the range 1 to 3 km. In this report, in an attempt to place constraints on the array aperture, we evaluate the beamforming azimuthal estimation error of such arrays and compare it to historical data on observed azimuth residuals. The analysis for perfectly correlated signals shows that for beam signal to noise >> 1, the rms error is proportional to wavelength and inversely proportional to array aperture, beam amplitude signal to noise, and the square root of the time-bandwidth product. For beam signal to noise << 1 the relations are the same except that the rms error is inversely proportional to the square of the amplitude signal to noise. In this report we use coherence values calculated from data from a large atmospheric nuclear test to parameterize the variance of the velocity and azimuth of infrasonic waves, and use these parameters to estimate the loss of signal coherence between two sensors as a function of signal period, distance between the sensors, and angle between the wavefront and the vector between the two sensors. We conclude that the loss of signal at 1 Hz for the 1 km array is 1.4 dB, probably acceptable, but for the 2 km array it is 3.8 dB, probably unacceptable if the array is to be used to monitor atmospheric events at regional distances. The addition of the 0.2 km aperture 3-element central array to the 2 km array overcomes this difficulty. These conclusions are preliminary because the underlying data is available only for much larger periods and inter-sensor distances. Analysis of additional signal data at more appropriate periods and spacings is critically important.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 07, 1997
- Accession Number
- ADA344356
Entities
People
- Robert R. Blandford