Global acoustic propagation and the link between hydroacoustics and infrasound

The ocean is nearly transparent for acoustic propagation at low frequencies, leading to the detection of signals (seismic events, volcanoes and man-made signals) at distances as large as the ocean basin. When the ocean depth approaches the acoustic wavelength (~ 1500m for 1 Hz energy) the potential for conversion of energy from ocean acoustic propagation to atmospheric infrasonic propagation exists. This was observed in an earthquake off the coast of Australia (Evers et al. GRL 2014) energy was observed as seismic waves, hydro-acoustic waves and then infrasound waves, the latter two on the International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization. In this paper, global scale hydro-acoustic propagation modeling from earthquakes and volcanoes will be presented with an emphasis on the locations where conversion to atmospheric propagation will is possible. Recent observations at the Aloha Cables Observatory (Butler JASA 2016) indicate the possibility of a loss of energy from 1-5 Hz as hydro-acoustic signals propagate over the Oahu-Kauai ridge (the Kauai “Keyhole”). Hydro-acoustic and infrasound modeling of this event will be presented.