Using Ultrasonic Reflection Resonance to Probe Stress Wave Velocity in Assemblies of Spherical Particles

A high-sensitivity method to measure acoustic wave speed in soils by analyzing the reflected ultrasonic signal from a resonating layered interface is proposed here. Specifically, an ultrasonic transducer which can be used to both transmit and receive signals is installed on a low-high acoustic-impedance layered structure of hard PVC and steel, which in turn is placed in contact with the soil deposit of interest. The acoustic impedance of the soil (the product of density and wave velocity) is deduced from analysis of the waves reflected back to the transducer. A system configuration design is enabled by developing an analytical model that correlates the objective wave speed with the measurable reflection coefficient spectrum. The physical viability of this testing approach is demonstrated by means of a one-dimensional compression device that probes the stress-dependence of compression wave velocity of different sizes of glass ballotini particles. Provided the ratio of the wavelength of the generated wave to the soil particle size is sufficiently large the data generated are in agreement with data obtained using conventional time-of-flight measurements. In principle, this high-sensitivity approach avoids the need for the wave to travel a long distance between multiple transmitter-receiver sensors as is typically the case in geophysical testing of soil. Therefore it is particularly suited to in-situ observation of soil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatial resolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted in soil.