For surveys in areas with difficult access, ground-based electromagnetic (EM) systems are difficult to operate, time-consuming, and expensive, whereas airborne EM (AEM) systems are unsafe and uneconomic for small survey areas. We have developed a new frequency-domain airborne electromagnetic (FAEM) system called the ground-airborne frequency-domain EM (GAFEM) system to achieve rapid data acquisition in complex terrains with high safety and low cost. The system consists of a grounded electrical source and an airborne receiver carried by an unmanned rotorcraft. The grounded electrical source transmits a 2n pseudorandom series obtaining signals at n frequencies in one flight, which can maintain the resolution without reducing efficiency. The airborne receiver based on the unmanned rotorcraft makes the system flexible, convenient, and economical to operate in areas with difficult access. The system can conduct surveys in far- and non-far-field regions and can obtain the apparent resistivity to present a quick overview of the subsurface structure. To demonstrate the GAFEM system’s effectiveness and practical detection potential, we have performed two field surveys that revealed the shallow structure with tunnels and the deep structure at source-receiver distances of 3 and 6 km, respectively. The GAFEM system effectively revealed the apparent-resistivity difference with a sufficient investigation depth at large source-receiver distances. Thus, the system can be extended to various important applications for rapid surveys or deep investigations in areas with difficult access.

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