Long-Distance Crosswell EM Logging of Copper Ore Using Borehole-Surface Current Injection in Slim Holes

Abstract

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 currentinjection-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 lowresistance 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 multianomaly 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.

A portable frequency‐domain electromagnetic detection system

“…[7][8][9][10][11] Compared with single-well electromagnetic detection, the frequency-domain electromagnetic detection method exhibits a notably enhanced lateral detection capacity, thus rendering it a technique of immense promise with broader prospects for application and heightened research and practical significance. [12][13][14][15] This detection method extends the detection horizon of the instrument from "around the wells" to "between the cross-wells," thus improving the accuracy of reservoir description. 16,17 The process of cross-well electromagnetic detection is shown in Figure 1.…”