Acoustic characterization of encapsulated microbubbles at seismic frequencies

Encapsulated microbubbles, whose diameters are on the order of microns, are widely used to provide acoustic contrast in biomedical applications. But well below the resonance frequencies of these microbubbles, any acoustic contrast is due solely to their relatively high compressibility compared to the surrounding medium. To estimate how well microbubbles may function as acoustic contrast agents in applications such as borehole logging or underground flow mapping, it must be determined how they behave both at atmospheric and down-well conditions, and how their presence affects the bulk acoustic properties of the surrounding medium, most crucially its specific acoustic impedance. Resonance tube experiments were performed on several varieties of acoustic contrast agents to determine their compressibility as a function of pressure and temperature, and the results are used to estimate the effect on sound propagation when they are introduced into rock formations.