The Source Effects in Short-Offset Semi-Airborne Electromagnetic Observation and a Conjugate-Source-based Method

Semi-Airborne Transient Electromagnetic Method (SATEM) is a geophysical method in which the transmitting device is deployed on the surface and a flight platform is used to carry sensors for aerial observation. To ensure the signal-to-noise ratio, the transmitting source usually has to be placed close to the survey area, which results in observation data that are more susceptible to source effects. In the research and application of this short-offset SATEM situation, we have found that when the observation point is close to the transmitting source, under certain conditions some of the data are more complexly affected by source effects than expected. Through the analysis of the mechanism of this phenomenon, we believe that the occurrence of this situation is related to various factors such as ground wave, discontinuity interface of the resistivity, and the location of the observation point. To address this issue, we propose a solution based on conjugate-source, which essentially utilizes the propagation paths of different excitation fields as constraints in inversion to overcome the above-mentioned effects. The result of a field comparative test showed that the detection method using conjugate-source can effectively suppress these source effects and obtain more reliable detection result. Theoretical research and experimental results will lay a solid foundation for subsequent research and applications.