Methods for monitoring hydroacoustic events using direct and reflected T waves in the Indian Ocean

Abstract

[1] The recent installation of permanent, three-element hydrophone arrays in the Indian Ocean offshore Diego Garcia and Cape Leeuwin, Australia, provides an opportunity to study hydroacoustic sources in more detail than previously possible. We developed and applied methods for coherent processing of the array data, for automated association of signals detected at more than one array, and for source location using only direct arrivals and using signals reflected from coastlines and other bathymetric features. During the 286-day study, 4725 hydroacoustic events were defined and located in the Indian and Southern oceans. Events fall into two classes: tectonic earthquakes and ice-related noise. The tectonic earthquakes consist of mid-ocean ridge, trench, and intraplate earthquakes. Mid-ocean ridge earthquakes are the most common tectonic events and often occur in clusters along transform offsets. Hydroacoustic signal levels for earthquakes in a standard catalog suggest that the hydroacoustic processing threshold for ridge events is one magnitude below the seismic network. Fewer earthquakes are observed along the Java Trench than expected because the large bathymetric relief of the source region complicates coupling between seismic and hydroacoustic signals, leading to divergent signal characteristics at different stations. We located 1843 events along the Antarctic coast resulting from various ice noises, most likely thermal fracturing and ice ridge forming events. Reflectors of signals from earthquakes are observed along coastlines, the mid-Indian Ocean and Ninety East ridges, and other bathymetric features. Reflected signals are used as synthetic stations to reduce location uncertainty and to enable event location with a single station.

Underwater acoustic records from the March 2009 eruption of Hunga Ha'apai-Hunga Tonga volcano in the Kingdom of Tonga

“…Nonetheless, these calibration results show that the method provides sufficient accuracy associate arrivals with the Hunga Ha'apai-Hunga Tonga volcanic edifice. The measured angular resolution and accuracy of the M3-array are consistent with published values for equivalent three-element arrays deployed with a similar aperture as T o f u a A r c part of the International Monitoring System (e.g., Graeber and Piserchia, 2004;Hanson and Bowman, 2006).…”

The marine soundscape of the Perth Canyon

“…Ice noise of broadband and tonal character with energy up to 100 Hz can be detected thousands of miles away at tropical latitudes (Gavrilov and Li, 2007;Tolstoy et al, 2004). Underwater volcanoes and oscillating tabular icebergs appear as harmonic tremors of up to about 50 Hz at long ranges, and earthquakes produce long-lasting impulses within the frequency band from fractions of 1 Hz to about 100 Hz (Dziak and Fox, 2002;Fox et al, 2001;Hanson and Bowman, 2006;Tolstoy et al, 2004).…”

Antarctic icebergs: A significant natural ocean sound source in the Southern Hemisphere

“…Hz [Hanson and Bowman, 2006] and Bransfield Strait 20 dB in 50-110 Hz [Matsumoto et al, 2007], where iceberg noise sources were at much closer ranges to the hydrophones. Interestingly, along the North American West Coast, Andrew et al [2011] show the same low-frequency noise seasonality with a few decibel of dynamic range and a wideband spectrum consistent with the characteristics of ice-generated noise observed at high southern latitudes [e.g., Dziak et al, 2013].…”