Borehole transient electromagnetic (TEM) techniques have been proven to be efficient for nondestructive evaluations (NDEs) of metal casings using eddy-current properties. However, physical limitations and bad borehole conditions restrict the use of eddy-current sensors, which makes downhole casing inspections very different from those of conventional NDE systems. In this paper, we present a uniform linear multi-coil array-based borehole TEM system for NDEs of downhole casings. On the basis of the borehole TEM signal model, a numerical multi-coil array approach using the Gauss–Legendre quadrature is derived. The TEM response can be divided into two independent parts related to the transmitting-receiving distance (TRD) and the observation time and casing thickness. Using this property, the signal received by the multi-coil array is weighted to cancel the influence of the TRDs of the different array elements to obtain the optimal response according to the linearly constrained minimum variance criterion, which can be shown to be identical to that of achieving the maximum signal-to-noise ratio. The effectiveness of the proposed method was verified by applying the uniform linear multi-coil array to a borehole TEM system for NDEs of oil-well casings. Field experiments were conducted, and the results demonstrate the effectiveness of the proposed method.

The inspection of corrosion under insulation (CUI) has been identified as a significant challenge in the petroleum and chemical process industries. As some of the most effective strategies, pulsed eddy current (PEC) techniques have proved effective for the measurement of the CUI of pipelines. In this paper, we propose a circular-arc array (CAA) to improve the measurement efficiency for the PEC inspection of thermally insulated pipelines. Based on the PEC system model for inspecting the CUI of pipelines, the magnetic field distribution of the CAA with multiple excitors was investigated. It is shown that the coverage of induced magnetic field gets much larger than that of the single excitor to realize high-efficiency measurements. Moreover, a sparsely distributed receiver array is designed to further improve the signal-to-noise ratio by eliminating the waviness effect due to multiple excitors. Finally, experiments were conducted, and the results demonstrated the effectiveness of the proposed method for the inspection of thermally insulated pipelines.

Transient electromagnetic (TEM) techniques are widely used in the field of geophysical prospecting. In borehole detection, the nondestructive inspection (NDI) of a metal pipe can be performed efficiently using the properties of eddy currents. However, with increasing concern for safety in oil and gas production, more than one string of pipe is used to protect wellbores, which complicates data interpretation. In this paper, an auxiliary sensor-based borehole TEM system for the NDI of multipipe strings is presented. On the basis of the characteristics of the borehole TEM model, we investigate the principle behind the NDI of multipipe strings using multiple time slices of induced electromotive force (EMF) in a single sensor. The results show that the detection performance of NDI is strongly influenced by eddy-current diffusion in the longitudinal direction. To solve this problem, we used time slices of the induced EMF in both the main and auxiliary sensors. The performance of the proposed system was verified by applying it to an oil well with a production casing and liner. Moreover, field experiments were conducted, and the results demonstrate the effectiveness of the proposed method.

In this article, we present a space–time pulsed eddy current (PEC) array for nondestructive testing (NDT) of wellbore casings using a multiple-input multiple-output (MIMO) technique. On the basis of a multilayered cylindrical wellbore model, an MIMO-PEC model is developed according to the phase shift characteristics of the PEC response corresponding to different transmitting–receiving (TR) distances. To achieve a more significant number of TR channels than in a traditional receiving array to reduce the complexity of the TR structure, a space–time transmission scheme is proposed to recover multiple TR channels. Moreover, a specific space–time MIMO array structure is designed to maximize the number of independent TR channels. The effectiveness of the proposed method is verified by applying the proposed MIMO array to a borehole PEC system for NDT of wellbore casings. The simulation and experimental results demonstrate that using the array structure and space–time transmission scheme proposed in this article, an MIMO structure consisting of ${M}$ transmitters and ${N}$ receivers can be equivalent to a transceiver system with a single transmitter and ${M}\,\,\times \,\,{N}$ receivers. Thus, MIMO technique can be effectively applied to PEC-NDT systems and drastically reduce the cost and complexity of the PEC system.

Pulsed eddy current (PEC) is a widely utilized technology for the nondestructive inspection of industrial tubes and pipes due to its rapid and accurate results. To improve the longitudinal resolution of PEC, multiple transmitting coils (MTCs) are used to realize magnetic focusing. However, this approach is difficult to apply to narrow downhole environments because of the complex transmitting array and electrical circuits. To address this issue, we present a synthesized transmitting coil (STC) that combines MTCs into a single coil with multiple connected sections using different winding directions and number of turns to adjust the magnetic field distribution. A theoretical derivation was presented for the analysis and interpretation of the magnetic field, and a figure of merit (FoM) was constructed to optimize the STC parameters. Numerical simulations and experiments were performed to validate the proposed STC for downhole casing inspection, and the experimental results showed good agreement with the simulation results.

Crosswell electromagnetic (EM) methods are widely used in subsurface geophysical prospecting because they can achieve more effective long-distance detection than single-well methods. However, a large-diameter borehole is required to increase the magnetic moment of the magnetic dipole source. For the long-distance detection of copper ores, which is usually performed in slim holes, we present a borehole-surface current-injection-based crosswell EM logging method. Considering the cost of deploying casing, we inject a low-frequency AC directly into the ground, and converging current is formed around low-resistance anomalies in the formation. Then, the distribution of the anomalies can be inferred by detecting the low-frequency alternating magnetic field of the converging current in the receiver well. Moreover, to further improve the detection performance, we design a placement scheme for the grounding electrode for multi-anomaly crosswell detection based on the Gauss–Newton inversion algorithm, where the EM responses for different grounding electrode locations are analyzed. Field experiments are conducted using two slim open holes spaced approximately 1000 m apart for the detection of two copper ores. Through the processing and interpretation of measured EM signals, the conductivity imaging results of the crosswell EM method indicate that the measured distribution of anomalies is consistent with prior knowledge obtained from numerous single-well loggings, demonstrating the feasibility of the proposed application for long-distance crosswell EM logging in slim open holes.