# 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
#### 4.4.3 The Influence of Shallow Aquifer Resistivity Changes  
Figure 19 shows the influence of the deep aquifer on each measuring point calculated when the resistivity of the shallow aquifer changes. The top interface of the shallow aquifer is fixed at 100 m, the thickness is fixed at 100 m, and the resistivity is set to 10 \(\Omega\)-m, 20 \(\Omega\)-m, 30 \(\Omega\)-m...90 \(\Omega\)-m. The polynomial fitting formula is shown in Equation (30), and the highest order is 4. The fitting difference between the sample point and the fitting curve is less than 5%, and the fitting curve essentially meets accuracy requirements. Figure 17: The influence of depth changes in shallow aquifers on the TEM detection ability of deep aquifers. Figure 18: The influence of thickness changes in shallow aquifers on the ability of TEM detection of deep aquifers. It can be seen from Figure 19b that although the relative change rate fitting curve shape of the receiving point is different when the resistivity of the shallow aquifer changes, the basic trend is consistent. From the numerical range of the first derivative of the fitting function, it can be seen that the influence of shallow aquifers on the detection of deep aquifers by TEM is clear, obviously, with increasing resistivity of the shallow aquifers. \[ \begin{array}{l}(0,0,0)\text{: }y\ =\ -1.61\times 10^{-5}x^{4}+3.8056\times 10^{- 3}x^{3}-0.33328x^{2}+13.919x-2.9807\\ (0,0,-300)\text{: }y\ =\ 2.33\times 10^{-6}x^{4}+2.7981\times 10^{-5}x^{3}-0.0854x^{2} +9.4074x+54.758\\ (0,0,-500)\text{: }y\ =\ -1.1\times 10^{-6}x^{4}+6.714\times 10^{-4}x^{3}-0.113 56x^{2}+8.1679x+54.888\end{array}  \tag{30}\]  
Changing the simulation results of one of the three elements reveals that the change in shallow aquifer thickness and resistivity shows a great influence on the shielding effect of deep aquifer detection, while the change in shallow aquifer depth shows a relatively small influence on the shielding effect of deep aquifer detection. 4.4 Influence of Shallow Aquifer Depth, Thickness, and Resistivity Changing Simultaneously on the Ability of the TEM to Detect Deep Aquifers  
Figure 20 represents a three-elements diagram while simultaneously changing the shallow aquifer. The model containing only the shallow aquifer is the background model (\(\partial B_{21}/\partial t\)), the EMF \(\partial B_{22}/\partial t\) containing both the shallow and deep aquifers is the present value, and the maximum value of the relative change rate (\(\frac{\max(\partial B_{22}/\partial t-\partial B_{21}/\partial t)\times 100}{ \partial B_{1}/\partial t}\)) is calculated. The lower left regions of Figure 20 show the model diagram, and the legend represents the maximum range of the relative change rate. The larger the maximum relative change rate is, the smaller the influence on the detection shielding of the deep aquifer. Figure 20a shows the distribution of the maximum relative change rate of the EMF in the deep aquifer under the influence of the three elements of the shallow aquifer when the receiving point is located on the surface, and the maximum relative change rate is between 18 and 273. Figure 20b shows the maximum relative change rate of the EMF in the deep aquifer under the influence of three elements when the receiving point is located at the top interface of the deep aquifer. The maximum relative change rate is between 18 and 389. Figure 20c shows the maximum distribution of the relative change rate of the EMF of the deep aquifer under the influence of the three elements when the receiving point is 100 m below the deep aquifer. The maximum value of the relative change rate ranges from 18 to 290. In summary, the existence of shallow aquifers exhibits a greater impact on the detection of deep aquifers by surface observation points, and the impact on deep measurement points is secondary.