# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 4 Electromagnetic Field Diffusion Characteristics Study of SUTEM
### Double-Aquifer Model  
Finally, we constructed a model with two aquifers to analyze the distribution characteristics of the electromagnetic field under the influence of two aquifers and the influence of shallow aquifers on the detection of deep aquifers by SUTEM. Figure 14 shows the distribution map of the x-component \(\partial B_{\rm x}/\partial t\) of the EMF of the double-aquifer model at different times. The parameter settings of the shallow aquifer and the background are consistent with the single-aquifer model. The thickness of the deep aquifer is 100 m, the buried depth of the top interface is 300 m, and the resistivity is 10 \(\Omega\)-m.  
Figure 14: Distribution of the double-aquifer model at different times. Figure 13: Time-varying curves and change rate curves of the \(z\) components of the EMF. Here, **(a)** is the \(\partial B_{21}/\partial t\) time-varying curve at different depths below the receiving point (0,0),0); **(b)** is the \(\partial B_{21}/\partial t\) change rate curves. At \(10^{-5}\) s to 1.9415 \(\times\)\(10^{-4}\) s, the extreme value distribution of \(\partial B_{x}/\partial t\) is consistent with the single-aquifer model. Affected by the deep aquifer, the \(\partial B_{x}/\partial t\) contour shows a significant deflection at 9.2492 \(\times\)\(10^{-4}\) s, and a horizontal contour with a value of 0 appears at the level of the shallow aquifer. At 4.4062 \(\times\)\(10^{-3}\) s, there exist horizontal contours of \(\partial B_{x}/\partial t\) with a value of 0 at the level of the shallow aquifer and the top interface of the deep aquifer. Extreme values of \(\partial B_{x}/\partial t\) are observed at the bottom interface of the deep aquifer and the top interface of the shallow aquifer. The \(\partial B_{x}/\partial t\) between the two low-resistivity aquifers is close to 0. At 2.0991 \(\times\)\(10^{-2}\) s, the horizontal contour line with a value of 0 in the shallow aquifer disappears, and extreme values of \(\partial B_{x}/\partial t\) exist at the bottom interface of the deep aquifer and the top interface of the shallow aquifer, while \(\partial B_{x}/\partial t\) at the bottom interface of the shallow aquifer and the top interface of the deep aquifer is close to 0. Figure 15 shows the distribution map of the z-component \(\partial B_{z}/\partial t\) of the EMF of the double-aquifer model at different times. Similar to the uniform half-space model, at \(10^{-5}\) s and 4.0754 \(\times\)\(10^{-5}\) s, the isoline with a value of 0 opens outward. With increasing time, the extreme value of \(\partial B_{z}/\partial t\) gradually moves down. At 1.9415 \(\times\)\(10^{-4}\) s, the extreme negative value of \(\partial B_{z}/\partial t\) outside the loop reaches the shallow aquifer, but the extreme positive value inside the loop does not reach the shallow aquifer. At 9.2492 \(\times\)\(10^{-4}\) s, the extreme positive value of \(\partial B_{z}/\partial t\) inside the loop reaches the shallow aquifer. The difference from the single-aquifer model is that the extreme value of \(\partial B_{z}/\partial t\) at 4.4062 \(\times\)\(10^{-3}\) s begins to spread to the deep aquifer. Until 2.0991 \(\times\)\(10^{-2}\) s, the extreme value of \(\partial B_{z}/\partial t\) is not already in the deep aquifer. Figure 16 shows the time-varying curves of the z-component \(\partial B_{z2}/\partial t\) of the EMF at different depths below the transmitting loop and the relative change rate curves with the single-aquifer model. Figure 16a shows the time-varying curves of \(\partial B_{z2}/\partial t\). Similar to the single-aquifer model, when the receiving point is at the surface, \(\partial B_{z2}/\partial t\) decays with time. When the receiving point is located underground, \(\partial B_{z2}/\partial t\) increases and then decreases  
Figure 15: Distribution of the double-aquifer model at different times. with increasing time. There exists a peak in the curve, and with the increase in the depth of the receiving point, the time of the peak appears later.