Design of a borehole transient electromagnetic receiving device based on STM32

Due to the large size of high-precision receiving circuits, existing borehole transient electromagnetic detection mostly adopts a separated structure with a surface receiver and a downhole probe. The signal transmitted through cables is prone to early-time high-frequency distortion and couples with downhole noise, and the dynamic range for weak signal acquisition is insufficient, which limits detection accuracy. To address these issues, a borehole transient electromagnetic receiving device based on STM32 suitable for narrow drilling spaces was designed, realizing the integration of the receiving antenna and hardware circuit. Because the borehole space limited the effective area of the receiving coil, the late-time secondary field signal was susceptible to background noise interference, which imposed higher requirements on the low-noise design of the miniaturized hardware system. A roadway–borehole three-dimensional electromagnetic model was established using COMSOL Multiphysics, and the optimal number of turns of the receiving coil was determined as 50 through simulation. An analog–digital separation design was adopted, and an ultra-low-noise power supply scheme consisting of cascaded DC/DC and a Low Dropout Linear Regulator (LDO) was constructed. A multi-stage low-noise analog conditioning chain of "low-noise preamplification–low-pass filtering–differential driving–programmable gain" was designed using low-noise chips, effectively suppressing digital noise coupling. Based on FreeRTOS, synchronous sampling at 50 kHz with a 24 bit ADC and multitask scheduling were achieved. Prototype test results showed that the background noise of the device was 490 nV at a sampling rate of 50 kHz, and the dynamic range reached 138 dB. In a surface-simulated borehole experiment, the device successfully identified a banded low-resistivity anomaly at depths of 8-18 m, verifying its capability to capture weak secondary field signals.