Magnetic Field Focusing Array for Nondestructive Evaluation of Oil and Gas Well Casings

Relying on eddy current changes with source distances, borehole transient electromagnetic (TEM) array techniques have achieved good results in nondestructive evaluation (NDE) of oil and gas well casings. However, the existing TEM array techniques cannot achieve sufficient longitudinal resolution of NDE. In this paper, a magnetic field focusing array for NDE of oil and gas well casings is presented. Based on the borehole TEM signal model, the eddy current field distribution is analyzed. The results show that the detection performance of NDE is strongly influenced by eddy current diffusion in the longitudinal direction which will be increased corresponding to the sampling time and transmitting power. To solve this problem, a uniform longitudinal linear transmitting array is adopted, and the transmitting weight of each element is optimized using the linear constraint minimum variance criterion. Thus, longitudinal focusing is achievable to significantly reduce the longitudinal diffusion range but improve the longitudinal resolution. Simulation results demonstrate that the proposed method effectively focuses the transmitting array’s eddy current field, improving the borehole TEM system’s resolution for NDE of oil and gas well casings.