Magnetic-Electric Dual-Drive Time-Domain Electromagnetic Wave Look-Ahead Focusing for Remote Detection Method Research

Borehole frequency-domain electromagnetic (EM) look- ahead detection requires long transmitter-receiver offsets and low frequencies to achieve long-range detection. However, this method suffers from several technical limitations, including weak signals from the target formations, complex instrument configurations with multiple sections, and complicated field operations. Borehole time-domain EM methods overcome these limitations by using short transmitter-receiver offsets to achieve reservoir-scale look-ahead focusing detection and have the potential to become a new mode of borehole EM detection. This paper proposes a new method for magnetic-electric dual-drive time-domain EM wave look-ahead focusing detection based on the "smoke ring theory" of full-space time-domain EM. By analyzing the coupling relationship between the detector and the formation boundary, the propagation mechanism of borehole time-domain EM waves is revealed, and an innovative measurement mode for look-ahead focusing detection is developed. A multi- scale time-domain finite element algorithm with adaptive meshing is employed to numerically simulate the response characteristics of distant formation interfaces under different excitation and reception modes, clarifying the detection performance and optimal focusing methods of the detector. The results show that the coupling relationship and response characteristics between the time-domain EM waves generated by horizontal electric sources and axial magnetic sources and the formation interfaces are the strongest. The measured signals are mainly contributed by the upcoming formation, providing geological information up to 50 meters ahead. By using multi- magnetic-moment magnetic sources and arrayed semi- annular electric sources, the magnetic and electric fields emitted by the excitation sources can be focused ahead, enhancing the multi-scale look-ahead focusing detection effects.