Characteristic analysis and optimal survey area definition for semi-airborne transient electromagnetics

Semi-airborne transient electromagnetics (SATEM) is a versatile tool in quick geophysical surveys at a low cost. This method usually uses a grounded-wire source on the surface to transmit electromagnetic field into the earth and a receiver hanging on unmanned aerial vehicles to acquire the secondary field in the air. Compared to conventional transient electromagnetic methods, the grounded-wire system has complications caused by the combined effects of the transmitter wire and the grounding points, with complications affecting the simplicity of the secondary response. In this study, we use the electric dipole integral algorithm to simulate the SATEM responses in a layered earth model and compare the responses with different offsets, receiver heights, and geometry positions of the receiver. With these analyses, we define the optimal survey area for a layered earth to be the area with a monotonic decay rate of the secondary field. We establish an ellipse boundary distribution function to determine the suitable and unsuitable survey areas, and areas outside the ellipse are the suitable or optimal survey areas. Two examples with short and long line sources are used to test the definition and demonstrate the usefulness of the definition. The ellipse boundary function can be used in real SATEM survey designs. •We analyze impact factors of semi-airborne transient electromagnetic (SATEM) including geometry, offsets, and flight height.•We use a novel parameter, the decay rate of electromagnetic field, to analyze suitable or unsuitable areas for SATEM survey.•We define an ellipse function to be the boundary of optimal survey areas of SATEM.