Design and Construction of a Specified Rough Interface for Calibrated Acoustic Measurements

Several mechanisms have been proposed to explain the observed penetration of an incident acoustic wave into sediment at angles below the compressional critical angle. A series of well-controlled laboratory measurements were made and compared with the results of a first-order perturbation theory model to evaluate the accuracy of one of these mechanisms, transmission due to interface roughness. An acrylic disc was used to mimic sediment in the laboratory measurements. Properties of the acrylic were determined in the initial measurements and then used to construct a numerical model. The model was validated by a later set of measurements and then extended for the rough interface case through the addition of first-order perturbation theory scattering terms. The extended model was used to select a roughness spectrum for use in a transmission-type scattering experiment. A realization of the roughness spectrum was machined into one face of the acrylic disc. Considerable care was required in the fabrication of the rough interface as its accuracy was essential to the success of the experiment. The results of the transmission-type scattering experiment compared well with the results of the model and clearly demonstrated subcritical scattering at the water-acrylic boundary due to interface roughness