Resolution enhancement of ultrasonic imaging based on AR-TFM method

In this paper, the multi-frequency windows autoregressive spectral extrapolation with total focusing method (AR-TFM) is presented and applied to raise the ultrasonic imaging resolution to from wavelength-scale subwavelength-scale. Singular value decomposition based on higher order cumulant statistics is employed to determine the AR order. Full matrix capture data is processed by AR spectrum extrapolation based on several effective frequency bands, and then extrapolated spectra are linearly combined and averaged. The TFM imaging is performed on the extrapolated full matrix capture data to enhance the robustness and applicability. The TFM and AR-TFM methods were used to test the carbon steel test blocks, and the test results were compared. The simulated and experimental results demonstrate that the diffraction limit is broken through by AR-TFM, realizing the identification of three side-drilled holes (SDHs) with 0.7λ (wavelength λ) central distance. The relative measurement errors of the depths and central distance of SDHs are no more than 1.8% and 15%, respectively.