# Influence of Source Shape on Semi-Airborne Transient Electromagnetic Surveys
## 3 Results
### Half-Space Model
#### 3.1.2 Magnetic Fields in the Air  
The SATEM method traditionally employs coils to measure the time derivative of the vertical magnetic field component \(dB_{z}/dt\)[26,27]. Recent advancements in instrumentation have enabled direct measurements of the vertical magnetic field component \(B_{z}\)[6]. In this study, we analyze the influence of the curved source geometry on both \(dB_{z}/dt\) and \(B_{z}\) measurements acquired in the airborne receiver. Figure 5 illustrates the \(B_{z}\) distribution in the oxy plane at various time instants for both linear and curved sources, where Figure 5a-c represent the curved source responses and Figure 5d-f depict the linear source responses. At an early time (t = 10\({}^{-5}\) s), the maximum  
Figure 4: Electric field (V/m) in oyz plane for the half-space model at different time instants. Panels (**a–c**) represent the curved source response, while panels (**d–f**) illustrate the linear source response. magnetic field intensity near the source follows the geometric configuration of the source, resulting in pronounced differences between the magnetic field distributions of curved and linear sources. As time progresses, the maximum magnetic field intensity gradually diffuses outward, diminishing the influence of the source geometry and progressively reducing the differences in magnetic field distribution near the source. At a late time (t = 0.01 s), the magnetic field distributions for curved and linear sources converge to near-identical patterns. In practical applications, magnetic field responses are typically recorded within a specific survey area at a certain offset from the source, as indicated by the green wireframe in Figure 2, representing the receiving area with offsets ranging from 200 m to 1000 m. Figure 6 presents the \(B_{z}\) distribution in the plane within this airborne receiving area at different time instants, with Figure 6a-c showing the curved source responses and Figure 6d-f displaying the linear source responses. Figure 6 clearly demonstrates that the source geometry significantly influences the magnetic field distribution in the proximal survey area. The magnetic field response of the linear source exhibits uniform distribution along the x-direction with a gradual attenuation along the y-direction. In contrast, the asymmetric configuration of the curved source produces non-uniform distributions along the x-direction, generating significant anomalies in the proximal survey area. At t = 10\({}^{-5}\) s, within the range x = \(-\)500 m to 0 m, the measurement profile at y = \(-\)200 m is positioned in close proximity to the curved source, manifesting as a region of maximum magnetic field intensity. Conversely, within the range x = 0 m to 500 m, the measurement profile at y = \(-\)200 m is situated farther from the curved source, and the maximum magnetic field has not yet propagated to this region. The magnetic field maximum diffuses progressively along the y-direction with time. At t = 2.5 \(\times\) 10\({}^{-4}\) s, within the range x = \(-\)500 m to 0 m, the maximum magnetic field has diffused to the region y = \(-\)400 m, while within the range x = 0 m to 500 m, the maximum magnetic field has just reached the vicinity of y = \(-\)200 m. At t = 0.01 s, as evident from comparing Figure 6c,f,  
Figure 5: \(B_{z}\) (A/m) of oxy plane in the air for the half-space model at different time instants. Panels (**a–c**) represent the curved source response, while panels (**d–f**) illustrate the linear source response. the maximum magnetic field has diffused to the maximum offset at y = \(-\)1000 m, and the magnetic field within the receiving region exhibits a uniform distribution along the x-direction, with minimal influence from the source geometry. Figure 7 illustrates the \(dB_{z}/dt\) distribution in the oxy plane at various time instants for both linear and curved sources, where Figure 7a-c represent the curved source responses and Figure 7d-f depict the linear source responses.