Effects of Soil Water Potential and Moisture Content on Sound Speed

To monitor and understand the temporal variations of sound speed due to the changes of soil physical properties such as soil water potential, moisture content, and soil temperature under natural conditions, a long‐term field soil survey has been conducted. In this study, transmitter and receiver acoustic probes, consisting of ten (five pairs) and five transducers respectively, were inserted into a prepared trench containing a soil‐sand mixture in an outdoor test site. Five time domain reflectometers, five tensiometers, and five thermocouples were buried during the trench‐filling at the same depths as the acoustic transducers. Measurements of sound speed, soil temperature, soil moisture content, and water potential were performed continuously, along with the measurements of surface temperature and precipitation over a period of 2 yr. Analysis of the data shows that there is a power law relationship between the sound speed and water potential. It is also found that the water potential is the dominant influence on the sound speed whereas the moisture content and temperature have relatively minor impacts. Brutsaert and Luthin's theory was employed to calculate the sound speed as a function of the effective stress. The theoretical predictions were compared with the experimental data and they are in good agreement. The study suggests that sound speed measurement could be used as a new and effective tool for water potential measurement.