A model for nonlinear gas bubble pulsation in marine sediments is presented. This model is then linearized to determine the resonance frequency and the damping terms for linear radial oscillations. The linear model is then used to predict the effects that such bubble pulsations will have on the sound speed and attenuation of acoustic waves propagating in gassy marine sediment. The results are compared for monodisperse populations against the predictions of a model of Anderson and Hampton and, furthermore, the additional abilities of the new model are discussed. These features include the removal of the sign ambiguities in the expressions, the straightforward implementation for acoustic propagation through polydisperse bubble populations, the capability to estimate bubble size distributions through a full acoustic inversion, and the capability to predict nonlinear effects.
The need to predict acoustic propagation through marine sediments that contain gas bubbles has become increasingly important for civil engineering and climate studies. There are relatively few in situ acoustic wave propagation studies of muddy intertidal sediments, in which bubbles of biogenic gas (generally methane, a potent greenhouse gas) are commonly found. We used a single experimental rig to conduct two in situ intertidal acoustical experiments to improve understanding of acoustic remote sensing of gassy sediments, eventually including gas bubble size distributions. In the first experiment, we measured sediment sound speed and attenuation between four aligned hydrophones for a quasi-plane wave propagating along the array. The second experiment involved a focused insonified sediment volume created by two transducers emitting coincident sound beams at different frequencies that generated bubblemediated acoustic signals at combination frequencies. The results from sediment core analyses, and comparison of in situ acoustic velocity and attenuation values with those of water-saturated sediments, together provide ample evidence for the presence of in situ gas bubbles in the insonified volumes of sediments. These datasets are suitable for linear and non-linear inversion studies that estimate in situ greenhouse gas bubble populations, needed for future acoustical remote sensing applications.
This paper presents the design, manufacturing and testing of a Dual Accelerometer Vector Sensor (DAVS). The device was built within the activities of the WiMUST project, supported under the Horizon 2020 Framework Programme, which aims to improve the efficiency of the methodologies used to perform geophysical acoustic surveys at sea by the use of Autonomous Underwater Vehicles (AUVs). The DAVS has the potential to contribute to this aim in various ways, for example, owing to its spatial filtering capability, it may reduce the amount of post processing by discriminating the bottom from the surface reflections. Additionally, its compact size allows easier integration with AUVs and hence facilitates the vehicle manoeuvrability compared to the classical towed arrays. The present paper is focused on results related to acoustic wave azimuth estimation as an example of its spatial filtering capabilities. The DAVS device consists of two tri-axial accelerometers and one hydrophone moulded in one unit. Sensitivity and directionality of these three sensors were measured in a tank, whilst the direction estimation capabilities of the accelerometers paired with the hydrophone, forming a vector sensor, were evaluated on a Medusa Class AUV, which was sailing around a deployed sound source. Results of these measurements are presented in this paper.
This paper presents the design, manufacturing and testing of a Dual Accelerometer Vector Sensor (DAVS). The device was built within the activities of the WiMust project, a EU project, supported under the Horizon 2020 Framework Programme, which aims to improve the efficiency of the methodologies used to perform geophysical acoustic surveys at sea by the use of Autonomous Underwater Vehicles (AUVs). The DAVS contributes to this aim in various ways, for example, owing to its spatial filtering capability, it can measure reflections at the desired direction therefore reducing the amount of post processing related to deghosting and multipath removal. Also its compact size allows easier integration with AUVs and hence facilitates the vehicle manoeuvrability compared to the classical towed arrays. The DAVS device consists of two tri-axial accelerometers and one hydrophone moulded in one unit. The device's directional estimation capabilities were evaluated on an AUV, which was sailing around a deployed sound source. Results of this experiment are presented in this paper.
The inversion of the acoustic properties of gassy sediments presents the optimum manner of determining the in situ distribution of sediment-based methane bubbles. An in situ device that measures both compressional wave attenuations and combination-frequency components in gassy sediment lying within 2 m of the seabed has been developed at the University of Southampton. This device was deployed at an inter-tidal site along the South coast of England. Compressional wave attenuations were measured from 10 to 100 kHz though the analysis of propagation signals transmitted from a variety of sources to a buried co-linear hydrophone array, with propagation distances spanning 0.5 to 2 m. Measured attenuations were inverted to infer in situ bubble size distributions using both established and new acoustic models for gassy sediment. The analysis and results of the combination-frequency component are described in a companion paper.
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