## 2 Experimental Principles and Method
### Induced Polarization Experiment  
The most commonly used means of detection in the induced polarization method are the time-domain and frequency-domain methods. Time-domain measurement has the advantages of high efficiency and convenient operation in the field. Through the measurement of a charge and discharge process, all time-domain induced polarization observation data can be obtained, which can greatly reduce fieldwork time and may improve working efficiency. Therefore, the time-domain measurement method was used for the experimental study of induced polarization. In the device used for observation in the experiment, a four-electrode arrangement was used for measurement. The outer side had two current electrodes. In order to reduce the influence of polarization from the current and potential electrodes on the measurement results, the current electrode, A/B, used graphite electrodes, as shown in Figure 2a. There were two potential electrodes on the inner side. In order to avoid the polarization of the electrodes influencing the observation data,  
Figure 1: Sand of samples with different grain size grades. (**a**) 0.1–0.2 mm; (**b**) 0.2–0.5 mm; (**c**) 0.5–1 mm; (**d**) 1–2 mm; (**e**) 2–3 mm; (**f**) 3–4 mm; (**g**) 4–6 mm; (**h**) 6–8 mm.  
there were two potential electrodes, M/N, on the inner side, using Ag/AgCl nonpolarized electrodes, as shown in Figure 2b. The electrolyte used was saturated with a KCL solution, which was replaced regularly; the purpose of this was to prevent the electrolyte from failing due to long-term use and to minimize the influence of the electrode's polarization on the measurement result.  
The experiment transmitter used was a Pentium WDFZ-10T excited polarization transmitter. The signal transmitter was connected to the current electrode, which could transmit DC square wave signals with different duty cycles and different power supply durations, and measured the current transmitted signal. The receiver used was a Horn3D full-function IP instrument with a sampling frequency of up to 250 Hz, and it collected full-waveform time series data.  
The transmission and reception periods were both 64 s. In order to eliminate the influence of DC offset caused by a unidirectional power supply, each cycle used square wave pulses of opposite polarity to supply power, with a duty cycle of 50%; that is, the power supply duration was the same as the power-off duration. At the same time, in order to reduce accidental errors in data processing, multiple cycles of power supply and power-off measurements were performed; the power supply current signal of the transmitter is shown in Figure 3a.  
In terms of data collection, the receiver used a late synchronization method without manually synchronizing the transmitted signal. The obtained observation signal is shown in Figure 3b. It can be seen there that the secondary field voltage slowly decayed within a certain period of time after the power supply signal was turned off.  
A larger, outdoor, square experiment tank with a size of 100 cm \(\times\) 100 cm \(\times\) 100 cm was used, as shown in Figure 4. The side walls and bottom were made of cement. Data were collected in full sequence, and time-domain decay information was obtained.  
Figure 3: Signal waveform curve. (**a**) Schematic of current signal; (**b**) measured full-waveform signal.  
Figure 2: The current electrode and potential electrode. (**a**) Graphite current electrode; (**b**) Ag/AgCl nonpolarized potential electrode.