Abstract
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.
Highlights
Transient electromagnetic (TEM) techniques have gained much attention over the past several decades owing to their use in a wide range of applications in geophysical prospecting, such as mineral and petroleum exploration [1], geotechnical and environmental investigations [2], and fundamental studies of stress and petrophysics [3]
On the basis of the linearly constrained minimum variance (LCMV)-based multi-coil array for borehole transient electromagnetic (TEM) system, we demonstrated that the influence of the transmitting-receiving distance (TRD) can be effectively eliminated, where the received signals of the uniform linear multi-coil array can be weighted to obtain the optimal response that would be received by a receiver with z = 0
Comparing Equations (36) and (38), we find that, when the optimization problem is subjected to WT X(z) = F to maximize the signal-to-noise ratio (SNR), we have μ = μ’, which means that the proposed LCMV-based multi-coil array method is equal to the maximum SNR of the borehole TEM
Summary
Transient electromagnetic (TEM) techniques have gained much attention over the past several decades owing to their use in a wide range of applications in geophysical prospecting, such as mineral and petroleum exploration [1], geotechnical and environmental investigations [2], and fundamental studies of stress and petrophysics [3]. On the basis of Doll’s theory in induction logging system [18], the TRD was replaced by two distances from the transmitting and receiving coils to “an elementary ring” Using this approximate theory, the multi-coil array sensors were successfully used for magnetic field focusing [19] to improve borehole detection performance [20,21]. We present a uniform linear multi-coil array-based borehole TEM system for NDEs of downhole casings. When a single receiver with a large number of turns or a multi-coil array is used, the TRD will strongly influence the interpretation of the NDE, where the change in the TRD cannot be ignored and must be compensated for to avoid model distortions in the borehole TEM system
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