Time domain electromagnetic survey in deep mine galleries for mineral exploration

Time-domain ElectroMagnetic (TDEM) methods are central to mineral exploration, yet the detectability of targets at depths of several kilometers remains limited or in most cases impossible for surface deployments. We evaluate a deep-gallery approach that uses a small transmitter loop in these infrastructures to enhance excitation of deep conductors. Through 3D modeling, a 250 m by 5 m gallery loop was designed and compared its primary field to that of a large 2000 m by 2000 m surface loop. We further investigated the secondary magnetic field generated by nearby conductors to assess detectability and response characteristics in whole-space conditions typical of underground settings. Field tests were conducted at the LaRonde mine (Abitibi, Québec) at ~ 3 km depth using a gallery transmitter (Tx) and a borehole receiver (Rx) configuration. Modeling indicates that, at depth, the small gallery loop can produce a stronger effective primary field than a large surface loop; survey design for this gallery loop should also prioritize increasing the number of turns over extending loop length. The secondary magnetic field modeling shows a polarity change in the horizontal components whether the conductor is located above, below, or on both sides of the loop, providing a diagnostic tool for interpretation. The measurements at LaRonde mine validate two known lenses and reveal an in-hole anomaly at 700 m, corroborating the feasibility and interpretive value of deep-gallery TDEM for exploring deep mineral deposits.