Ultrasonic guided waves are widely used in the nondestructive testing (NDT) of aboveground pipelines. However, their application in buried pipeline inspection is significantly hindered by severe soil-induced attenuation. This study proposes a method for detecting defects in buried pipelines using nonlinear chirp signals encoded with orthogonal complementary Golay code pairs. By adjusting the proportion of low-frequency and high-frequency components in the excitation signal, the attenuation of guided waves in buried pipelines is effectively reduced. Meanwhile, the use of coded sequences increases the energy of the excitation signal, and the excellent autocorrelation properties of broadband signals enhance the time-domain resolution of defect echoes. The fundamental principles of coded excitation based on nonlinear chirp signals and pulse compression methods are first introduced. MATLAB simulations are then employed to verify the approach's effectiveness in the characterization of defect echoes under various conditions and signal-to-noise ratios (SNR). A subsequent comparative analysis, using finite element (FE) simulations for buried pipelines, demonstrates that nonlinear chirp signals with a higher proportion of low-frequency components exhibit better resistance to attenuation. By fine-tuning the chirp parameters, higher defect reflectivity can be achieved than with conventional tone bursts for various defect types in buried pipelines. FE simulations further illustrate the superiority of the proposed method over tone bursts in terms of excitation signal amplitude, defect echo reflectivity, and defect location accuracy. Finally, defect detection experiments on buried pipelines with multiple defects confirm that the nonlinear chirp signals with carefully selected parameters exhibit lower attenuation rates. In the same testing environment, the coded nonlinear chirp signals outperform tone bursts by providing higher excitation amplitudes, greater defect echo reflectivity with an increase of up to 81.45 percent, and enhanced time-domain resolution. The proposed method effectively reduces ultrasonic guided wave attenuation in buried pipelines while increasing defect echo reflectivity and extending the effective detection range.

In this article, we propose a method using T(0,1) guided waves combined with coil coding technique to detect defects in buried liquid-filled pipes implemented by an electromagnetic acoustic transducer (EMAT). Due to its non-dispersive properties and the fact that there is no energy loss in nonviscoelastic fluids, the T(0,1) mode is selected for pipe defects detection. The electromagnetic device that generates the circumferential magnetic field is optimized to excite the pure T(0,1) mode. To realize energy enhancement and defect location identification, the electromagnetic acoustic coil is spatially encoded by 11-bit Barker code and the receiver coil is multiplexed consisting of a spatial coded coil and a unit coil. The defect detection is accomplished through time-of-flight (TOF) time-frequency analysis, and the defect location identification is achieved by digital signal processing methods (cross correlation and convolution). The feasibility of this method is verified by the finite element (FE) model and experimental analysis, indicating the defect locating error in a liquid-filled pipes is less than 1%. Overall, the proposed method achieves a high-precision flaw detection and location identification.