# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 4 Electromagnetic Field Diffusion Characteristics Study of SUTEM
### Single-Aquifer Model
Based on the characteristics of the SUTEM data uniform half-space, we selected only the z-component \(\partial B_{z1}/\partial t\) of the EMF at different depths to further investigate the case of a single aquifer. Figure 13 shows the time-varying curves of \(\partial B_{z1}/\partial t\) at different depths below the transmitting loop and the relative change rate of these curves with the uniform half-space model. Figure 13a shows the variation curve of \(\partial B_{z1}/\partial t\) with time. When the receiving point is located on the surface, \(\partial B_{z1}/\partial t\) decays with time. When the receiving point is located underground, \(\partial B_{z1}/\partial t\) increases and then decreases with increasing time. There exists a peak in the curve that appears later in time as the depth increases. Figure 13b shows the relative change rate curves of \(\partial B_{z}/\partial t\) between the single-aquifer model and the uniform half-space model, where the ordinate label represents the calculation of the relative change rate where \(\partial B_{z0}/\partial t\) is the z-component of the EMF calculated by the uniform half-space, and \(\partial B_{z1}/\partial t\) is the z-component of the EMF calculated by the single-aquifer model. The diagram shows that the initial value of the relative change  
Figure 11: Distribution of the single-aquifer model at different times. rate curve of the surface receiving point is 0. With increasing time, this value increases negatively, then increases positively, and finally decreases gradually. The relative change rate curve shows extreme positive and negative values. When the receiving point is located at the top interface of the aquifer (z = \(-100\) m), the initial value of the change rate curve is negative and remains unchanged for a certain period of time. After this, the change rate of the receiving point at the top interface of the aquifer increases positively and then decreases rapidly. There is a positive extrema, and the positive extrema are greater than the positive extrema of the surface receiving point. When the receiving point is located at the bottom interface of the aquifer (z = \(-200\) m), the initial value of the change rate curve is near \(-100\) and remains unchanged for a certain period of time. This change pattern is consistent with the shape of the receiving point at z = \(-100\) m, and the positive extreme value is greater than the positive extreme value recorded at the surface receiving point. When the receiving point is located at different depths below the aquifer, the change rate is constant in the early period of time, with a value near \(-100\). The change rate curve is consistent with the shape of the receiving point at z = \(-200\) m. As the distance from the aquifer increases, the positive peak gradually decreases and appears later. In summary, when considering a single aquifer, the positive peak value of the data received by the surface receiving point (z = 0 m) is larger than that of the underground receiving point (z = \(-300\) m) at the same distance from the aquifer. However, all the receiving points below the aquifer are affected by the aquifer in the early stage, and the change rates are consistent and all approximately equal to \(-100\). The above results show that the influence of the secondary field generated by the aquifer on the underground receiving point influences the system even during the early stage. Thus, the data received by the receiving point below the aquifer contain more aquifer information. Figure 12: Distribution of the single-aquifer model at different times.