Abstract
Developments in acoustic theory of soil have allowed acoustic methods to be used to investi-gate soil physical properties. There have been few studies on the measurement of air conductivity. In ad-dition, some research has suggested that the charac-teristic scale of the air phase structure of soil could be evaluated using acoustic methods; however, there is little understanding of its value and the relationship to the soil water condition. Therefore, our study objec-tives were to develop an acoustic method for measur-ing air conductivity and provide the characteristic scale of the air phase structure using acoustic mea-surements. Combining acoustic theory with the rela-tionship between volumetric air content and tortuosity, we determined the formulae to calculate volumetric air content and air conductivity from acoustic impedance. The Tottori dune sand was used as sample. The re-sults showed that the estimate of the volumetric air content obtained by the acoustic method was less than 15% of that obtained by the traditional gravimetric method over the range of 14‒34%. The estimates of conductivity were within 3-fold that obtained by tradi-tional air conductivity measurement. The results also showed that, in some cases, a sand sample can be re-garded as an assembled mass of air, which has a char-acteristic scale of about 0.6‒3.7cm and roughly propor-tional to the volumetric air content.
