Short Note Hydrophone Calibration at Very Low Frequencies

International audienceWe propose a new method to study the response of a hydrophone at very low frequencies. In our method, the hydrophone is placed in a calibration chamber filled with water and, by instantaneously changing the water height, an abrupt pressure increase of about 1000 Pa is produced. The pressure variation mathematically corresponds to an input signal close to a step function. The response is recorded after filtering and digitizing so that we obtain the response of the complete system. We also report on the development of an automatic method to determine the number of poles and zeros and their values that describe the observed response. We apply our method to the RAFOS II hydrophone, used on the Mobile Earthquake Recorder in Marine Areas by Independent Divers (MERMAID) floats. As an illustration of the method, an instrumental response in terms of poles and zeros is used to correct seis-mograms from the 7 April 2014 (M w 4.8) Barcelonnette earthquake, recorded by three MERMAIDs deployed in the Mediterranean Sea, and to express the observed signals in pascals

Long term evolution and internal architecture of a high-energy banner ridge from seismic survey of Banc du Four (Western Brittany, France)

International audienceThe recent completion of a coupled seismic and swath bathymetric survey, conducted across the sand ridge system of the Banc du Four located on the Atlantic continental shelf of Brittany (Mer d’Iroise, France), provided new data for the study of the long term evolution of deep tidal sand ridges. Five seismic units are distinguished within the ridge, separated by pronounced major bounding surfaces. The basal unit is interpreted to be shoreface deposits forming the core of the ridge. It is overlaid by a succession of marine sand dunes fields forming the upper units. Sandwave climbing, which combines progradation and accretion, is the major process controlling the growth of the ridge. The elevation of the preserved dune foresets reaches values of about 20–30 m within the ridge. The foresets indicate a combination of giant dunes characterized by numerous steep (up to 20°) clinoforms corresponding to a high-energy depositional environment. Moreover, the presence of scour pits linked to the 3D geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines. All of the radiocarbon ages of the biogenic surficial deposits of the Banc du Four range from 10,036 to 2748 cal years B.P. and suggest the Banc du Four has grown during the last sea-level rise. The apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non-conservation of recent deposits. In contrast, the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge. Moreover, the long-term evolution of the ridge appears strongly controlled by the morphology of the igneous basement. The multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy, consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement.Therefore, the Banc du Four should be thought of as a representative example of a large-scale high-energy banner bank

Initial results from a hydroacoustic network to monitor submarine lava flows near Mayotte Island

In 2019, a new underwater volcano was discovered at 3500 m below sea level (b.s.l.), 50 km east of Mayotte Island in the northern part of the Mozambique Channel. In January 2021, the submarine eruption was still going on and the volcanic activity, along with the intense seismicity that accompanies this crisis, was monitored by the recently created REVOSIMA (MAyotte VOlcano and Seismic Monitoring) network. In this framework, four hydrophones were moored in the SOFAR channel in October 2020. Surrounding the volcano, they monitor sounds generated by the volcanic activity and the lava flows. The first year of hydroacoustic data evidenced many earthquakes, underwater landslides, large marine mammal calls, along with anthropogenic noise. Of particular interest are impulsive signals that we relate to steam bursts during lava flow emplacement. A preliminary analysis of these impulsive signals (ten days in a year, and only one day in full detail) reveals that lava emplacement was active when our monitoring started, but faded out during the first year of the experiment. A systematic and robust detection of these specific signals would hence contribute to monitor active submarine eruptions in the absence of seafloor deep-tow imaging or swath-bathymetry surveys of the active area

Geoacoustic inversion with two source-receiver arrays in shallow water

International audienceA geoacoustic inversion scheme based on a double beamforming algorithm in shallow water is proposed and tested. Double beamforming allows identification of multi-reverberated eigenrays propagating between two vertical transducer arrays according to their emission and reception angles and arrival times. Analysis of eigenray intensities yields the bottom reflection coefficient as a function of angle of incidence. By fitting the experimental reflection coefficient with a theoretical prediction, values of the acoustic parameters of the waveguide bottom can be extracted. The procedure was initially tested in a small-scale tank experiment for a waveguide with a Plexiglas bottom. Inversion results for the speed of shear waves in Plexiglas are in good agreement with the table values. A similar analysis was applied to data collected during an at-sea experiment in shallow coastal waters of the Mediterranean. Bottom reflection coefficient was fitted with the theory in which bottom sediments are modeled as a multi-layered system. Retrieved bottom parameters are in quantitative agreement with those determined from a prior inversion scheme performed in the same area. The present study confirms the interest in processing source-receiver array data through the double beamforming algorithm, and indicates the potential for application of eigenray intensity analysis to geoacoustic inversion problems

