Mobile Pulsed Electromagnetic Soundings: Feasibility and Potential of UAV-Based Survey Technology

Abstrsct—The effectiveness of TEM-FAST technology using a compact 25 m × 12.5 m single-loop combined receiving-transmitting antenna for measuring transient responses in microsecond period range is demonstrated by the example of transient electromagnetic sounding (TEM) of moraine sediments. The depth of investigation in sandy clay deposits with resistivity 30–300 Ω m is at least 50 m, with a dead zone thickness of at most 6 m. The on-foot soundings used an antenna fixed at four flexible poles at a height of 2.5 m above the ground. The portable measurement system with a total weight of 2.5 kg was moved by a field crew of four people along the profiles at a speed of ~1 m/s. Every 20 seconds after processing 13 000 pulses, the system recorded the measurement results and coordinates in autopilot mode. The repetition rate of current pulses with amplitude I = 3.3 A was f = 3.2 kHz, transient responses were monitored at time gates t = 4–64 µs. Vibration noise generated during motion due to antenna loop deformations in the Earth’s magnetic field reduced the depth of sounding to 50–60 m. The areal sounding data were inverted to design a 3D geoelectric model of the subsurface, which was then used to assess the capabilities of the TEM-FAST acquisition system in flying mode. Transient responses corresponding to different antenna heights above the terrain have been synthesized. Despite the decrease in response amplitudes when raising antennas to a height of 30 m, the depth of the soundings remained within 50 m. Considering the drone-borne survey technology using commercially available lightweight unmanned aerial vehicles (UAVs) with real-time dynamic positioning systems, we propose to tow the TEM acquisition system by a convoy of UAVs flying or hovering above the ground surface in autopilot mode. With decimeter positioning accuracy of antenna corners, the noise floor of measurements of transient responses will not exceed levels obtained in mobile on-foot surveys, and the depth of investigation (DOI) will not drop below 50 m.