**2. The Analysis of TEM Signal**

The TEM system consists of a transmitting coil, a transmitter, a receiving coil, and a receiver. Its working principle is shown in Figure 1. A bipolar step pulse current is sent through the transmitting coil by the transmitter. Meanwhile, a primary field is created, surrounded, and transmitted in the form of smoke rings. Due to Faraday's law of electromagnetic induction, geological bodies are excited by the primary field, and the eddy current is induced underground with the secondary field when the pulse is on. When the pulse is turned off, the secondary field starts decaying at different rates, which are related to the conductivity of the subsurface layers. The voltage in the receiving coil is generated by the secondary field and observed in the receiver. After TEM data interpretation, the abnormality and subsurface layers can be known [18].

To further analyze the characteristics of TEM signals, we use the TEM forward modeling to generate ideal signals for time-frequency analysis. Since the actual geological layering is very complex, it is difficult to fully cover the condition of multi-layer forward modeling. Because the TEM has the characteristics of a strong response in a low-resistance medium and a weak response in a high-resistance medium, the actual geological response will be higher than the pure high-resistance response and lower than the pure low-resistance response. Therefore, we set the high-resistance uniform half-space to 100 Ω·m and adjust the transmitting magnetic moment to 1 × 104 <sup>A</sup>·m2, and the receiving equivalent area to 128 m2 [19]. The forward modeling based on the sinusoidal numerical filtering algorithm is used to obtain the forward response within 1 ms, as shown in Figure 2 [20,21]. It is observed that the early TEM forward response is five orders of magnitude higher than the late response. Therefore, to identify the late signal, not only a high-amplification amplifier is required, but also the noise floor of the amplifier is limited; otherwise, it is extremely difficult to obtain useful information on TEM from the circuit noise. Since the noise power spectrum is mainly used to measure the noise characteristics of LNA, we calculate the power spectral density (PSD) of the forward response in 100 Ω·m pure high

resistance as a reference index for designing the LNA. The results in Figure 3 show that the low-frequency PSD of the TEM signal is larger and the high-frequency PSD is smaller and below 1nV/√Hz. Therefore, when designing the amplifier, it is necessary to ensure that the amplifier has a stable gain and an extremely low noise floor in the whole frequency band to obtain high-quality TEM data and lay the foundation for subsequent data denoising and data interpretation.

**Figure 1.** Schematic diagram of TEM survey.

**Figure 2.** The TEM forward response.

**Figure 3.** The PSD of TEM forward response.

## **3. Optimal Design of JFET LNA**
