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Long-Range Detection of Hydroacoustic Signals from Large Icebergs in the Ross Sea, Antarctica

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Long-Range Detection of Hydroacoustic Signals from Large Icebergs in the Ross Sea, Antarctica Earth and Planetary Science Letters 203 (2002) 519^534 www.elsevier.com/locate/epsl Long-range detection of hydroacoustic signals from large icebergs in the Ross Sea, Antarctica Jacques Talandier a, Olivier Hyvernaud b, Emile A.Okal c;Ã, Pierre-Franck Piserchia a a De¤partement Analyse et Surveillance de l’Environnement, Commissariat a' l’Energie Atomique, B.P. 12, F-91680 Bruye'res-le-Cha“tel, France b Laboratoire de De¤tection et Ge¤ophysique, Commissariat a' l’Energie Atomique, B.P. 640, F-98713 Papeete, Tahiti, French Polynesia c Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA Abstract Hydroacoustic signals detected in late 2000 by seismic stations in Polynesia are shown to originate from huge icebergs which at the time were drifting in the Ross Sea after calving off the Ross Ice Shelf.The signals present a broad variety of spectral characteristics, most of them featuring prominent eigenfrequencies in the 4^7 Hz range, often complemented by overtones.Most epicenters, obtained by combining observations of distant hydroacoustic and regional seismic records, follow the spatio^temporal evolution of the drift of iceberg B-15B.Most of the signals are generated during a 36-day time window when it is speculated that B-15B collided with smaller icebergs or was scraping the ocean floor on the shallow continental shelf.We speculate on the possible physical nature of the resonator generating the signals, which could correspond to an elastic mode of the iceberg, or to the oscillation of fluid-filled cracks in the ice. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: acoustical methods; seismic waves; icebergs 1. Introduction their sources remains speculative.However, their locations, obtained from a combination of distant We report in this paper the detection by the T phases and of seismic waves recorded at land- Polynesian Seismic Network (Re¤seau Sismique based Antarctic stations, correlate perfectly with Polyne¤sien; hereafter RSP) of hydroacoustic sig- the positions of the bergs, as tracked by satellite nals (T phases) originating from huge icebergs imagery, thus clearly associating the acoustic which were drifting in the Ross Sea in Late sources with mechanical processes inside the ice- 2000, after calving o¡ the Ross Ice Shelf around bergs.In the context of the monitoring of the May 2000.The signals o¡er a broad variety of Comprehensive Nuclear-Test Ban Treaty (CTBT), spectral characteristics and the exact nature of these events provide new insight into the nature of sources contributing energy into the hydroacous- tic spectrum.They further illustrate the powerful * Corresponding author.Tel.: +1-847-491-3238; synergy achieved by combining datasets of a dif- Fax: +1-847-491-8060. ferent nature, i.e. regional seismic waves and T E-mail address: [email protected] (E.A. Okal). phases recorded at island stations. 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0012-821X(02)00867-1 EPSL 6386 25-9-02 Cyaan Magenta Geel Zwart 520 J. Talandier et al. / Earth and Planetary Science Letters 203 (2002) 519^534 The RSP has been described in many previous does £uctuate with time, a property already ob- publications [1].The operation, at its sites, of re- served during the Hollister swarm [4], with some cording channels equipped with ¢lters boosting of the Ross events featuring more substantial var- magni¢cation in the 2^10 Hz range [2] helped in- iations of frequency with time (e.g. Event 7, see troduce the concept of the T-phase stations later below, Section 4), which give the spectrograms a mandated by the CTBT [3].The detection capa- contorted, ‘snake-shaped’ aspect.Finally, several bilities of the RSP were highlighted during the sequences end with singular signals exhibiting an Hollister swarm of 1991^1993, leading Talandier increase of frequency with time (suggestive of the and Okal [4] to the identi¢cation of a major vol- shrinking of the resonator), which can be slow canic edi¢ce linking the Paci¢c^Antarctic Ridge and gradual (Event 4) or fast and abrupt (Event to the Louisville Ridge, later discovered and sur- 3, Fig.1B ).Event 13 ( Fig.1D ) terminates through veyed by Ge¤li et al. [5], during a cruise of the N/O a complex evolution of prominent frequencies L’Atalante. with time, featuring several episodes of frequency decrease, a property termed ‘gliding’ and identi- ¢ed during episodes of volcanic tremor [7], which 2. Detection in general terms would suggest a geometrical ex- pansion of the resonator [8]. 