The I55US infrasonic array is located in Antarctica on the Ross Ice Shelf in Windless Bight about 25 kilometers to the northeast of McMurdo Station. The geographic coordinates at the center of the microphone array for year 2004 are: 77.7416 degrees South latitude and 167.5820 degrees East longitude. The shelf ice area to the south of Ross Island is essentially flat because the ice is floating on the Ross Sea beneath it. The individual elevations of each of the 8 microphone sites are within one meter of the average elevation of 41 meters. The Ross Ice Shelf is about 450 meters thick under the I55US infrasonic microphone   array. The array is carried along with the movement of the ice shelf about 64.25 meters per year in a southwesterly direction. Thus it is getting closer, at a rate of about 17.6 centimeters a day, to Hut Point Peninsula and McMurdo station. There is very little rotation of the array pattern or differential motion among its 8 sensor sites. The entire array shape does not change significantly but rather moves as an undistorted shape with the general ice shelf motion. The exact coordinates of the I55US array are surveyed every year and the results are sent to CTBTO and the G.I. at UAF for use in the data analysis software.

The relatively windless area of Windless Bight to the south of Ross Island, where I55US is located, was initially found by the British Antarctic Expedition in 1912 on a winter sledging journey from Hut Point to Cape Crosier on the northeast side of Ross Island. It was so windless that they could travel by the light of a “naked candle flame” during their winter-night trek. An infrasonic array was operated at this same location from 1976 to 1965 by UAF with great success. The average RMS wind-noise level in Windless Bight for the year 1983 was found to be 0.096Pa. Katabatic wind studies, using infrared satellite imaging of the Ross Ice Shelf show that Windless Bight on the south side of Ross Island is not subjected to katabatic wind flow from either the Tran Antarctic Mountains or the vast Ross Ice Shelf to the southeast. There is radio line-of-sight communication from the I55US site to the McMurdo area for the telemetry of   the microphone data.

In January of 2001, during the end of the austral summer, the I55US array was installed during a 47 day working period that ended with the UAF team leaving on the penultimate flight out of Antarctica for that season.  The 8 microphone array was laid out on the Ross Ice Shelf surface with 5 microphones in a pentagonal pattern with each of these five microphones 1000 meters from the center of the pentagon. An inner equilateral triangle pattern of three microphones was places at the center of the array with each of these three sensors at a distance of 100 meters from the array center. The I55US microphone array has East-West symmetry about the geographic North-South line. The microphone array was installed using surface-laid wires from a centrally located hybrid diesel/photovoltaic generator to supply 65 volts dc to each individual microphone site. Successive surveys of the I55US array geometry have shown that all changes in the separation distances between any microphone pairs in the array are always negative.  The shelf ice is under compressive stress. Thus the wires laid on the surface from each microphone site to the central power supply should not be broken in successive years as they become more deeply  buried in the ice during the annual snow accumulation.

An aerial photograph of the location of the I55US array in Windless Bight is shown in Figure 1 below with the eight microphone sites indicated by the labels I55H1 through I55H8. The power supply building is at the center of the array. Mount Erebus volcano can be seen 23 kilometers to the north of the array. Small eruptions in the lava lake in the crater of Erebus occur on the average about once a week.  These produce excellent infrasonic signals at I55US in the pass band from 0.1 to 10 Hz.

Figure 2 is a radar image of Ross Island and Windless Bight that clearly shows the stream lines of the glacier ice flow down off Mt Terror on to the Ross Shelf Ice. The flow of the ice carries the I55US array toward McMurdo on Hut Point Peninsula toward the left in the image. The array moves without distortion 64.2 meters per year. 

A map is shown in Figure 3 of the locations of the microphone sites in the I55US array. 

The coordinates of the microphone sites are given in the table below in kilometers with respect to the location of microphone H1, that is the origin of the coordinate system, at zero Easting and zero Northing.

