Analytical reverse time migration with new imaging conditions for one-sided nondestructive evaluation of concrete elements using shear waves.

Cross-correlation imaging condition of reverse time migration generates high-amplitude artifacts in the reconstructed images. In addition, due to geometrical spreading and scattering attenuations, this imaging condition assigns amplitudes to the points of the reconstructed image that are not a true representative of the reflection coefficient of the scanned medium at those points. These all can lead to a reduction in quality and misinterpretation of the reconstructed images. In this paper, we proposed new imaging conditions to mitigate high-amplitude artifacts and to assign more accurate amplitudes to an RTM image by considering geometrical spreading and scattering attenuation in concrete members when horizontal shear waves are used for imaging. We used data obtained by transmitting horizontal shear waves to 3D synthetic homogeneous and concrete specimens and demonstrated the effectiveness of the new imaging conditions.

Numerical investigation of the effect of heterogeneity on the attenuation of shear waves in concrete.

Although ultrasonic transducers that emit horizontal shear waves are widely used in practice, no investigation has been conducted to study the attenuation of shear waves in concrete due to scattering by aggregates and air voids. Horizontal shear waves preserve more energy in comparison with longitudinal waves when propagating in concrete in low frequencies. However, the lower wavelength of shear waves increases the potential of their scattering attenuation in concrete. In this paper, we developed a 3D numerical tool and used it to study the scattering attenuation of horizontal shear waves in concrete in the 20-150 kHz frequency range. We showed that the scattering attenuation in this frequency range strongly depends on the size and material properties of aggregates. The shape of the aggregates and the presence of air voids slightly affected the scattering attenuation.

Analytical reverse time migration: An innovation in imaging of infrastructures using ultrasonic shear waves.

The emergence of ultrasonic dry point contact (DPC) transducers that emit horizontal shear waves has enabled efficient collection of high-quality data in the context of a nondestructive evaluation of concrete structures. This offers an opportunity to improve the quality of evaluation by adapting advanced imaging techniques. Reverse time migration (RTM) is a simulation-based reconstruction technique that offers advantages over conventional methods, such as the synthetic aperture focusing technique. RTM is capable of imaging boundaries and interfaces with steep slopes and the bottom boundaries of inclusions and defects. However, this imaging technique requires a massive amount of memory and its computation cost is high. In this study, both bottlenecks of the RTM are resolved when shear transducers are used for data acquisition. An analytical approach was developed to obtain the source and receiver wavefields needed for imaging using reverse time migration. It is shown that the proposed analytical approach not only eliminates the high memory demand, but also drastically reduces the computation time from days to minutes.