# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 4 Electromagnetic Field Diffusion Characteristics Study of SUTEM
### Analysis of the Influence Degree of Different Parameters of Shallow Aquifers
#### 4.4.1 The Influence of Shallow Aquifer Depth Changes  
In this model, the thickness of the shallow aquifer is fixed at 50 m, the resistivity is fixed at 10 \(\Omega\cdot\)m, and the top interface is set at 10 m, 30 m, 50 m, 70 m, 100 m, 130 m, 150 m, 170 m, and 200 m. The EMF z-component \(\partial B_{z1}/\partial t\) calculated by the model containing only shallow aquifers is used as the background data, and the EMF z-component \(\partial B_{z2}/\partial t\) calculated by the model containing both shallow and deep aquifers is used as the present data. The relative change rate is calculated by the relative change rate calculation formula, and the maximum value of the relative change rate is extracted to draw the curves of the change rate with depth, as shown in Figure 17. The ordinate label in Figure 17a represents the calculation of the maximum value of the relative change rate. The solid dots, rectangular frames, and solid triangles in Figure 17 represent the maximum relative change rates obtained by simulation when the receiving points are located on the surface, the top interface of the deep aquifer, and below the deep aquifer, respectively. The solid line, dotted line, and point horizontal line represent the curves of the relative change rate with the depth of the shallow aquifer drawn by fitting the polynomial of Equation (28). When the depth of the shallow aquifer changes, the highest order of the polynomial fitting curve is 2, and the fitting difference (RMS) between the sample points and polynomial curve is less than 1%. Figure 17b shows the first derivative curve of the fitted polynomial. The following determinations are given:  
* When the receiving point is located on the surface, the constant term of the first derivative curve is positive, indicating that with the increase in the depth of the top interface of the shallow aquifer, the influence of the deep aquifer on the surface receiving point gradually increases, and the shielding effect of the shallow aquifer on the deep aquifer gradually decreases.
* When the receiving point is located underground, the relative change rate is greater than that of the surface receiving point, indicating that the influence of the shallow aquifer on the underground receiving point is less than that of the surface receiving point. The initial value of the first derivative of the fitting curve of the underground receiving point is negative and then positive, indicating that the influence of the shallow aquifer on the underground receiving point increases first and then decreases with increasing depth.
* According to the first derivative curve, this range is between \(-\)0.06 and 0.16, and the value is very small, indicating that the change in shallow aquifer depth demonstrates a relatively small impact on all the receiving points. \[\begin{array}{l}(0,0,0)\colon y\ =\ 0.0001431x^{2}+0.009534x+156.08\\ (0,0,-300)\colon y\ =\ 0.0003738x^{2}-0.047261x+200.19\\ (0,0,-500)\colon y\ =\ 0.0002456x^{2}-0.032708x+172.18\end{array}\] (28)