Infrasound station I59US, Hawaii , is part of the global infrasound network of the International Monitoring System (IMS). Due to the station’s location leeward of the massive volcanoes and in a dense tropical forest, the Hawaii station has very low ambient noise levels and is one of the most sensitive stations of the IMS. The station consists of four Chaparral 5 microphones with a passband of 0.05-8 Hz and a dynamic range exceeding 120 dB. Three of the sensors are arranged as a triangle with a 2km baseline, with the fourth sensor near the center of the triangle. Sensor data is recorded by 24-bit digitizers and sent in real time via radio telemetry to the Infrasound Laboratory in Keahole Point, West Hawaii. IS59 (Figure 1) started operating on May 25, 2000.
Figure 1. Location of IS59 and Infrasound Laboratory (ISLA) on the island of Hawaii. H1-4 are the locations of the infrasonic microphones.
The three volcanoes of Mauna Loa, Mauna Kea, and Hualalai block the prevailing trade winds from reaching Kona, thereby substantially reducing wind noise. There is a thermally driven diurnal wind flow pattern in Kona, with cold mountain winds in the evening and sea breezes in the day, but the wind speeds associated with this flow are generally low. The IS59 array site is further protected by wind (and cultural) noise by a dense tropical forest consisting of tall (~20-30m) trees (Figure 2), tree ferns (2-3 m, Figure 3), and ground cover.
Figure 2. Access road to IS59 in forested conservation district.
Figure 3. Tree ferns and ground cover around instrument vault.
In the forest we built a central power and communication node, and four instrument vaults to house the microphones and digitizers. Although forests make for acoustically quiet sites, falling hardwood trees can hit the vaults (Figure 4) and block access roads (Figure 5). Therefore, we built our vaults from concrete and steel, and carry two chainsaws with us during maintenance visits.
Figure 4. Hardwood ohia branch in the foreground bounced off green steel vault lid in the background. The lid was slightly bent.
Figure 5. There has been a delay in today’s maintenance schedule …
Since Kona rarely gets heavy winds, dozens of trees may fall when strong storms do occur, thereby turning us into amateur lumberjacks. All in a day’s work.
Other problems introduced by trees is that they block radio transmissions and sunlight. So we had to erect a 30m mast (Figure 6) to send the station data from the field to the ISLA laboratory. We also deployed an array of solar panels at a central location (close to H1) to power all the electronics for the station (Figure 7).
Figure 6. Communication mast near the center of the array.
Figure 7. Solar panels for powering the array electronics.
Through the forest we wove the communication and power cables, which delivered power from the solar panels to the instrument vaults and returned digital data from the microphones to the radio link atop the mast.
The electronics in each vault consist of a Chaparral 5 infrasonic microphone, a digitizer with autentication/encryption, a modem for transmitting the digital data and for command and control of the digitizer, and power regulation, surge suppression, and electronic noise reduction circuits (Figure 8).
Figure 8. Typical vault contents.
At each vault we also installed a wind noise suppression filter. These wind noise filters are intended to spatially average out turbulent eddies.
Multiplexed field data is sent via spread spectrum radio telemetry to the ISLA, where it is demultiplexed, repackaged, and sent to Vienna via satellite (Figure 9).
Figure 9. Equipment for transmitting digital station data.
Data is also stored and archived at the ISLA for quality control as well as for R&D.