# Influence of Source Shape on Semi-Airborne Transient Electromagnetic Surveys
###### Abstract  
The semi-airborne transient electromagnetic (SATEM) method has garnered increasing attention and research interest due to its superior detection depth and high efficiency. Theoretically, the SATEM method employs a long straight grounded wire as its transmitter source; however, in practical applications, various source shapes emerge due to terrain constraints. This paper investigates the influence of source shape on SATEM data. A three-dimensional (3D) block model is established, and a model order reduction algorithm is applied to calculate the 3D spatial distribution of electromagnetic fields generated by both an ideal linear source and a curved source. Numerical simulation results reveal that: (1) in the early stage, maximum values of electric and magnetic fields near the source are distributed along the source shape; this influence diminishes with time, and at the late stage, the spatial electromagnetic field distributions generated by linear and curved sources converge, exhibiting similar patterns regardless of the source geometry; (2) the source shape primarily affects early responses in small-offset areas while having minimal influence on late responses in large-offset regions; (3) for deep detection applications conducted in large-offset areas, the influence of the source shape can be disregarded; however, for shallow detection with receivers positioned in small-offset regions, the source shape effects should be taken into consideration. three-dimensional simulation; geophysical survey; mineral exploration 2025154389  
11 March  
In theory, the SATEM method employs a long straight wire grounded at both ends to form a closed circuit with the earth. A stable current flows through this wire, generating a consistent primary magnetic field. When the current is abruptly terminated, eddy currents are induced in subsurface conductive bodies, which subsequently generate secondary magnetic fields [15]. The temporal variations in these secondary magnetic fields are then measured by receivers mounted on airborne platforms. In practical field operations, however, the ideal linear configuration of the transmitter wire is often compromised by topographical constraints and accessibility issues. Consequently, the wire must conform to available terrain features or follow existing roads, resulting in irregular geometric configurations rather than the theoretical straight-line arrangement. These deviations from the ideal model significantly alter the underground induced current distribution patterns and, by extension, the characteristics of the secondary magnetic fields. If data interpretation continues to rely on algorithms developed for straight-wire source models, the accuracy of geological interpretations may be substantially compromised. Therefore, comprehensive consideration of the transmitter wire geometry and its influence on electromagnetic field observations becomes crucial for accurate interpretations of SATEM survey data. Extensive research has been conducted to investigate the influence of the source geometry on electromagnetic field measurements. [PERSON] and [PERSON] [16] examined the discrepancies between frequency-domain electromagnetic fields generated by finite-length wire sources versus point dipoles. Their findings revealed that these differences can be substantial over distances extending to several times the wire length. [PERSON] et al. [17] investigated the impact of the transient electromagnetic source geometry on measurement outcomes with a one-dimensional (1D) model. The results demonstrated that the source geometry effects exceed 10% at small transmitter-receiver offsets, with a diminishing influence as the offset increases. [PERSON] et al. [18] conducted a comparative analysis of linear and arcuate sources with transient electromagnetic observation of equatorial and axial 1D models. The results show that the influence of the equatorial electric and magnetic field gradually increases with the decrease in the offset. [PERSON] et al.