The CTBT/IMS infrasonic arrays at both I53US in Fairbanks and I55US in Antarctica have 8 microphones that are laid out in a pentagon-triangle pattern as described below.  Model 5 Chaparral microphones, each with its own Geotech digitizer, are used at each sensor site.  Commercial power is available at each microphone site in the I53US array.  The Chaparral microphone, the Geotech digitizer, a Dc-to-Dc power panel and a Free-Wave radio site are all housed at each in a 40 x 40 inch square insulated box made of ¼ inch thick steel to protect the equipment as pictured in Figure 1. There is a 15 foot high steel mast attached to each instrument box with a GPS unit and the telemetry antenna mounted at the top. The four pipes radiating from the box at ground level are used to connect the microphone to a noise-reducing pipe array.

Wind noise-reduction at each microphone site is accomplished using the standard CTBT design of a set of four rosettes of 24 radial pipes vented at the end of each pipe as shown in Figure 2. Each site’s wind-reducing system of pipes is 18 meters in width with a total of 96 low impedance inlets. There are 24 inlets installed on each rosette of pipes. The design of the total noise reducing pipe array used at each microphone is shown in Figure 3. The low impedance vent at the end of each radial pipe is shown in Figure 4.

The Model 5 Chaparral microphones are built with a special intake manifold on top that will mate with a portable field calibration device. The manifold also provides four import fittings for connection to the noise-reduction pipe array. In Figure 5 below the Chaparral microphone’s   white manifold is shown in the foreground of the picture with a port hole on the top for use in the insertion of the portable calibrator. The calibrator itself is shown in Figure 5 in the background of the picture mounted on another Chaparral microphone.  At both I55US and I53US all the Chaparral microphones were calibrated in-situ after installation at the microphone sites. The calibrations were done at the three frequencies of 1.0 Hz, 0.10 Hz, and 0.02 Hz using a sinusoidal pressure input of 0.1 Pa peak-to-peak.

In Figure 6 there is a diagram of the geometry of the eight-sensor I53US infrasonic array. The array pattern is an outer pentagon of 5 microphones and an   inner centered-triangle of three microphones. The 5 microphones at the apexes of the pentagon are each approximately 1000 meters from the center of the array while the inner 3 microphones are each approximately 100 meters from the center.

The center of the array is about 1500 meters to the NW of the G.I. building. Local topography to some extent prevented the establishment of a perfect pentagon array pattern with an equilateral triangle pattern at the center. The northernmost microphone I53H1 is taken as the reference site with coordinates (0 east, 0 north). The East and North coordinates of the 8 microphones are given in kilometers in the table below.

In  wave number space  the output Y(k)  of a microphone  array  to an infrasonic signal X(k)  impinging upon the array is given by the convolution of X(k) with the array impulse response H(k). The array impulse response H(k) is the array’s response to a unit impulse input at t = 0. In order to make an unambiguous detection of an incoming infrasonic signal impinging upon a microphone array it is necessary to design the array pattern so that it’s impulse response has most of the power in it’s central maximum at K = 0. If there is a lot of power distributed among many other maxima in the H(k) impulse response diagram in K space then  spatial aliasing is possible and may affect the detection capability of the array. The pentagon-triangle array design for I53US and I55US, as shown in Figure 6, has an excellent impulse function as can be seen in Figure 7. This array design has nearly circular symmetry so that the uncertainties in the determination of trace-velocity and azimuth of arrival of an incoming infrasonic signal are nearly independent of azimuth. In Figure 7 the principal maxima in the middle of the diagram, that can be seen at coordinates X scale units = 50 and Y scale units = 50, is at the origin in K space at Kx = 0 and Ky =0.