Shadow effect and source overprint effect of short-offset transient electromagnetic method

In artificial-source electromagnetic explorations, shadow effect and source overprint effect are one of the key factors to affect the detection reliability. Compared with the frequency-domain electromagnetic methods, transient electromagnetic (TEM) shadow effect and source overprint effect are more complex, especially for the short-offset TEM (SOTEM) observed in the near-source region. However, there is less relevant research. Therefore, in the paper, we first realize 3D SOTEM simulations based on the vector time-domain finite element method, then further analyze the generation of SOTEM shadow effect and source overprint effect from view of the diffused electromagnetic fields, as well as their influence on ground Ex and dBz/dt responses. Results show that the attraction or repulsion of the electrical inhomogeneity would affect the diffusion speed of the electromagnetic fields, which is the essential of the two source effects. Besides, from view of the ground Ex and dBz/dt responses, the relative anomaly of SOTEM shadow effect is almost twice of those of SOTEM source overprint effect, i.e. SOTEM shadow effect is easier to be recorded, especially for the dBz/dt responses. In addition, due to the shadow effect, SOTEM may fail to identify the real target anomaly. While the influence of the source overprint effect for recognizing the target anomaly mainly occurs on all-time dBz/dt responses and early-time Ex responses (before about 10 ms), that is, observing late-time SOTEM Ex responses can effectively avoid such influence. Hence, to fully ensure avoiding the influence of the source overprint effect, we suggest adopting high-power transmission to increase the late-time signal-to-noise ratio in the field measurement.