A small coil transient electromagnetic system for quick subsurface mapping

Transient Electromagnetic (TEM) methods are widely used in near-surface geophysical exploration. Traditional ground-based TEM systems utilize large transmitter loops (25x25 to 50x50 m²) to investigate depths between 200 and 300 meters, yielding 15-20 soundings per day. To enhance efficiency in shallower investigations (0 - 100 m), we introduce a compact TEM system with a small coil setup for rapid deployment and mobility, increasing data collection rates along extensive transects.

 

The novel system comprises a 3x3 m transmitter loop with two turns and an equivalently sized offset receiver loop with four turns, separated by 10 m to minimize coil coupling and ensure unbiased signals. Operating as a single-moment setup, it achieves a peak current of 5 Amp or 10 Amp, turning off in approximately 7 µs. Unbiased measurement begins at 10 µs post turn-off, extending to a late gate of 3 ms. Both transmitter and receiver are integrated into a portable unit powered by lightweight lithium-ion batteries. A dedicated mobile application for Android and iOS devices controls the system, facilitating real-time monitoring of data curves and system parameters like current and temperature.

With this system, two people can collect a 400 m profile in under 60 minutes, significantly faster than Electrical Resistivity Tomography (ERT) methods. The presentation will cover the system's layout, operational methodology, depth capabilities, and validation against the Danish National TEM Test Site. Comparative analyses will underscore its efficiency and effectiveness in aquifer layer mapping, offering a compelling alternative to traditional ground-based systems.