Infrasound array IS39 is located on the island of Babeldaob in the Republic of Palau. Palau is located southwest of Guam, east of the Philippines, and north of Papau New Guinea (Figure 1). The archipelago comprises several hundred volcanic islands, Babeldaob being the largest, and a few coral atolls and extends about 400 miles in a northeast-southwest direction.
Figure 1. Location maps. Top: Location of the Republic of Palau. Bottom: Location of IS39 array.
Few of the islands are inhabited and less than 20,000 people are lucky enough to call Palau home. The local people have a very "laid back" way of life, enjoy fishing in the warm ocean waters, and can be seen constantly chewing betel nuts. Tourists from around the world come to Palau for the incredible scuba diving and tasty fruit bats (Figure 2). Tropical jungle forest covers much of the islands and the average rainfall is in upwards of 150" per year. Temperature varies little, with the average temperature near 82, and the humidity is always high.
Figure 2. Yum! After a hard days work in the jungle, there's nothing Milton likes better than fresh fruit bat soup.
IS39 is a seven-element array deployed in a triangular geometry with four elements in central cluster (Fig. 4). The baseline of the array is approximately 2 km. The infrasound data is collected at 20 hz, while the meteorologic data is recorded at 1 hz.
Figure 4. Array layout. Blue dots indicate the 7 elements, while the purple dots show the central and repeater power/communication sites.
Each element consists of one MB-2000 pressure sensor, a Geotech DR-24 digitizer, power regulation and surge suppression electronics, and a GPS timing system. These items are housed in a fiberglass enclosure supported and protected by a heavy-duty aluminum framework. Figure 5 shows a typical enclosure, which is elevated above ground to prevent flooding. The enclosure is designed to be corrosion and UV resistant and strong enough to withstand falling trees. The electronics within the enclosure are shown in Figure 6.
Figure 5. Typical element enclosure.
Figure 6. Electronics inside enclosure.
Wind speed, wind direction, and temperature are also recorded at H1, the central site (Figure 7).
Figure 7. Meteorologic instruments. The three-pronged wind sensor records the wind speed and direction. The temperature sensor is in the lower left.
The winds near the array are relatively light, as the array is located in dense tropical jungle and surrounded by valleys and cliffs. In fact, the wind speed rarely goes above 1 m/s. In order to reduce the pressure changes associated with wind, an 18m wind-noise reducing pipe array constructed from PVC components is deployed at each element as well (Figure 8).
Cultural noise near the array is minor due to its semi-remote location. Because of the low amounts of cultural noise and wind at IS39, the background noise are very low. This creates a good environment for infrasound recording. Signals from distant locations, such as the December 26 Sumatra Earthquake/Tsunami and recent Anatahan volcano eruptions, have been recorded, as well as many local explosions and storms.
Figure 8. Wind-noise reducing pipe array quadrants. Pipe arrays consist of four quadrants, each with 24 inlet ports, as shown above. The pipes are supported with footings made from ¾" PVC tees and short sections of ¾" pipe. Screened inlet ports are constructed from PVC plastic to ensure corrosion resistance, and are located on the edges of the pipe array. Each of the quadrants has a central secondary manifold, which connects to another section of PVC pipe that in turn connects to the primary manifold. A pipe from the primary manifold enters the enclosure and is connected to the sensor.
Power for the elements, meteorological system, and communications comes from a photovoltaic system. The central power and communications site (Figure 9) consists of twelve 12 VDC, 120-Watt solar panels wired into a 4-in-series / 4-in-parallel set configuration to achieve four 48 VDC circuits. This site powers elements H1, H5, H6, and H7.
Figure 9. Central power and commuications site.
The repeater site (Figure 10) has twelve of the 120 Watt solar panels wired to produce three 48 VDC circuits and powers site H2. The power and communications sites at H3 and H4 each have two 120 Watt solar panels wired in parallel to produce 12 VDC. Each solar panel is pointed south and is slightly angled to receive maximum sunlight. Twelve 12V batteries at both the central and repeater power sites are used to provide storage of power at 48 VDC (3 banks of 4 batteries wired in series). At H3, and H4, 4 batteries are hooked up in parallel to produce 12 VDC.
Figure 10. Repeater power and electronics site.
Data collected at each element is transmitted through fiber optic lines to its respective power and communications site. From here the data is transmitted via radio telemetry to the Palau Community College Research/Demonstration (PCC R/D) station in the state of Ngeremlengui, which acts as the data hub (Fig. 11). The data is then transmitted via satellite to Vienna , Austria to be analyzed by the CTBTO. ISLA also retains a copy of the data for quality control and analysis.
Figure 11. PCC R/D station. Here the data is collected and then sent to the CTBTO via satellite.
Figure 12. The nearby beautiful rock islands.