Development of a two-frequency technique for gas-bubbles sizing in marine sediment.
University of Southampton, Institute of Sound and Vibration Research,
The aim of this research is to develop an acoustic method to estimate the size distribution of spherical or near-spherical gas bubbles in intertidal sediments. For this purpose, a new inversion method was developed which employs two frequencies. This method is based on monitoring the scattered spectral components, which are generated from nonlinear bubble pulsations under acoustic excitation. Independent transmission acoustic measurements along a hydrophone array were undertaken and inverted following the well-established method of inferring bubble populations from transmission measurements. The first, i.e. the two-frequency, inversion method is able to detect and measure only ‘acoustically spherical’ bubbles (i.e. bubbles, which can be modelled as monopole sources), whereas the latter inversion method (which will be called the transmission method) assumes that all attenuation originates from ‘acoustically spherical’ bubbles. Hence, comparison of the two bubble population estimates can be employed to assess the acoustical effects of the nonspherical gas occlusions. In situ data were taken at two different intertidal sites of the South coast of England. The first one is a closed muddy region with high organic content and low sand content. The second one has a dynamic environment and the sediment is muddy, with higher sand content and low organic content.
Pre-requisite for the application of the new bubble sizing method is knowledge of the nonlinear bubble response in sediments under acoustic excitation. Current models for bubbles in sediments are limited to the linear regime. A new model suitable for both linear and nonlinear pulsations of bubbles has been developed. The model requires as input parameters properties of the gas-free water saturated sediment. These unknown parameters were estimated using both laboratory measurements and values found in the literature for similar types of marine sediment.
Water tank tests were carried out to test the performance of the devices and to crosscheck the method against the two-frequency method of inversion of attenuation data, i.e. the transmission method. The experimental conditions in water guaranteed the near-sphericity of the bubbles present. As such, the transmission data provide unambiguous information on the bubble population of the bubbly water. Therefore, good agreement between the two methods proves the ability of the two-frequency method to predict the bubble size distributions. Both set-ups were deployed in the field and the data were inverted in the same manner as the water tank data. The computed bubble distributions showed rather good agreement for the first site, which was a strong indication that most of the gas at the site exists in spherical form. At the second site, a poorer agreement was observed which suggests that only a proportion of the gas has spherical form. These observations are limited to sizes resonant at the frequencies used for the experimental fieldwork, i.e. small gas pockets equal to or at maximum ten times bigger than the size of the grains. The results
of this study suggest that using the two-frequency technique, estimation of the size distribution of the ‘acoustically spherical’ bubbles is possible for the gas situated a few centimetres below seafloor. However the implementation of the two-frequency technique is not sufficient to draw conclusions on the acoustical effects of the gas in non-spherical form. That is, parallel use of the transmission method is needed in order to draw conclusion on the gas phase present in the sediment.
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