Characterization of the acoustic output of single marine-seismic airguns and clusters: The Svein Vaage dataset.

The acoustical output of marine-seismic airguns is determined from recordings of the sound pressure made on hydrophones suspended below a floating barge from which the airguns are also deployed. The signals from multiple types of airguns are considered and each type is operated over a range of deployment depths and chamber pressures. The acoustical output is characterized in terms of a "source waveform" with dimensions of the pressure-times-distance and in an infinite idealized medium, could be divided by the source-receiver distance to give the sound pressure at that receiver. In more realistic environments, the source waveform may be used to predict the pressure at any arbitrary receiver position simply by the application of a time-domain transfer function describing the propagation between the source and receiver. The sources are further characterized by metrics such as the peak source waveform and energy source level. These metrics are calculated in several frequency bands so that the resulting metrics can be used to characterize the acoustical output of the airguns in terms of their utility for seismic image-processing or possible effects on marine life. These characterizations provide reference data for the calibration of models that predict the airguns' acoustical output. They are validated via comparisons of the acoustic pressure measured on far-field hydrophones and predicted using the source waveforms. Comparisons are also made between empirically derived expressions relating the acoustic metrics to the chamber volume, chamber pressure, and deployment depth and similar expressions from the literature.

Bathymetric diffraction of basin-scale hydroacoustic signals.

The ocean is nearly transparent to low frequency sound permitting the observation of distant events such as earthquakes or explosions at fully basin scales. For very low frequency the ocean acts as a shallow-water waveguide and lateral variability in bathymetry can lead to out-of-plane effects. In this paper, data from the International Monitoring System of the Comprehensive Test Ban Treaty Organization (CTBTO) is used to present two cases where robustly localized seismic events in locations clearly within the two-dimensional (2-D) shadow of a continent or large island generate T-phase signals that are received on a hydro-acoustic station. A fully three- dimensional parabolic equation model is used to demonstrate that lateral variability of the bathymetry can lead to diffraction, explaining both observations. The implications of this are that the CTBTO network has greater coverage than predicted by 2-D models and that inclusion of diffraction in future processing can improve the automatic global association of hydroacoustic events.

On the coherent components of low-frequency ambient noise in the Indian Ocean.

This letter demonstrates that the dominant coherent component of low-frequency (1 Hz < f < 20 Hz) ambient noise propagating between hydrophone pairs of the same hydroacoustic station, deployed in the deep sound channel of the Indian Ocean, is directional and mainly originates from Antarctica. However, the amplitude of the peak coherent noise arrivals, obtained using a 4-month-long averaging interval, was relatively low given the small hydrophones spacing hydrophones (<2 km). Hence, extracting similar coherent arrivals between two distinct hydroacoustic stations separated instead by thousands of kilometers for noise-based acoustic thermometry purposes seems unlikely, even using a year-long averaging.

Geoacoustic inversion using multipath pulse shape.

Experimental data, measured in a shallow water region of the Mediterranean Sea, are used to show that the variation of received intensity with time is well described by existing expressions [Harrison and Nielsen, J. Acoust. Soc. Am. 121, 1362-1373 (2007)]. These expressions indicate that the effect of the sea-water sound speed profile can be neglected for times greater than the peak intensity arrival. Beyond this time, intensity is shown to decay at a rate determined by the seabed acoustic properties in a manner very similar to that for an isovelocity water column. It is shown that a method of determining seabed acoustic properties, previously restricted to isovelocity water columns [Prior and Harrison, J. Acoust. Soc. Am. 116, 1341-1344 (2004)], can consequently be used in the presence of a sound-speed profile. The method relates the decay rate of smeared multipath arrivals to the angular derivative of seabed reflection loss. Two datasets are studied and the method is used to describe average seabed properties and to detect changes in seabed type. The seabed descriptions thus derived are used to predict total received intensity as a function of source-receiver separation. Agreement between the propagation measurements and predictions is shown to be within measurement uncertainties.