Volume 128, Issue 2, August 2010
Index of content:
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
A phase comparison technique for sound velocity measurement in strongly dissipative liquids under pressure128(2010); http://dx.doi.org/10.1121/1.3455857View Description Hide Description
An accurate technique for the sound velocity measurement in strongly dissipative liquids is elaborated. This technique is based upon high sensitive phase detection. Each medium, at a given temperature and pressure, is characterized by a specific phase shift due to the propagation of the ultrasonicwave within the analyzed medium. By tuning the insonation frequency of the ultrasonic signal, a succession of consecutive nulls of the output dc voltage generated by the phase detector is observed. Thus from the obtained series of frequency values, the soundvelocity is computed. Numerous organic liquids, such as alcohols and alkanes, have been used to validate this experimental procedure. As the developed method is well suited for the sound velocity measurement in strongly dissipative liquids, measurements of compressional wavevelocity in heavy oil are also carried out over the temperature range to . The experimental results agree well with those found in the literature. The accuracy of the developed method is estimated at about ±0.3%.
Fast inverse scattering solutions using the distorted Born iterative method and the multilevel fast multipole algorithm128(2010); http://dx.doi.org/10.1121/1.3458856View Description Hide Description
The distorted Born iterative method (DBIM) computes iterative solutions to nonlinear inverse scattering problems through successive linear approximations. By decomposing the scattered field into a superposition of scattering by an inhomogeneous background and by a material perturbation, large or high-contrast variations in medium properties can be imaged through iterations that are each subject to the distorted Born approximation. However, the need to repeatedly compute forward solutions still imposes a very heavy computational burden. To ameliorate this problem, the multilevel fast multipole algorithm (MLFMA) has been applied as a forward solver within the DBIM. The MLFMA computes forward solutions in linear time for volumetric scatterers. The typically regular distribution and shape of scattering elements in the inverse scattering problem allow the method to take advantage of data redundancy and reduce the computational demands of the normally expensive MLFMA setup. Additional benefits are gained by employing Kaczmarz-like iterations, where partial measurements are used to accelerate convergence. Numerical results demonstrate both the efficiency of the forward solver and the successful application of the inverse method to imaging problems with dimensions in the neighborhood of ten wavelengths.