Using Teleseismic P-Wave Arrivals to Calibrate the Clock Drift of Autonomous Underwater Hydrophones

Networks of autonomous underwater hydrophones (AUHs) are successfully employed for monitoring the low‐level seismicity of mid‐oceanic ridges by detecting hydroacoustic phases known as T waves. For a precise localization of a seismic event from T‐wave arrival times, all AUHs must be synchronized. To this effect, at the beginning of the experiment, all instrument clocks are set to GPS time, which serves as a common reference. However, during the experiment, the instrument clock often deviates from GPS time, and, because the amount of deviation differs from one instrument to another, the synchronization of the AUHs deteriorates, as the experiment progresses in time. Just after the instrument recovery, the time difference (called “skew”) between the instrument and the GPS clocks is measured. Assuming that the skew varies linearly with time, the correction of a time series for the clock drift is a straightforward procedure. When the final skew cannot be determined, correcting for the clock drift is not possible, and any event localization becomes problematic. In this article, we demonstrate that the clock‐drift rate (assumed to be time‐independent) can be successfully estimated from arrival times of teleseismic P waves, commonly recorded by AUHs. Using a ray‐tracing code, and accounting for the uncertainties in event hypocenter locations, origin times, and the Earth seismic‐velocity model, confidence intervals of the estimated drift rates are deduced. The validity of the approach is tested on data from two AUHs with known clock drifts. Our results show that a reliable estimation is possible for skews as small as 4 s per two years (corresponding to a drift rate of about 5.5  ms⋅day−1⁠). This method can also be applied to correct data of other recording instruments subject to internal‐clock drift, such as ocean‐bottom seismometers, when the skew is unknown

Automatic recognition of T and teleseismic P waves by statistical analysis of their spectra: An application to continuous records of moored hydrophones

International audienceA network of moored hydrophones is an effective way of monitoring seismicity of oceanic ridges since it allows detection and localization of underwater events by recording generated T waves. The high cost of ship time necessitates long periods (normally a year) of autonomous functioning of the hydrophones, which results in very large data sets. The preliminary but indispensable part of the data analysis consists of identifying all T wave signals. This process is extremely time consuming if it is done by a human operator who visually examines the entire database. We propose a new method for automatic signal discrimination based on the Gradient Boosted Decision Trees technique that uses the distribution of signal spectral power among different frequency bands as the discriminating characteristic. We have applied this method to automatically identify the types of acoustic signals in data collected by two moored hydrophones in the North Atlantic. We show that the method is capable of efficiently resolving the signals of seismic origin with a small percentage of wrong identifications and missed events: 1.2% and 0.5% for T waves and 14.5% and 2.8% for teleseismic P waves, respectively. In addition, good identification rates for signals of other types (iceberg and ship generated) are obtained. Our results indicate that the method can be successfully applied to automate the analysis of other (not necessarily acoustic) databases provided that enough information is available to describe statistical properties of the signals to be identified

The August 2010 earthquake swarm at North FAMOUS–FAMOUS segments, Mid-Atlantic Ridge: geophysical evidence of dike intrusion

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Observer des ondes sismiques en milieu marin

Il y a une dizaine d'années, l'un de nous (Gusst Nolet) travaillant avec Frederik Simons à l'Université de Princeton, avait observé une onde sismique P d'un séisme lointain de magnitude 6. Bien qu'il n'est pas rare de voir un séisme de cette ampleur, dont environ 200 se produisent chaque année, leur méthode d'observation était unique : aidé par des collègues de la Scripps Institution of Oceanography, ils avaient utilisé un flotteur SOLO, à 700m sous la surface de la mer près de San Diego, Californie, qu'ils avaient équipé, avec pas mal de bricolage improvisé, d'un hydrophone. L'importance de cette observation historique ne doit pas être sous-estimée, car elle aurait le mérite d'ouvrir les océans à des observations sismiques à grande échelle

Observer des ondes sismiques en milieu marin

Il y a une dizaine d'années, l'un de nous (Gusst Nolet) travaillant avec Frederik Simons à l'Université de Princeton, avait observé une onde sismique P d'un séisme lointain de magnitude 6. Bien qu'il n'est pas rare de voir un séisme de cette ampleur, dont environ 200 se produisent chaque année, leur méthode d'observation était unique : aidé par des collègues de la Scripps Institution of Oceanography, ils avaient utilisé un flotteur SOLO, à 700m sous la surface de la mer près de San Diego, Californie, qu'ils avaient équipé, avec pas mal de bricolage improvisé, d'un hydrophone. L'importance de cette observation historique ne doit pas être sous-estimée, car elle aurait le mérite d'ouvrir les océans à des observations sismiques à grande échelle