Hydroacoustic signals generated by parked and drifting icebergs in the Southern Indian and Pacific Oceans

Abstract

SUMMARY We report the detection, principally by the French Polynesian seismic network, of hydroacoustic signals generated inside large icebergs, either ‘parked’ along the Wilkes coast of Antarctica in the Indian Ocean, or drifting in the Southern Pacific Ocean between latitudes of 55° and 65°S, during the years 2002–2004. The signals can be classified into two very broad families, based on the nature of their spectra. A first group features prominently monochromatic signals, whose frequency can, however, fluctuate with time during a single sequence of emission (typically lasting a few to a few tens of minutes). Such signals are generally reminiscent of those detected in 2000 in the Ross Sea and are generated principally in the Indian Ocean ‘iceberg parking lot’, between longitudes 144°E and 156°E. A new family of signals features a much broader spectrum, superimposed on a number of preferential frequencies suggesting the background activation of a number of resonators; these signals occur both in the parking lot and in the Southern Pacific. Further variations in spectra are documented inside each family. On the basis of similar in situ observations on Ross Sea icebergs under project SOUTHBERG, the first family is generally interpreted as expressing a stick‐and‐slip process during collisions between large iceberg masses. The second family of signals are observed during exceptional episodes of the otherwise silent drift of the icebergs in the deep Pacific Basin, some of which correlate with their passage over the various fronts defining the oceanographic southern convergence zone. Finally, a most recent episode of activity, generally similar to the above first family, was detected on 2004 December 3–4, at the ocean entry of the Dibble Ice Tongue, 600 km west of the parking lot along the coast of Antarctica. It is interpreted as resulting from collisions between large drifting icebergs and fragments of the ice tongue calved off during its disintegration, as documented by satellite imagery.

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

“…Consequently, other noise sources typically disrupt any seasonal pattern related to ice, and the temporal and spectral noise characteristics observed at HA03N therefore do not match those observed in open waters at EEP-NW (Figures 3b and 3c). For example, at EEP-NW sound energy was consistently higher in the 10-13 Hz range than in the 30-36 Hz range (Figure 3c), in keeping with the spectra of ice-generated noise [e.g., Dziak et al, 2013, Talandier et al, 2006. Whereas, at HA03N, prior to the arrival of C19a and B15a, noise in the 30-36 Hz range was typically higher than in the 10-13 Hz range.…”

“…Harmonics tremors are generated when the icebergs shoal or collide with other icebergs, whereas short-duration bursts, which are much more common [Talandier et al, 2006], are generally associated with iceberg breakup in the open sea and probably caused by ''edge wasting'' and ''rapid disintegration'' processes [Scambos et al, 2008]. By monitoring these sounds with a deep-water hydrophone array, iceberg locations and source levels can be estimated.…”

“…Antarctic icebergs are known to generate two types of sounds: one is a long-duration harmonic tremor and the other is a broadband burst [Talandier et al, 2006;Chapp et al, 2005;MacAyeal et al, 2008]. Harmonics tremors are generated when the icebergs shoal or collide with other icebergs, whereas short-duration bursts, which are much more common [Talandier et al, 2006], are generally associated with iceberg breakup in the open sea and probably caused by ''edge wasting'' and ''rapid disintegration'' processes [Scambos et al, 2008].…”

Deep ocean sound speed characteristics passively derived from the ambient acoustic noise field

“…The SOFAR channel is a low-velocity layer in the deep ocean, that is, the average channel axis depth is 1.5 km, which allows low-frequency sound to be detected over long ranges (Munk & Forbes 1989). The efficiency of the SOFAR channel for sound propagation has already been used in studies related to earthquakes (Evers et al 2014;De Groot-Hedlin 2005;Guilbert et al 2005), icebergs (Chapp et al 2005;Talandier et al 2006;Evers et al 2013), explosions (Munk & Forbes 1989;Prior et al 2011), marine mammals ) and underwater volcanoes . Guided wave propagation contributes to the limited acoustical attenuation by the SOFAR channel.…”

Helicopter location and tracking using seismometer recordings

“…2000; Steel, 2009), (ii) a change in acoustic velocity (De Angelis & McNutt, 2007;Benoit & McNutt, 1997) or (iii) a change in dimension of the resonating body (De Angelis & McNutt, 2007;Jousset et al, 2003). Repeating processes such as frictional faulting (Dmitrieva et al, 2013;Hotovec et al, 2013;Lipovsky & Dunham, 2015), hybrid events (Neuberg et al, 2000) or merging low frequency events (Steel, 2009) are analogous to the helicopter generated tremor and are analogous to stick-slip motion between two adjacent icebergs (Talandier et al, 2006;MacAyeal et al, 2008) in glaciology. Distinguishing if a frequency change is due to a lateral movement of the source as for helicopter generated tremor or a change in source repetition time as suggested for volcanic tremor is challenging.…”