ULF (0.1-30 Hz) band magnetometer receiver was developed for detecting electromagnetic-wave precursors of earthquakes and volcanic eruption. The receiver attained a high sensitivity of about 1pT/√Hz (10Hz) for ground based observation of the flux anomaly. This paper describes the design and analysis of the induction coil sensor magnetometer. The nonlinear magnetization characteristics of the ferromagnetic Permalloy core are expressed by a negative feedback model in this paper. The sensor performances such as sensitivity are analyzed by the linear model. The sensor performances are measured by the effective cross-section area. The sensitivity calibration method and the test bench are shown by making use of the Helmholtz coil. The minimum detectable flux density is determined by the ambient flux noise density.
