Abstract
Borehole transient electromagnetic (TEM) array has proven to be efficient for the downhole nondestructive testing (NDT) of metal casings through the eddy-current property. However, restricted by bad downhole conditions, the simple increase of the receiving array is not sufficient enough for improving the downhole NDT performance. In this paper, we present a multiple-transmit focusing (MTF)-based borehole TEM system for the NDT of downhole casings. On the basis of the borehole TEM signal model, the response of the multiple-transmit array is derived by employing the matrix form of the borehole TEM response. It was shown that the excited magnetic field by the multiple-transmit array can be focused by weighting the current of each transmitter. Using this property, a modified linear constrained minimum variance-based multiple-transmitting array weighting method was proposed to realize the MTF. Moreover, a subarray partition approach was proposed to simplify the MTF realization, where the subarray weighting and mean square error were also analyzed. The MTF method performance was verified by applying it to a borehole TEM system for the NDTs of downhole casings. Finally, simulations and experiments were conducted, and the results demonstrated the effectiveness of the proposed method.
Highlights
Owing to their significant feature of accessibility to targets, borehole measurements have been widely used in many research fields, such as mineral detection [1], [2], oil & gas exploration [3], [4], and geotechnical and environmental investigations [5]
nondestructive testing (NDT) techniques, the design of eddy-current sensors as well as their signal processing for borehole measurements are strongly restricted by harsh environments, including narrow underground confined spaces [9], the large temperature range [10], and cumbrous metal tool housings against high wellbore pressure [11], which greatly affect the performance of borehole NDTs [12], [13]
It is obvious that the solutions of the transmitting current in (18) and (23) are different, where the linear constrained minimum variance (LCMV)-based optimal weight has a right multiplication vector, while the maximum signal-to-noise ratio (SNR)-based optimal weight only has a single variable as a coefficient
Summary
Owing to their significant feature of accessibility to targets, borehole measurements have been widely used in many research fields, such as mineral detection [1], [2], oil & gas exploration [3], [4], and geotechnical and environmental investigations [5]. In this study, inspired by the multi-coil array techniques with array weighting and magnetic field focusing, VOLUME 8, 2020 a multiple-transmit focusing (MTF)-based borehole TEM array system for the NDT of downhole casings was proposed. MTF FOR THE BOREHOLE TEM SYSTEM the magnetic field distribution as well as the induced EMF can be changed by adjusting the transmitting current value of each transmitter with different TRDs. In [19], the receiving weighting vector were applied to a multiple receiving array and were optimized to improve the detection performance.
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