# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 4 Electromagnetic Field Diffusion Characteristics Study of SUTEM
### Single-Aquifer Model  
We constructed a model with only one aquifer to study its influence on the electromagnetic field diffusion and data at different depths. Figure 11 shows the distribution map of the x-component \(\partial B_{x}/\partial t\) of the EMF of a single-aquifer model at different times. The gray shadow in the figure is a low-resistivity aquifer. The thickness of the aquifer is 100 m, the buried depth of the top interface is 100 m, the resistivity is 10 \(\Omega\cdot\)m, and the background resistivity is 100 \(\Omega\cdot\)m. At 10\({}^{-5}\) s, the extreme value distribution of \(\partial B_{x}/\partial t\) is consistent with that of the uniform half-space, but due to the influence of the low-resistivity aquifer, the \(\partial B_{x}/\partial t\) value below is significantly lower than that of the uniform half-space and is close to 0. At 4.0754 \(\times\) 10\({}^{-5}\) s, the extreme value of \(\partial B_{x}/\partial t\) is affected by the aquifer, and the downward diffusion speed is slower than that of the uniform half-space. Subsequently, the extreme value of \(\partial B_{x}/\partial t\) accumulates at the level of the aquifer for a long time, and the downward diffusion speed is much smaller than that of the uniform half-space. From 9.2492 \(\times\) 10\({}^{-4}\) s to 2.0991 \(\times\) 10\({}^{-2}\) s, the horizontal isoline of \(\partial B_{x}/\partial t\) with a value of 0 appears at a position of the aquifer. Different from the distribution of \(\partial B_{x}/\partial t\) in the uniform half-space, the extreme value above the horizontal contour (with a value of 0) in the single-aquifer model has remained above the aquifer. Therefore, it can be seen that when there is a single aquifer under the ground, the \(\partial B_{x}/\partial t\) signal received by the receiving point below the aquifer is very small in the early stage, and there is no inflection point of positive and negative transformation in the middle and late stages. The data received above the aquifer are larger than the data received in a uniform half-space. Figure 12 shows the distribution map of the z component \(\partial B_{z}/\partial t\) of the EMF of the single-aquifer model at different times. At 10\({}^{-5}\) s and 4.0754 \(\times\) 10\({}^{-5}\) s, the extreme value distribution of \(\partial B_{z}/\partial t\) is consistent with that of the uniform half-space. However, due to  
Figure 10: Decay curves of the \(x\) and \(z\) components of the EMF with time. Here, (**a**) is the \(\partial B_{x0}/\partial t\) time-varying curve at different depths below the receiving point (250,0,0) and (**b**) is the \(\partial B_{z0}/\partial t\) time-varying curve at different depths below the receiving point (0,0,0). the influence of the low-resistivity aquifer, the isoline with a value of 0 below the aquifer is different from that of the uniform half-space. The secondary field generated by the low-resistivity aquifer is larger than the secondary field generated by the medium at the vertical position of 100-200 m in the uniform half-space, resulting in different opening directions of the isoline with a value of 0. At 1.9415 \(\times\) 10\({}^{-4}\) s, the extreme value of \(\partial B_{z}/\partial t\) outside the transmitting loop reaches the aquifer, but the extreme value inside the transmitting loop remains located above the aquifer. Different from the distribution of \(\partial B_{z}/\partial t\) in the uniform half-space, at 9.2492 \(\times\) 10\({}^{-4}\) s, there remains a negative value in the horizontal direction of the aquifer in the range of \(-\)500-500 m, mainly due to the slow propagation of the electromagnetic field in the low-resistivity layer. In addition, the extreme positive value of \(\partial B_{z}/\partial t\) converges in the aquifer. Until 2.0991 \(\times\) 10\({}^{-2}\) s, the extreme value of \(\partial B_{z}/\partial t\) remains located at the level of the aquifer, which is different from the result of the uniform half-space at the same time.