Magnetic Sensing System for Potential Applications in Deep Earth Extremes for Long-Term Continuous Monitoring

Abstract

Long-term monitoring of weak changes in the magnetic field in the deep part of the Earth, allows to detect anomalies related to the pre-natural disaster stage at the onset of the deformation of the earth’s crust. However, standard electronic instruments are unable to work under high temperatures and high-pressure harsh environments for a long time. To address this problem, an in-well magnetic field measurement system, including a three-axis “residence times difference” fluxgate magnetometer, a heat exchange system with a coolant, and a control module, is designed. The measured maximum sensor sensitivity at room temperature is 0.02 s/(A/m). The rms input noise is within ±2 nT, corresponding to a noise spectral density of 200 pT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> @1 Hz, that is sufficient to accurately detect the fluctuations of the geomagnetic field. The feasibility of the system was verified in laboratory at four temperatures, from 120°C to 210°C, corresponding to 2000 m - 5000m subsurface depths. When, in the experiments performed at 210 °C, the magnetic field was changed of some hundreds of nT, the sensor outputs responded very well in the three directions, with a dynamic detectability of few tens of nT. The control module maintains the internal temperature in the range 25 °C - 40 °C when the external temperature varies between 120 °C and 210°C, allowing a good long-term stability, as demonstrated by aging tests in laboratory. Finally, field experiments were conducted to verify the engineering feasibility of the system for ten days at 100 m depth and 15~18 °C.