Construction and Experimental Verification of an Integrated Dual-Mode Overhauser Magnetometer for Marine Magnetism Survey

Magnetic field sensing is of great significance for marine geoscience research and industrial engineering. This article proposes a high-precision and dual-mode Overhauser marine magnetometer, which integrates base station monitoring mode and towed mobile survey mode. To accurately extract the frequency of weak free induction decay (FID) signals in complex marine environments, a precision narrowband resonant pickup circuit is adopted to enhance the signal, and a short-time precision frequency measuring method is presented. In addition, in terms of long-range underwater power supply and communication, we propose an interactive communication method for the Overhauser marine magnetometer based on low-voltage power line carrier, which not only provides power supply but also supports reliable duplex communication at 2.4 kbps baud rate within 3 km in the case of twin-core cable. The effectiveness and superiority of the developed Overhauser marine magnetometer were validated through extensive laboratory and field experiments. Results from a standard magnetic metrology laboratory indicate that the proposed magnetometer covers a field range from 20 to $100 \mu \text{T}$ , while the indicated value error is less than 0.09 nT and the noise is less than 0.06 nT. The geomagnetic observation experiment demonstrates the reliability and the advanced performance of the magnetometer under base station monitoring mode. Furthermore, the comparison results of the marine magnetism survey under towed mobile survey mode show that the agreement of the magnetometer and the commercial instrument installed on board is 0.95, which proves the feasibility and effectiveness of the proposed instrument for the marine magnetism survey.