# Electromagnetic Field Distribution and Data Characteristics of SUTEM of Multilayer Aquifers
## 2 Principle of SUTEM  
TEM, also known as the time domain electromagnetic method (TDEM), uses an ungrounded loop or grounded-wire source to send a primary pulse magnetic field to the ground. As shown in Figure 1a, the coil is supplied with current I during the time segment of t0-t1, and the transmitting loop generates a primary field. When the primary field disappears instantly, the induced eddy current excited by the underground geological body will produce a time-varying induced electromagnetic field (secondary field) [20], as shown in Figure 1b. During the intermittent period of the primary pulsed magnetic field, that is, t1-t2, the secondary fields can be observed by using the receiver loop or grounded electrodes. Through the extraction and analysis of these responses, the purpose of detecting underground geological bodies is achieved. In actual work, positive and negative square waves with a duty cycle of 50% are usually used for the power supply, and observation is performed only during the interval of the primary pulse magnetic field. Through multiple superposition and filtering, the random noise and periodic noise of the surrounding environment can be eliminated. In the TEM simulation, only the data during the interval of one pulse magnetic field are calculated. Since the transmitting current and the area of the receiving coil are normalized during the processing of the measured data, the numerical simulation is the data after the normalization of the current and the receiving coil.  
The difference between ground TEM and SUTEM is that the receiving point and the transmitting loop are not in the same horizontal plane, similar to semi-airborne TEM [21]. The difference between SUTEM and semi-airborne TEM is that the receiving points are located in the tunnel of the underground coal mining face. Due to the influence of the tunnel space, the quantity of observation data in SUTEM is much smaller than that of semi-airborne TEM. The construction layout of the SUTEM system is shown in Figure 2. The rectangular transmitting loop is arranged at a surface position corresponding to the coal mining working face to be detected, and the measuring points are arranged at the upper and lower tunnels of the working face.  
According to the Biot-Savart law [22], when a constant current is applied to the rectangular transmitting loop, a constant magnetic field is formed in the space around the transmitting coil. After the constant current is turned off, the primary field originally existing around the transmitting coil disappears rapidly. According to [PERSON]'s law of electromagnetic induction, the rock and ore at various locations underground will produce a secondary field to prevent this rapid disappearance of the primary field. Because the size of the primary field is inversely proportional to the square of the distance between the field point and the transmitting coil, the secondary field generated by the deep geological body at the initial time is much smaller than  
field is near the transmitting coil. With the passage of time, the secondary field spreads to the deep underground, and the secondary field value also decreases rapidly, i.e., the \"smoke ring effect\" [23]. When the secondary field contains deep geological structure information and propagates to the ground, the signal becomes very small. However, when the receiving point is located in the tunnel of the coal mining face, the secondary field as generated by the nearby geological body is mainly received in the early propagation stage. The secondary field received in the middle and later stages contains shallow and deeper geological information, respectively. Compared with ground TEM, SUTEM is more sensitive to the reflection of deep geological information, yet because the receiving point is underground, the volume effect is greater than that of the ground TEM.