I55US microphone coordinates relative to H1 in kilometers

Sensor    H1          H2           H3           H4             H5          H6        H7           H8     

East       0.00       0.9604    0.5835   - 0.5550    - 0.9499   0.0087    0.0975     - 0.0803

North     0.00     - 0.6754   -1.7992   -1.8202    -0.6850    -0.8937   -1.0475    -1.0502

The circular symmetry of the I55US array is nearly perfect because we were able to install the sensor sites very close to the exact locations of the apexes of the pentagon and those of the centered equilateral triangle. The fact that Windless Bight is a flat plane of floating Shelf ice with no significant topographic features to obstruct the installation of the array led to an array with nearly perfect impulse response diagram for the I55US array as can be seen in Figure 4 below. The central maximum is larger by a factor of two than any other of the maxima in the wave number plane K.

The installation of the I55US array was facilitated by the ease of transport of all the equipment by truck from McMurdo station to the edge of the Ross Ice Shelf from which point it could then be hauled by snow tractor and sleds into Windless Bight as shown below in Figure 5.

At each microphone site there is an insulated yellow vault to house the microphone, the digitizer, the Free-Wave radio and the ac to dc power panel. The heat dissipated by the electronics in each vault is enough to keep the equipment within its operating specifications at the coldest temperatures experienced in Windless Bight. The equipment vaults are covered with snow to insulate them from the most severe cold.   At each site there is a 30 foot guyed steel tower for mounting the Free-Wave antenna for the telemetry of the data to McMurdo Station. The site GPS receiver unit is mounted on top of the tower. A typical microphone site, H2, with Vee Cliffs on Mount Terror in the background, is shown in Figure 6. Ice avalanches at the Vee Cliffs are a frequent source of infrasonic signals throughout the year at I55US. The yellow instrument vault at the base of the antenna tower can be seen before it was buried by drifting snow. The sixteen black noise-reducing array pipes can be seen on the surface of the snow.  

At each microphone there is a noise-reduction system that consists of 16 surface-laid pipes in a radial pattern from the microphone at its center. The pipes are made of high density polyethylene. They are  1.34 inches in inside diameter, 8.5 meters long, and are vented every meter with a small hole about 0.5 mm in diameter. The summing manifolds, connectors, and sector feel hoses add additional length so the outer diameter of the pipe ends is 18 meters. The acoustic impedance of the holes is matched to that of the pipe itself. Thus there are 96 vents in the entire pipe- array at each microphone.  The system is designed such that, during the annual servicing of the array the 8.5 meter  pipes can be pulled out from under the accumulated snow by using special pipe connectors at the end of the pipe. The noise-reduction pipe system as well as the microphone vault can be seen in Figure 6. A view into an open microphone vault is shown in Figure 7 with the portable calibrator attached to the top of the Chaparral microphone. The digitizer can be seen in the lower left on the bottom of the vault.

The hybrid power supply for I55US was custom built by Northern Power Systems in an Arctic-type self-contained trailer with sled runners on its base so that it could be towed out across the ice shelf to Windless Bight by a tractor after it was off loaded from the ship at McMurdo station. The power trailer contains 2 diesel-electric generators, a large bank of batteries, and all the electronic controls for the generators and the photovoltaic systems. The two photovoltaic panels are installed on the top of the trailer facing east and west for maximum solar illumination. In Figure 8 the power trailer is shown on location at the center of the microphone array during the installation of the two solar panels.

The digital output of all eight Chaparral Model 5 microphones as well as the meteorological data is telemetered to McMurdo Station from each microphone site. A picture of the data-receiving hub room at McMurdo station for the I55US infrasonic array is shown in Figure 9. The data flow from I55US to UAF in Fairbanks and CTBTO in Vienna began on February 1, 2001. The quality of the infrasonic data as well as that from the hybrid power supply in Windless Bight is monitored at the McMurdo CTBT infrasonic hub site.   Infrasonic and meteorological data as well as Power-Supply operational data are sent by satellite in real-time to the CTBTO in Vienna and to the Geophysical Institute in Fairbanks for analysis and operational quality-control monitoring. The I55US infrasonic station in Windless Bight was certified by the CTBTO on August 22, 2003.

We would like to acknowledge the United States Antarctic Program for providing excellent logistical and operational support during the installation and subsequent operation of the CTBT/IMS infrasonic array I55US in Windless Bight.