# Double Trapezoidal Wave Transmitting System with Controllable Turn-Off Edge
## 1 Introduction  
Time domain transient electromagnetic method (TEM) is a common mineral exploration method. It is widely used because of its high lateral resolution, sensitive response to low resistivity, and the ability to distinguish small underground heterogeneous bodies. In the measurement of the nonpolarized region, the transient electromagnetic signal is approximately a monotone attenuation e-exponential curve that contains the resistivity information of the earth [1, 2]. However, a negative sign often appears in the transient electromagnetic signal measured in the polarization region. In 1980, [PERSON] first interpreted the negative transient electromagnetic response of the data in TEM [3]. In 1982, [PERSON] theoretically proved that if the earth's resistivity does not have dispersion characteristics, the transient electromagnetic response during the off-time after excitation will not change sign at any time, which stimulates the study of the influence of induced polarization characteristics of the earth on the transient electromagnetic response [4]. The above research showed that the induced polarization effect is the key to the negative sign in TEM. Recently, more studies focused on the induced polarization effect in TEM measurement. [PERSON] discussed the influence of the induced polarization effect on TEM inversion through numerical simulation experiments [5]. [PERSON] developed a procedure to invert time domain induced polarization (IP) data for inductive sources [6]. The Cole-Cole model proposed by [PERSON] is considered to be an effective equivalent model for describing the induced polarization effect of the earth, and the extended version, i.e., the filament model, proved useful in the analysis of physical processes [7]. The circuit of the filament model is shown in Figure 1, where the energy storage lies in a simple capacitor. [PERSON] discussed the excitation of transient induced polarization responses using current and voltage sources and proposed that the induced polarization effect of the earth should be observed using an ungrounded loop [8]. [PERSON] reported TEM measurements employing a LT SQUID system as a B-sensor in a polarizable and resistive survey area and proposed that the flux changes imposed in the ground by switching off the primary field in TEM can be considered as a pure voltage source (emf); the sign reversal in TEM is due to the discharging of in-ground capacitance (C) [9]. The turn-off edge of an emission current in an ungrounded loop directly affects the measurement result of the induced polarization effect. The conventional transmitting system of TEM measurement is currently used to measure the induced polarization effect [10; 11]. TEM is based on step pulse electromagnetic excitation. The transmitting system tends to reduce the turn-off time to enhance the induced response. There are many methods to realize fast turn-off times such as RCD circuits and voltage clamps [12; 13]. According to the law of electromagnetic induction, the turn-off rate of an emission current, the change rate of a magnetic field, and the voltage of a voltage source (emf) are positively proportional. The voltage at both ends of the capacitor (C) increases continuously during the current turn-off period. Extending the turn-off time of the emission current means increasing the charging time of the capacitance and reducing the charging voltage at the same time. The energy in capacitance is the product of voltage and duration that provides charging. Turn-off time can compensate for the influence of charging voltage weakening the polarization effect. We propose that extending the turn-off time weakens the excitation of the induced field. As a result, the polarization effect can be measured more easily under this balance. In this study, we designed the double trapezoidal wave transmitting system with a controllable turn-off edge for the induced polarization effect in TEM measurement. The system can change the turn-off time by changing the clamp voltage during the turn-off process of the emission current.