Bender Elements: Performance and Signal Interpretation

TLDR

Bender elements are convenient shear wave transducers for soil cells, generating both P‑ and S‑waves with quasicircular in‑plane S‑wave directivity, and their resonant frequency depends on element geometry, anchor efficiency, and soil stiffness. The study implements experimental and analytical methods to investigate electromagnetic coupling prevention, directivity, resonant frequency, first‑arrival detection, and near‑field effects in bender element installations. These methods include cross‑correlation of successive reflections to self‑heal travel‑time and distance uncertainties, and matching measured signals to analytical solutions to account for near‑field effects and directly derive shear‑wave velocity. Electromagnetic coupling is critical in highly conductive soils but can be mitigated by shielding, grounding, or using parallel‑type bender elements.

Abstract

Bender elements are convenient shear wave transducers for instrumenting soil cells due to optimal soil–transducer coupling and compatible operating frequency. Experimental and analytical methods are implemented in this study to explore various aspects of bender element installations including: electromagnetic coupling prevention, directivity, resonant frequency, detection of first arrival, and near field effects. It is shown that electromagnetic coupling effects are critical in soils with high electrical conductivity and can be minimized by shielding and grounding, or by using parallel-type bender elements. Bender elements generate both P- and S-waves. The in-plane S-wave directivity is quasicircular. The resonant frequency of bender element installations depends on the geometry of the bender element, the anchor efficiency, and the soil stiffness. The cross correlation of subsequent reflections is a self-healing measurement procedure which resolves uncertainties in both travel time and travel distance. Near field effects can be effectively taken into consideration by matching the measured signal with the analytical solution, directly rendering shear wave velocity.