Abstract:  With the depletion of shallow resources, refined exploration of deep metal deposits has become a key focus in geophysical prospecting. Underground electromagnetic (EM) methods demonstrate distinctive potential in deep mineral exploration owing to their proximity to target bodies, strong anti-interference capability, and high-resolution imaging. Internationally, these methods have been established as core technologies for exploring deep (>500 m) massive sulfide and disseminated deposits. This paper systematically reviews recent advances in borehole, crosswell, and borehole-to-surface EM methods both domestically and internationally, analyzing the developments of high-power transmitters, high-sensitivity receivers, and multicomponent observation systems, and summarizing numerical modeling, inversion algorithms, and intelligent data processing under complex well conditions. Within the workflow of deep mineral exploration, application strategies of underground EM techniques at different stages are summarized, including constraining metallogenic background, guiding metallogenic models, supporting target prediction, and locating ore bodies. Results indicate that underground EM methods are evolving toward multi-source integration and multi-parameter fusion, forming deep-exploration frameworks of “multi-parameter cascading with near-far integration” and “strong-source close-receiving with dense irradiation and wide-area coverage”, providing technical support for deep mineral exploration.

Key words: underground electromagnetic method, borehole-crosswell-borehole-to-surface integration, multi-parameter fusion, intelligent inversion, deep metal mineral, metallogenic model