A long baseline fluxgate gradiometer with electrical alignment and its calibration method

Summary form only given. In this paper, the developed fluxgate gradiometer probe and its calibration method are presented. Method Both the sensing and reference elements were composed of three toroidal core fluxgate magnetometers, which were orthogonally oriented to each other, and implemented in an aluminum straight pipe with a diameter and length of 40 mm and 1.24 m, respectively. The sensing element was at the head of the pipe, and the reference element was 1 m away from the sensing element. The excitation coils for six individual toroidal cores, made of permalloy, were connected in series. The core was excited by a DC-biased sinusoidal current at a frequency of 100 kHz, and the demodulation of the induced secondary voltage from the sensing coil was operated in the fundamental mode [2][3]. The demodulated signal from the fluxgate driving circuit was connected to flux locked loops (FLL), which are commonly used for superconducting quantum interference devices, to expand their dynamic range and achieve linearity [4]. The double-integrator type FLL was applied, and the frequency band of the FLL was adjusted to 0.1 Hz-10 Hz for the sensing element and to DC-10 Hz for the reference element. The probe and the electronics were connected by a 50-m-long waterproof multi-core cable. For calibration, an array of coils was prepared to emit a fundamental magnetic field. Three coils, each 30 mm in diameter, were orthogonally combined into one bobbin.