**6. Conclusions**

In order to improve the survey capability and accuracy of the TEM system, we have designed a low-noise differential amplifier for the TEM receiver by using the low-noise JFET IF3602 according to the power spectrum characteristics of the TEM forward response. Additionally, effectively increase the effective length and quality of the TEM observed data.

Firstly, based on the model of TEM forward response, we created the theoretical signal by homogeneous half-space. Secondly, we analyzed the power spectrum characteristics of the TEM signal to determine the requirements of the TEM amplifier noise floor and frequency bands. Thirdly, we designed a source-coupled differential amplifier circuit using the IF3602 low-noise JFET. We applied a current source to replace the resistance in the conventional long-tail circuit for providing stable current output for the left and right branches to simplify the JFET static working point settings. Besides, the parameters of the low-noise circuit were optimized combined with the theoretical model of the circuit and noise. Results of the noise test indicate that the designed circuit achieves good noise characteristics performance with a minimum of 0.60nV/√Hz ··· @10kHz, which is significantly lower than the performance of the corresponding frequency point of the LT1028 LNA. It can improve the ability of the system to distinguish TEM signals. Compared with the LT1028 LNA, the designed IF3602 low-noise circuit can increase the effective data points of the observed data by about 2.6 times and the theoretical detection depth by about 1.6 times under the same experimental conditions. In addition, under the condition of the same number of superpositions, the signal amplified by the IF3602 LNA is of high quality and smoother, which proves that reducing the noise floor of the preamplifier can not only improve the data quality but also greatly help improve the survey capability and detection efficiency.

Therefore, this study is of great significance for the design of preamplifier circuits in TEM receivers. In future work, we will continue to explore new circuit design schemes, such as parallel noise reduction design, to further improve the detection performance of the TEM receivers.

**Author Contributions:** S.W.: JFET amplifier design, experiment design, writing, discussion. Y.Z. (Yuqi Zhao): noise floor test, LNA circuit making, discussion. Y.S.: the PSD programing, discussion, English editing. W.W.: discussion, experiment assistance, English editing. J.C.: discussion, TEM data processing. Y.Z. (Yang Zhang): research guidance, editing, discussion. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Natural Science Foundation of China (Grant Nos. 41827803), Jilin University Youth Teacher Student Cross disciplinary Cultivation Project (2022-JCXK-32) and the Engineering Research Center of Geothermal Resources Development Technology and Equipment, Ministry of Education, Jilin University.

**Data Availability Statement:** Upon reasonable request, the data supporting this investigation are available from the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

