Measurement of attenuation and speed of sound in soils

The potential application of this work is the detection and imaging of buried objects using acoustic methodology. To image buried artifacts, it is vital to know speed and attenuation of sound in the particular soil being examined because they vary in different soil types and at different moisture contents. To that end, our research involved six soils representing a range of properties expected to influence acoustic response. Clay ranged from 2 to 38%, silt from 1 to 82%, sand from 2 to 97%, and organic matter from 0.1 to 11.7%. Signals from an acoustic source were passed through soil samples and detected by an acoustically coupled hydrophone. From a total of 231 evaluations, we determined the acoustic attenuation coefficient and the propagation speed of sound in the soil samples as a function of four levels of soil moisture and two levels of compaction. Attenuation coefficients determined over frequencies of 2 to 6 kHz ranged from 0.12 to 0.96 dB cm^-1 kHz^-1. Lower attenuation tended to be in loose dry samples. Correlation coefficients were 0.35 (P = 0.01) and 0.31 (P = 0.03) between attenuation and soil water content and soil bulk density, respectively. Propagation speeds ranged from 86 to 260 m s^-1. The correlation coefficient with speed was -0.28 (P = 0.05) for soil water content and -0.42 (P = 0.002) for total porosity. Given the acoustic properties, it is theoretically possible to detect an object down to ∼40 cm below the soil surface.