In this study, a new nonlinear acoustic technique, the phase shift method, is developed to measure the hysteretic nonlinearity parameter for a field soil. The technique is based on measuring the variation of phase difference between two transducers, i.e. the phase shift, induced by changing sound level. The hysteretic nonlinear parameter can be extracted from the measured phase shift as a function of sound level, or dynamic strain. With this technique, a long-term soil survey is conducted to study the variations of soil properties due to climate and seasonal changes. The hysteretic nonlinear parameter and sound speed of the soil as functions of temperature, moisture, surface tension, rain precipitation, and time are studied.
Understanding the variability of the grounds acoustic properties will lead to a reduction in false alarms associated with acoustic landmine detection. Experimental measurements of the acoustic-to-seismic transfer functions performed at a US Army eastern temperate site reveal frequency modulation scales in the acoustic-to-seismic transfer function. These modulations have different spatial dependencies along and across the mine lanes. It was hypothesized that these are due to spatial dependencies of the acoustic parameters in the ground layers. It also was speculated that downward gradients in these parameters are due to additional soil strain produced by the wheels of vehicles repeatedly moving down the lane. The measured transfer functions for a few sites were analyzed. It is shown that an elastic layered model of the ground with downward gradients of sound speed in the ground layers successfully models the features observed in the experimental data. Direct time-of-flight measurements of sound speeds in and out of the wheeled tracks confirm the results obtained from the acoustic-to-seismic transfer function analysis.