2.1. General characteristics of the signals 2.2. Detection of T phases at other sites Starting in August 2000, the RSP stations at Tubuai (TBI), Vaihoa (VAH), Taravao (TVO), A systematic e¡ort was made to complement and Rikitea (RKT) detected irregular activity the RSP dataset with records at other Paci¢c seis- originating from the Southern Ocean.We report mic stations (Fig.2 ).When available, the IRIS here on a group of 13 sequences (hereafter the station at Rarotonga (RAR) provided increased ‘Ross Events’) with most of the activity concen- azimuthal aperture; it was, however, inoperative trated in November and December 2000.Their from 04 to 18 December 2000.Unfortunately, no principal properties are listed in Table 1, and other seismic station could contribute T-phase sig- Fig.1A^D shows representative spectrograms of nals, and this for a variety of reasons, which we signals recorded at VAH, a station located on the deem important to discuss, as they give insight southern shore of the atoll of Rangiroa.In such into the performance of seismic stations as hydro- conditions, the acoustic-to-seismic conversion is acoustic detectors, notably in the context of the particularly e⁄cient [6], and given the short monitoring of the CTBT.These include blockage post-conversion path (not exceeding 100 m), the (by the large Campbell Plateau and New Zealand) seismic signal at the T-phase station is essentially at Noumea (NOUC), by the Samoa Islands at equivalent to the acoustic one inside the SOFAR, Johnston (JOHN), and by Rangiroa and the Tua- within the frequency window of interest motu Plateau at the ocean-bottom observatory (2 9 f 9 16 Hz).The examples shown document H2O, where neither a seismic nor a pressure sig- a remarkable variability among events.In most nal could be detected; unfavorable station loca- instances, the energy is concentrated at one or tions involving long post-conversion seismic paths several prominent frequencies, which suggests at Tasmania University (TAU), Kipapa (KIP) that its source must involve the oscillation of and Christmas (XMAS); a probable combination some kind of resonator, a model also supported of those two e¡ects at Van Inlet (VIB), blocked by the often long duration of the signals, e.g. by the Tuamotu Plateau east of VAH (Fig.2B ); Event 4, which lasted for 3 h.However, some and a high level of background noise at the small sequences feature signi¢cantly broader spectra island of Pitcairn (PTCN).Finally, no signal was (e.g. Event 12), and some, like Event 2 (Fig. identi¢ed at Easter Island (RPN), where T-phase 1A), last no more than 2 min.In addition, the detection has been previously recognized as irreg- frequency of the largely monochromatic signals ular [9^11].In this respect, and despite the gener- EPSL 6386 25-9-02 Cyaan Magenta Geel Zwart J. Talandier et al. / Earth and Planetary Science Letters 203 (2002) 519^534 521 Table 1 Principal characteristics of the 13 events studied Number Date and time Epicenter Duration Peak-to-peak Remarks velocity at VAH D M (J) (‡S) (‡E) (min) (Wm/s) 1 15 AUG (228) 78.21 3168.61 1.3 1.23 Edge of Ross Ice Shelf. No data at RAR, RKT. 22:28:18.6 Monochromatic signal (3.5 Hz) with overtones; sharp impulsive beginning. 2 08 NOV (313) 72.10 170.16 2 2.18 Borchgrevink Land. Relatively monochromatic; 22:18:31.3 frequency £uctuates; no overtones. 3 12 NOV (317) 75.76 3175.75 8 0.62 Central Ross Sea. Generally weak at Antarctic 01:13:26.6 stations.Narrow spectrum centered around 4 Hz. Strong ¢nal pu¡ with frequency increasing with time. 4 12 NOV (317) 75.8 3175.8 3 h 0.24 Central Ross Sea. Only traces in Polynesia 06:00^09:00 (except VAH); no signal at SBA and Erebus. Long signal, generally monochromatic (2.5^4 Hz), with frequency £uctuations but few overtones. 5 14 NOV (319) 75.92 3175.60 3 0.81 Central Ross Sea. No data: Erebus; no signal: 01:01:56.6 SBA.Relatively broad spectrum on acoustic records.Dominant frequency 2.5Hz at VNDA. 6 19 NOV (324) 75.8 3175.8 10 0.42 No signal: RSP except VAH. No data: Erebus, W02:30 SBA.Broad spectrum (2^16 Hz).Assumed in same area as Events 3^5, based on times at VNDA and VAH. 7 21 NOV (326) 75.8 3175.8 10 0.24 No signal: Polynesia except VAH. No data: 15:22 Erebus, SBA.Fluctuation of dominant f, leading to ‘snake-shaped’ spectrogram.Assumed at same location as Events 4^6, based on VNDA and VAH times. 8 22 NOV (327) 75.85 3176.28 10 0.34 Central Ross Sea. No data: Erebus, SBA. Same 21:32:13.0 general characteristics as Event 7. 9 05 DEC (340) 75.08 3177.75 8 0.30 Central Ross Sea. No data: Erebus; no signal: 03:21:19.8 SBA.Broad spectrum (2^10 Hz), with dominant f = 2.9 Hz.
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