State‐of‐the‐art geophysics for metal exploration
Authors: Arai Eiichi 
Published: July 2021

The new discovery of the economical metal deposit is getting more difficult year by year because the unexplored areas tend to be more remote areas than ever, and the metal prospective zones are deepening in the recent decades. Therefore, the advent of the state‐of‐the‐art or advanced geophysics technology to peer the deeper parts of the ground with higher accuracy and spatial resolution has been aspired in metal exploration industry. The author reports and introduces the drone‐based magnetic survey, the three‐dimensional time‐domain IP inversion algorithm using Cole–Cole parameter, the time domain electromagnetic method using superconducting quantum interference device (SQUID), and passive seismic survey as the state‐of‐the‐art or advanced geophysics technology for metal exploration.

In the early stage of metal exploration, airborne magnetic survey has been used in a wide variety of exploration programs for large areas to map basement structures and to delineate other magnetic structures. The drone‐based magnetic survey is recently getting prevailing in the metal exploration because it can be conducted at much lower cost with simpler clearance for its flight by the governmental authority, and with higher spatial resolution than the conventional fixed‐wing or heliborne magnetic survey.

In the target identification stage, induced polarization (IP) method has been recognized as one of the extremely powerful tools in the discovery of many new metal deposits. The historical various studies on complex resistivity measurements for metal sulphides, graphite, magnetite and pyrrhotite have revealed that there are significant differences in the spectral responses among metal sulphides, graphite, magnetite and pyrrhotite which is capable of being parametrized using the empirical Cole–Cole parameter model. JOGMEC developed the three‐dimensional time‐domain IP inversion code using Cole–Cole parameter model so as to interpret the difference of IP sources using Cole–Cole parameters, and has applied this code to its exploration activities. Transient electromagnetic method (TEM) also has a well‐established place in metal exploration. Since recent exploration targets are generally concealed by thick conductive overburdens such as weathered layers and black shale, these geological covers preclude induction currents generated by the TEM transmitter loop on the surface from diffusing into the exploration targets plentifully beneath the conductive covers. B‐field measurement using SQUID magnetic sensor is one of the technologies which enable TEM to investigate deeper than dB/dt measurement by the conventional induction coil system, because B‐field decays slower than dB/dt and time decay of B‐field reaches the system noise floor at later time than dB/dt if dynamic ranges of the both systems are almost identical. JOGMEC had developed the TEM system using a high‐temperature superconductor (HTS) SQUID, and has been using it in its metal exploration.

The other current trend of mining geophysics is application of seismic method to metal exploration. In the target identification stage, deep geochemistry using aircore drilling is much utilized over the areas covered by transported cover. In order to design the aircore drilling survey, passive seismic survey has been utilized to evaluate thickness of transported cover in order to make plans of specification (length) of the aircore drilling survey, which is also a new trend in mining geophysics.