Infrasound Signals Observed at I53US and I55US John V. Olson, Charles R. Wilson, C. A Szuberla and D. Osborne Infrasound Group, University of Alaska Fairbanks Presented at the Infrasound Technology Workshop De Bilt, The NetherlandsOctober 2002 This presentation does not necessarily reflect the policies or views of the United States Government.

October 2002Infrasound Technology Workshop2 I53US: Fairbanks, Alaska In May 1999 a four-element, CTBT prototype array was installed in Fairbanks near the Geophysical Institute on the campus of the University of Alaska. The eight-element I53US array is currently in place in the same wooded location near the Geophysical Institute. The data contain a wide variety of both man-made and natural signals.

October 2002Infrasound Technology Workshop3 University of Alaska Infrasound Sites

October 2002Infrasound Technology Workshop4 Fairbanks DOE Array Geometry and Impulse Response The UAF CTBT prototype test array is comprised of four microphones in a centered triangle geometry. Data are sampled at 100 Hz. The impulse response shows the possibility of spatial aliasing at frequencies above 1 Hz.

October 2002Infrasound Technology Workshop5 Fairbanks DOE Array A prototype CTBT array, funded by DOE, was established on the UAF campus in May 1999 and has operated continuously since that time. The array has enabled various studies of microphone and pipe arrays to be undertaken.

October 2002Infrasound Technology Workshop6 I53US CTBT Array Geometry and Impulse Response The UAF CTBT installations are constructed using 8 sensors in the form of a pentagram surrounding a triangle. This provides broad-band coverage of the k- spectrum. Data are sampled at 20 Hz.

October 2002Infrasound Technology Workshop7 I53US Sensor Vault

October 2002Infrasound Technology Workshop8 I55US: Windless Bight, Antarctica I55US, the CTBT infrasound site, is located on the Ross ice shelf approximately 30 km from McMurdo base.

October 2002Infrasound Technology Workshop9 CTBT Array Geometry and Impulse Response The UAF CTBT installations are constructed using 8 sensors in the form of a pentagram surrounding a triangle. This provides broad-band coverage of the k- spectrum. Data are taken at 20 Hz.

October 2002Infrasound Technology Workshop10 Signals with Impulsive Waveforms Man-made –Aircraft takeoffs, overflights, landings (props, jets, helicopters) –Military cannons, howitzers –The crash of a military aircraft –Bomb explosions –Rocket Launches Natural –Lightening –Auroral infrasound –Bolides –Volcanoes –Earthquakes (seismic and acoustic signals)

October 2002Infrasound Technology Workshop11 Helicopter over flight A common occurrence each day is the over flight of aircraft of all types. Shown at the left is a spectrogram from one channel of a helicopter over flight. The spectrum shows a series of lines consistent with the Fourier analysis of a pulse-train. It also shows the classic Doppler shift in frequency as the helicopter passed over the array.

October 2002Infrasound Technology Workshop12 Army Howitzers On July 4, 2002 the US Army helped celebrate by firing a series of howitzer rounds, one for each state in the Union. The howitzers were located in a park in central Fairbanks approximately 5km from the GI array.

October 2002Infrasound Technology Workshop13 The Crash of a Jet Aircraft in Alaska On July 25, 2001 a British Jaguar jet crashed near Eagle, AK some 90 miles from Fairbanks. A signal was received at the UAF array at a time consistent with acoustic propagation speeds. The bearing of the arrival derived from the waveform delays agreed well with the location of the crash. Estimates of the explosive force were consistent with the aircraft size and fuel load.

October 2002Infrasound Technology Workshop14 USAF Bomb Explosions On March 27, 2001 the GI received phone calls from citizens in Delta Junction, AK (~90 miles from UAF) concerning a possible series of earthquakes. Signals were found in both the GI seismic network and in the Infrasound database showing three clear explosions. Later reports indicated that these were from USAF bombing tests near Delta Junction.

October 2002Infrasound Technology Workshop15 Rocket Launch from Poker Flat Rocket Range launch time: ~09:39 UT 1 st stage burnout: 6.4s 2 nd stage ignition: 16.0s 2 nd stage burnout: 19.5s 3 rd stage ignition: 25.0s 3 rd stage burnout: 57.4s 4 th stage ignition: 76.0s 4 th stage burnout: 94.6s first arrival: To=09:40:32 (ignition pulse) 2 nd arrival: To+16 (2 nd stage ignition) 3 rd arrival: To+21 (reflection?) 4 th arrival: To+40 (etc)

October 2002Infrasound Technology Workshop16 Bolide signal observed at Fairbanks 04/23/01

October 2002Infrasound Technology Workshop17 Tentative Epicenter from Azimuths and Travel Times: 28 o N, 132 o W Tentative Origin Time: 06:00 UT April 23, 2001 Bolide Event

October 2002Infrasound Technology Workshop18 Thunderclaps

October 2002Infrasound Technology Workshop19 Volcanic signal from Erebus Mt. Erebus, located approximately 30 km from the infrasound site, is an active volcano producing occasional small eruptions.

October 2002Infrasound Technology Workshop20 I55US: Vee Cliffs Ice Avalanche Mt. Terror

October 2002Infrasound Technology Workshop21 October 23, 2002 M6.2 Earthquake (central Alaska) I53US Array UAF Array Maximum peak-to-peak amplitude ~ 6.8 Pa equivalent

October 2002Infrasound Technology Workshop22 Signals with Continuous Waveforms Microbaroms (associated with marine storms) Mountain associated waves Volcanic eruptions

October 2002Infrasound Technology Workshop23 Microbaroms Associated with Marine Storms observed in Alaska microbaroms are narrowband waves with a 5 second period, observed world-wide and produced by marine storms. can be the dominant signal in the winter infrasound data Thought to be produced by atmospheric interaction with wind- driven standing water waves

October 2002Infrasound Technology Workshop24 Microbarom Signals, April 6, 2001 microbaroms are narrowband waves with a 5 second period, observed world-wide and produced by marine storms. can be the dominant signal in the winter infrasound data Thought to be produced by atmospheric interaction with wind- driven standing water waves

October 2002Infrasound Technology Workshop25 Microbaroms at I55US Windless Bight

October 2002Infrasound Technology Workshop26 Summary We are currently compiling an event catalog from the three data sets. At Fairbanks we detect over 30 coherent events per day in the frequency band above 0.5 Hz. The microbarom band (0.1 – 0.3 Hz) is always active. At Windless Bight, we see relatively low amplitude microbarom signals. Mt. Erebus produces pulses at a regular rate. Other coherent signals are relatively rare. We have not yet identified long period waves (Mountain Associated Waves, Auroral Infrasound) in the Fairbanks data sets even though such signals are known to exist.