Dynamic Multimodal Response of Bender Element Transmitter

Abstract

The response of a bender transmitter to sinusoidal excitations in air, liquids, and transparent soil is investigated using a novel laser vibrometer technique. The purpose of the study is to investigate how benders vibrate in different media so that the bender–soil interaction can be better understood. This may help in more accurate determination of the travel time of shear waves in soil and in more reliable prediction of the wave velocity. The study is important because, currently, there is no standardization of the bender element (BE) test available because the bender’s behavior is not well understood when it is embedded in soil. BE test results depend on the critical assumption that the bender transducer behaves as a cantilever beam. It is shown for the first time through experimental modal analysis that benders actually vibrate as plates and higher modes of plate vibration play a significant role in bender–soil interaction. The participation of the different modes depends on the confining stresses present in the soil sample. The hydrodynamic interactions of the bender transmitter in different liquids show that the added mass effect of the liquids impact the frequency and, more importantly, the damping of the bender vibration. The changes in vibration mode and damping with a change in confinement have practical implications because the signal-to-noise ratio of bender vibration gets altered by these changes (thereby increasing the error in the estimation of shear waves), which impacts the interpretation of the BE test results. Thus, the study provides important insights into the bender behavior in terms of its displacement, velocity, resonant frequencies, damping ratios, and mode shapes.