The exploitation of acoustic-to-seismic coupling for the determination of soil properties
Laboratory measurements of three predicted wave types (two compressional or P-waves and one shear S-wave) have been made in artificial soils. The Type-I P and S-wave are predicted to be most sensitive to the macroscopic elastic properties of the frame, whilst the Type II P-wave is predicted to be most sensitive to the hydrodynamic material properties. A loudspeaker source has been used for the preferential excitation of the Type II P-wave whilst preferential excitement of the Type-I P-wave has been accomplished using a mechanical shaker. Probe microphone measurements of the Type-II wave allowed the flow resistivity and tortuosity of the material to be determined using a rigid frame model, whilst deduction of elastic moduli has been made from signals received at buried geophones. It has been shown that microphone signals include Type-I P-wave energy in a high flow resistivity soil. Acoustically deduced soil properties are consistent with mechanically derived values. A systematic investigation of outdoor measurements of acoustic-to-seismic coupling ratio has been made. From the measurements, it has been found that the geophone-ground coupling has a great effect upon the measured coupling ratio. In-situ calibration methods have been developed to overcome this problem. whilst the novel use of a Laser Doppler Vibrometer has been proposed to provide a completely non-invasive method of measuring motion in soils. The measured coupling ratio has been compared with theoretical predictions, using a modified Bio-Sto11 formulation. The model can be used to predict values of flow resistivity, porosity, bulk and shear moduli and layer depths. Reasonable agreement has been obtained between the model and data. Procedures that exploit acoustic-to-seismic coupling data and models to determine soil properties have been developed and used to measure the soil properties of friable agricultural soils where more standard investigation techniques have proved unsuitable.