Comparison of torsional and SH-waves in isotropic and anisotropic media based on laboratory measurements

Abstract Utilization of the torsional wave signal has been reported in medical testing and engineering detection. A torsional wave is a shear-wave train whose vibrating direction (polarization) forms a torsional motion. However, there is a lack of understanding of its characteristics and potential for subsurface investigations. Here, compressional (P), shear (S), and torsional (T) transducers are constructed using different piezoelectric-transformer (PZT) chips in the laboratory, and the characteristics of the torsional (T) wave propagating in isotropic and fracture-induced anisotropic media are analyzed in line with conventional P- and S-waves, covering waveform, amplitude, arrival time, and velocity at different propagating angles. The results indicate that the velocity and anisotropy values measured using the T transducer are almost the same as the SH-wave. For propagation in isotropic and anisotropic media, the wavefield generated by the T transducer is very simple with no mode conversions and source-generated interferences, which is similar to the SH-wavefield, while the SV-wavefield comprises a series of complex events from mode conversions and interferences. For propagating in fluid-saturated fractured rocks, the torsional wavefield also comprises a fast and slow wave train, which is referred to as torsional wave splitting, and is similar to the shear-wave splitting from S transducers. Furthermore, the anisotropy parameters measured using P, S, and T transducers show a good correlation with the fracture density, and the P-wave anisotropy is higher in air-saturated rocks than water-saturated rocks, while the shear-wave anisotropy measured by the S and T transducers is sensitive to fracture density. Like the P transducer, the T transducer is polarization-independent, or directional invariant. Therefore, utilizing the T transducer may simplify the field logistics during 3D shear-wave surveys. However, field torsional wave generation is a major issue that needs further study.