Semi-airborne transient electromagnetic method: Geometrical distortion effects of grounded source under undulating terrain conditions and optimization of observation systems

As mineral exploitation is gradually shifting to complex terrain areas, the traditional transient electromagnetic (TEM) method has encountered challenges such as inconvenient deployment and low efficiency in these areas. In contrast, the semi-airborne TEM (SATEM) method, which combines a ground-based transmitter and an airborne receiver, can significantly improve data acquisition efficiency and adaptability in complex areas. However, due to the undulating terrains, the grounded source often deviates from the ideal straight-line pattern. The geometrical distortion effects of the grounded source on electromagnetic responses remain under-studied. Through 3D finite-element forward modeling, this study systematically analyzed the electromagnetic field distortions caused by the curvature and vertical undulation of the grounded source under typical undulating terrains, such as mountain peaks and valleys. The results indicate that the non-ideal geometry of the grounded source is the primary cause of early asymmetric electromagnetic responses and false anomalies. Mountain peaks concentrate the electromagnetic energy in the shallow parts of the mountains, suppressing the detection depth, whereas valleys enhance subsurface coupling due to the lower position of the grounded source, thus expanding the early detection range. More importantly, adopting a terrain-following flight line (i.e., maintaining a constant receiver height relative to the ground surface) can effectively mitigate the false anomalies of apparent resistivity caused by the geometrical distortion of the grounded source and the terrain coupling. Accordingly, this study proposes a SATEM-optimized data acquisition scheme for complex terrains, providing theoretical support and technical guidance for improving the accuracy of deep resource exploration.