IMPROVED MATERIALS CHARACTERIZATION BY PRESSURE-DEPENDENT ULTRASONIC-ATTENUATION IN AIR-FILLED PERMEABLE SOLIDS

Recently developed airborne ultrasonic inspection techniques can suppl ement other methods routinely used for materials characterization of p ermeable solids. In particular, the velocity and attenuation of the sl ow compressional wave transmitted through thin plates of a few millime ter thickness can be used to assess the tortuosity and dynamic permeab ility of the specimen. The main advantage of the ultrasonic method ove r conventional flow resistivity, electrical conductivity, and other me asurements is that it can be used to study the heterogeneity of the po re structure at scales comparable to the grain size. In the 100-500 kH z frequency range slow wave images can be obtained with resolution on the order of 1 mm or better. However, due to substantial viscous and s cattering losses, the sensitivity of the method is relatively low ther efore, the technique is limited to materials of at least 10% connected porosity and permeability higher than 200 mo. It is demonstrated in t his letter that varying the air pressure significantly enhances the ca pabilities of slow wave inspection. Using high-pressure air saturation significantly reduces the absorption losses so that better resolution can be achieved by increasing the frequency. Alternatively, materials of lower permeability or specimens of higher thickness can be inspect ed at the same frequency. In addition, scattering losses can be elimin ated by subtracting images taken at the same frequency but at differen t pressures. (C) 1996 American Institute of Physics.