Acoustic Tomography Applied to Water Flow in Unsaturated Soils

The water content of unsaturated soils influences soil properties like shear strength, elasticity, and the velocity of acoustic waves. They are also suited to investigate the spatial–temporal variations of soil water. In this study we present noninvasive two‐dimensional tomography based on acoustic pulse transmission with a spatial resolution of 60 by 60 mm. Ten acoustic transducers were placed in a horizontal plane along the wall of a soil column of an undisturbed loess soil. Acoustic pulses were emitted into the soil and monitored after traveling through the soil column. Two infiltration experiments followed by water redistribution were performed while acoustic signals were monitored at 60‐s intervals. The water content was measured with a time domain reflectometer (TDR). Travel times of the acoustic signals were converted to acoustic velocity distributions using a tomography algorithm. The acoustic velocities were transformed to water contents, resulting in 21 time series. Some of the measurements indicated fast (i.e., preferential) flow paths, whose positions varied between the two infiltration experiments.